WEBVTT 00:00:00.000 --> 00:00:02.640 align:middle line:84% The following content is provided under a Creative 00:00:02.640 --> 00:00:04.000 align:middle line:90% Commons license. 00:00:04.000 --> 00:00:06.400 align:middle line:84% Your support will help MIT OpenCourseWare 00:00:06.400 --> 00:00:10.080 align:middle line:84% continue to offer high quality educational resources for free. 00:00:10.080 --> 00:00:13.320 align:middle line:84% To make a donation or view additional materials 00:00:13.320 --> 00:00:17.280 align:middle line:84% from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:17.280 --> 00:00:21.000 align:middle line:90% at ocw.mit.edu. 00:00:21.000 --> 00:00:22.660 align:middle line:84% PROFESSOR: So for the next two days, 00:00:22.660 --> 00:00:25.680 align:middle line:84% I'm going to talk about marine sediments, which 00:00:25.680 --> 00:00:27.680 align:middle line:90% is the field that I work in. 00:00:27.680 --> 00:00:33.680 align:middle line:84% And I think of it as a forgotten corner of chemical oceanography. 00:00:33.680 --> 00:00:37.640 align:middle line:84% And so I start out these two lectures admitting right off 00:00:37.640 --> 00:00:39.160 align:middle line:84% that I'm not going to make anybody 00:00:39.160 --> 00:00:41.620 align:middle line:84% a sediment geochemist in two lectures. 00:00:41.620 --> 00:00:43.840 align:middle line:90% So I have two real simple goals. 00:00:43.840 --> 00:00:47.200 align:middle line:84% The first one is to make sure you 00:00:47.200 --> 00:00:50.920 align:middle line:84% remember that the ocean does have a bottom-- 00:00:50.920 --> 00:00:55.560 align:middle line:84% JGOFS aside, where they eliminated the sea floor-- 00:00:55.560 --> 00:00:57.560 align:middle line:84% there really is a bottom to the ocean. 00:00:57.560 --> 00:01:02.553 align:middle line:84% And second is that it has some important effects, 00:01:02.553 --> 00:01:04.470 align:middle line:84% that reactions in the surface of the sediments 00:01:04.470 --> 00:01:05.650 align:middle line:90% have some important effects. 00:01:05.650 --> 00:01:07.710 align:middle line:84% And I'd like you to look at surface sediments 00:01:07.710 --> 00:01:10.925 align:middle line:84% in the same way you look at the air/sea boundary 00:01:10.925 --> 00:01:12.550 align:middle line:84% as the interface between the atmosphere 00:01:12.550 --> 00:01:15.410 align:middle line:84% is the ocean, same way as you look at estuaries 00:01:15.410 --> 00:01:17.870 align:middle line:84% as the interface between rivers and the ocean, 00:01:17.870 --> 00:01:20.710 align:middle line:84% or as hydrothermal vent systems as the interface 00:01:20.710 --> 00:01:22.590 align:middle line:84% between the deep Earth and the ocean. 00:01:22.590 --> 00:01:24.510 align:middle line:84% The sediments are the interface between-- 00:01:24.510 --> 00:01:27.510 align:middle line:84% the sediment-water interface is the interface between the water 00:01:27.510 --> 00:01:30.890 align:middle line:84% column and the accumulating sediments. 00:01:30.890 --> 00:01:34.530 align:middle line:84% So with that in mind I'm going to ask, 00:01:34.530 --> 00:01:36.330 align:middle line:84% why would we study this part of the ocean? 00:01:36.330 --> 00:01:38.122 align:middle line:84% After all, the ocean is a huge thing that's 00:01:38.122 --> 00:01:39.750 align:middle line:90% several kilometers thick. 00:01:39.750 --> 00:01:44.430 align:middle line:84% And the region that I focus on is about 10 to 20 centimeters 00:01:44.430 --> 00:01:45.010 align:middle line:90% thick. 00:01:45.010 --> 00:01:46.990 align:middle line:90% So it's a very small piece. 00:01:46.990 --> 00:01:49.030 align:middle line:90% But it does have some-- 00:01:49.030 --> 00:01:50.610 align:middle line:84% the reactions there are important. 00:01:50.610 --> 00:01:53.290 align:middle line:84% And if we look at it in the way I've shown here, 00:01:53.290 --> 00:01:54.750 align:middle line:84% where the inputs for the reactions 00:01:54.750 --> 00:01:57.030 align:middle line:84% are the particle flux to the sea floor, 00:01:57.030 --> 00:01:59.050 align:middle line:84% you can think of the surface of the sediments 00:01:59.050 --> 00:02:02.370 align:middle line:84% as being a layer in which reactions occur. 00:02:02.370 --> 00:02:03.810 align:middle line:84% Those reactions can have a couple 00:02:03.810 --> 00:02:05.570 align:middle line:90% of important consequences. 00:02:05.570 --> 00:02:08.970 align:middle line:84% The first is that they both use up 00:02:08.970 --> 00:02:11.410 align:middle line:84% solutes that are present in bottom water. 00:02:11.410 --> 00:02:15.210 align:middle line:84% And they produce solutes that can be returned to bottom water. 00:02:15.210 --> 00:02:17.450 align:middle line:84% So that solute exchange between the surface 00:02:17.450 --> 00:02:21.550 align:middle line:84% of the sediments and the deep ocean, and the water column, 00:02:21.550 --> 00:02:24.010 align:middle line:84% be it deep or coastal ocean, can have important effects 00:02:24.010 --> 00:02:26.490 align:middle line:90% on ocean chemistry. 00:02:26.490 --> 00:02:29.130 align:middle line:84% The second way it matters is that, say 00:02:29.130 --> 00:02:31.087 align:middle line:84% you're a paleoceanographer, and you're 00:02:31.087 --> 00:02:33.170 align:middle line:84% interested in studying the record that's preserved 00:02:33.170 --> 00:02:34.970 align:middle line:90% in accumulating sediments. 00:02:34.970 --> 00:02:37.770 align:middle line:84% Well, then the particle flux to the sea floor 00:02:37.770 --> 00:02:41.330 align:middle line:84% has to pass through this layer where lots of reactions 00:02:41.330 --> 00:02:45.090 align:middle line:84% occur before those particles become part 00:02:45.090 --> 00:02:46.890 align:middle line:90% of the accumulating sediments. 00:02:46.890 --> 00:02:49.090 align:middle line:84% And as we'll see, the composition 00:02:49.090 --> 00:02:51.050 align:middle line:84% of accumulating sediments is very 00:02:51.050 --> 00:02:53.490 align:middle line:84% different from the composition of the particle flux 00:02:53.490 --> 00:02:54.745 align:middle line:90% to the sea floor. 00:02:54.745 --> 00:02:56.370 align:middle line:84% So if you're interested in interpreting 00:02:56.370 --> 00:02:59.280 align:middle line:84% the sedimentary record, it's important 00:02:59.280 --> 00:03:02.320 align:middle line:84% not to forget that there's this layer at the sediment surface 00:03:02.320 --> 00:03:04.140 align:middle line:84% that the particles have to pass through. 00:03:04.140 --> 00:03:08.756 align:middle line:90% 00:03:08.756 --> 00:03:14.000 align:middle line:84% So with that little preamble in mind, we're going to start-- 00:03:14.000 --> 00:03:16.080 align:middle line:84% which I hope ties in with what you've 00:03:16.080 --> 00:03:20.360 align:middle line:84% been doing up to this point-- with a brief discussion 00:03:20.360 --> 00:03:23.960 align:middle line:84% of the composition of the particle flux to the sea floor. 00:03:23.960 --> 00:03:27.580 align:middle line:84% After that, we'll talk about the preservation-- 00:03:27.580 --> 00:03:29.680 align:middle line:84% we're going to focus on the biogenic components 00:03:29.680 --> 00:03:31.200 align:middle line:90% of the particle flux. 00:03:31.200 --> 00:03:34.120 align:middle line:84% After that, we'll talk about the preservation rates 00:03:34.120 --> 00:03:38.840 align:middle line:84% in a general way of biogenic components of the particle flux. 00:03:38.840 --> 00:03:40.680 align:middle line:84% Once we've done that, I'll remind you 00:03:40.680 --> 00:03:43.600 align:middle line:84% of a couple of the consequences of early diagenesis, which 00:03:43.600 --> 00:03:45.800 align:middle line:84% we're not going to talk about in any detail, 00:03:45.800 --> 00:03:50.960 align:middle line:84% but I stick them in here as a motivation for the things 00:03:50.960 --> 00:03:52.640 align:middle line:90% I'm going to talk about later. 00:03:52.640 --> 00:03:54.740 align:middle line:84% And after we get through those preambles, 00:03:54.740 --> 00:03:57.240 align:middle line:84% we're going to get a little bit more specific, 00:03:57.240 --> 00:04:00.740 align:middle line:84% and I'll talk in turn about reactions 00:04:00.740 --> 00:04:03.820 align:middle line:84% that organic matter undergoes in the surface sediments. 00:04:03.820 --> 00:04:05.580 align:middle line:84% That will take up the rest of today. 00:04:05.580 --> 00:04:07.620 align:middle line:84% Then in the lecture on Tuesday, I'll 00:04:07.620 --> 00:04:11.620 align:middle line:84% talk about calcium carbonate and biogenic silica, the other two 00:04:11.620 --> 00:04:14.123 align:middle line:84% biogenic components of the particle flux. 00:04:14.123 --> 00:04:15.540 align:middle line:84% And for each of those, we'll first 00:04:15.540 --> 00:04:19.860 align:middle line:84% talk about mechanism for decomposition or dissolution 00:04:19.860 --> 00:04:20.980 align:middle line:90% of the phase. 00:04:20.980 --> 00:04:26.060 align:middle line:84% And then we'll talk about what sort of measurements 00:04:26.060 --> 00:04:27.980 align:middle line:84% we make to quantify the reactions 00:04:27.980 --> 00:04:31.640 align:middle line:84% and to support our descriptions of the mechanisms. 00:04:31.640 --> 00:04:34.480 align:middle line:90% 00:04:34.480 --> 00:04:37.180 align:middle line:84% So first, the particle flux to the sea floor. 00:04:37.180 --> 00:04:42.900 align:middle line:84% And this is just a very brief overview. 00:04:42.900 --> 00:04:46.420 align:middle line:84% On the left-hand side, these are all-- on the left-hand side 00:04:46.420 --> 00:04:51.060 align:middle line:84% are fluxes calculated from sediment trap data 00:04:51.060 --> 00:04:54.330 align:middle line:84% in milligrams per meter squared per day. 00:04:54.330 --> 00:04:58.730 align:middle line:84% On the very left-hand side is the total mass flux. 00:04:58.730 --> 00:05:01.170 align:middle line:84% And this range of total mass fluxes 00:05:01.170 --> 00:05:06.610 align:middle line:84% from about 50 to 250 milligrams of material 00:05:06.610 --> 00:05:08.770 align:middle line:84% per square meter of sediment surface per day 00:05:08.770 --> 00:05:10.370 align:middle line:84% is about representative of what you'll 00:05:10.370 --> 00:05:14.016 align:middle line:90% see over most of the ocean. 00:05:14.016 --> 00:05:17.530 align:middle line:84% To the right of that, the flux is broken down 00:05:17.530 --> 00:05:18.670 align:middle line:90% into four components. 00:05:18.670 --> 00:05:21.150 align:middle line:84% The first three, calcium carbonate, organic matter, 00:05:21.150 --> 00:05:24.450 align:middle line:84% and opal are the biogenic components of the particle flux. 00:05:24.450 --> 00:05:27.810 align:middle line:84% And the fourth one on the far right-hand side in this picture 00:05:27.810 --> 00:05:30.090 align:middle line:90% is the lithogenic component. 00:05:30.090 --> 00:05:33.170 align:middle line:84% So that's the terrestrial material, 00:05:33.170 --> 00:05:37.290 align:middle line:84% which has a lithogenic origin, that falls to the sea floor. 00:05:37.290 --> 00:05:40.050 align:middle line:84% I want you to notice a couple of things. 00:05:40.050 --> 00:05:45.065 align:middle line:84% First, in the Atlantic Ocean, which is the top-- well, 00:05:45.065 --> 00:05:47.690 align:middle line:84% before I get into that, on the right-hand side is the same data 00:05:47.690 --> 00:05:50.910 align:middle line:84% but plotted in composition terms. 00:05:50.910 --> 00:05:54.070 align:middle line:84% So it as weight percent of the different components 00:05:54.070 --> 00:05:56.950 align:middle line:90% of the flux. 00:05:56.950 --> 00:06:01.910 align:middle line:84% So if we look now at the right-hand side, the thing-- 00:06:01.910 --> 00:06:04.030 align:middle line:84% first, if you compare the Atlantic Ocean 00:06:04.030 --> 00:06:06.130 align:middle line:84% to the central equatorial Pacific Ocean, 00:06:06.130 --> 00:06:09.327 align:middle line:84% you can see that the lithogenic component of the flux 00:06:09.327 --> 00:06:10.910 align:middle line:84% is much more important in the Atlantic 00:06:10.910 --> 00:06:13.630 align:middle line:84% Ocean than the Pacific, which is what you'd expect, 00:06:13.630 --> 00:06:16.350 align:middle line:84% because the central equatorial Pacific is far removed 00:06:16.350 --> 00:06:20.190 align:middle line:84% from any continental sources of material. 00:06:20.190 --> 00:06:24.190 align:middle line:84% Again, looking at the lithogenic component, 00:06:24.190 --> 00:06:27.650 align:middle line:84% you'll see that this one marked eastern equatorial Atlantic, 00:06:27.650 --> 00:06:30.242 align:middle line:84% the lithogenic component is considerably bigger than the one 00:06:30.242 --> 00:06:31.450 align:middle line:90% that's marked North Atlantic. 00:06:31.450 --> 00:06:36.150 align:middle line:84% That's because this eastern equatorial Atlantic site is just 00:06:36.150 --> 00:06:38.470 align:middle line:90% west of the Sahara desert. 00:06:38.470 --> 00:06:42.990 align:middle line:84% And so there is a lot of input of Saharan dust at this site. 00:06:42.990 --> 00:06:45.950 align:middle line:84% And so the general take-home message 00:06:45.950 --> 00:06:47.695 align:middle line:84% is that lithogenic components of the flux 00:06:47.695 --> 00:06:49.070 align:middle line:84% are more important the closer you 00:06:49.070 --> 00:06:52.460 align:middle line:84% are to the continental sources of the lithogenic material. 00:06:52.460 --> 00:06:54.980 align:middle line:84% The second thing to notice from all of this 00:06:54.980 --> 00:06:57.940 align:middle line:84% is that in the deep ocean, the majority 00:06:57.940 --> 00:07:00.260 align:middle line:84% of the flux of particles to the seafloor 00:07:00.260 --> 00:07:03.100 align:middle line:84% is made up of biogenic components of the flux. 00:07:03.100 --> 00:07:09.100 align:middle line:84% And on a mass basis, the most important component 00:07:09.100 --> 00:07:14.380 align:middle line:84% of the biogenic flux is calcium carbonate. 00:07:14.380 --> 00:07:17.060 align:middle line:84% Where calcium carbonate in these three examples 00:07:17.060 --> 00:07:22.620 align:middle line:84% makes up 50% to 60% of the mass flux to the seafloor, 00:07:22.620 --> 00:07:25.320 align:middle line:84% organic matter and biogenic silica are opal. 00:07:25.320 --> 00:07:29.240 align:middle line:84% Each make up about 10%, or a little bit more. 00:07:29.240 --> 00:07:35.660 align:middle line:90% 00:07:35.660 --> 00:07:39.460 align:middle line:84% But I want to show you one more set of data. 00:07:39.460 --> 00:07:42.380 align:middle line:84% And the reason to show you this is so that you don't go away 00:07:42.380 --> 00:07:44.240 align:middle line:84% thinking that everywhere in the ocean, 00:07:44.240 --> 00:07:46.060 align:middle line:84% calcium carbonate is the dominant component 00:07:46.060 --> 00:07:48.180 align:middle line:84% of the particle flux to the seafloor. 00:07:48.180 --> 00:07:52.170 align:middle line:84% And the counterexample is in the Southern Ocean, 00:07:52.170 --> 00:07:53.730 align:middle line:84% where you may have studied already 00:07:53.730 --> 00:07:56.290 align:middle line:84% that south of the polar front, going 00:07:56.290 --> 00:07:59.890 align:middle line:84% towards the Antarctic continent, surface 00:07:59.890 --> 00:08:03.070 align:middle line:84% ocean productivity is dominated by diatom productivity. 00:08:03.070 --> 00:08:07.970 align:middle line:84% And as a result of that, close to the Antarctic continent, 00:08:07.970 --> 00:08:09.690 align:middle line:84% the flux to the sea floor is dominated 00:08:09.690 --> 00:08:12.850 align:middle line:84% not by calcium carbonate, but by biogenic silica. 00:08:12.850 --> 00:08:15.610 align:middle line:84% And what I've shown here are some data 00:08:15.610 --> 00:08:20.630 align:middle line:84% collected by Sus Honjo on the AESOPS transect, 00:08:20.630 --> 00:08:23.570 align:middle line:84% which went along 170 degrees west 00:08:23.570 --> 00:08:27.250 align:middle line:84% from the Antarctic continent, across the polar front, 00:08:27.250 --> 00:08:30.330 align:middle line:84% and north into the part of the Southern 00:08:30.330 --> 00:08:32.210 align:middle line:84% Ocean that's north of the polar front. 00:08:32.210 --> 00:08:37.610 align:middle line:84% And this site, MS-5 here, corresponds 00:08:37.610 --> 00:08:40.070 align:middle line:84% to this left-hand point on the graph. 00:08:40.070 --> 00:08:41.450 align:middle line:90% It's south of the polar front. 00:08:41.450 --> 00:08:43.289 align:middle line:84% And you can see that in this graph, which 00:08:43.289 --> 00:08:48.170 align:middle line:84% shows the components of the particle flux to the sea floor, 00:08:48.170 --> 00:08:50.810 align:middle line:84% it's greatly dominated by biogenic silica. 00:08:50.810 --> 00:08:53.510 align:middle line:84% But as you move further north, and eventually cross 00:08:53.510 --> 00:08:56.070 align:middle line:84% the polar front, the biogenic silica 00:08:56.070 --> 00:08:58.890 align:middle line:84% is essentially displaced by calcium carbonate. 00:08:58.890 --> 00:09:01.470 align:middle line:84% And calcium carbonate becomes the dominant component, 00:09:01.470 --> 00:09:04.190 align:middle line:84% as it is over much of the sea floor. 00:09:04.190 --> 00:09:07.550 align:middle line:84% So the main messages from this are 00:09:07.550 --> 00:09:13.110 align:middle line:84% that in the deep ocean, away from the continents, 00:09:13.110 --> 00:09:15.350 align:middle line:84% the majority of the flux to the seafloor 00:09:15.350 --> 00:09:17.390 align:middle line:84% is made up of biogenic materials. 00:09:17.390 --> 00:09:20.550 align:middle line:84% And that over much of the ocean, with the exception 00:09:20.550 --> 00:09:23.150 align:middle line:84% of the Southern Ocean, the dominant component 00:09:23.150 --> 00:09:26.230 align:middle line:84% of the biogenic flux to the seafloor on a mass basis 00:09:26.230 --> 00:09:29.090 align:middle line:90% is calcium carbonate. 00:09:29.090 --> 00:09:33.830 align:middle line:84% So I'm just going to summarize here the preservation 00:09:33.830 --> 00:09:35.290 align:middle line:90% rates of biogenic components. 00:09:35.290 --> 00:09:38.790 align:middle line:84% So if you go around and measure the accumulation 00:09:38.790 --> 00:09:41.490 align:middle line:84% rate of biogenic components in sediments, 00:09:41.490 --> 00:09:45.270 align:middle line:84% and compare that to the rain rate to the seafloor and divide 00:09:45.270 --> 00:09:46.550 align:middle line:90% the-- 00:09:46.550 --> 00:09:49.740 align:middle line:84% well, then you can use those two measurements to calculate 00:09:49.740 --> 00:09:52.700 align:middle line:84% what fraction of the flux to the sea floor for a given component 00:09:52.700 --> 00:09:55.220 align:middle line:84% is recycled, and what fraction is actually 00:09:55.220 --> 00:09:56.720 align:middle line:90% buried in the sediments. 00:09:56.720 --> 00:10:03.180 align:middle line:84% And what you find is that for organic matter in the deep sea, 00:10:03.180 --> 00:10:08.140 align:middle line:84% preservation rates are always less than 3%, and in many cases, 00:10:08.140 --> 00:10:09.620 align:middle line:90% are 1%. 00:10:09.620 --> 00:10:12.740 align:middle line:84% So the organic matter that falls to the sea floor 00:10:12.740 --> 00:10:15.560 align:middle line:90% is almost all remineralized. 00:10:15.560 --> 00:10:19.820 align:middle line:84% It's almost all recycled, with the carbon returned to the water 00:10:19.820 --> 00:10:21.020 align:middle line:90% column as CO2. 00:10:21.020 --> 00:10:25.180 align:middle line:90% 00:10:25.180 --> 00:10:28.820 align:middle line:84% Sticking with organic matter for a minute, 00:10:28.820 --> 00:10:32.820 align:middle line:84% the fraction recycled can become somewhat smaller 00:10:32.820 --> 00:10:35.380 align:middle line:84% on continent and continental margin sediments, 00:10:35.380 --> 00:10:38.540 align:middle line:84% where the burial rates are faster. 00:10:38.540 --> 00:10:45.300 align:middle line:84% But in most instances, the recycling rates still 00:10:45.300 --> 00:10:48.700 align:middle line:84% are up in the 90% range, but in some cases, 00:10:48.700 --> 00:10:51.920 align:middle line:84% which are pretty rare, they can be less than-- the recycling 00:10:51.920 --> 00:10:54.000 align:middle line:90% rate can be less than 50%. 00:10:54.000 --> 00:10:56.058 align:middle line:84% But the overall message still stays the same, 00:10:56.058 --> 00:10:57.600 align:middle line:84% is that almost all the organic matter 00:10:57.600 --> 00:11:00.480 align:middle line:84% that falls to the seafloor is recycled rather than being 00:11:00.480 --> 00:11:02.360 align:middle line:90% preserved in the sediments. 00:11:02.360 --> 00:11:06.400 align:middle line:84% For calcium carbonate, as we'll talk about on Tuesday, 00:11:06.400 --> 00:11:08.780 align:middle line:84% the picture is a little more complicated, 00:11:08.780 --> 00:11:14.320 align:middle line:84% where there's a depth in the ocean above which 00:11:14.320 --> 00:11:16.780 align:middle line:84% the predominant calcium carbonate mineral in the ocean, 00:11:16.780 --> 00:11:18.920 align:middle line:90% which is calcite, is preserved. 00:11:18.920 --> 00:11:23.360 align:middle line:84% And below that depth, the water is undersaturated with respect 00:11:23.360 --> 00:11:24.340 align:middle line:90% to calcite. 00:11:24.340 --> 00:11:26.080 align:middle line:84% And at those depths, calcium carbonate 00:11:26.080 --> 00:11:28.640 align:middle line:90% tends to be remineralized. 00:11:28.640 --> 00:11:31.640 align:middle line:84% And that depth is called the calcite saturation horizon. 00:11:31.640 --> 00:11:33.900 align:middle line:84% So above the calcite saturation horizon, 00:11:33.900 --> 00:11:36.720 align:middle line:84% preservation rates of calcium carbonate in the sediment 00:11:36.720 --> 00:11:40.440 align:middle line:84% range from about 30% to 90% of the flux of calcium carbonate 00:11:40.440 --> 00:11:41.800 align:middle line:90% to the seafloor. 00:11:41.800 --> 00:11:45.060 align:middle line:84% Below the calcite saturation horizon, as you might expect, 00:11:45.060 --> 00:11:48.990 align:middle line:84% the preservation rate drops to zero. 00:11:48.990 --> 00:11:50.770 align:middle line:84% So that's a picture for calcium carbonate. 00:11:50.770 --> 00:11:55.350 align:middle line:84% For biogenic silica, the median preservation rate 00:11:55.350 --> 00:11:58.070 align:middle line:84% throughout the oceans-- so if you compare the burial 00:11:58.070 --> 00:12:02.390 align:middle line:84% rate to the flux to the sea floor-- is only about 10%. 00:12:02.390 --> 00:12:05.250 align:middle line:84% In 80% of the areas where mass balances have been made, 00:12:05.250 --> 00:12:08.470 align:middle line:84% the preservation rate is less than 15%. 00:12:08.470 --> 00:12:12.990 align:middle line:84% And so throughout the oceans, the preservation rate 00:12:12.990 --> 00:12:15.190 align:middle line:90% of biogenic silica is very low. 00:12:15.190 --> 00:12:19.030 align:middle line:84% And so the recycling rate is very high. 00:12:19.030 --> 00:12:22.870 align:middle line:84% And so the overall message here is 00:12:22.870 --> 00:12:26.390 align:middle line:84% that the biogenic components of the flux to the sea floor 00:12:26.390 --> 00:12:30.590 align:middle line:84% are, to a large extent, recycled back. 00:12:30.590 --> 00:12:33.110 align:middle line:84% So only a relatively small fraction 00:12:33.110 --> 00:12:36.230 align:middle line:84% of the biogenic flux of particles to the sea floor 00:12:36.230 --> 00:12:39.470 align:middle line:90% is preserved in the sediments. 00:12:39.470 --> 00:12:43.430 align:middle line:84% Now, I have one specific example, 00:12:43.430 --> 00:12:45.210 align:middle line:84% just to show the sorts of measurements 00:12:45.210 --> 00:12:48.170 align:middle line:84% that are made to make these mass balance calculations. 00:12:48.170 --> 00:12:50.010 align:middle line:84% And this is from a study that was 00:12:50.010 --> 00:12:53.930 align:middle line:84% part of the JGOFS project in the central equatorial Pacific. 00:12:53.930 --> 00:12:57.850 align:middle line:84% And as part of this study, they measured sediment accumulation 00:12:57.850 --> 00:13:02.610 align:middle line:84% rates and the accumulation rate of biogenic components 00:13:02.610 --> 00:13:04.490 align:middle line:90% of the sediments. 00:13:04.490 --> 00:13:07.530 align:middle line:84% They also measured remineralization rates 00:13:07.530 --> 00:13:12.090 align:middle line:84% by measuring fluxes of dissolved reactants and products 00:13:12.090 --> 00:13:14.570 align:middle line:84% of decomposition and dissolution reactions 00:13:14.570 --> 00:13:16.850 align:middle line:84% across the sediment-water interface. 00:13:16.850 --> 00:13:19.890 align:middle line:84% So for instance, if calcium carbonate is dissolved, 00:13:19.890 --> 00:13:22.530 align:middle line:84% that results in a flux of CO2 out of the sediments 00:13:22.530 --> 00:13:23.670 align:middle line:90% into the water column. 00:13:23.670 --> 00:13:25.410 align:middle line:84% So if you measure that flux, you can 00:13:25.410 --> 00:13:28.610 align:middle line:84% calculate the rate of recycling of the calcium carbonate 00:13:28.610 --> 00:13:29.490 align:middle line:90% at the seafloor. 00:13:29.490 --> 00:13:32.130 align:middle line:84% And the same sorts of arguments go for biogenic silica 00:13:32.130 --> 00:13:33.410 align:middle line:90% and organic matter. 00:13:33.410 --> 00:13:36.830 align:middle line:84% So you can measure burial, remineralization rate, 00:13:36.830 --> 00:13:38.970 align:middle line:84% and they also measured the rain rates of particles 00:13:38.970 --> 00:13:42.170 align:middle line:84% to the seafloor using sediment traps. 00:13:42.170 --> 00:13:45.480 align:middle line:84% So if the system's in steady state, where 00:13:45.480 --> 00:13:48.040 align:middle line:84% the input to the sea floor equals 00:13:48.040 --> 00:13:52.040 align:middle line:84% the sum of the remineralization rate and the burial rate, 00:13:52.040 --> 00:13:54.920 align:middle line:84% then the remineralization rate plus the burial rate 00:13:54.920 --> 00:13:57.592 align:middle line:84% should be equal to the rain rate to the sea floor, 00:13:57.592 --> 00:13:59.800 align:middle line:84% because only when that's true will the concentrations 00:13:59.800 --> 00:14:01.633 align:middle line:84% in the sediments-- or the accumulation rates 00:14:01.633 --> 00:14:03.160 align:middle line:90% in the sediments be constant. 00:14:03.160 --> 00:14:08.560 align:middle line:84% AUDIENCE: So how do you measure like a carbonate flux 00:14:08.560 --> 00:14:10.040 align:middle line:90% in the sediments? 00:14:10.040 --> 00:14:10.560 align:middle line:90% Just-- 00:14:10.560 --> 00:14:12.300 align:middle line:90% PROFESSOR: Which flux? 00:14:12.300 --> 00:14:13.280 align:middle line:90% The burial rate or-- 00:14:13.280 --> 00:14:15.560 align:middle line:84% AUDIENCE: Now, how do you-- the remineralization rate. 00:14:15.560 --> 00:14:18.420 align:middle line:84% PROFESSOR: It's a little bit complicated. 00:14:18.420 --> 00:14:20.638 align:middle line:84% What you would do is measure the-- 00:14:20.638 --> 00:14:22.180 align:middle line:84% there's a few things you can measure. 00:14:22.180 --> 00:14:23.620 align:middle line:84% We'll actually talk about this more later. 00:14:23.620 --> 00:14:23.960 align:middle line:90% AUDIENCE: Oh, OK. 00:14:23.960 --> 00:14:24.627 align:middle line:90% No, that's fine. 00:14:24.627 --> 00:14:28.320 align:middle line:84% PROFESSOR: But just when calcium carbonate dissolves, 00:14:28.320 --> 00:14:31.640 align:middle line:84% it releases total CO2 into the water and alkalinity 00:14:31.640 --> 00:14:32.720 align:middle line:90% into the water. 00:14:32.720 --> 00:14:36.400 align:middle line:84% You can measure the flux of those two across the interface 00:14:36.400 --> 00:14:39.680 align:middle line:84% and calculate the dissolution rate of calcium carbonate 00:14:39.680 --> 00:14:40.720 align:middle line:90% in the sediments. 00:14:40.720 --> 00:14:43.130 align:middle line:84% We'll talk about that more on Tuesday. 00:14:43.130 --> 00:14:46.796 align:middle line:90% 00:14:46.796 --> 00:14:50.150 align:middle line:84% And so these guys, this group of investigators, 00:14:50.150 --> 00:14:52.870 align:middle line:84% measured burial rates for organic carbon 00:14:52.870 --> 00:14:57.470 align:middle line:84% in the black, calcium carbonate in the hatched bars, 00:14:57.470 --> 00:15:00.390 align:middle line:84% and opal in the-- or biogenic silica 00:15:00.390 --> 00:15:04.190 align:middle line:90% in the unfilled bars here. 00:15:04.190 --> 00:15:06.430 align:middle line:84% For burial rates, the burial rate of organic carbon 00:15:06.430 --> 00:15:09.030 align:middle line:84% was so small that it doesn't show up on this scale. 00:15:09.030 --> 00:15:12.670 align:middle line:84% And so what they did was they had separate measurements 00:15:12.670 --> 00:15:14.870 align:middle line:84% of burial, remineralization, and rain, 00:15:14.870 --> 00:15:18.750 align:middle line:84% and they added together the burial and the remineralization 00:15:18.750 --> 00:15:22.110 align:middle line:84% rate, and said in steady state, that should equal the rain rate. 00:15:22.110 --> 00:15:24.590 align:middle line:84% And what they found was a balance 00:15:24.590 --> 00:15:28.270 align:middle line:84% for organic carbon and biogenic silica. 00:15:28.270 --> 00:15:30.970 align:middle line:84% And from this, they conclude that-- what I said before, 00:15:30.970 --> 00:15:33.910 align:middle line:84% which is that these components of the particles' flux 00:15:33.910 --> 00:15:36.710 align:middle line:84% are very nearly totally remineralized in the surface 00:15:36.710 --> 00:15:37.890 align:middle line:90% sediments is true. 00:15:37.890 --> 00:15:39.430 align:middle line:90% It held at this site. 00:15:39.430 --> 00:15:44.330 align:middle line:84% And for calcium carbonate, they actually measured a larger flux 00:15:44.330 --> 00:15:47.130 align:middle line:84% from dissolution of calcium carbonate 00:15:47.130 --> 00:15:49.530 align:middle line:84% than they measured a rain rate of calcium carbonate 00:15:49.530 --> 00:15:52.290 align:middle line:90% to the sea floor. 00:15:52.290 --> 00:15:54.390 align:middle line:84% You can say two things from this, 00:15:54.390 --> 00:15:56.477 align:middle line:84% and I'd say the community is split. 00:15:56.477 --> 00:15:58.310 align:middle line:84% One is to say, well, these are close enough, 00:15:58.310 --> 00:16:01.090 align:middle line:84% and actually, they show that calcium carbonate is basically 00:16:01.090 --> 00:16:03.410 align:middle line:84% totally remineralized at this site. 00:16:03.410 --> 00:16:05.457 align:middle line:84% Or you can take it at face value and say, 00:16:05.457 --> 00:16:07.290 align:middle line:84% what this means is that the sediments aren't 00:16:07.290 --> 00:16:08.270 align:middle line:90% in steady state. 00:16:08.270 --> 00:16:10.610 align:middle line:84% And more calcium carbonate is dissolving now 00:16:10.610 --> 00:16:12.350 align:middle line:84% than it's falling to the sea floor now. 00:16:12.350 --> 00:16:16.230 align:middle line:90% 00:16:16.230 --> 00:16:18.570 align:middle line:84% But anyway, I include this just to give you 00:16:18.570 --> 00:16:21.317 align:middle line:84% an example of a specific case in which these mass balances were 00:16:21.317 --> 00:16:23.650 align:middle line:84% calculated, and tell you how the measurements were made, 00:16:23.650 --> 00:16:25.910 align:middle line:84% and the sorts of comparisons that are done. 00:16:25.910 --> 00:16:31.250 align:middle line:90% 00:16:31.250 --> 00:16:38.010 align:middle line:84% Now, as I've said over and over again, 00:16:38.010 --> 00:16:41.080 align:middle line:84% the remineralization rate of these biogenic components 00:16:41.080 --> 00:16:42.840 align:middle line:84% of the particle flux to the sea floor 00:16:42.840 --> 00:16:45.960 align:middle line:84% tend to be very much greater than their burial rates. 00:16:45.960 --> 00:16:48.460 align:middle line:84% And the question is, should we care? 00:16:48.460 --> 00:16:50.040 align:middle line:90% Is that important to us? 00:16:50.040 --> 00:16:52.960 align:middle line:84% And to answer that question, I've 00:16:52.960 --> 00:16:56.000 align:middle line:84% said, well, suppose that you're a paleoceanographer, 00:16:56.000 --> 00:16:58.103 align:middle line:84% and you want to measure accumulation rates 00:16:58.103 --> 00:16:59.520 align:middle line:84% of some component of the sediments 00:16:59.520 --> 00:17:03.040 align:middle line:84% in the past and use that to infer rain rates to the sea 00:17:03.040 --> 00:17:05.000 align:middle line:90% floor in the past. 00:17:05.000 --> 00:17:08.560 align:middle line:84% So in order to look at the problem, 00:17:08.560 --> 00:17:10.500 align:middle line:90% we'll assume steady state. 00:17:10.500 --> 00:17:12.280 align:middle line:84% And I put a star there because obviously, 00:17:12.280 --> 00:17:16.073 align:middle line:84% if we're looking about changes in accumulation rates over time, 00:17:16.073 --> 00:17:18.240 align:middle line:84% the flux to the sea floor, or the accumulation rate, 00:17:18.240 --> 00:17:19.700 align:middle line:90% isn't constant over time. 00:17:19.700 --> 00:17:22.240 align:middle line:84% So by assuming steady state, what we mean 00:17:22.240 --> 00:17:26.359 align:middle line:84% is that the flux is constant over a long enough time 00:17:26.359 --> 00:17:31.080 align:middle line:84% for the sediments to come into a steady state for that flux rate 00:17:31.080 --> 00:17:32.440 align:middle line:90% to the sea floor. 00:17:32.440 --> 00:17:34.300 align:middle line:84% And once that steady state has been reached, 00:17:34.300 --> 00:17:35.760 align:middle line:84% then the flux could change again, 00:17:35.760 --> 00:17:38.080 align:middle line:84% and the system would reach a new steady state. 00:17:38.080 --> 00:17:42.397 align:middle line:84% But we're talking about one of those periods of constant flux, 00:17:42.397 --> 00:17:43.980 align:middle line:84% which is long enough for the sediments 00:17:43.980 --> 00:17:45.320 align:middle line:90% to reach a steady state. 00:17:45.320 --> 00:17:49.540 align:middle line:84% So if that's true, if the system is in steady state for a given 00:17:49.540 --> 00:17:52.940 align:middle line:84% component of the particle flux, the rain rate to the sea floor 00:17:52.940 --> 00:17:55.020 align:middle line:84% must equal the sum of the accumulation 00:17:55.020 --> 00:17:59.740 align:middle line:84% rate in the sediments plus the rate at which the component is 00:17:59.740 --> 00:18:00.880 align:middle line:90% remineralized. 00:18:00.880 --> 00:18:05.140 align:middle line:84% So if you write that in symbols, R, the rain rate, 00:18:05.140 --> 00:18:07.760 align:middle line:84% equals A, the accumulation rate, plus D, 00:18:07.760 --> 00:18:10.540 align:middle line:84% which I've called the diagenetic reaction rate. 00:18:10.540 --> 00:18:13.380 align:middle line:84% If you divide this equation through by R, 00:18:13.380 --> 00:18:16.260 align:middle line:84% and then rearrange it, you get rain rate 00:18:16.260 --> 00:18:20.620 align:middle line:84% equals some quantity multiplied by the accumulation rate. 00:18:20.620 --> 00:18:24.500 align:middle line:84% So the way you might use this is if you're measuring accumulation 00:18:24.500 --> 00:18:27.860 align:middle line:84% rates at times in the past, and you 00:18:27.860 --> 00:18:30.470 align:middle line:84% want to infer past rain rates from that. 00:18:30.470 --> 00:18:32.220 align:middle line:84% You will need to multiply the accumulation 00:18:32.220 --> 00:18:35.660 align:middle line:84% rate by this quantity that's in the brackets here. 00:18:35.660 --> 00:18:39.810 align:middle line:84% So in order to make that inference of rain rates 00:18:39.810 --> 00:18:41.370 align:middle line:84% from accumulation rates, you need 00:18:41.370 --> 00:18:45.010 align:middle line:84% to know the quantity that's in the brackets. 00:18:45.010 --> 00:18:46.970 align:middle line:84% So in looking at this equation, I've 00:18:46.970 --> 00:18:48.690 align:middle line:90% divided it into three cases. 00:18:48.690 --> 00:18:53.330 align:middle line:84% First is suppose that the recycling rate is very much 00:18:53.330 --> 00:18:54.890 align:middle line:90% smaller than the rain rate. 00:18:54.890 --> 00:18:57.590 align:middle line:84% This would be true, say, for the lithogenic flux, 00:18:57.590 --> 00:18:59.810 align:middle line:84% which is essentially completely preserved. 00:18:59.810 --> 00:19:01.490 align:middle line:84% It essentially doesn't undergo reactions 00:19:01.490 --> 00:19:02.930 align:middle line:90% in the surface sediments. 00:19:02.930 --> 00:19:08.747 align:middle line:84% So in that case, this D/R term would be very small, 00:19:08.747 --> 00:19:10.330 align:middle line:84% and the rain rate would be essentially 00:19:10.330 --> 00:19:13.130 align:middle line:90% equal to the accumulation rate. 00:19:13.130 --> 00:19:16.490 align:middle line:84% If this D/R term were very small, 00:19:16.490 --> 00:19:18.410 align:middle line:84% then still, this quantity in the brackets 00:19:18.410 --> 00:19:20.270 align:middle line:90% would be essentially equal to 1. 00:19:20.270 --> 00:19:23.970 align:middle line:84% And you could make an easy inference of rain rate 00:19:23.970 --> 00:19:26.210 align:middle line:84% from your measured accumulation rates. 00:19:26.210 --> 00:19:31.290 align:middle line:84% The second simple case is if this recycling rate were not 00:19:31.290 --> 00:19:34.290 align:middle line:84% insignificant compared to the rain rate, but it's constant. 00:19:34.290 --> 00:19:36.880 align:middle line:84% If that's true, then you could measure 00:19:36.880 --> 00:19:40.902 align:middle line:84% the ratio of the recycling rate to the rain rate, which 00:19:40.902 --> 00:19:43.360 align:middle line:84% would be a constant, and then this quantity in the brackets 00:19:43.360 --> 00:19:44.380 align:middle line:90% would be a constant. 00:19:44.380 --> 00:19:46.480 align:middle line:84% And it would again be easy to infer 00:19:46.480 --> 00:19:48.500 align:middle line:84% rain rates from measured accumulation rates. 00:19:48.500 --> 00:19:51.080 align:middle line:84% But what if you have a case, which 00:19:51.080 --> 00:19:52.560 align:middle line:84% is the one that seems to hold over 00:19:52.560 --> 00:19:55.880 align:middle line:84% much of the oceans for these biogenic components of the flux, 00:19:55.880 --> 00:19:59.640 align:middle line:84% where the recycling rate is very nearly equal to the rain rate? 00:19:59.640 --> 00:20:03.600 align:middle line:84% If that's the case, then small changes in this recycling 00:20:03.600 --> 00:20:07.240 align:middle line:84% efficiency, or the ratio of D/R in this equation, 00:20:07.240 --> 00:20:10.000 align:middle line:84% would lead to large changes in the rain rate 00:20:10.000 --> 00:20:13.800 align:middle line:84% that you'd infer from the accumulation rate 00:20:13.800 --> 00:20:17.320 align:middle line:84% independent of any changes in the accumulation rate. 00:20:17.320 --> 00:20:20.160 align:middle line:84% And so changes in this quantity in the brackets 00:20:20.160 --> 00:20:24.120 align:middle line:84% could mask changes in accumulation rates. 00:20:24.120 --> 00:20:27.000 align:middle line:84% I've just shown that graphically here, 00:20:27.000 --> 00:20:29.560 align:middle line:84% where I've plotted the quantity in the brackets 00:20:29.560 --> 00:20:33.080 align:middle line:84% on the vertical axis, and the ratio of the recycling 00:20:33.080 --> 00:20:37.020 align:middle line:84% rate to the rain rate to the sea floor on the horizontal axis, 00:20:37.020 --> 00:20:39.460 align:middle line:90% going from the simple-- 00:20:39.460 --> 00:20:42.300 align:middle line:84% what I called case 1, where not much of the material 00:20:42.300 --> 00:20:43.540 align:middle line:90% is recycled. 00:20:43.540 --> 00:20:48.840 align:middle line:84% And then, in that case, this quantity varies a little bit, 00:20:48.840 --> 00:20:49.600 align:middle line:90% but not too much. 00:20:49.600 --> 00:20:51.780 align:middle line:84% So you're pretty safe inferring rain rates 00:20:51.780 --> 00:20:52.880 align:middle line:90% from accumulation rates. 00:20:52.880 --> 00:20:59.580 align:middle line:84% But if you get to the case where the recycling efficiency, D/R, 00:20:59.580 --> 00:21:04.180 align:middle line:84% exceeds 80% then small changes in D/R 00:21:04.180 --> 00:21:07.440 align:middle line:84% lead to very large changes in the inferred accumulation rate. 00:21:07.440 --> 00:21:11.140 align:middle line:84% So this curve becomes very steep. 00:21:11.140 --> 00:21:13.780 align:middle line:84% And so if you're a paleoceanographer, 00:21:13.780 --> 00:21:16.300 align:middle line:84% it's probably a good idea to find 00:21:16.300 --> 00:21:19.460 align:middle line:84% some proxy for the biogenic components of the flux 00:21:19.460 --> 00:21:21.940 align:middle line:84% to the sea floor, because measuring those components 00:21:21.940 --> 00:21:24.020 align:middle line:90% directly is-- 00:21:24.020 --> 00:21:25.580 align:middle line:84% measurements of accumulation rates 00:21:25.580 --> 00:21:27.340 align:middle line:84% of those biogenic components directly 00:21:27.340 --> 00:21:29.200 align:middle line:84% are going to be difficult to interpret. 00:21:29.200 --> 00:21:35.080 align:middle line:90% 00:21:35.080 --> 00:21:40.650 align:middle line:84% Now, some motivations for not pulling the plug on me 00:21:40.650 --> 00:21:43.050 align:middle line:84% now, and listening for a little bit 00:21:43.050 --> 00:21:46.190 align:middle line:84% longer, consequences of early diagenesis. 00:21:46.190 --> 00:21:48.410 align:middle line:84% First, we talked about-- we've just 00:21:48.410 --> 00:21:50.130 align:middle line:84% talked about low rates of preservation 00:21:50.130 --> 00:21:53.930 align:middle line:84% of biogenic components and what that means for interpreting 00:21:53.930 --> 00:21:56.410 align:middle line:90% the sedimentary record. 00:21:56.410 --> 00:21:58.790 align:middle line:84% Second, another long timescale-- in fact, 00:21:58.790 --> 00:22:01.010 align:middle line:90% a very long timescale process-- 00:22:01.010 --> 00:22:04.370 align:middle line:84% is how early diagenesis in marine sediments 00:22:04.370 --> 00:22:06.970 align:middle line:84% affects the atmospheric oxygen level. 00:22:06.970 --> 00:22:12.170 align:middle line:84% And here, it's a simple enough picture. 00:22:12.170 --> 00:22:16.530 align:middle line:84% Oxygen is consumed on the continents by weathering. 00:22:16.530 --> 00:22:19.190 align:middle line:84% If the oxygen level in the atmosphere is to be constant, 00:22:19.190 --> 00:22:21.350 align:middle line:84% then the excess of photosynthesis, 00:22:21.350 --> 00:22:23.810 align:middle line:84% which releases oxygen to the atmosphere, 00:22:23.810 --> 00:22:27.650 align:middle line:84% over respiration, which removes oxygen, 00:22:27.650 --> 00:22:32.230 align:middle line:84% must balance that consumption of the oxygen from the atmosphere 00:22:32.230 --> 00:22:33.170 align:middle line:90% by weathering. 00:22:33.170 --> 00:22:36.330 align:middle line:90% 00:22:36.330 --> 00:22:39.710 align:middle line:90% Did I say that clearly enough? 00:22:39.710 --> 00:22:42.150 align:middle line:84% For the oxygen level to be constant in the atmosphere, 00:22:42.150 --> 00:22:44.390 align:middle line:84% processes which consume oxygen have 00:22:44.390 --> 00:22:45.870 align:middle line:84% to be balanced by processes which 00:22:45.870 --> 00:22:48.390 align:middle line:84% produce oxygen. Oxygen is produced 00:22:48.390 --> 00:22:52.650 align:middle line:84% by the excess of photosynthesis over respiration. 00:22:52.650 --> 00:22:54.730 align:middle line:84% It's consumed by weathering processes. 00:22:54.730 --> 00:22:56.430 align:middle line:90% So those need to be equal. 00:22:56.430 --> 00:23:01.430 align:middle line:84% In the ocean, photosynthesis occurs in the surface ocean. 00:23:01.430 --> 00:23:03.790 align:middle line:84% Much of the organic matter-- most of the organic matter 00:23:03.790 --> 00:23:07.310 align:middle line:84% that's produced is respired, is consumed 00:23:07.310 --> 00:23:08.770 align:middle line:84% by respiration in the water column, 00:23:08.770 --> 00:23:11.747 align:middle line:84% but a little bit is left over to accumulate in the sediments. 00:23:11.747 --> 00:23:13.830 align:middle line:84% And the fraction that accumulates in the sediments 00:23:13.830 --> 00:23:17.990 align:middle line:84% is determined, to some extent, by processes 00:23:17.990 --> 00:23:21.130 align:middle line:84% that recycle organic matter in the sediments. 00:23:21.130 --> 00:23:26.310 align:middle line:84% And in addition to that, once oxygen is used up 00:23:26.310 --> 00:23:28.630 align:middle line:84% in the sediments, then other electron acceptors 00:23:28.630 --> 00:23:29.690 align:middle line:90% become important. 00:23:29.690 --> 00:23:33.820 align:middle line:84% And two of those, the most important, are sulfate and iron. 00:23:33.820 --> 00:23:36.740 align:middle line:84% When sulfate and iron are used to oxidize organic matter 00:23:36.740 --> 00:23:39.420 align:middle line:84% anaerobically in sediments, then the products 00:23:39.420 --> 00:23:41.700 align:middle line:84% are reduced iron and reduced sulfur. 00:23:41.700 --> 00:23:43.460 align:middle line:84% Some of that reduced iron and sulfur 00:23:43.460 --> 00:23:46.180 align:middle line:84% is buried as pyrite, as iron sulfides. 00:23:46.180 --> 00:23:48.000 align:middle line:84% To the extent that those are buried, 00:23:48.000 --> 00:23:52.380 align:middle line:84% it represents a source of oxygen to the atmosphere 00:23:52.380 --> 00:23:55.180 align:middle line:84% in the same way that photosynthesis does. 00:23:55.180 --> 00:23:58.820 align:middle line:84% So processes in sediments, the burial of organic matter 00:23:58.820 --> 00:24:01.120 align:middle line:84% and the burial of reduced iron and sulfur, 00:24:01.120 --> 00:24:04.060 align:middle line:84% which are produced by sedimentary processes, 00:24:04.060 --> 00:24:07.260 align:middle line:84% are significant components in the long term atmospheric oxygen 00:24:07.260 --> 00:24:08.860 align:middle line:90% cycle. 00:24:08.860 --> 00:24:10.860 align:middle line:84% If we move from these longtime scale things 00:24:10.860 --> 00:24:13.260 align:middle line:84% to the contemporary ocean, there have 00:24:13.260 --> 00:24:19.140 align:middle line:84% been studies of deep water oxygen consumption done. 00:24:19.140 --> 00:24:22.740 align:middle line:84% And when measurements of oxygen consumption in the sediments are 00:24:22.740 --> 00:24:26.740 align:middle line:84% included in those studies, what they show is that about half 00:24:26.740 --> 00:24:29.640 align:middle line:84% of the oxygen consumption below 1,000 meters in the ocean 00:24:29.640 --> 00:24:33.240 align:middle line:84% actually occurs in the sediments rather than in the water column. 00:24:33.240 --> 00:24:34.880 align:middle line:84% So sedimentary oxygen consumption 00:24:34.880 --> 00:24:38.840 align:middle line:84% is an important piece of the deep water oxygen balance. 00:24:38.840 --> 00:24:42.360 align:middle line:84% And then the topic that has been of most interest 00:24:42.360 --> 00:24:48.160 align:middle line:84% to people in the last few years is that about half 00:24:48.160 --> 00:24:52.240 align:middle line:84% of the denitrification that occurs in the oceans today-- 00:24:52.240 --> 00:24:54.400 align:middle line:84% or actually more than half, even-- 00:24:54.400 --> 00:24:56.180 align:middle line:90% occurs in sediments. 00:24:56.180 --> 00:25:00.080 align:middle line:84% So you might ask yourself why that's true. 00:25:00.080 --> 00:25:03.960 align:middle line:84% What condition has to hold in the ocean for denitrification 00:25:03.960 --> 00:25:06.960 align:middle line:90% to occur? 00:25:06.960 --> 00:25:09.300 align:middle line:84% What has to be true of the water? 00:25:09.300 --> 00:25:12.440 align:middle line:90% 00:25:12.440 --> 00:25:14.100 align:middle line:84% I bet you've studied this already. 00:25:14.100 --> 00:25:16.473 align:middle line:90% AUDIENCE: [INAUDIBLE] anaerobic? 00:25:16.473 --> 00:25:17.140 align:middle line:90% PROFESSOR: Yeah. 00:25:17.140 --> 00:25:18.438 align:middle line:90% The oxygen has to be gone. 00:25:18.438 --> 00:25:20.480 align:middle line:84% So where in the ocean-- where in the water column 00:25:20.480 --> 00:25:23.720 align:middle line:84% does denitrification occur throughout the oceans? 00:25:23.720 --> 00:25:25.238 align:middle line:90% AUDIENCE: Oxygen minimum zones. 00:25:25.238 --> 00:25:26.280 align:middle line:90% PROFESSOR: OK, which are? 00:25:26.280 --> 00:25:27.790 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:25:27.790 --> 00:25:32.070 align:middle line:84% PROFESSOR: OK, but there's only a very small number of places 00:25:32.070 --> 00:25:34.670 align:middle line:84% where the oxygen minimum is intense enough to actually have 00:25:34.670 --> 00:25:38.830 align:middle line:90% an anoxic water column. 00:25:38.830 --> 00:25:40.510 align:middle line:90% OK, I'll tell you. 00:25:40.510 --> 00:25:43.990 align:middle line:84% The Arabian Sea and the Eastern Tropical Pacific north and south 00:25:43.990 --> 00:25:44.850 align:middle line:90% of the equator. 00:25:44.850 --> 00:25:48.750 align:middle line:84% So there's a very limited number of very limited in extent 00:25:48.750 --> 00:25:53.230 align:middle line:84% places where denitrification can occur in the water column. 00:25:53.230 --> 00:25:56.230 align:middle line:84% So why are sediments a better place for denitrification 00:25:56.230 --> 00:25:57.590 align:middle line:90% to occur? 00:25:57.590 --> 00:26:03.790 align:middle line:84% And what I've shown here is a picture, on the left, 00:26:03.790 --> 00:26:06.070 align:middle line:84% a profile of a continental margin. 00:26:06.070 --> 00:26:11.250 align:middle line:84% And continental margin has a continental shelf, 00:26:11.250 --> 00:26:13.110 align:middle line:90% which is very shallow and flat. 00:26:13.110 --> 00:26:16.270 align:middle line:84% And seaward of the continental shelf, the continental margin 00:26:16.270 --> 00:26:20.850 align:middle line:84% slopes relatively rapidly down to the abyssal plain. 00:26:20.850 --> 00:26:25.260 align:middle line:84% So it goes from a depth of about 100 meters to a depth of about 00:26:25.260 --> 00:26:34.500 align:middle line:84% 4,000 meters over a horizontal scale of maybe 100 kilometers. 00:26:34.500 --> 00:26:37.460 align:middle line:84% So that's a region where the depth of the sea floor 00:26:37.460 --> 00:26:39.200 align:middle line:90% is changing a lot very fast. 00:26:39.200 --> 00:26:41.620 align:middle line:84% And what I've shown on the right is a plot 00:26:41.620 --> 00:26:45.420 align:middle line:84% of the oxygen penetration depth in the sediments as a function 00:26:45.420 --> 00:26:47.540 align:middle line:90% of water column depth. 00:26:47.540 --> 00:26:50.740 align:middle line:84% So the thickness of the water column overlying the sediments. 00:26:50.740 --> 00:26:53.300 align:middle line:84% And I've done it for two continental margins. 00:26:53.300 --> 00:26:57.820 align:middle line:84% In the open symbols are the continental margins 00:26:57.820 --> 00:27:01.180 align:middle line:84% in the eastern and western North Atlantic Ocean. 00:27:01.180 --> 00:27:05.540 align:middle line:84% And the filled triangles are along the continental margin 00:27:05.540 --> 00:27:10.340 align:middle line:84% in the Eastern Pacific Ocean, the Northeast Pacific Ocean. 00:27:10.340 --> 00:27:13.660 align:middle line:84% So the first thing to notice is that this oxygen penetration 00:27:13.660 --> 00:27:15.660 align:middle line:90% depth scale is in millimeters. 00:27:15.660 --> 00:27:22.740 align:middle line:84% And so say, at a water depth of 2,000 meters in the Atlantic 00:27:22.740 --> 00:27:25.260 align:middle line:84% Ocean-- so if you're a spot in this picture on the left, 00:27:25.260 --> 00:27:27.267 align:middle line:84% which is about where my arrow is there, 00:27:27.267 --> 00:27:29.600 align:middle line:84% if you go ahead and measure the oxygen penetration depth 00:27:29.600 --> 00:27:32.700 align:middle line:84% in the sediments, oxygen penetrates to a depth of about 5 00:27:32.700 --> 00:27:33.680 align:middle line:90% centimeters. 00:27:33.680 --> 00:27:36.960 align:middle line:84% As you go up the slope, if you measure oxygen penetration 00:27:36.960 --> 00:27:39.640 align:middle line:84% depths in the sediments, it gets shallower and shallower, 00:27:39.640 --> 00:27:42.400 align:middle line:84% until when you get to about 400 meters, 00:27:42.400 --> 00:27:44.280 align:middle line:84% oxygen penetration depth in the sediments 00:27:44.280 --> 00:27:46.640 align:middle line:90% is less than a centimeter. 00:27:46.640 --> 00:27:49.440 align:middle line:84% So that means only the top-- this much of the sediments 00:27:49.440 --> 00:27:51.700 align:middle line:84% actually have oxygen in the pore waters. 00:27:51.700 --> 00:27:54.920 align:middle line:84% If you do the same thing in the Pacific Ocean, 00:27:54.920 --> 00:27:57.740 align:middle line:84% you find oxygen penetration depths are much shallower. 00:27:57.740 --> 00:27:59.840 align:middle line:84% So why would oxygen penetration depths 00:27:59.840 --> 00:28:02.800 align:middle line:84% be shallower in sediments on the continental margin 00:28:02.800 --> 00:28:04.020 align:middle line:90% in the Pacific ocean? 00:28:04.020 --> 00:28:07.520 align:middle line:90% 00:28:07.520 --> 00:28:09.000 align:middle line:90% In the Eastern Pacific-- 00:28:09.000 --> 00:28:10.300 align:middle line:90% Northeastern Pacific? 00:28:10.300 --> 00:28:12.880 align:middle line:90% 00:28:12.880 --> 00:28:15.300 align:middle line:84% This has to be stuck in your brain somewhere, 00:28:15.300 --> 00:28:16.660 align:middle line:90% just waiting to come out. 00:28:16.660 --> 00:28:19.600 align:middle line:90% 00:28:19.600 --> 00:28:21.320 align:middle line:84% What are the oxygen concentrations 00:28:21.320 --> 00:28:23.830 align:middle line:84% in the oxygen minimum in the Atlantic 00:28:23.830 --> 00:28:25.210 align:middle line:90% compared to in the Pacific? 00:28:25.210 --> 00:28:27.950 align:middle line:90% 00:28:27.950 --> 00:28:30.070 align:middle line:90% They're much higher. 00:28:30.070 --> 00:28:33.670 align:middle line:84% So oxygen concentrations in bottom waters at these Pacific 00:28:33.670 --> 00:28:36.550 align:middle line:84% sites are much lower than they are at corresponding depths 00:28:36.550 --> 00:28:38.110 align:middle line:90% in the Atlantic Ocean. 00:28:38.110 --> 00:28:43.162 align:middle line:84% So oxygen penetration depths in the sediments are much smaller. 00:28:43.162 --> 00:28:44.870 align:middle line:84% So if you go along the continental margin 00:28:44.870 --> 00:28:47.870 align:middle line:84% in the Pacific Ocean, oxygen penetration depths vary from 00:28:47.870 --> 00:28:52.990 align:middle line:84% about 2 centimeters at a depth of 4,000 meters and decrease 00:28:52.990 --> 00:28:58.030 align:middle line:84% in the oxygen minimum to a depth of about half a centimeter. 00:28:58.030 --> 00:28:59.710 align:middle line:90% So this means in-- 00:28:59.710 --> 00:29:01.910 align:middle line:84% remember, in the water column, there's 00:29:01.910 --> 00:29:04.670 align:middle line:84% three places in the ocean where oxygen concentrations 00:29:04.670 --> 00:29:07.670 align:middle line:84% in the water get low enough for denitrification to occur. 00:29:07.670 --> 00:29:11.070 align:middle line:84% In the sediments, oxygen penetration depths 00:29:11.070 --> 00:29:13.630 align:middle line:84% along all of the continental margins 00:29:13.630 --> 00:29:17.390 align:middle line:84% are ranged from 5 to half a centimeter. 00:29:17.390 --> 00:29:20.670 align:middle line:84% So there's this region very close to the water column that's 00:29:20.670 --> 00:29:22.210 align:middle line:90% anoxic. 00:29:22.210 --> 00:29:25.130 align:middle line:84% And as long as the flux of organic matter to the sediments 00:29:25.130 --> 00:29:27.610 align:middle line:84% is high enough, denitrification can occur there. 00:29:27.610 --> 00:29:30.490 align:middle line:84% And the result of that is that over half the denitrification 00:29:30.490 --> 00:29:32.730 align:middle line:84% in the oceans occurs in the sediments. 00:29:32.730 --> 00:29:36.450 align:middle line:84% So if you're interested in oceanic nitrogen balances, 00:29:36.450 --> 00:29:38.270 align:middle line:84% you shouldn't forget about the sediments. 00:29:38.270 --> 00:29:44.810 align:middle line:90% 00:29:44.810 --> 00:29:48.447 align:middle line:84% So now that we've talked in general about reactions 00:29:48.447 --> 00:29:50.530 align:middle line:84% in the sediments, we'll get a little more specific 00:29:50.530 --> 00:29:54.050 align:middle line:84% and focus on, for the rest of today, 00:29:54.050 --> 00:29:57.130 align:middle line:84% the oxidation of organic matter in the sediments. 00:29:57.130 --> 00:29:59.850 align:middle line:84% So you might ask how sedimentary processes 00:29:59.850 --> 00:30:01.710 align:middle line:84% are different from water column processes. 00:30:01.710 --> 00:30:04.090 align:middle line:90% Well, they must be different. 00:30:04.090 --> 00:30:08.110 align:middle line:84% Remember that-- I'm sure you've studied this-- 00:30:08.110 --> 00:30:12.250 align:middle line:84% that most of the organic matter that's fixed by photosynthesis 00:30:12.250 --> 00:30:15.530 align:middle line:84% in the surface ocean, something like-- 00:30:15.530 --> 00:30:20.170 align:middle line:84% well, well over 90%, is respired in the upper 1,000 meters 00:30:20.170 --> 00:30:21.383 align:middle line:90% of the water column. 00:30:21.383 --> 00:30:23.800 align:middle line:84% So relatively little gets to the seafloor, and what's left 00:30:23.800 --> 00:30:27.298 align:middle line:84% is presumably the relatively unreactive components. 00:30:27.298 --> 00:30:28.840 align:middle line:84% So the question is-- and I've already 00:30:28.840 --> 00:30:31.920 align:middle line:84% told you that these are almost completely remineralized 00:30:31.920 --> 00:30:33.310 align:middle line:90% in the sediments. 00:30:33.310 --> 00:30:35.060 align:middle line:84% So they're unreactive in the water column, 00:30:35.060 --> 00:30:37.560 align:middle line:84% but they hit the seafloor and conditions have 00:30:37.560 --> 00:30:39.780 align:middle line:90% changed so that they'll react. 00:30:39.780 --> 00:30:43.500 align:middle line:84% So how are processes in the sediments different? 00:30:43.500 --> 00:30:47.160 align:middle line:84% First is the really obvious one, that there's 00:30:47.160 --> 00:30:48.680 align:middle line:84% a lot more particles in sediments 00:30:48.680 --> 00:30:50.240 align:middle line:84% than there are in the water column. 00:30:50.240 --> 00:30:53.120 align:middle line:84% So the concentration of particles in the water column 00:30:53.120 --> 00:30:55.180 align:middle line:84% is on the order of a milligram per liter, 00:30:55.180 --> 00:30:59.480 align:middle line:84% but surface sediments are 40% to 70% particles by weight. 00:30:59.480 --> 00:31:04.540 align:middle line:84% So they're 30% to 60% water, 40% to 70% particles. 00:31:04.540 --> 00:31:06.080 align:middle line:90% So there's a lot more particles. 00:31:06.080 --> 00:31:08.880 align:middle line:84% And along with those particles are a lot more bacteria. 00:31:08.880 --> 00:31:12.720 align:middle line:84% So it's a great place for reactions to occur. 00:31:12.720 --> 00:31:15.280 align:middle line:84% The second is a question of time. 00:31:15.280 --> 00:31:19.660 align:middle line:84% So if you consider a water column that's about 3,500 meters 00:31:19.660 --> 00:31:22.880 align:middle line:84% long, and if you consider the large particle flux, 00:31:22.880 --> 00:31:25.620 align:middle line:84% which falls at a rate of something like 50 meters a day, 00:31:25.620 --> 00:31:28.340 align:middle line:84% then those particles will reside in the water column for about 70 00:31:28.340 --> 00:31:29.420 align:middle line:90% days. 00:31:29.420 --> 00:31:32.580 align:middle line:84% Now, if you consider the surface sediments, and here, 00:31:32.580 --> 00:31:37.220 align:middle line:84% I've taken as the layer of the sediments in which these "rapid" 00:31:37.220 --> 00:31:40.380 align:middle line:84% reactions occur-- "rapid" in quotes-- 00:31:40.380 --> 00:31:42.460 align:middle line:84% to be what we call the mixed layer, which 00:31:42.460 --> 00:31:45.140 align:middle line:84% is the layer at the surface of that live in the sediments, 00:31:45.140 --> 00:31:49.220 align:middle line:84% in the deep sea, in the pelagic ocean, away from the continents, 00:31:49.220 --> 00:31:52.300 align:middle line:84% the depth of the mixed layer is about 8 centimeters. 00:31:52.300 --> 00:31:55.140 align:middle line:84% And in the open ocean, sedimentation rates are about 00:31:55.140 --> 00:31:57.060 align:middle line:90% a centimeter per 1,000 years. 00:31:57.060 --> 00:32:00.540 align:middle line:84% So a particle will reside in that mixed layer where reactions 00:32:00.540 --> 00:32:02.660 align:middle line:90% occur for about 8,000 years. 00:32:02.660 --> 00:32:06.220 align:middle line:84% In the coastal ocean, mixed layers are thicker, 00:32:06.220 --> 00:32:07.560 align:middle line:90% say 20 centimeters. 00:32:07.560 --> 00:32:09.860 align:middle line:84% Sedimentation rates are faster, say a tenth 00:32:09.860 --> 00:32:11.620 align:middle line:90% of a centimeter per year. 00:32:11.620 --> 00:32:13.660 align:middle line:84% So a particle will reside in that mixed layer 00:32:13.660 --> 00:32:15.660 align:middle line:90% for about 200 years. 00:32:15.660 --> 00:32:18.570 align:middle line:84% So whereas a particle resides in the water column 00:32:18.570 --> 00:32:22.730 align:middle line:84% for on the order of months, it resides in that surface layer 00:32:22.730 --> 00:32:25.910 align:middle line:84% of the sediments for hundreds to thousands of years, 00:32:25.910 --> 00:32:29.437 align:middle line:84% so there's a lot longer for reactions to occur there. 00:32:29.437 --> 00:32:31.770 align:middle line:84% Reactions that are too slow to occur in the water column 00:32:31.770 --> 00:32:35.650 align:middle line:84% have plenty of time to happen in the sediments. 00:32:35.650 --> 00:32:38.050 align:middle line:84% And a final way, and this is actually 00:32:38.050 --> 00:32:41.330 align:middle line:84% a key to the occurrence of denitrification in sediments, 00:32:41.330 --> 00:32:46.090 align:middle line:84% is that the reactants for the processes that 00:32:46.090 --> 00:32:48.730 align:middle line:84% occur in the sediments are supplied from above. 00:32:48.730 --> 00:32:50.832 align:middle line:84% So the particles fall to the sediment surface. 00:32:50.832 --> 00:32:53.290 align:middle line:84% Once they fall there, they can be mixed through the surface 00:32:53.290 --> 00:32:53.910 align:middle line:90% sediments. 00:32:53.910 --> 00:32:55.970 align:middle line:84% But they arrive at the sediment surface 00:32:55.970 --> 00:32:59.715 align:middle line:84% and they react with solutes, and in particular, you 00:32:59.715 --> 00:33:02.250 align:middle line:84% can think of oxygen, that diffuse from the water column 00:33:02.250 --> 00:33:03.930 align:middle line:90% into the sediments. 00:33:03.930 --> 00:33:08.290 align:middle line:90% So you can see that reactions-- 00:33:08.290 --> 00:33:10.970 align:middle line:84% there tends to be a layered structure 00:33:10.970 --> 00:33:16.310 align:middle line:84% to zones of reactions that occur in sediments, where first-- 00:33:16.310 --> 00:33:18.840 align:middle line:90% 00:33:18.840 --> 00:33:21.880 align:middle line:90% that's as far as-- 00:33:21.880 --> 00:33:24.920 align:middle line:84% reactants are supplied from above, 00:33:24.920 --> 00:33:27.280 align:middle line:84% and the sequence of reactions that occurs 00:33:27.280 --> 00:33:29.480 align:middle line:84% tends to make a sort of layered structure 00:33:29.480 --> 00:33:32.960 align:middle line:84% in the sediment column-- to a first approximation. 00:33:32.960 --> 00:33:36.200 align:middle line:90% So why is that important? 00:33:36.200 --> 00:33:38.940 align:middle line:84% When you think about how organic matter is oxidized, 00:33:38.940 --> 00:33:42.080 align:middle line:84% and in writing the equation in this funny way, 00:33:42.080 --> 00:33:44.240 align:middle line:84% I've assumed that you've already written out 00:33:44.240 --> 00:33:47.680 align:middle line:84% equations for the oxidation of organic matter 00:33:47.680 --> 00:33:53.720 align:middle line:90% by oxygen, nitrogen, by nitrate. 00:33:53.720 --> 00:33:55.860 align:middle line:84% AUDIENCE: Only [INAUDIBLE] respiration. 00:33:55.860 --> 00:33:57.240 align:middle line:90% PROFESSOR: OK. 00:33:57.240 --> 00:33:57.920 align:middle line:90% Well, then-- 00:33:57.920 --> 00:34:00.360 align:middle line:84% AUDIENCE: Carrying sea nitrate per [INAUDIBLE] 00:34:00.360 --> 00:34:01.983 align:middle line:90% comes after [INAUDIBLE]. 00:34:01.983 --> 00:34:04.400 align:middle line:84% PROFESSOR: Well, then this equation will look a little odd 00:34:04.400 --> 00:34:07.400 align:middle line:90% to you, but it's-- 00:34:07.400 --> 00:34:10.840 align:middle line:84% AUDIENCE: But it'll be very useful for your homework. 00:34:10.840 --> 00:34:13.960 align:middle line:84% PROFESSOR: And anticipating that you've already seen these, 00:34:13.960 --> 00:34:16.340 align:middle line:84% I didn't include any stoichiometric information 00:34:16.340 --> 00:34:16.840 align:middle line:90% at all. 00:34:16.840 --> 00:34:18.465 align:middle line:84% So you're going to have to go somewhere 00:34:18.465 --> 00:34:20.480 align:middle line:84% else to get stoichiometry of these reactions. 00:34:20.480 --> 00:34:23.940 align:middle line:84% But what I've done is written the reactants 00:34:23.940 --> 00:34:27.460 align:middle line:84% on the left-hand side here, and products on the right-hand side, 00:34:27.460 --> 00:34:30.620 align:middle line:84% where the reactant obviously is organic matter. 00:34:30.620 --> 00:34:33.420 align:middle line:90% And that can react with oxygen. 00:34:33.420 --> 00:34:35.440 align:middle line:84% When organic matter reacts with oxygen, 00:34:35.440 --> 00:34:37.780 align:middle line:84% the nitrogen in the organic matter 00:34:37.780 --> 00:34:40.739 align:middle line:84% is-- the carbon is oxidized to CO2. 00:34:40.739 --> 00:34:42.739 align:middle line:84% The nitrogen is oxidized to nitrate. 00:34:42.739 --> 00:34:46.780 align:middle line:90% 00:34:46.780 --> 00:34:50.219 align:middle line:84% And just to give one more example, 00:34:50.219 --> 00:34:53.620 align:middle line:84% the reason I'm going through this will become clear, I hope. 00:34:53.620 --> 00:34:56.900 align:middle line:84% If organic matter is oxidized by nitrate, 00:34:56.900 --> 00:35:02.020 align:middle line:84% then the nitrogen in the organic matter ends up as N2. 00:35:02.020 --> 00:35:05.180 align:middle line:90% And the same for the nitrate-- 00:35:05.180 --> 00:35:06.700 align:middle line:84% for the electrons-- nitrate that's 00:35:06.700 --> 00:35:10.920 align:middle line:84% used as an electron acceptor, it ends up as N2. 00:35:10.920 --> 00:35:15.010 align:middle line:84% When manganese, iron, and sulfate 00:35:15.010 --> 00:35:18.610 align:middle line:84% are used as electron acceptors by anaerobic bacteria 00:35:18.610 --> 00:35:21.450 align:middle line:84% for organic matter oxidation, then the nitrogen 00:35:21.450 --> 00:35:24.190 align:middle line:84% in the organic matter is released as ammonia. 00:35:24.190 --> 00:35:26.530 align:middle line:90% It's not further oxidized. 00:35:26.530 --> 00:35:28.730 align:middle line:84% And in addition, the manganese oxides 00:35:28.730 --> 00:35:32.110 align:middle line:84% are released to the water as dissolved manganese. 00:35:32.110 --> 00:35:33.770 align:middle line:90% Iron is dissolved iron. 00:35:33.770 --> 00:35:36.850 align:middle line:84% And the sulfur is released as sulfide. 00:35:36.850 --> 00:35:40.050 align:middle line:84% And the reason for going through this 00:35:40.050 --> 00:35:45.170 align:middle line:84% is that you can use measurements of solutes in pore waters 00:35:45.170 --> 00:35:47.850 align:middle line:84% to tell you which of these reactions 00:35:47.850 --> 00:35:50.370 align:middle line:84% is occurring where in the sediment column. 00:35:50.370 --> 00:35:53.610 align:middle line:84% You can measure oxygen profiles to tell you 00:35:53.610 --> 00:35:56.750 align:middle line:84% the rate of consumption of organic matter by oxygen, 00:35:56.750 --> 00:35:58.730 align:middle line:84% and to tell you where in the sediment column 00:35:58.730 --> 00:36:00.570 align:middle line:90% oxygen is being used. 00:36:00.570 --> 00:36:05.610 align:middle line:84% You can use nitrate as an indicator of oxic respiration 00:36:05.610 --> 00:36:08.210 align:middle line:84% because nitrate is produced, and a denitrification 00:36:08.210 --> 00:36:12.430 align:middle line:84% because nitrate is consumed, and so on down the list. 00:36:12.430 --> 00:36:14.150 align:middle line:84% And the one thing I'll point out is 00:36:14.150 --> 00:36:16.750 align:middle line:84% you can use ammonia profiles in sediments 00:36:16.750 --> 00:36:21.510 align:middle line:84% as an indicator of the combined effect of oxidation 00:36:21.510 --> 00:36:24.970 align:middle line:84% of organic matter by manganese and iron oxides, 00:36:24.970 --> 00:36:27.150 align:middle line:84% and by sulfate sulfur, because it's 00:36:27.150 --> 00:36:29.790 align:middle line:84% a product of each of those reactions. 00:36:29.790 --> 00:36:33.150 align:middle line:90% 00:36:33.150 --> 00:36:36.450 align:middle line:84% Now, I've written a very simple-- well, simple. 00:36:36.450 --> 00:36:38.810 align:middle line:84% If I wrote out one of these equations on its own, 00:36:38.810 --> 00:36:42.990 align:middle line:84% it would be a simple picture of a reaction with organic matter 00:36:42.990 --> 00:36:46.950 align:middle line:84% plus electron acceptor equals oxidized products. 00:36:46.950 --> 00:36:49.790 align:middle line:84% But in reality, you have to be aware 00:36:49.790 --> 00:36:51.710 align:middle line:84% that the true mechanisms for these reactions 00:36:51.710 --> 00:36:53.590 align:middle line:90% are much more complicated. 00:36:53.590 --> 00:36:58.930 align:middle line:84% Bacteria don't bite off pieces of particulate organic matter. 00:36:58.930 --> 00:37:03.110 align:middle line:84% Instead, they release enzymes which 00:37:03.110 --> 00:37:05.270 align:middle line:84% break down the organic matter into pieces 00:37:05.270 --> 00:37:08.390 align:middle line:84% that are small enough for them to incorporate in their cells. 00:37:08.390 --> 00:37:14.140 align:middle line:84% And anaerobic bacteria, at least, can't take organic matter 00:37:14.140 --> 00:37:16.340 align:middle line:84% and oxidize it all the way to CO2. 00:37:16.340 --> 00:37:18.273 align:middle line:84% So it's not a single bacterium that's 00:37:18.273 --> 00:37:19.940 align:middle line:84% carrying out the whole process, but it's 00:37:19.940 --> 00:37:21.980 align:middle line:90% a consortium of bacteria. 00:37:21.980 --> 00:37:26.140 align:middle line:84% But the overall effect is these overall reactions 00:37:26.140 --> 00:37:28.380 align:middle line:84% with organic matter plus electron acceptor 00:37:28.380 --> 00:37:30.080 align:middle line:90% goes to oxidized products. 00:37:30.080 --> 00:37:32.660 align:middle line:84% And the reason for writing them in that way 00:37:32.660 --> 00:37:36.500 align:middle line:84% is that writing them that way allows 00:37:36.500 --> 00:37:40.500 align:middle line:84% you to calculate the free energy yield of each of the reactions. 00:37:40.500 --> 00:37:44.380 align:middle line:84% And what you find is that the order in which these reactions 00:37:44.380 --> 00:37:48.580 align:middle line:84% occur is the order of decreasing free energy yield 00:37:48.580 --> 00:37:50.520 align:middle line:90% for this overall reaction. 00:37:50.520 --> 00:37:55.677 align:middle line:84% And I was assuming you had done this already, but you haven't. 00:37:55.677 --> 00:37:57.260 align:middle line:84% But if you do this calculation, you'll 00:37:57.260 --> 00:38:00.940 align:middle line:84% find that the free energy yield for the oxidation 00:38:00.940 --> 00:38:08.660 align:middle line:84% of organic matter by oxygen, to CO2 plus nitrate-- 00:38:08.660 --> 00:38:12.560 align:middle line:84% to CO2 plus nitrate has a higher free energy yield 00:38:12.560 --> 00:38:14.200 align:middle line:84% than for the reaction of organic matter 00:38:14.200 --> 00:38:17.840 align:middle line:84% with nitrate, which in turn has a higher free energy 00:38:17.840 --> 00:38:22.080 align:middle line:84% yield than that of organic matter with manganese oxides, 00:38:22.080 --> 00:38:27.100 align:middle line:84% and so on, going down the ladder to iron oxides and sulfate. 00:38:27.100 --> 00:38:31.680 align:middle line:84% So the order of reactions that you would predict based 00:38:31.680 --> 00:38:34.120 align:middle line:84% on the free energy yield of the reactions 00:38:34.120 --> 00:38:40.220 align:middle line:84% is oxygen first, nitrate second, manganese oxides, iron oxides, 00:38:40.220 --> 00:38:41.280 align:middle line:90% sulfate. 00:38:41.280 --> 00:38:44.560 align:middle line:84% And if all those electron acceptors are used up 00:38:44.560 --> 00:38:47.720 align:middle line:84% and there's nothing left but smaller organic molecules, 00:38:47.720 --> 00:38:51.640 align:middle line:90% then organic matter-- 00:38:51.640 --> 00:38:53.400 align:middle line:84% carbon reacts with organic carbon 00:38:53.400 --> 00:38:55.660 align:middle line:90% and produces methane, actually. 00:38:55.660 --> 00:38:59.330 align:middle line:90% 00:38:59.330 --> 00:39:03.280 align:middle line:84% So I've essentially asserted that that's 00:39:03.280 --> 00:39:05.180 align:middle line:84% the order in which these reactions occur. 00:39:05.180 --> 00:39:06.800 align:middle line:90% So how do we know? 00:39:06.800 --> 00:39:09.350 align:middle line:84% We can find out the rates of these reactions 00:39:09.350 --> 00:39:11.710 align:middle line:84% and where in the sediment column each of the reactions 00:39:11.710 --> 00:39:14.710 align:middle line:84% occurs by measuring benthic fluxes, so 00:39:14.710 --> 00:39:18.210 align:middle line:84% exchanges across the sediment-water interface 00:39:18.210 --> 00:39:20.510 align:middle line:84% of dissolved reactants and products 00:39:20.510 --> 00:39:22.190 align:middle line:90% of these different processes. 00:39:22.190 --> 00:39:27.550 align:middle line:84% And we can measure pore water profiles of both reactants, 00:39:27.550 --> 00:39:31.310 align:middle line:84% those being oxygen and nitrate and sulfate, 00:39:31.310 --> 00:39:37.350 align:middle line:84% and products of the reaction, which can be CO2. 00:39:37.350 --> 00:39:41.870 align:middle line:90% They can be nitrate or ammonia. 00:39:41.870 --> 00:39:45.470 align:middle line:84% And we can also measure the reduced electron acceptors, 00:39:45.470 --> 00:39:49.790 align:middle line:84% especially manganese, iron, and sulfide. 00:39:49.790 --> 00:39:52.070 align:middle line:84% Don't use solid phase measurements as a rule 00:39:52.070 --> 00:39:57.910 align:middle line:84% to determine these reactions, because, one, they're-- 00:39:57.910 --> 00:40:00.750 align:middle line:84% concentrations in the solid phase tend to be quite high. 00:40:00.750 --> 00:40:03.750 align:middle line:84% So solid phase profiles are very insensitive to the occurrence 00:40:03.750 --> 00:40:05.190 align:middle line:90% of these reactions. 00:40:05.190 --> 00:40:07.550 align:middle line:84% And second is that solids are mixed 00:40:07.550 --> 00:40:10.600 align:middle line:84% by very complicated particle mixing processes, 00:40:10.600 --> 00:40:13.000 align:middle line:84% so it's very difficult to interpret the profiles. 00:40:13.000 --> 00:40:15.860 align:middle line:84% So we measure solute profiles as sensitive indicators 00:40:15.860 --> 00:40:18.320 align:middle line:84% of these reactions' occurrence and their rates. 00:40:18.320 --> 00:40:21.620 align:middle line:90% 00:40:21.620 --> 00:40:26.020 align:middle line:84% So what do these pore water profiles look like? 00:40:26.020 --> 00:40:31.100 align:middle line:84% What I've shown here is pore water dissolved oxygen profiles 00:40:31.100 --> 00:40:34.700 align:middle line:84% at three places in the ocean, which 00:40:34.700 --> 00:40:38.500 align:middle line:84% were intended to show the range of oxygen profiles 00:40:38.500 --> 00:40:39.980 align:middle line:90% that you would observe. 00:40:39.980 --> 00:40:43.060 align:middle line:84% And all these measurements were done by in-situ microelectrode 00:40:43.060 --> 00:40:44.140 align:middle line:90% profiling. 00:40:44.140 --> 00:40:46.700 align:middle line:84% These in-situ instruments measure 00:40:46.700 --> 00:40:48.200 align:middle line:90% oxygen electrochemically. 00:40:48.200 --> 00:40:50.380 align:middle line:84% So you have an oxygen-sensitive electrode 00:40:50.380 --> 00:40:54.780 align:middle line:84% that's mounted on an instrument that sits on the sea floor 00:40:54.780 --> 00:40:57.020 align:middle line:84% and has a plate that lowers down by very 00:40:57.020 --> 00:41:02.500 align:middle line:84% small and well-controlled steps into the sediments. 00:41:02.500 --> 00:41:05.420 align:middle line:84% So you can use the electrode that's attached to that plate 00:41:05.420 --> 00:41:08.520 align:middle line:84% to measure dissolved oxygen profile in the pore waters 00:41:08.520 --> 00:41:12.360 align:middle line:84% on a very, very fine depth scale of millimeters 00:41:12.360 --> 00:41:15.920 align:middle line:90% or even less than millimeters. 00:41:15.920 --> 00:41:19.660 align:middle line:84% So I've shown pore water oxygen profiles at three locations. 00:41:19.660 --> 00:41:21.640 align:middle line:90% One is an open ocean place. 00:41:21.640 --> 00:41:23.320 align:middle line:84% It happens to be in the Atlantic, 00:41:23.320 --> 00:41:25.680 align:middle line:84% at a water depth of 3,200 meters, 00:41:25.680 --> 00:41:28.980 align:middle line:84% where the total organic matter oxidation rate, 00:41:28.980 --> 00:41:31.720 align:middle line:84% which is essentially equal to the rain rate of organic matter 00:41:31.720 --> 00:41:34.600 align:middle line:84% to the sea floor, was 14 micromoles per centimeter 00:41:34.600 --> 00:41:36.080 align:middle line:90% squared per year. 00:41:36.080 --> 00:41:38.280 align:middle line:84% And under these conditions, you can 00:41:38.280 --> 00:41:41.480 align:middle line:84% see that over a depth of 7 centimeters 00:41:41.480 --> 00:41:44.960 align:middle line:84% in the sediment column, oxygen dropped from its bottom water 00:41:44.960 --> 00:41:48.660 align:middle line:84% value of about 250 micromolar, down to about 100. 00:41:48.660 --> 00:41:50.960 align:middle line:84% So there was lots of oxygen left even 00:41:50.960 --> 00:41:53.480 align:middle line:84% at a depth of 7 centimeters in the sediments, 00:41:53.480 --> 00:41:58.580 align:middle line:84% these are what we would call deep sea oxic sediments. 00:41:58.580 --> 00:42:01.120 align:middle line:84% The next one, moving to the right, 00:42:01.120 --> 00:42:04.480 align:middle line:84% is at a water depth of 600 meters on the Northwest Atlantic 00:42:04.480 --> 00:42:05.890 align:middle line:90% continental margin. 00:42:05.890 --> 00:42:08.550 align:middle line:84% And here, the rain rate of organic matter to the sea floor 00:42:08.550 --> 00:42:11.850 align:middle line:84% was about three times what it was at that deep water site, 00:42:11.850 --> 00:42:15.110 align:middle line:84% about 45 micromoles per centimeter squared per year. 00:42:15.110 --> 00:42:17.590 align:middle line:84% And here again, this is an oxygen profile 00:42:17.590 --> 00:42:20.470 align:middle line:84% measured with an in-situ microelectrode profiler. 00:42:20.470 --> 00:42:21.967 align:middle line:84% Starting in bottom water, hitting 00:42:21.967 --> 00:42:23.550 align:middle line:84% the sediment-water interface, and then 00:42:23.550 --> 00:42:26.870 align:middle line:84% the oxygen concentration in the pore waters of the sediments 00:42:26.870 --> 00:42:30.310 align:middle line:84% drops off to zero at a depth of a little less 00:42:30.310 --> 00:42:31.990 align:middle line:90% than 2 centimeters. 00:42:31.990 --> 00:42:33.990 align:middle line:84% So now, with three times the flux 00:42:33.990 --> 00:42:35.650 align:middle line:84% of organic matter to the seafloor, 00:42:35.650 --> 00:42:38.910 align:middle line:84% there's enough organic matter so that the oxygen is completely 00:42:38.910 --> 00:42:43.870 align:middle line:84% consumed by a depth of about 1.5 centimeters in the sediments. 00:42:43.870 --> 00:42:46.470 align:middle line:84% This would be the oxygen penetration depth 00:42:46.470 --> 00:42:49.110 align:middle line:84% that I showed on that earlier plot. 00:42:49.110 --> 00:42:54.430 align:middle line:84% And if we go to a more extreme case of a shallow water 00:42:54.430 --> 00:42:56.870 align:middle line:84% coastal sediment, where the organic matter 00:42:56.870 --> 00:43:00.790 align:middle line:90% flux to the seafloor is about-- 00:43:00.790 --> 00:43:03.530 align:middle line:84% well, 10 times what it was at the margin site. 00:43:03.530 --> 00:43:06.970 align:middle line:84% So now, 350 micromoles per centimeter squared per year. 00:43:06.970 --> 00:43:10.510 align:middle line:84% This is another in-situ microelectrode oxygen profile. 00:43:10.510 --> 00:43:14.350 align:middle line:84% The oxygen penetration depth is about 2.5 millimeters. 00:43:14.350 --> 00:43:17.610 align:middle line:84% So here, the sediments become anoxic very close 00:43:17.610 --> 00:43:19.290 align:middle line:84% to the interface, because there's 00:43:19.290 --> 00:43:22.070 align:middle line:84% a very large flux of organic matter to the seafloor. 00:43:22.070 --> 00:43:27.530 align:middle line:90% 00:43:27.530 --> 00:43:34.050 align:middle line:84% So that's what pore water profiles in general look like. 00:43:34.050 --> 00:43:36.940 align:middle line:84% So that you have a measurement of the solute concentration 00:43:36.940 --> 00:43:39.190 align:middle line:84% in the pore waters that are included in the sediments, 00:43:39.190 --> 00:43:41.890 align:middle line:84% and the concentration changes with depth 00:43:41.890 --> 00:43:46.210 align:middle line:84% going from the sediment-water interface down. 00:43:46.210 --> 00:43:48.650 align:middle line:84% How do you interpret profiles like that? 00:43:48.650 --> 00:43:50.450 align:middle line:84% First, we'll talk a little bit about 00:43:50.450 --> 00:43:52.410 align:middle line:90% a qualitative interpretation. 00:43:52.410 --> 00:43:54.950 align:middle line:84% If we assume that the system is in steady state, 00:43:54.950 --> 00:43:56.370 align:middle line:84% so that the concentration profile 00:43:56.370 --> 00:44:00.970 align:middle line:84% is not changing over time, then-- 00:44:00.970 --> 00:44:04.380 align:middle line:84% well, also assume constant porosity and diffusivity, 00:44:04.380 --> 00:44:06.840 align:middle line:90% but we won't worry about that. 00:44:06.840 --> 00:44:08.640 align:middle line:84% So what I've shown on the left-hand side 00:44:08.640 --> 00:44:11.760 align:middle line:84% is a hypothetical solute profile of a solute that 00:44:11.760 --> 00:44:13.360 align:middle line:84% happens to be increasing with depth 00:44:13.360 --> 00:44:15.480 align:middle line:84% below the sediment-water interface. 00:44:15.480 --> 00:44:22.720 align:middle line:84% And we'll consider what happens between depth XA and depth XB. 00:44:22.720 --> 00:44:27.640 align:middle line:84% If we draw a straight line between the concentration points 00:44:27.640 --> 00:44:30.160 align:middle line:84% corresponding to those two depths-- 00:44:30.160 --> 00:44:32.320 align:middle line:84% it's shown in the dashed line here. 00:44:32.320 --> 00:44:36.860 align:middle line:84% And what you can see is that at every depth over that interval, 00:44:36.860 --> 00:44:38.920 align:middle line:84% the actual concentration of the solute 00:44:38.920 --> 00:44:42.600 align:middle line:84% is greater than the concentration 00:44:42.600 --> 00:44:44.520 align:middle line:90% along that straight line. 00:44:44.520 --> 00:44:46.560 align:middle line:84% Now, the concentration along that straight line 00:44:46.560 --> 00:44:49.680 align:middle line:84% is what you'd get if you just mixed water from point A 00:44:49.680 --> 00:44:53.920 align:middle line:84% and from point B. The fact that the concentration is higher 00:44:53.920 --> 00:44:56.520 align:middle line:84% at every point tells you that over this interval, 00:44:56.520 --> 00:44:59.560 align:middle line:84% the solute is being added to the pore waters over that depth 00:44:59.560 --> 00:45:00.800 align:middle line:90% interval. 00:45:00.800 --> 00:45:05.060 align:middle line:84% If we take the opposite case, and here, 00:45:05.060 --> 00:45:06.900 align:middle line:84% a solute that happens to be decreasing 00:45:06.900 --> 00:45:09.340 align:middle line:84% with depth below the sediment-water interface. 00:45:09.340 --> 00:45:14.060 align:middle line:84% Again, point A, depth A to depth B. 00:45:14.060 --> 00:45:17.980 align:middle line:84% And if we connect the points on the concentration versus depth 00:45:17.980 --> 00:45:20.540 align:middle line:84% profile by a straight line, we find 00:45:20.540 --> 00:45:24.260 align:middle line:84% that the concentration of this solute is less at every depth 00:45:24.260 --> 00:45:26.320 align:middle line:84% than it would be along that straight line. 00:45:26.320 --> 00:45:28.340 align:middle line:84% The straight line is the mixing line, 00:45:28.340 --> 00:45:31.460 align:middle line:84% which is the line that you would get if the water were 00:45:31.460 --> 00:45:33.820 align:middle line:90% being mixed with no reaction. 00:45:33.820 --> 00:45:36.140 align:middle line:84% And since the concentration of this solute 00:45:36.140 --> 00:45:40.260 align:middle line:84% is less at every depth than the concentration 00:45:40.260 --> 00:45:42.740 align:middle line:84% along this mixing line, then the solute 00:45:42.740 --> 00:45:47.640 align:middle line:84% must be removed from pore waters over this depth interval. 00:45:47.640 --> 00:45:52.260 align:middle line:84% The reason for going through of complicated picture, 00:45:52.260 --> 00:45:55.240 align:middle line:84% rather than just saying, oh, the solute is increasing with depth, 00:45:55.240 --> 00:45:56.860 align:middle line:84% therefore, it must be produced, is 00:45:56.860 --> 00:46:00.290 align:middle line:84% because you can have situations where the solute is increasing, 00:46:00.290 --> 00:46:02.590 align:middle line:84% but over some depth range in its profile, 00:46:02.590 --> 00:46:04.070 align:middle line:90% it's actually being consumed. 00:46:04.070 --> 00:46:08.650 align:middle line:84% And in that case, you could apply this same criterion. 00:46:08.650 --> 00:46:11.810 align:middle line:84% Draw a straight line between the points bounding the interval 00:46:11.810 --> 00:46:13.090 align:middle line:90% you're interested in. 00:46:13.090 --> 00:46:16.890 align:middle line:84% If the concentration profile falls above that straight line, 00:46:16.890 --> 00:46:18.950 align:middle line:84% then the solute must be added over the interval. 00:46:18.950 --> 00:46:20.550 align:middle line:84% If it falls below that straight line, 00:46:20.550 --> 00:46:22.650 align:middle line:84% then it must be consumed over the interval. 00:46:22.650 --> 00:46:25.210 align:middle line:84% So that's the basis for qualitative interpretations 00:46:25.210 --> 00:46:28.250 align:middle line:90% of pore water profiles. 00:46:28.250 --> 00:46:29.570 align:middle line:90% Why are these useful? 00:46:29.570 --> 00:46:32.850 align:middle line:84% Well, I've shown you what pore water oxygen profiles look like. 00:46:32.850 --> 00:46:36.210 align:middle line:84% Here's a hypothetical picture of what some other solutes might 00:46:36.210 --> 00:46:37.770 align:middle line:90% look like as well. 00:46:37.770 --> 00:46:42.530 align:middle line:84% So in the dashed line up here is a hypothetical pore water oxygen 00:46:42.530 --> 00:46:44.050 align:middle line:90% profile. 00:46:44.050 --> 00:46:47.290 align:middle line:84% And over the whole range of this profile, 00:46:47.290 --> 00:46:49.730 align:middle line:84% if you apply that straight-line test, 00:46:49.730 --> 00:46:52.370 align:middle line:84% you'll find that oxygen is consumed over that interval. 00:46:52.370 --> 00:46:55.730 align:middle line:84% Over this interval where oxygen is consumed, 00:46:55.730 --> 00:46:58.210 align:middle line:84% if you apply that same straight-line test 00:46:58.210 --> 00:47:00.520 align:middle line:84% to the nitrate profile, you'll find 00:47:00.520 --> 00:47:02.700 align:middle line:84% that in the region where oxygen is being consumed, 00:47:02.700 --> 00:47:04.440 align:middle line:90% nitrate is produced. 00:47:04.440 --> 00:47:07.560 align:middle line:84% Remember that I said that when oxygen is used as an electron 00:47:07.560 --> 00:47:09.840 align:middle line:84% acceptor for organic matter oxidation, 00:47:09.840 --> 00:47:13.360 align:middle line:84% the organic nitrogen is oxidized all the way to nitrate. 00:47:13.360 --> 00:47:16.000 align:middle line:90% So nitrate is produced. 00:47:16.000 --> 00:47:19.720 align:middle line:84% Below this interval, the nitrate profile in this case 00:47:19.720 --> 00:47:22.000 align:middle line:90% has a straight line component. 00:47:22.000 --> 00:47:24.140 align:middle line:84% That means that over that depth interval, 00:47:24.140 --> 00:47:27.080 align:middle line:84% the nitrate would be neither consumed nor produced. 00:47:27.080 --> 00:47:30.640 align:middle line:90% It's just diffusing down. 00:47:30.640 --> 00:47:35.000 align:middle line:84% It shows manganese profile where if you apply the same tests, 00:47:35.000 --> 00:47:40.460 align:middle line:84% manganese is produced over a depth region in the sediments. 00:47:40.460 --> 00:47:42.763 align:middle line:84% But at the very top of the profile, 00:47:42.763 --> 00:47:44.680 align:middle line:84% you note that manganese doesn't go all the way 00:47:44.680 --> 00:47:45.980 align:middle line:90% to the sediment-water interface. 00:47:45.980 --> 00:47:48.355 align:middle line:84% Instead, there's a region at the top of the profile where 00:47:48.355 --> 00:47:50.840 align:middle line:84% the manganese that's diffusing up from below 00:47:50.840 --> 00:47:54.440 align:middle line:84% is removed from the pore waters by oxidation to manganese 00:47:54.440 --> 00:47:55.180 align:middle line:90% oxides. 00:47:55.180 --> 00:47:59.100 align:middle line:84% And you can apply these same tests. 00:47:59.100 --> 00:48:00.980 align:middle line:84% But the main point of this is that you 00:48:00.980 --> 00:48:03.700 align:middle line:84% can look at a suite of pore water profiles, 00:48:03.700 --> 00:48:09.020 align:middle line:84% analyze the depth intervals over which the solutes are either 00:48:09.020 --> 00:48:11.740 align:middle line:84% produced or consumed, and use that to infer 00:48:11.740 --> 00:48:14.740 align:middle line:84% the depth in the sediments over which the reactions that either 00:48:14.740 --> 00:48:17.000 align:middle line:84% produce or consume those solutes occur. 00:48:17.000 --> 00:48:19.900 align:middle line:90% 00:48:19.900 --> 00:48:23.220 align:middle line:90% So if we go on from that-- 00:48:23.220 --> 00:48:24.920 align:middle line:84% we're going to do that in just a minute. 00:48:24.920 --> 00:48:27.060 align:middle line:84% But before we do, you should know 00:48:27.060 --> 00:48:30.120 align:middle line:84% that people don't just look at them and infer reaction zones. 00:48:30.120 --> 00:48:33.780 align:middle line:84% They also use pore water profiles of solutes 00:48:33.780 --> 00:48:35.860 align:middle line:90% to calculate reaction rates. 00:48:35.860 --> 00:48:38.820 align:middle line:84% And they do that by calculating fluxes. 00:48:38.820 --> 00:48:43.900 align:middle line:84% And in this case, we've assumed it's a simple sediment where 00:48:43.900 --> 00:48:46.260 align:middle line:84% the transport process is diffusion 00:48:46.260 --> 00:48:48.200 align:middle line:90% of solutes in the pore waters. 00:48:48.200 --> 00:48:51.020 align:middle line:84% And the diffusion occurs perpendicular 00:48:51.020 --> 00:48:52.400 align:middle line:90% to the sediment water interface. 00:48:52.400 --> 00:48:56.490 align:middle line:90% So it occurs up and down 00:48:56.490 --> 00:49:01.490 align:middle line:84% Now, you probably haven't seen Fick's first law. 00:49:01.490 --> 00:49:04.310 align:middle line:84% So this is how you calculate a diffusive flux. 00:49:04.310 --> 00:49:07.250 align:middle line:90% 00:49:07.250 --> 00:49:09.170 align:middle line:84% There is a diffusive flux if there 00:49:09.170 --> 00:49:12.250 align:middle line:90% is a concentration gradient. 00:49:12.250 --> 00:49:14.170 align:middle line:84% So in this case, the concentration gradient 00:49:14.170 --> 00:49:19.370 align:middle line:84% is the change in concentration with depth. 00:49:19.370 --> 00:49:21.770 align:middle line:84% And there's a proportionality constant 00:49:21.770 --> 00:49:25.930 align:middle line:84% that you multiply that gradient by to calculate the flux, 00:49:25.930 --> 00:49:28.130 align:middle line:84% and that's the diffusion coefficient. 00:49:28.130 --> 00:49:32.330 align:middle line:84% So the diffusive flux in water equals diffusion coefficient 00:49:32.330 --> 00:49:35.010 align:middle line:84% multiplied by the concentration gradient. 00:49:35.010 --> 00:49:39.890 align:middle line:84% And this minus sign means that the diffusive transport is down 00:49:39.890 --> 00:49:41.350 align:middle line:90% the concentration gradient. 00:49:41.350 --> 00:49:44.810 align:middle line:84% So for instance, if you look at this profile, from that point 00:49:44.810 --> 00:49:47.810 align:middle line:84% there is this point here, the concentration 00:49:47.810 --> 00:49:50.230 align:middle line:84% decreases going toward the sediment-water interface. 00:49:50.230 --> 00:49:52.050 align:middle line:84% That means the diffusive flux is toward 00:49:52.050 --> 00:49:54.570 align:middle line:90% the sediment-water interface. 00:49:54.570 --> 00:49:56.070 align:middle line:84% There's an additional thing you have 00:49:56.070 --> 00:49:57.790 align:middle line:84% to worry about in sediments, which 00:49:57.790 --> 00:50:02.070 align:middle line:84% is, How much of the area actually of sediments 00:50:02.070 --> 00:50:05.170 align:middle line:84% is water compared to how much is solids? 00:50:05.170 --> 00:50:09.110 align:middle line:84% which are sediment grains, because the solutes are only 00:50:09.110 --> 00:50:12.110 align:middle line:90% diffusing through the water. 00:50:12.110 --> 00:50:14.750 align:middle line:84% There's a factor called the porosity 00:50:14.750 --> 00:50:17.870 align:middle line:84% that you have to multiply this flux by to account for the fact 00:50:17.870 --> 00:50:21.590 align:middle line:84% that part of the path is blocked by sediment grains. 00:50:21.590 --> 00:50:23.830 align:middle line:84% So the solutes that are diffusing through the water 00:50:23.830 --> 00:50:27.670 align:middle line:84% can only diffuse through part of the cross-sectional area. 00:50:27.670 --> 00:50:28.670 align:middle line:90% Is that clear? 00:50:28.670 --> 00:50:31.430 align:middle line:84% So the diffusive flux of a solute in sediments 00:50:31.430 --> 00:50:35.270 align:middle line:84% is equal to the porosity, which is the volume fraction of water 00:50:35.270 --> 00:50:38.270 align:middle line:84% in the sediments multiplied by diffusion coefficient 00:50:38.270 --> 00:50:44.723 align:middle line:84% for the solute multiplied by the concentration gradient. 00:50:44.723 --> 00:50:49.470 align:middle line:84% And to take this just one step further, 00:50:49.470 --> 00:50:51.750 align:middle line:84% this is actually a typical profile 00:50:51.750 --> 00:50:57.180 align:middle line:84% for the nitrate concentration in sediments, where, 00:50:57.180 --> 00:51:02.340 align:middle line:84% in the surface of the sediment, organic matter is oxidized 00:51:02.340 --> 00:51:05.780 align:middle line:84% by oxygen, and it continues to be oxidized by oxygen until all 00:51:05.780 --> 00:51:08.340 align:middle line:90% the oxygen is used up. 00:51:08.340 --> 00:51:10.780 align:middle line:84% When the organic matter is oxidized by oxygen, 00:51:10.780 --> 00:51:13.160 align:middle line:84% nitrate is released into the pore waters. 00:51:13.160 --> 00:51:16.580 align:middle line:84% So if you first do that quantitative interpretation 00:51:16.580 --> 00:51:21.860 align:middle line:84% I talked about, you find that this dashed line here 00:51:21.860 --> 00:51:26.163 align:middle line:84% is the layer in the sediments in which nitrate is being produced. 00:51:26.163 --> 00:51:27.580 align:middle line:84% It's actually important to realize 00:51:27.580 --> 00:51:31.580 align:middle line:84% that it goes a little bit below the nitrate maximum. 00:51:31.580 --> 00:51:34.260 align:middle line:84% And the layer below that is the layer in which nitrate 00:51:34.260 --> 00:51:36.500 align:middle line:90% is consumed by denitrification. 00:51:36.500 --> 00:51:40.380 align:middle line:84% We can calculate the rate at which nitrate 00:51:40.380 --> 00:51:46.220 align:middle line:84% is produced in this oxic layer by using a mass balance, where 00:51:46.220 --> 00:51:48.900 align:middle line:84% we say the rate of production in the layer 00:51:48.900 --> 00:51:52.720 align:middle line:84% has to be equal to the net flux out of the layer. 00:51:52.720 --> 00:51:56.040 align:middle line:84% If the concentration is constant within that sediment layer, 00:51:56.040 --> 00:51:58.480 align:middle line:84% then the fluxes out have to be exactly 00:51:58.480 --> 00:52:00.860 align:middle line:84% equal to the rate at which the solute is produced. 00:52:00.860 --> 00:52:05.080 align:middle line:84% So if we calculate the diffusive flux up out 00:52:05.080 --> 00:52:08.480 align:middle line:84% of the sediments at point 1, we calculate 00:52:08.480 --> 00:52:12.560 align:middle line:84% the rate at which nitrate is consumed by denitrification 00:52:12.560 --> 00:52:15.995 align:middle line:84% at point 2, then the sum of-- the absolute values of those two 00:52:15.995 --> 00:52:18.120 align:middle line:84% fluxes has to be equal to the rate at which nitrate 00:52:18.120 --> 00:52:20.720 align:middle line:90% is produced in this oxic layer. 00:52:20.720 --> 00:52:24.880 align:middle line:84% And we can use stoichiometry from that calculated nitrate 00:52:24.880 --> 00:52:27.280 align:middle line:84% flux to tell us the rate of organic matter oxidation 00:52:27.280 --> 00:52:32.520 align:middle line:84% by oxygen But the point of this is just to show you 00:52:32.520 --> 00:52:33.560 align:middle line:90% how it's done. 00:52:33.560 --> 00:52:35.160 align:middle line:84% And if you're really lucky, Scott 00:52:35.160 --> 00:52:39.470 align:middle line:84% will come up with a problem that makes you do it. 00:52:39.470 --> 00:52:41.220 align:middle line:84% AUDIENCE: [INAUDIBLE] on your own problem. 00:52:41.220 --> 00:52:45.150 align:middle line:90% 00:52:45.150 --> 00:52:47.520 align:middle line:84% PROFESSOR: So I've essentially asserted 00:52:47.520 --> 00:52:50.950 align:middle line:84% that the order in which electron acceptors are used for oxidation 00:52:50.950 --> 00:52:52.550 align:middle line:84% processes in the sediments is oxygen 00:52:52.550 --> 00:52:57.270 align:middle line:84% first, nitrate second, and manganese, 00:52:57.270 --> 00:53:00.070 align:middle line:90% then iron, then sulfate. 00:53:00.070 --> 00:53:03.615 align:middle line:84% So is there evidence that this sequence is actually followed? 00:53:03.615 --> 00:53:05.990 align:middle line:84% First, you have to go to a place where the organic matter 00:53:05.990 --> 00:53:09.830 align:middle line:84% flux is large enough so that the oxygen is completely 00:53:09.830 --> 00:53:11.330 align:middle line:90% consumed in the sediments. 00:53:11.330 --> 00:53:13.387 align:middle line:84% That's the first thing that you have to do. 00:53:13.387 --> 00:53:15.470 align:middle line:84% If you go to such a place with a large enough flux 00:53:15.470 --> 00:53:19.590 align:middle line:84% of organic matter, you could measure an oxygen profile, 00:53:19.590 --> 00:53:22.790 align:middle line:84% like here, and you find that the oxygen 00:53:22.790 --> 00:53:24.710 align:middle line:84% decreases from its bottom water value 00:53:24.710 --> 00:53:27.750 align:middle line:84% and goes to zero at about 1 centimeter depth. 00:53:27.750 --> 00:53:30.670 align:middle line:84% If, at the same place, you measure a pore water nitrate 00:53:30.670 --> 00:53:34.030 align:middle line:84% profile, and here, zero is the sediment-water interface. 00:53:34.030 --> 00:53:35.750 align:middle line:84% And that's measuring the concentration 00:53:35.750 --> 00:53:38.990 align:middle line:84% of nitrate in the pore waters as you go down into the sediments. 00:53:38.990 --> 00:53:43.310 align:middle line:84% And the depth scale here is from 0 to 5 centimeters. 00:53:43.310 --> 00:53:47.950 align:middle line:84% See that nitrate is produced in this upper layer, 00:53:47.950 --> 00:53:51.260 align:middle line:84% and then immediately below that layer in which nitrate 00:53:51.260 --> 00:53:53.820 align:middle line:84% is produced, the profile shape tells 00:53:53.820 --> 00:53:55.560 align:middle line:90% you nitrate is being consumed. 00:53:55.560 --> 00:53:58.460 align:middle line:84% So just below this oxic layer of the sediments 00:53:58.460 --> 00:54:01.940 align:middle line:84% is a layer in which nitrate is used by denitrification 00:54:01.940 --> 00:54:03.660 align:middle line:90% to oxidize organic matter. 00:54:03.660 --> 00:54:07.660 align:middle line:84% If you go a little bit deeper, and here the indicator 00:54:07.660 --> 00:54:11.620 align:middle line:84% is pore water dissolved ammonia concentrations-- remember, 00:54:11.620 --> 00:54:13.660 align:middle line:84% ammonia is a product of the oxidation 00:54:13.660 --> 00:54:18.840 align:middle line:84% of organic matter by manganese, iron, and sulfate. 00:54:18.840 --> 00:54:22.940 align:middle line:84% You see that the pore water ammonia profile 00:54:22.940 --> 00:54:24.780 align:middle line:84% shows that ammonia is actually consumed 00:54:24.780 --> 00:54:29.060 align:middle line:84% in the sediments above a depth of about, oh, 3 centimeters 00:54:29.060 --> 00:54:29.860 align:middle line:90% or so. 00:54:29.860 --> 00:54:31.500 align:middle line:84% But below that, the profile shape 00:54:31.500 --> 00:54:33.237 align:middle line:84% tells you that ammonia is produced. 00:54:33.237 --> 00:54:35.320 align:middle line:84% If you look at where nitrate goes to zero up here, 00:54:35.320 --> 00:54:37.340 align:middle line:84% it's right about at 3 centimeters. 00:54:37.340 --> 00:54:40.140 align:middle line:84% So you have a pretty distinct sequence 00:54:40.140 --> 00:54:43.900 align:middle line:84% where oxygen is consumed first, then nitrate is consumed, 00:54:43.900 --> 00:54:46.940 align:middle line:84% then reactions which release ammonia into the pore water 00:54:46.940 --> 00:54:47.560 align:middle line:90% occur. 00:54:47.560 --> 00:54:51.720 align:middle line:84% So that's some evidence for the sequence of reactions 00:54:51.720 --> 00:54:54.920 align:middle line:90% which I asserted earlier. 00:54:54.920 --> 00:54:59.720 align:middle line:84% And for completeness sake, I'll show you an extreme example 00:54:59.720 --> 00:55:01.240 align:middle line:90% of a coastal sediment. 00:55:01.240 --> 00:55:05.620 align:middle line:84% Now, remember the rates of organic matter oxidation, 00:55:05.620 --> 00:55:07.720 align:middle line:84% or the flux of organic matter to the seafloor, 00:55:07.720 --> 00:55:11.520 align:middle line:84% at a deep sea site, is something like 15 micromoles of carbon 00:55:11.520 --> 00:55:13.400 align:middle line:90% per square centimeter per year. 00:55:13.400 --> 00:55:16.520 align:middle line:84% On a continental margin site, it might be as much as 50. 00:55:16.520 --> 00:55:19.960 align:middle line:84% But here, these are results from a site which is actually 00:55:19.960 --> 00:55:23.000 align:middle line:84% in Boston Harbor, where the rain rate 00:55:23.000 --> 00:55:25.880 align:middle line:84% of organic matter to the sediments 00:55:25.880 --> 00:55:28.880 align:middle line:84% is about 850 micromoles of carbon. 00:55:28.880 --> 00:55:32.200 align:middle line:84% So the oxygen penetration depth at this site 00:55:32.200 --> 00:55:34.420 align:middle line:84% is on the order of 2 millimeters. 00:55:34.420 --> 00:55:36.120 align:middle line:90% It's very shallow. 00:55:36.120 --> 00:55:39.120 align:middle line:84% Below that, there's essentially no nitrate visible, 00:55:39.120 --> 00:55:41.180 align:middle line:84% because first, this is a very shallow water. 00:55:41.180 --> 00:55:43.200 align:middle line:84% It's in about 5 meters water depth. 00:55:43.200 --> 00:55:45.160 align:middle line:84% So there's very little nitrate present there, 00:55:45.160 --> 00:55:46.880 align:middle line:84% because it's in the euphotic zone. 00:55:46.880 --> 00:55:50.390 align:middle line:84% And second, because denitrification is so fast 00:55:50.390 --> 00:55:53.230 align:middle line:84% that the nitrate that's produced by oxic organic matter 00:55:53.230 --> 00:55:56.410 align:middle line:84% decomposition is essentially immediately used up. 00:55:56.410 --> 00:56:01.870 align:middle line:84% And so you go right into the use of manganese 00:56:01.870 --> 00:56:03.330 align:middle line:90% as an electron acceptor. 00:56:03.330 --> 00:56:04.950 align:middle line:90% And you can see that manganese-- 00:56:04.950 --> 00:56:06.750 align:middle line:90% there's the bottom water value. 00:56:06.750 --> 00:56:10.950 align:middle line:84% If my arrow wouldn't keep going away, I'd show it to you. 00:56:10.950 --> 00:56:14.310 align:middle line:84% The bottom water value is very close to zero. 00:56:14.310 --> 00:56:16.590 align:middle line:84% And right away, in the first pore water sample, 00:56:16.590 --> 00:56:18.730 align:middle line:84% there's dissolved manganese present. 00:56:18.730 --> 00:56:20.750 align:middle line:84% So manganese is used as an electron acceptor 00:56:20.750 --> 00:56:23.030 align:middle line:84% right near the sediment-water interface. 00:56:23.030 --> 00:56:26.310 align:middle line:84% If you look closely at this iron profile, 00:56:26.310 --> 00:56:29.245 align:middle line:84% see there's a very thin layer-- it's very hard to see. 00:56:29.245 --> 00:56:30.870 align:middle line:84% But if I expanded it, you'd see there's 00:56:30.870 --> 00:56:34.470 align:middle line:84% very thin layer where iron is consumed, diffuses up, and is 00:56:34.470 --> 00:56:35.530 align:middle line:90% oxidized. 00:56:35.530 --> 00:56:38.990 align:middle line:90% It can be oxidized by manganese. 00:56:38.990 --> 00:56:42.190 align:middle line:84% And below that, iron is released into the pore waters 00:56:42.190 --> 00:56:47.290 align:middle line:84% by decomposition of organic matter using iron oxides. 00:56:47.290 --> 00:56:50.628 align:middle line:84% One point of interest, if you're used to looking at water column 00:56:50.628 --> 00:56:52.170 align:middle line:84% data, you think of manganese and iron 00:56:52.170 --> 00:56:56.270 align:middle line:84% as trace metals with nanomolar concentrations. 00:56:56.270 --> 00:56:59.010 align:middle line:84% If you look at these reducing sediments, 00:56:59.010 --> 00:57:01.570 align:middle line:84% manganese concentrations are in the tens of micromoles 00:57:01.570 --> 00:57:04.810 align:middle line:84% per liter, and iron actually reaches very nearly millimolar 00:57:04.810 --> 00:57:05.710 align:middle line:90% concentrations. 00:57:05.710 --> 00:57:11.050 align:middle line:84% So they're not trace metals in these sedimentary pore waters. 00:57:11.050 --> 00:57:14.670 align:middle line:84% We go down here and look at the sulfate profile-- 00:57:14.670 --> 00:57:16.150 align:middle line:90% don't look at the open circles. 00:57:16.150 --> 00:57:17.530 align:middle line:90% That's a different place. 00:57:17.530 --> 00:57:19.950 align:middle line:84% If you look at these triangles, see, 00:57:19.950 --> 00:57:24.810 align:middle line:84% sulfate reduction begins at a depth of about 2 and 1/2 or 3 00:57:24.810 --> 00:57:28.530 align:middle line:84% centimeters, which is right about there. 00:57:28.530 --> 00:57:31.090 align:middle line:84% So sulfate reduction and iron reduction actually 00:57:31.090 --> 00:57:35.610 align:middle line:84% occur in overlapping regions of the sediments, which is actually 00:57:35.610 --> 00:57:39.210 align:middle line:84% what you would predict if you calculate the free energy 00:57:39.210 --> 00:57:42.830 align:middle line:90% yield of the reactions. 00:57:42.830 --> 00:57:44.600 align:middle line:84% And you see an interesting thing-- 00:57:44.600 --> 00:57:46.480 align:middle line:84% so again, the sequence is followed, 00:57:46.480 --> 00:57:49.360 align:middle line:84% where you have oxic decomposition, 00:57:49.360 --> 00:57:51.440 align:middle line:84% nitrate's not important here-- then 00:57:51.440 --> 00:57:54.320 align:middle line:84% decomposition using manganese oxides and iron oxides 00:57:54.320 --> 00:57:55.340 align:middle line:90% and sulfate. 00:57:55.340 --> 00:57:57.880 align:middle line:84% But you see an interesting thing, 00:57:57.880 --> 00:58:01.260 align:middle line:84% if you look at pore water dissolved sulfide profiles, 00:58:01.260 --> 00:58:03.480 align:middle line:84% where I've said that sulfide is the reduced 00:58:03.480 --> 00:58:06.440 align:middle line:90% product of sulfate reduction. 00:58:06.440 --> 00:58:09.320 align:middle line:84% But if you look at this, you see sulfide concentrations 00:58:09.320 --> 00:58:13.600 align:middle line:84% are pegged at zero until the depth at which the iron 00:58:13.600 --> 00:58:15.480 align:middle line:90% disappears from pore waters. 00:58:15.480 --> 00:58:20.240 align:middle line:84% And then sulfide concentrations increase pretty dramatically. 00:58:20.240 --> 00:58:23.960 align:middle line:84% The reason for that is that sulfate reduction and iron 00:58:23.960 --> 00:58:27.520 align:middle line:84% reduction occur over pretty much the same interval 00:58:27.520 --> 00:58:28.760 align:middle line:90% in these sediments. 00:58:28.760 --> 00:58:33.760 align:middle line:84% And that iron, reduced iron, has very insoluble sulfides. 00:58:33.760 --> 00:58:36.720 align:middle line:84% And so the sulfide is produced by sulfate reduction 00:58:36.720 --> 00:58:38.760 align:middle line:84% reacts with the iron and precipitates out 00:58:38.760 --> 00:58:42.240 align:middle line:84% as iron sulfides until all the iron is gone. 00:58:42.240 --> 00:58:46.270 align:middle line:84% And then sulfide starts to be released into the pore waters. 00:58:46.270 --> 00:58:48.770 align:middle line:84% Anyway, this is an example-- so I've shown you two examples. 00:58:48.770 --> 00:58:55.230 align:middle line:84% One continental margin site and one extreme coastal sediment 00:58:55.230 --> 00:58:58.890 align:middle line:84% example, showing you the range of conditions you'd observe. 00:58:58.890 --> 00:59:02.550 align:middle line:84% And if you remember back to the very first oxygen profile, 00:59:02.550 --> 00:59:05.030 align:middle line:84% I showed you an oxygen profile in an oxic sediment 00:59:05.030 --> 00:59:07.290 align:middle line:84% where it never even got close to going to zero. 00:59:07.290 --> 00:59:09.050 align:middle line:90% That's a deep sea condition. 00:59:09.050 --> 00:59:14.470 align:middle line:90% 00:59:14.470 --> 00:59:17.190 align:middle line:84% To summarize this little section about electron acceptors that 00:59:17.190 --> 00:59:19.630 align:middle line:84% are used for organic matter oxidation, 00:59:19.630 --> 00:59:26.270 align:middle line:84% I've put together a table which shows, for, first, this table up 00:59:26.270 --> 00:59:30.032 align:middle line:84% here, electron acceptors in pelagic sediments. 00:59:30.032 --> 00:59:31.990 align:middle line:84% So those are away from the continental margins, 00:59:31.990 --> 00:59:33.350 align:middle line:90% out in the open ocean. 00:59:33.350 --> 00:59:36.830 align:middle line:84% And these are all examples from the top tour 00:59:36.830 --> 00:59:40.110 align:middle line:84% from the equatorial Pacific and the last ones 00:59:40.110 --> 00:59:44.170 align:middle line:84% from the equatorial Atlantic, with organic matter oxidation 00:59:44.170 --> 00:59:48.160 align:middle line:84% rates that are pretty low, 12 to 20 micromoles 00:59:48.160 --> 00:59:49.910 align:middle line:84% of carbon per centimeter squared per year. 00:59:49.910 --> 00:59:52.170 align:middle line:84% So those are typical values for the deep sea. 00:59:52.170 --> 00:59:54.650 align:middle line:84% And you find that under these conditions, 00:59:54.650 --> 00:59:56.370 align:middle line:84% essentially all the organic matter 00:59:56.370 --> 00:59:59.930 align:middle line:84% oxidation occurs using oxygen as an electron acceptor. 00:59:59.930 --> 01:00:01.770 align:middle line:90% It's the first one used. 01:00:01.770 --> 01:00:03.970 align:middle line:84% There's plenty of it in the bottom water 01:00:03.970 --> 01:00:05.570 align:middle line:90% in all these locations. 01:00:05.570 --> 01:00:09.290 align:middle line:84% And essentially, what happens is the supply of organic matter 01:00:09.290 --> 01:00:11.990 align:middle line:84% runs out before the oxygen runs out. 01:00:11.990 --> 01:00:15.370 align:middle line:84% So nearly all the organic matter decomposition 01:00:15.370 --> 01:00:17.590 align:middle line:84% uses oxygen as an electron acceptor, 01:00:17.590 --> 01:00:19.370 align:middle line:90% just like in the water column. 01:00:19.370 --> 01:00:22.090 align:middle line:84% But if you go to places on continental margins 01:00:22.090 --> 01:00:25.290 align:middle line:84% where the supply of organic matter to the seafloor 01:00:25.290 --> 01:00:28.250 align:middle line:84% is great enough so that all the oxygen is used up, 01:00:28.250 --> 01:00:30.410 align:middle line:84% then you get a little bit different story. 01:00:30.410 --> 01:00:33.890 align:middle line:84% And here, I've got examples from the North Atlantic Ocean 01:00:33.890 --> 01:00:36.050 align:middle line:84% and from the North Pacific Ocean. 01:00:36.050 --> 01:00:39.250 align:middle line:84% In the North Atlantic Ocean, oxygen still 01:00:39.250 --> 01:00:43.440 align:middle line:84% accounts for 70% to 95% of all the organic matter 01:00:43.440 --> 01:00:45.520 align:middle line:90% oxidation in the sediments. 01:00:45.520 --> 01:00:49.280 align:middle line:84% Nitrate accounts for a little bit, and of the remainder, 01:00:49.280 --> 01:00:52.080 align:middle line:84% most of it is accounted for by sulfate reduction. 01:00:52.080 --> 01:00:54.120 align:middle line:84% So under these continental margin conditions 01:00:54.120 --> 01:00:57.360 align:middle line:84% in the Atlantic, oxygen is the main most important electron 01:00:57.360 --> 01:01:00.040 align:middle line:90% acceptor, followed by nitrate. 01:01:00.040 --> 01:01:02.340 align:middle line:84% Manganese and iron have a very limited importance, 01:01:02.340 --> 01:01:04.780 align:middle line:84% but sulfate reduction can be significant. 01:01:04.780 --> 01:01:09.880 align:middle line:84% If you go to the Northeast Pacific Ocean, 01:01:09.880 --> 01:01:14.080 align:middle line:84% the story is a little bit different because-- 01:01:14.080 --> 01:01:15.300 align:middle line:90% why would it be different? 01:01:15.300 --> 01:01:17.192 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:01:17.192 --> 01:01:17.457 align:middle line:90% 01:01:17.457 --> 01:01:19.040 align:middle line:84% PROFESSOR: I didn't hear you, but I'll 01:01:19.040 --> 01:01:21.000 align:middle line:84% bet you said because oxygen concentrations 01:01:21.000 --> 01:01:23.700 align:middle line:84% in the bottom water are much lower in the Northeast Pacific. 01:01:23.700 --> 01:01:26.280 align:middle line:84% And in fact, there's an intense oxygen minimum. 01:01:26.280 --> 01:01:28.805 align:middle line:90% And so in the oxygen minimum-- 01:01:28.805 --> 01:01:30.680 align:middle line:84% that's what's shown in this line in the table 01:01:30.680 --> 01:01:35.120 align:middle line:84% here-- oxygen only accounts for 5% to 50% 01:01:35.120 --> 01:01:37.840 align:middle line:84% and nitrate accounts for most of the rest. 01:01:37.840 --> 01:01:40.280 align:middle line:84% Iron and manganese again are not very important, 01:01:40.280 --> 01:01:42.660 align:middle line:84% but sulfate reduction is significant. 01:01:42.660 --> 01:01:46.480 align:middle line:84% The Northeast Pacific deeper than the oxygen minimum, 01:01:46.480 --> 01:01:49.740 align:middle line:84% you get a picture that's much more like the Atlantic. 01:01:49.740 --> 01:01:51.500 align:middle line:90% Oxygen is the most important. 01:01:51.500 --> 01:01:54.300 align:middle line:84% In this case, nitrate is second, and sulfate reduction 01:01:54.300 --> 01:01:55.820 align:middle line:90% can also be important. 01:01:55.820 --> 01:01:58.620 align:middle line:84% So why would nitrate be more important under these conditions 01:01:58.620 --> 01:02:00.400 align:middle line:84% in the Pacific than the Atlantic? 01:02:00.400 --> 01:02:03.100 align:middle line:90% 01:02:03.100 --> 01:02:05.580 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:02:05.580 --> 01:02:09.940 align:middle line:84% PROFESSOR: What else is there in the bottom waters? 01:02:09.940 --> 01:02:11.780 align:middle line:90% There's more nitrate. 01:02:11.780 --> 01:02:14.760 align:middle line:84% So nitrate can diffuse in from the bottom waters. 01:02:14.760 --> 01:02:15.900 align:middle line:90% Actually, if you look at-- 01:02:15.900 --> 01:02:18.060 align:middle line:84% I showed you a nitrate profile from the Atlantic 01:02:18.060 --> 01:02:20.620 align:middle line:84% earlier, where there's actually a nitrate maximum 01:02:20.620 --> 01:02:23.300 align:middle line:84% in the sediments, meaning that some of the nitrate that's 01:02:23.300 --> 01:02:25.860 align:middle line:84% produced by oxic organic matter decomposition 01:02:25.860 --> 01:02:29.300 align:middle line:84% leaves the sediments and isn't used for denitrification. 01:02:29.300 --> 01:02:32.660 align:middle line:84% If you look at nitrate pore water profiles in the Pacific, 01:02:32.660 --> 01:02:35.180 align:middle line:84% you see there is no subsurface maximum. 01:02:35.180 --> 01:02:37.800 align:middle line:84% Concentration decreases right from the interface, 01:02:37.800 --> 01:02:39.490 align:middle line:84% which means that bottom water nitrate is 01:02:39.490 --> 01:02:40.870 align:middle line:90% diffusing into the sediments. 01:02:40.870 --> 01:02:43.130 align:middle line:84% So that's an added source of nitrate. 01:02:43.130 --> 01:02:47.130 align:middle line:84% So nitrate is a little bit more important in the Pacific. 01:02:47.130 --> 01:02:51.750 align:middle line:84% But anyway, all these deep ocean and continental margin sites, 01:02:51.750 --> 01:02:53.770 align:middle line:84% oxygen is the most important electron acceptor, 01:02:53.770 --> 01:02:57.130 align:middle line:84% except in the oxygen minimum, followed by nitrate and sulfate. 01:02:57.130 --> 01:03:01.210 align:middle line:84% Now, in coastal sediments, very shallow water sediments, 01:03:01.210 --> 01:03:03.570 align:middle line:84% is a different story where the dominant electron 01:03:03.570 --> 01:03:07.130 align:middle line:90% acceptors are iron and sulfur. 01:03:07.130 --> 01:03:08.610 align:middle line:90% But there's a catch. 01:03:08.610 --> 01:03:12.590 align:middle line:84% And the catch is that the reduced products-- 01:03:12.590 --> 01:03:15.610 align:middle line:90% so reduced iron sulfides-- 01:03:15.610 --> 01:03:18.627 align:middle line:84% can be mixed back up to the sediment surface. 01:03:18.627 --> 01:03:20.710 align:middle line:84% And at the sediment surface, what happens to them? 01:03:20.710 --> 01:03:22.330 align:middle line:90% They're oxidized by oxygen. 01:03:22.330 --> 01:03:26.570 align:middle line:84% So even though the pathway for organic matter decomposition 01:03:26.570 --> 01:03:29.530 align:middle line:84% is mostly anaerobic, where iron and sulfate 01:03:29.530 --> 01:03:31.950 align:middle line:84% are the electron acceptors used by the bacteria, 01:03:31.950 --> 01:03:35.570 align:middle line:84% actually the ultimate electron acceptor is still oxygen, 01:03:35.570 --> 01:03:37.270 align:middle line:84% because very little of the reduced iron 01:03:37.270 --> 01:03:38.650 align:middle line:90% and sulfur are buried. 01:03:38.650 --> 01:03:41.730 align:middle line:84% They're mostly mixed back to the surface to be re-oxidized. 01:03:41.730 --> 01:03:43.330 align:middle line:84% So they're used over and over again. 01:03:43.330 --> 01:03:45.610 align:middle line:84% And oxygen is the ultimate electron acceptor. 01:03:45.610 --> 01:03:53.610 align:middle line:90% 01:03:53.610 --> 01:03:55.150 align:middle line:84% Now you've heard more than you ever 01:03:55.150 --> 01:03:57.210 align:middle line:84% wanted to know about pore water profiles. 01:03:57.210 --> 01:03:59.910 align:middle line:84% So we'll talk for a minute about organic carbon 01:03:59.910 --> 01:04:01.790 align:middle line:90% burial in marine sediments. 01:04:01.790 --> 01:04:05.470 align:middle line:84% I said way back at the beginning, 01:04:05.470 --> 01:04:08.070 align:middle line:84% what the long-term importance of organic matter burial 01:04:08.070 --> 01:04:14.470 align:middle line:84% in sediments is, and that's that it represents the net-- 01:04:14.470 --> 01:04:17.690 align:middle line:84% the excess of photosynthesis over respiration in the ocean. 01:04:17.690 --> 01:04:20.375 align:middle line:84% And so it's a part of the long-term oxygen balance. 01:04:20.375 --> 01:04:22.750 align:middle line:84% So there's good reason for studying organic carbon burial 01:04:22.750 --> 01:04:26.110 align:middle line:84% in marine sediments, and to understand 01:04:26.110 --> 01:04:29.030 align:middle line:84% what makes it change from place to place, 01:04:29.030 --> 01:04:30.990 align:middle line:84% and what may make it change over time. 01:04:30.990 --> 01:04:39.140 align:middle line:84% So what's written in this table are three summaries 01:04:39.140 --> 01:04:41.347 align:middle line:84% of organic matter burial in marine sediments. 01:04:41.347 --> 01:04:43.180 align:middle line:84% The first one is an old one, and it actually 01:04:43.180 --> 01:04:44.420 align:middle line:90% doesn't tell you much. 01:04:44.420 --> 01:04:48.340 align:middle line:84% The second one, the point of these 01:04:48.340 --> 01:04:52.380 align:middle line:84% is to show you that of the total organic carbon that's 01:04:52.380 --> 01:04:55.860 align:middle line:84% buried in sediments, by far, the majority of it 01:04:55.860 --> 01:05:01.140 align:middle line:84% is buried in the Berner compilation 01:05:01.140 --> 01:05:03.580 align:middle line:84% terrigenous deltaic shelf sediments. 01:05:03.580 --> 01:05:09.140 align:middle line:84% And when those were broken down by Hedges and Keil 01:05:09.140 --> 01:05:12.660 align:middle line:84% into deltaic sediments plus sediments on the upper shelves 01:05:12.660 --> 01:05:13.780 align:middle line:90% and slopes. 01:05:13.780 --> 01:05:15.580 align:middle line:84% So what that tells you, if you want 01:05:15.580 --> 01:05:19.137 align:middle line:84% to understand organic carbon burial in marine sediments 01:05:19.137 --> 01:05:21.220 align:middle line:84% in the locations that are most important, you need 01:05:21.220 --> 01:05:23.940 align:middle line:84% to understand organic carbon burial in continental margin 01:05:23.940 --> 01:05:26.580 align:middle line:84% sediments, because that's where most of the organic matter 01:05:26.580 --> 01:05:27.720 align:middle line:90% burial occurs. 01:05:27.720 --> 01:05:30.900 align:middle line:90% 01:05:30.900 --> 01:05:37.650 align:middle line:84% So you could ask yourself, what determines the organic carbon 01:05:37.650 --> 01:05:40.250 align:middle line:90% accumulation rate in sediments? 01:05:40.250 --> 01:05:45.350 align:middle line:84% And way back when, two camps arose. 01:05:45.350 --> 01:05:47.990 align:middle line:84% One camp said, well, what determined-- 01:05:47.990 --> 01:05:49.790 align:middle line:84% I think of these as the wishful thinkers-- 01:05:49.790 --> 01:05:54.410 align:middle line:84% that what determines the organic matter burial rate in sediments 01:05:54.410 --> 01:05:56.810 align:middle line:84% is the productivity of the overlying waters. 01:05:56.810 --> 01:05:59.550 align:middle line:84% So if you measure organic carbon burial rates in sediments, 01:05:59.550 --> 01:06:02.730 align:middle line:84% that'll give you a picture of productivity in the overlying 01:06:02.730 --> 01:06:04.530 align:middle line:90% waters. 01:06:04.530 --> 01:06:07.690 align:middle line:84% The others say, no, that's not true. 01:06:07.690 --> 01:06:10.810 align:middle line:84% And they go back to what I discussed earlier, which 01:06:10.810 --> 01:06:14.042 align:middle line:84% is the fact that in marine sediments, 01:06:14.042 --> 01:06:16.250 align:middle line:84% most of the organic matter that falls to the seafloor 01:06:16.250 --> 01:06:17.730 align:middle line:90% is remineralized. 01:06:17.730 --> 01:06:22.250 align:middle line:84% So variations in the inputs can be 01:06:22.250 --> 01:06:25.490 align:middle line:84% masked by variations in the remineralization rate, 01:06:25.490 --> 01:06:28.650 align:middle line:84% or the recycling efficiency of the organic matter. 01:06:28.650 --> 01:06:33.670 align:middle line:84% So how did these two camps, one that says local productivity 01:06:33.670 --> 01:06:36.230 align:middle line:84% determines organic carbon accumulation in sediments, 01:06:36.230 --> 01:06:40.470 align:middle line:84% and one that says variable preservation 01:06:40.470 --> 01:06:44.510 align:middle line:84% determines variations in organic carbon accumulation. 01:06:44.510 --> 01:06:46.550 align:middle line:84% And there's actually an important piece 01:06:46.550 --> 01:06:50.190 align:middle line:84% to the variable preservation story. 01:06:50.190 --> 01:06:53.150 align:middle line:84% And that is that when this argument first arose, 01:06:53.150 --> 01:06:56.030 align:middle line:84% the variable preservation was attributed entirely 01:06:56.030 --> 01:06:59.110 align:middle line:84% to variations in bottom water oxygen concentration. 01:06:59.110 --> 01:07:01.610 align:middle line:84% They said that if the bottom waters are anoxic, 01:07:01.610 --> 01:07:04.150 align:middle line:84% organic matter is preserved to a much greater extent 01:07:04.150 --> 01:07:06.030 align:middle line:90% than if bottom waters are oxic. 01:07:06.030 --> 01:07:08.590 align:middle line:90% 01:07:08.590 --> 01:07:10.230 align:middle line:84% So you could simplify this discussion 01:07:10.230 --> 01:07:11.790 align:middle line:90% into pictures like this. 01:07:11.790 --> 01:07:16.750 align:middle line:84% The top one is a picture of the organic carbon concentration 01:07:16.750 --> 01:07:19.850 align:middle line:84% in sediments around the world, where the darker the color, 01:07:19.850 --> 01:07:22.430 align:middle line:84% the higher the organic matter concentration. 01:07:22.430 --> 01:07:25.510 align:middle line:84% I put a star there because, remember, up until now, 01:07:25.510 --> 01:07:27.910 align:middle line:84% I've talked about organic matter accumulation rates. 01:07:27.910 --> 01:07:32.457 align:middle line:90% 01:07:32.457 --> 01:07:34.040 align:middle line:84% And that's what I really want to know. 01:07:34.040 --> 01:07:35.820 align:middle line:84% I want to know the accumulation rate. 01:07:35.820 --> 01:07:37.340 align:middle line:84% It turns out that if you're looking 01:07:37.340 --> 01:07:40.860 align:middle line:84% at gross large-scale features, the concentration variations 01:07:40.860 --> 01:07:45.860 align:middle line:84% give you a reasonable picture of accumulation rate variations. 01:07:45.860 --> 01:07:50.660 align:middle line:84% But there's a catch, and you can see the catch 01:07:50.660 --> 01:07:54.540 align:middle line:84% if you look at these areas of lightly hatched 01:07:54.540 --> 01:07:56.680 align:middle line:84% low organic carbon concentration. 01:07:56.680 --> 01:07:59.540 align:middle line:90% 01:07:59.540 --> 01:08:02.560 align:middle line:84% Have you had your hydrothermal lecture yet? 01:08:02.560 --> 01:08:08.140 align:middle line:84% So you know that those follow mid-ocean ridge system. 01:08:08.140 --> 01:08:11.260 align:middle line:84% Those are all shallow places in the water. 01:08:11.260 --> 01:08:14.300 align:middle line:84% And as we'll see later, those are places where 01:08:14.300 --> 01:08:16.420 align:middle line:90% calcium carbonate accumulates. 01:08:16.420 --> 01:08:21.700 align:middle line:84% And so what these low organic carbon concentrations tell you 01:08:21.700 --> 01:08:23.779 align:middle line:84% is that the organic matter is being diluted 01:08:23.779 --> 01:08:25.032 align:middle line:90% by a lot of calcium carbonate. 01:08:25.032 --> 01:08:26.740 align:middle line:84% It doesn't tell you that the accumulation 01:08:26.740 --> 01:08:28.479 align:middle line:84% rate of organic matter is lower there. 01:08:28.479 --> 01:08:29.819 align:middle line:90% That's an aside. 01:08:29.819 --> 01:08:31.960 align:middle line:84% The main point here that I want you to be aware of, 01:08:31.960 --> 01:08:34.040 align:middle line:84% because I don't want you to get trapped 01:08:34.040 --> 01:08:37.819 align:middle line:84% into equating concentrations and accumulation rates. 01:08:37.819 --> 01:08:41.399 align:middle line:90% You can run into problems. 01:08:41.399 --> 01:08:47.120 align:middle line:84% And I used to be an economist and mixing stocks and flows 01:08:47.120 --> 01:08:51.359 align:middle line:84% was grounds to get you failed out of all your classes. 01:08:51.359 --> 01:08:53.680 align:middle line:90% So I don't want you to do that. 01:08:53.680 --> 01:08:55.880 align:middle line:84% But having said, that I'm going to proceed to mix 01:08:55.880 --> 01:08:58.340 align:middle line:90% stocks and flows. 01:08:58.340 --> 01:08:59.160 align:middle line:90% So these regions-- 01:08:59.160 --> 01:09:01.040 align:middle line:84% AUDIENCE: You can still do that in fisheries research. 01:09:01.040 --> 01:09:01.998 align:middle line:90% PROFESSOR: Oh, can you? 01:09:01.998 --> 01:09:04.279 align:middle line:90% [LAUGHTER] 01:09:04.279 --> 01:09:07.180 align:middle line:84% So anyway, the point here is that the black spots, 01:09:07.180 --> 01:09:09.779 align:middle line:84% the black ones, which are high organic carbon concentrations, 01:09:09.779 --> 01:09:11.380 align:middle line:84% are along the continental margins. 01:09:11.380 --> 01:09:13.200 align:middle line:84% And I've highlighted three spots. 01:09:13.200 --> 01:09:15.120 align:middle line:90% Number one is the Arabian Sea. 01:09:15.120 --> 01:09:17.680 align:middle line:84% Number two is the Eastern Tropical Pacific. 01:09:17.680 --> 01:09:21.080 align:middle line:84% And number three is the West African Coast. 01:09:21.080 --> 01:09:23.279 align:middle line:84% What's shown in the next plot down 01:09:23.279 --> 01:09:27.840 align:middle line:84% is a map of oxygen concentrations at the oxygen 01:09:27.840 --> 01:09:30.830 align:middle line:84% minimum in the water column, where the black ones are 01:09:30.830 --> 01:09:32.609 align:middle line:90% very low oxygen concentrations. 01:09:32.609 --> 01:09:36.770 align:middle line:84% And you see that I've labeled the same spots, the Arabian Sea, 01:09:36.770 --> 01:09:42.050 align:middle line:84% the West African Coast, and the Eastern Tropical Pacific. 01:09:42.050 --> 01:09:44.149 align:middle line:84% And they're not only areas of high organic carbon 01:09:44.149 --> 01:09:45.790 align:middle line:84% concentration and accumulation rates, 01:09:45.790 --> 01:09:49.590 align:middle line:84% they're also areas of low bottom water oxygen concentration. 01:09:49.590 --> 01:09:52.830 align:middle line:84% Well, it turns out that if you take this one step further 01:09:52.830 --> 01:09:55.430 align:middle line:84% and look at a map of primary productivity-- 01:09:55.430 --> 01:09:59.530 align:middle line:84% this is an ancient map, which I'm sure will make Scott cringe, 01:09:59.530 --> 01:10:02.310 align:middle line:90% but it does the purpose here-- 01:10:02.310 --> 01:10:04.150 align:middle line:84% see that those same spots labeled 01:10:04.150 --> 01:10:09.310 align:middle line:84% 1, 2, and 3 are also areas of high primary productivity. 01:10:09.310 --> 01:10:12.008 align:middle line:84% And so what you see is there's a correspondence between regions 01:10:12.008 --> 01:10:13.550 align:middle line:84% of high productivity and high percent 01:10:13.550 --> 01:10:15.550 align:middle line:90% organic carbon in sediments. 01:10:15.550 --> 01:10:18.850 align:middle line:84% So that bolsters the argument of the overlying water 01:10:18.850 --> 01:10:20.450 align:middle line:90% Productivity is what counts. 01:10:20.450 --> 01:10:24.950 align:middle line:84% But those are also regions of low bottom water oxygen. 01:10:24.950 --> 01:10:26.790 align:middle line:84% And so that also bolstered the argument 01:10:26.790 --> 01:10:30.250 align:middle line:84% of the variable preservation rate camp. 01:10:30.250 --> 01:10:33.090 align:middle line:84% And in addition, it's been shown by a number of studies 01:10:33.090 --> 01:10:35.010 align:middle line:84% that some naturally occurring molecules 01:10:35.010 --> 01:10:37.570 align:middle line:84% require oxygen for decomposition. 01:10:37.570 --> 01:10:39.930 align:middle line:84% And so you might reasonably ask yourself, 01:10:39.930 --> 01:10:42.410 align:middle line:84% does the sedimentary organic matter accumulation 01:10:42.410 --> 01:10:44.730 align:middle line:90% rate actually depend-- 01:10:44.730 --> 01:10:46.290 align:middle line:84% the variations, are they determined 01:10:46.290 --> 01:10:50.010 align:middle line:84% by variations in overlying water productivity 01:10:50.010 --> 01:10:52.250 align:middle line:84% or variations in the preservation rate 01:10:52.250 --> 01:10:55.610 align:middle line:90% of the organic matter, or both? 01:10:55.610 --> 01:10:58.510 align:middle line:84% What I'm going to show is not an answer to that question. 01:10:58.510 --> 01:11:01.590 align:middle line:84% I think I've already made clear where I stand on the issue. 01:11:01.590 --> 01:11:03.610 align:middle line:84% I think preservation is important. 01:11:03.610 --> 01:11:12.730 align:middle line:84% But is actually a recent-- well, fairly recent improvement 01:11:12.730 --> 01:11:16.650 align:middle line:84% of the argument of how bottom water oxygen concentrations 01:11:16.650 --> 01:11:20.250 align:middle line:84% affect organic matter preservation 01:11:20.250 --> 01:11:26.560 align:middle line:84% in the sediments, where it used to be just oxic versus anoxic. 01:11:26.560 --> 01:11:29.120 align:middle line:84% This reaches a middle ground, this interpretation 01:11:29.120 --> 01:11:30.180 align:middle line:90% that I'm going to show. 01:11:30.180 --> 01:11:35.503 align:middle line:84% And what these people did was study-- 01:11:35.503 --> 01:11:36.920 align:middle line:84% was come up with an idea that they 01:11:36.920 --> 01:11:39.360 align:middle line:90% called oxygen exposure time. 01:11:39.360 --> 01:11:42.660 align:middle line:84% And in their original work on this, 01:11:42.660 --> 01:11:45.400 align:middle line:84% they studied two areas, which were actually 01:11:45.400 --> 01:11:46.840 align:middle line:84% very good for this purpose because 01:11:46.840 --> 01:11:48.540 align:middle line:90% of their contrasting properties. 01:11:48.540 --> 01:11:50.720 align:middle line:84% They studied the Washington margin, 01:11:50.720 --> 01:11:53.200 align:middle line:84% so that's in the Northeastern Pacific 01:11:53.200 --> 01:11:55.440 align:middle line:90% off the coast of Washington. 01:11:55.440 --> 01:11:57.280 align:middle line:84% And the Mexico margin, so that's, 01:11:57.280 --> 01:12:00.720 align:middle line:84% again, Northeast Pacific, but off the coast of Mexico. 01:12:00.720 --> 01:12:04.040 align:middle line:84% If you compare these two regions, 01:12:04.040 --> 01:12:05.640 align:middle line:84% the Washington margin, which is shown 01:12:05.640 --> 01:12:09.040 align:middle line:84% by the squares in these plots that I've shown below it, 01:12:09.040 --> 01:12:10.760 align:middle line:84% is a region that has higher productivity 01:12:10.760 --> 01:12:15.240 align:middle line:84% than the Mexico margin, but a less intense oxygen minimum. 01:12:15.240 --> 01:12:16.760 align:middle line:84% The Mexico margin, which is shown 01:12:16.760 --> 01:12:20.020 align:middle line:84% by the circles in these plots, has lower productivity, 01:12:20.020 --> 01:12:22.060 align:middle line:84% but a very intense oxygen minimum. 01:12:22.060 --> 01:12:28.390 align:middle line:84% So on the left here is a plot of profile of oxygen in the water 01:12:28.390 --> 01:12:31.170 align:middle line:84% column in these regions, where the Washington margin, 01:12:31.170 --> 01:12:32.750 align:middle line:84% there is an oxygen minimum, but it's 01:12:32.750 --> 01:12:39.390 align:middle line:84% deeper and not as intense in this region from zero 01:12:39.390 --> 01:12:43.370 align:middle line:84% to 600 or 700 meters as it is in the Mexico margin, 01:12:43.370 --> 01:12:46.470 align:middle line:84% where the water column is anoxic. 01:12:46.470 --> 01:12:50.390 align:middle line:84% What's shown on the right are concentrations of organic carbon 01:12:50.390 --> 01:12:51.930 align:middle line:90% in the sediments. 01:12:51.930 --> 01:12:56.390 align:middle line:84% So on the Washington margin, the organic carbon concentrations 01:12:56.390 --> 01:12:58.190 align:middle line:90% are around 2%. 01:12:58.190 --> 01:13:00.790 align:middle line:84% In the Mexico margin, the concentrations 01:13:00.790 --> 01:13:04.470 align:middle line:84% of organic matter in the sediment run around 10%. 01:13:04.470 --> 01:13:06.710 align:middle line:84% So you can already see where this argument 01:13:06.710 --> 01:13:10.070 align:middle line:84% is going in the simple productivity versus preservation 01:13:10.070 --> 01:13:15.110 align:middle line:84% discussion, where the site that has an intense oxygen 01:13:15.110 --> 01:13:18.057 align:middle line:84% minimum, but lower productivity has a much higher oxygen 01:13:18.057 --> 01:13:20.390 align:middle line:84% concentration in the sediments than the site with higher 01:13:20.390 --> 01:13:23.150 align:middle line:84% productivity, but a less intense oxygen minimum, 01:13:23.150 --> 01:13:26.090 align:middle line:84% saying that preservation rate is a key in determining 01:13:26.090 --> 01:13:27.830 align:middle line:90% organic carbon distributions. 01:13:27.830 --> 01:13:32.330 align:middle line:90% 01:13:32.330 --> 01:13:34.930 align:middle line:84% What these guys did was, in addition to measuring 01:13:34.930 --> 01:13:37.310 align:middle line:84% organic carbon concentrations in the sediments, 01:13:37.310 --> 01:13:40.050 align:middle line:84% they also measured oxygen profiles 01:13:40.050 --> 01:13:42.010 align:middle line:90% in the sediment pore waters. 01:13:42.010 --> 01:13:44.310 align:middle line:84% And with each of those oxygen profiles, 01:13:44.310 --> 01:13:47.090 align:middle line:84% they determined the depth below the sediment-water interface 01:13:47.090 --> 01:13:49.290 align:middle line:84% at which the oxygen goes to zero. 01:13:49.290 --> 01:13:52.370 align:middle line:84% And they called that depth the oxygen penetration depth. 01:13:52.370 --> 01:13:54.370 align:middle line:84% What they were interested in doing 01:13:54.370 --> 01:13:57.210 align:middle line:84% was calculating how long a particular particle 01:13:57.210 --> 01:14:00.810 align:middle line:84% would be exposed to oxygen after it reaches the sea floor. 01:14:00.810 --> 01:14:02.850 align:middle line:84% And they did that by also measuring 01:14:02.850 --> 01:14:06.330 align:middle line:84% the sediment accumulation rate at all these places, 01:14:06.330 --> 01:14:10.010 align:middle line:84% and dividing this oxygen penetration depth 01:14:10.010 --> 01:14:11.287 align:middle line:90% by the sedimentation rate. 01:14:11.287 --> 01:14:12.870 align:middle line:84% And the result of that would give you, 01:14:12.870 --> 01:14:15.090 align:middle line:84% on average, how long a particle spends 01:14:15.090 --> 01:14:18.450 align:middle line:84% in the oxic part of the sediment column. 01:14:18.450 --> 01:14:21.970 align:middle line:84% And they called that quantity the oxygen exposure time. 01:14:21.970 --> 01:14:24.440 align:middle line:84% So the longer the oxygen exposure time, 01:14:24.440 --> 01:14:27.640 align:middle line:84% the longer a sediment particle, or a bit 01:14:27.640 --> 01:14:29.480 align:middle line:84% of sedimentary organic matter, is 01:14:29.480 --> 01:14:32.480 align:middle line:84% in a region where it's exposed to dissolved oxygen. 01:14:32.480 --> 01:14:36.980 align:middle line:84% And once they made these calculations, 01:14:36.980 --> 01:14:41.320 align:middle line:84% they compared the oxygen exposure time for particles 01:14:41.320 --> 01:14:44.440 align:middle line:84% at a given site to the organic carbon burial 01:14:44.440 --> 01:14:46.040 align:middle line:90% efficiency at that site. 01:14:46.040 --> 01:14:48.080 align:middle line:84% And the burial efficiency is here 01:14:48.080 --> 01:14:51.320 align:middle line:84% equal to the burial rate divided by the rain rate. 01:14:51.320 --> 01:14:57.320 align:middle line:84% And the prediction would be that if oxygen matters-- 01:14:57.320 --> 01:15:01.040 align:middle line:84% so if organic matter is oxidized more readily by oxygen 01:15:01.040 --> 01:15:03.160 align:middle line:84% than by other electron acceptors, 01:15:03.160 --> 01:15:05.480 align:middle line:84% then the longer the oxygen exposure time, 01:15:05.480 --> 01:15:10.506 align:middle line:84% the lower the organic carbon burial efficiency will be. 01:15:10.506 --> 01:15:13.560 align:middle line:84% And they came up with a compilation 01:15:13.560 --> 01:15:16.360 align:middle line:84% of data that looked like this, where 01:15:16.360 --> 01:15:19.300 align:middle line:84% they plotted organic carbon burial efficiency. 01:15:19.300 --> 01:15:28.340 align:middle line:84% And here the range is from 0 to 60% versus oxygen exposure time. 01:15:28.340 --> 01:15:30.200 align:middle line:84% The vertical axis is linear scale. 01:15:30.200 --> 01:15:32.580 align:middle line:84% So burial efficiency is on a linear scale. 01:15:32.580 --> 01:15:35.220 align:middle line:84% Oxygen exposure time is on a log scale. 01:15:35.220 --> 01:15:38.220 align:middle line:84% And a very important piece of their conclusion 01:15:38.220 --> 01:15:42.580 align:middle line:84% is that they measured oxygen exposure time over a five 01:15:42.580 --> 01:15:44.260 align:middle line:90% order of magnitude range. 01:15:44.260 --> 01:15:46.718 align:middle line:84% And it's safe to say they wouldn't 01:15:46.718 --> 01:15:48.260 align:middle line:84% have come to any conclusion if they'd 01:15:48.260 --> 01:15:50.220 align:middle line:90% measured a much smaller range. 01:15:50.220 --> 01:15:52.340 align:middle line:84% They're actually very upfront about that, because 01:15:52.340 --> 01:15:56.740 align:middle line:84% in this inset here, they plot both on linear scales, 01:15:56.740 --> 01:15:59.980 align:middle line:84% and you see that all their data fall in a big clump. 01:15:59.980 --> 01:16:01.860 align:middle line:84% They don't start to separate out and let 01:16:01.860 --> 01:16:04.740 align:middle line:84% you see a pattern until you plot it on a log scale 01:16:04.740 --> 01:16:08.180 align:middle line:84% and look at this very large scale, this very large change 01:16:08.180 --> 01:16:10.700 align:middle line:90% in oxygen exposure time. 01:16:10.700 --> 01:16:13.300 align:middle line:84% What you see, if you look at this overall picture, 01:16:13.300 --> 01:16:18.020 align:middle line:84% is that there is indeed, over this large range of oxygen 01:16:18.020 --> 01:16:22.130 align:middle line:84% exposure times, a decrease on average in organic carbon burial 01:16:22.130 --> 01:16:26.970 align:middle line:84% efficiency the longer the oxygen exposure time is 01:16:26.970 --> 01:16:30.330 align:middle line:84% But if you look at any particular spot, 01:16:30.330 --> 01:16:32.730 align:middle line:90% the variations are really big. 01:16:32.730 --> 01:16:35.730 align:middle line:84% So my conclusion from this was that it's 01:16:35.730 --> 01:16:39.030 align:middle line:84% a promising idea that oxygen does matter, 01:16:39.030 --> 01:16:41.730 align:middle line:84% and that you can determine how much it matters by measuring 01:16:41.730 --> 01:16:46.467 align:middle line:84% this oxygen exposure time thing, but that you really 01:16:46.467 --> 01:16:48.050 align:middle line:84% need more information, because there's 01:16:48.050 --> 01:16:49.508 align:middle line:84% a lot of variability in the results 01:16:49.508 --> 01:16:51.930 align:middle line:84% that isn't explained by this simple model. 01:16:51.930 --> 01:16:54.770 align:middle line:84% And in fact, they've gone on in subsequent years 01:16:54.770 --> 01:16:56.430 align:middle line:84% to make it a lot more complicated. 01:16:56.430 --> 01:16:59.010 align:middle line:90% 01:16:59.010 --> 01:17:01.530 align:middle line:84% One interesting note is that this actually 01:17:01.530 --> 01:17:04.410 align:middle line:84% provides a feedback between atmospheric oxygen 01:17:04.410 --> 01:17:07.133 align:middle line:84% levels and the organic carbon burial rate. 01:17:07.133 --> 01:17:08.550 align:middle line:84% When you get into these arguments, 01:17:08.550 --> 01:17:10.810 align:middle line:84% they're very hand-wavy, but they're 01:17:10.810 --> 01:17:13.250 align:middle line:90% interesting to think about. 01:17:13.250 --> 01:17:16.490 align:middle line:84% If you suppose oxygen exposure time matters, 01:17:16.490 --> 01:17:18.710 align:middle line:84% and, say, there's an increase in weathering 01:17:18.710 --> 01:17:21.410 align:middle line:84% rates on the continent, or something that increases-- 01:17:21.410 --> 01:17:25.213 align:middle line:84% that tends to decrease the level of oxygen in the atmosphere, 01:17:25.213 --> 01:17:27.630 align:middle line:84% the level of oxygen in the atmosphere decreases, then less 01:17:27.630 --> 01:17:29.630 align:middle line:84% will be mixed down into the deep ocean. 01:17:29.630 --> 01:17:32.990 align:middle line:84% So the bottom water oxygen will have lower-- 01:17:32.990 --> 01:17:35.470 align:middle line:84% bottom water will have lower oxygen concentration. 01:17:35.470 --> 01:17:37.990 align:middle line:84% And other things being equal, the penetration depth 01:17:37.990 --> 01:17:40.470 align:middle line:84% of oxygen in the sediments will be smaller. 01:17:40.470 --> 01:17:43.410 align:middle line:84% If that's true, and if this argument is true, 01:17:43.410 --> 01:17:45.950 align:middle line:84% then oxygen exposure times will be smaller. 01:17:45.950 --> 01:17:48.150 align:middle line:84% So the preservation rate of organic matter 01:17:48.150 --> 01:17:53.710 align:middle line:84% will increase, which, remember, means a larger 01:17:53.710 --> 01:17:55.890 align:middle line:84% excess of photosynthesis over respiration, 01:17:55.890 --> 01:17:58.510 align:middle line:84% which tends to increase the atmospheric oxygen level. 01:17:58.510 --> 01:18:03.070 align:middle line:84% So there's a feedback between these inorganic carbon burial 01:18:03.070 --> 01:18:05.090 align:middle line:90% and continental weathering. 01:18:05.090 --> 01:18:08.870 align:middle line:90% 01:18:08.870 --> 01:18:15.390 align:middle line:84% So with that, I'll quit my discussion of organic matter 01:18:15.390 --> 01:18:19.140 align:middle line:90% decomposition in sediments.