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 --> 00:00:20.700 align:middle line:90% at ocw.mit.edu. 00:00:20.700 --> 00:00:22.880 align:middle line:84% PROFESSOR: I talked about some general overview 00:00:22.880 --> 00:00:23.620 align:middle line:90% kinds of things. 00:00:23.620 --> 00:00:25.960 align:middle line:84% And then we talked specifically about organic matter 00:00:25.960 --> 00:00:27.120 align:middle line:90% for a while. 00:00:27.120 --> 00:00:32.800 align:middle line:84% And in today's, we'll talk about the two, 00:00:32.800 --> 00:00:35.760 align:middle line:84% by mass, most important biogenic components 00:00:35.760 --> 00:00:40.360 align:middle line:84% of marine sediments, those being biogenic opal and calcium 00:00:40.360 --> 00:00:41.400 align:middle line:90% carbonate. 00:00:41.400 --> 00:00:48.880 align:middle line:84% And just by way of introduction, before we get into the details, 00:00:48.880 --> 00:00:52.360 align:middle line:84% say that, if you remember from before 00:00:52.360 --> 00:00:55.240 align:middle line:84% and we'll go over this again a little bit later, 00:00:55.240 --> 00:00:58.360 align:middle line:84% most of the biogenic opal that falls to the sea floor actually 00:00:58.360 --> 00:01:00.020 align:middle line:90% dissolves. 00:01:00.020 --> 00:01:02.740 align:middle line:84% But nonetheless, there has been in the past 00:01:02.740 --> 00:01:04.660 align:middle line:84% and there continues to be interest 00:01:04.660 --> 00:01:11.800 align:middle line:84% in biogenic opal and, in fact, in the silicon to-- well, 00:01:11.800 --> 00:01:15.140 align:middle line:84% the opal to organic carbon ratio in sediments as paleo 00:01:15.140 --> 00:01:17.260 align:middle line:90% productivity indicators. 00:01:17.260 --> 00:01:20.980 align:middle line:84% So there's lots of study of biogenic opal in sediments going 00:01:20.980 --> 00:01:27.315 align:middle line:84% on because of interest in it as a paleo proxy for productivity. 00:01:27.315 --> 00:01:29.440 align:middle line:84% AUDIENCE: I just want to let you know I'm here now. 00:01:29.440 --> 00:01:31.620 align:middle line:90% I'm sorry I'm late. 00:01:31.620 --> 00:01:32.600 align:middle line:90% It's Kristen. 00:01:32.600 --> 00:01:35.017 align:middle line:90% Hi. 00:01:35.017 --> 00:01:35.600 align:middle line:90% PROFESSOR: OK. 00:01:35.600 --> 00:01:39.820 align:middle line:84% And the second one we'll talk about is calcium carbonate. 00:01:39.820 --> 00:01:43.860 align:middle line:84% And the reason why calcium carbonate is important 00:01:43.860 --> 00:01:47.100 align:middle line:84% is because it's an important part of the marine carbon cycle. 00:01:47.100 --> 00:01:49.540 align:middle line:84% And we'll talk about that a little bit more 00:01:49.540 --> 00:01:51.460 align:middle line:84% as we get into calcium carbonate. 00:01:51.460 --> 00:01:53.200 align:middle line:84% So we're going to do them in turn. 00:01:53.200 --> 00:01:56.000 align:middle line:84% First, we'll talk about biogenic silica, or biogenic opal. 00:01:56.000 --> 00:01:58.240 align:middle line:84% And then we'll talk about calcium carbonate. 00:01:58.240 --> 00:02:03.080 align:middle line:84% And I have to warn you that last Thursday's lecture was 00:02:03.080 --> 00:02:04.080 align:middle line:90% pretty long. 00:02:04.080 --> 00:02:06.080 align:middle line:90% And today's is pretty long, too. 00:02:06.080 --> 00:02:08.320 align:middle line:84% You have to appreciate my position, 00:02:08.320 --> 00:02:11.500 align:middle line:84% where I spend my life studying sediment diagenesis, 00:02:11.500 --> 00:02:14.320 align:middle line:84% and the whole thing gets crammed into two lectures. 00:02:14.320 --> 00:02:16.640 align:middle line:84% And so I tend to stick much more stuff in there 00:02:16.640 --> 00:02:19.980 align:middle line:84% than really belongs, which can have two things. 00:02:19.980 --> 00:02:20.900 align:middle line:90% It can confuse you. 00:02:20.900 --> 00:02:22.600 align:middle line:90% And second, it can confuse me. 00:02:22.600 --> 00:02:26.960 align:middle line:84% So if I get confused, you make sure to stop me and tell me 00:02:26.960 --> 00:02:28.660 align:middle line:90% that I'm being confusing. 00:02:28.660 --> 00:02:29.160 align:middle line:90% OK. 00:02:29.160 --> 00:02:34.320 align:middle line:90% 00:02:34.320 --> 00:02:36.200 align:middle line:90% OK. 00:02:36.200 --> 00:02:38.560 align:middle line:84% So to start with biogenic opal, and first you 00:02:38.560 --> 00:02:39.660 align:middle line:90% might ask what it is. 00:02:39.660 --> 00:02:41.000 align:middle line:90% It's mineral phase. 00:02:41.000 --> 00:02:42.940 align:middle line:90% And it's amorphous silica. 00:02:42.940 --> 00:02:50.800 align:middle line:84% So it's SiO2 with water included in the mineral matrix. 00:02:50.800 --> 00:02:53.120 align:middle line:84% It's precipitated in the surface ocean 00:02:53.120 --> 00:02:56.790 align:middle line:84% by phytoplankton, diatoms, and silicoflagellates, 00:02:56.790 --> 00:02:58.290 align:middle line:90% and also by protozoans. 00:02:58.290 --> 00:03:01.230 align:middle line:84% And the main ones are radiolaria. 00:03:01.230 --> 00:03:04.690 align:middle line:84% A fraction of opal dissolves throughout the water column. 00:03:04.690 --> 00:03:08.190 align:middle line:84% A fraction of the opal that is formed in the surface ocean 00:03:08.190 --> 00:03:09.550 align:middle line:90% falls to the sea floor. 00:03:09.550 --> 00:03:11.010 align:middle line:84% And once it reaches the sea floor, 00:03:11.010 --> 00:03:13.550 align:middle line:90% it's efficiently recycled. 00:03:13.550 --> 00:03:15.910 align:middle line:84% And overall, about 3% of the opal 00:03:15.910 --> 00:03:17.470 align:middle line:84% that's formed in the surface ocean 00:03:17.470 --> 00:03:20.010 align:middle line:84% is ultimately preserved in sediments. 00:03:20.010 --> 00:03:26.230 align:middle line:90% 00:03:26.230 --> 00:03:31.812 align:middle line:84% In studying the early diagenesis of opal, 00:03:31.812 --> 00:03:33.270 align:middle line:84% the first thing you want to know is 00:03:33.270 --> 00:03:37.830 align:middle line:84% what's the solubility of this mineral phase. 00:03:37.830 --> 00:03:41.470 align:middle line:90% And I-- OK. 00:03:41.470 --> 00:03:43.950 align:middle line:90% Now-- OK. 00:03:43.950 --> 00:03:46.630 align:middle line:84% So the solubility of biogenic opal in seawater 00:03:46.630 --> 00:03:50.310 align:middle line:84% was studied a long time ago by David Heard. 00:03:50.310 --> 00:03:52.030 align:middle line:84% And the experiments he carried out 00:03:52.030 --> 00:03:53.950 align:middle line:84% were first he collected sediment cores 00:03:53.950 --> 00:03:55.770 align:middle line:90% from the equatorial Pacific. 00:03:55.770 --> 00:03:58.970 align:middle line:84% He separated the biogenic opal from those cores, 00:03:58.970 --> 00:04:02.850 align:middle line:84% cleaned up the opal so they'd have a clean surface 00:04:02.850 --> 00:04:05.290 align:middle line:84% to come into equilibrium with seawater. 00:04:05.290 --> 00:04:08.690 align:middle line:84% He placed his cleaned biogenic opal in seawater 00:04:08.690 --> 00:04:11.570 align:middle line:84% and controlled the temperature and pH. 00:04:11.570 --> 00:04:14.890 align:middle line:84% He let the opal sit in the seawater for several days. 00:04:14.890 --> 00:04:19.170 align:middle line:84% And at the end of his several day incubations 00:04:19.170 --> 00:04:21.649 align:middle line:84% of the opal in seawater, he measured the silicic acid 00:04:21.649 --> 00:04:24.130 align:middle line:90% concentration in seawater. 00:04:24.130 --> 00:04:28.610 align:middle line:84% And he presumed that the opal had come into equilibrium 00:04:28.610 --> 00:04:31.610 align:middle line:84% in seawater, so that the silicic acid concentration 00:04:31.610 --> 00:04:35.450 align:middle line:84% he was measuring in his experimental solutions 00:04:35.450 --> 00:04:39.010 align:middle line:84% was the equilibrium solubility of the opal. 00:04:39.010 --> 00:04:41.530 align:middle line:84% And what he's found is shown in this plot 00:04:41.530 --> 00:04:45.530 align:middle line:84% over here, which is a plot of on the y-axis 00:04:45.530 --> 00:04:48.950 align:middle line:84% is silicic acid concentration in his beakers 00:04:48.950 --> 00:04:52.010 align:middle line:84% with opal and seawater in them in micromoles per liter. 00:04:52.010 --> 00:04:55.670 align:middle line:84% And he also measured solubility as a function of pH. 00:04:55.670 --> 00:05:00.470 align:middle line:84% The pH range is about 6 to 9 here. 00:05:00.470 --> 00:05:01.910 align:middle line:90% Two things stand out. 00:05:01.910 --> 00:05:04.110 align:middle line:84% The first is that there is a strong temperature 00:05:04.110 --> 00:05:05.770 align:middle line:90% dependence of opal solubility. 00:05:05.770 --> 00:05:11.230 align:middle line:84% Opal is considerably-- is about 50% more soluble at 23 degrees 00:05:11.230 --> 00:05:14.710 align:middle line:84% than it is at deep ocean temperatures of about 3 degrees. 00:05:14.710 --> 00:05:17.190 align:middle line:84% And the second thing, and here's the number that you'll 00:05:17.190 --> 00:05:19.270 align:middle line:84% need to remember as we go along, is 00:05:19.270 --> 00:05:23.030 align:middle line:84% that the solubility of biogenic opal in seawater at 3 degrees 00:05:23.030 --> 00:05:27.890 align:middle line:84% is about 900 micromoles per liter. 00:05:27.890 --> 00:05:30.470 align:middle line:84% Now, the second piece of this puzzle 00:05:30.470 --> 00:05:35.350 align:middle line:84% is the silicic acid concentration in seawater. 00:05:35.350 --> 00:05:37.590 align:middle line:84% And what I've shown here is a bunch 00:05:37.590 --> 00:05:42.870 align:middle line:84% of contour plots of silicic acid concentrations in seawater 00:05:42.870 --> 00:05:45.270 align:middle line:84% that I lifted from Broecker and Peng. 00:05:45.270 --> 00:05:49.390 align:middle line:84% In the upper left is in the Western Atlantic. 00:05:49.390 --> 00:05:52.890 align:middle line:84% And following clockwise, it goes to the Indian Ocean. 00:05:52.890 --> 00:05:56.650 align:middle line:84% And then down below, it's in the North Pacific. 00:05:56.650 --> 00:05:58.910 align:middle line:84% You can see that in the Atlantic, 00:05:58.910 --> 00:06:00.310 align:middle line:90% concentrations increase. 00:06:00.310 --> 00:06:02.850 align:middle line:84% In the deep Atlantic, concentrations of silicic acid 00:06:02.850 --> 00:06:04.890 align:middle line:84% increase going from north to south. 00:06:04.890 --> 00:06:07.410 align:middle line:84% And the whole range in deep water is about 20 00:06:07.410 --> 00:06:10.730 align:middle line:84% to a maximum of about 120 micromoles per liter. 00:06:10.730 --> 00:06:13.130 align:middle line:84% The concentrations in the deep Indian Ocean 00:06:13.130 --> 00:06:16.610 align:middle line:84% are a little bit higher, but not a whole lot. 00:06:16.610 --> 00:06:19.482 align:middle line:84% They go up to a maximum of about 140 micromoles per liter. 00:06:19.482 --> 00:06:21.690 align:middle line:84% And then the highest concentrations in the deep ocean 00:06:21.690 --> 00:06:23.810 align:middle line:84% are reached in the North Pacific. 00:06:23.810 --> 00:06:27.370 align:middle line:84% And those are only about 200 micromoles per liter. 00:06:27.370 --> 00:06:31.730 align:middle line:84% So throughout the deep oceans, the seawater 00:06:31.730 --> 00:06:37.050 align:middle line:84% is greatly undersaturated with respect to biogenic opal. 00:06:37.050 --> 00:06:39.940 align:middle line:84% And so we've just said that some opal 00:06:39.940 --> 00:06:41.190 align:middle line:90% is preserved in the sediments. 00:06:41.190 --> 00:06:42.565 align:middle line:84% You might reasonably ask yourself 00:06:42.565 --> 00:06:44.810 align:middle line:84% why any opal is preserved since seawater 00:06:44.810 --> 00:06:48.150 align:middle line:84% is so undersaturated with respect to biogenic opal. 00:06:48.150 --> 00:06:51.030 align:middle line:90% 00:06:51.030 --> 00:06:54.870 align:middle line:90% So opal is a mineral phase. 00:06:54.870 --> 00:06:57.390 align:middle line:84% And bottom water is undersaturated with respect 00:06:57.390 --> 00:06:57.930 align:middle line:90% to opal. 00:06:57.930 --> 00:06:59.710 align:middle line:84% So the question is, what would you expect 00:06:59.710 --> 00:07:02.470 align:middle line:90% to observe in pore waters? 00:07:02.470 --> 00:07:06.310 align:middle line:84% And how would you expect the silicic acid concentration 00:07:06.310 --> 00:07:10.070 align:middle line:84% in pore waters to vary with depth below the sediment water 00:07:10.070 --> 00:07:12.910 align:middle line:84% interface as a result of the presence of this mineral that's 00:07:12.910 --> 00:07:15.350 align:middle line:84% set under saturation in the water? 00:07:15.350 --> 00:07:18.190 align:middle line:84% And the answer is you would get profiles that look something 00:07:18.190 --> 00:07:22.390 align:middle line:84% like this, where on the top is concentration, 00:07:22.390 --> 00:07:25.350 align:middle line:84% going from low concentration in bottom water. 00:07:25.350 --> 00:07:29.590 align:middle line:84% And I've run it up to what's a hypothetical saturation 00:07:29.590 --> 00:07:32.630 align:middle line:84% concentration with respect to the mineral phase 00:07:32.630 --> 00:07:33.830 align:middle line:90% that's dissolving. 00:07:33.830 --> 00:07:38.110 align:middle line:84% And on the x-axis is depth below the sediment water interface 00:07:38.110 --> 00:07:39.670 align:middle line:90% starting at 0. 00:07:39.670 --> 00:07:41.470 align:middle line:84% And I haven't specified a scale, but this 00:07:41.470 --> 00:07:44.270 align:middle line:84% might go down to about 20 centimeters 00:07:44.270 --> 00:07:46.110 align:middle line:90% depth in the sediments. 00:07:46.110 --> 00:07:51.700 align:middle line:84% And what occurs when a mineral that's undersaturated 00:07:51.700 --> 00:07:54.740 align:middle line:84% is included in sediments is immediately 00:07:54.740 --> 00:07:56.780 align:middle line:90% the mineral starts to dissolve. 00:07:56.780 --> 00:07:58.680 align:middle line:84% And what's produced-- in this case, 00:07:58.680 --> 00:08:01.820 align:middle line:84% silicic acid, which is produced by the dissolution of opal-- 00:08:01.820 --> 00:08:06.260 align:middle line:84% tends to diffuse out of the sediments into bottom water. 00:08:06.260 --> 00:08:09.140 align:middle line:84% So at any depth in the sediments, 00:08:09.140 --> 00:08:11.540 align:middle line:84% the concentration that's reached is a balance 00:08:11.540 --> 00:08:14.340 align:middle line:84% between this diffusion out, which 00:08:14.340 --> 00:08:16.820 align:middle line:84% takes away silicic acid, and the addition 00:08:16.820 --> 00:08:20.380 align:middle line:84% of silicic acid to the pore waters by dissolution of opal. 00:08:20.380 --> 00:08:22.500 align:middle line:84% And as you go deeper in the sediments, 00:08:22.500 --> 00:08:25.420 align:middle line:84% the concentration of silicic acid in the pore waters 00:08:25.420 --> 00:08:28.020 align:middle line:84% will increase and asymptotically approach 00:08:28.020 --> 00:08:30.820 align:middle line:90% the saturation concentration. 00:08:30.820 --> 00:08:38.179 align:middle line:84% So you might think that the silicic acid concentration 00:08:38.179 --> 00:08:40.539 align:middle line:84% in pore waters would look very similar 00:08:40.539 --> 00:08:44.340 align:middle line:84% throughout the oceans, where opal falls to the sea floor. 00:08:44.340 --> 00:08:45.320 align:middle line:90% It dissolves. 00:08:45.320 --> 00:08:49.400 align:middle line:84% And the silicic acid level in the pore waters 00:08:49.400 --> 00:08:51.600 align:middle line:84% asymptotically approaches saturation, which 00:08:51.600 --> 00:08:54.760 align:middle line:90% is 900 micromoles per liter. 00:08:54.760 --> 00:09:00.840 align:middle line:84% So the question is, what do you observe in actual pore waters? 00:09:00.840 --> 00:09:05.080 align:middle line:84% And here's a sampling of what you observe, 00:09:05.080 --> 00:09:07.960 align:middle line:84% where I've lifted pore water silicic acid 00:09:07.960 --> 00:09:10.920 align:middle line:84% profiles from the North Atlantic up here, 00:09:10.920 --> 00:09:13.600 align:middle line:84% the central equatorial Pacific, the Peru margin 00:09:13.600 --> 00:09:15.120 align:middle line:90% in the Southern Ocean. 00:09:15.120 --> 00:09:19.065 align:middle line:84% And I hope these are easy to read, 00:09:19.065 --> 00:09:20.940 align:middle line:84% where if you look at the North Atlantic ones, 00:09:20.940 --> 00:09:23.460 align:middle line:90% the concentration starts at 0. 00:09:23.460 --> 00:09:27.000 align:middle line:84% That's the sediment water interface at about a little 00:09:27.000 --> 00:09:29.660 align:middle line:84% more than 20 micromoles per liter and increases. 00:09:29.660 --> 00:09:32.440 align:middle line:84% It reaches an asymptotic value, as you 00:09:32.440 --> 00:09:34.660 align:middle line:84% would expect from that simple picture I showed you. 00:09:34.660 --> 00:09:38.000 align:middle line:84% But the catch is that this asymptotic value 00:09:38.000 --> 00:09:41.920 align:middle line:84% is only about 100 to 120 micromoles per liter. 00:09:41.920 --> 00:09:45.800 align:middle line:84% This is in the North Atlantic at an oligotrophic site. 00:09:45.800 --> 00:09:48.860 align:middle line:84% If we go to the equatorial Pacific upwelling region, 00:09:48.860 --> 00:09:53.700 align:middle line:84% again, you see the simple increasing profile 00:09:53.700 --> 00:09:55.780 align:middle line:84% where the silicic acid concentration 00:09:55.780 --> 00:09:59.440 align:middle line:84% starts at bottom water value and approaches an asymptotic value. 00:09:59.440 --> 00:10:03.540 align:middle line:84% But here, it's about 600 micromoles per liter. 00:10:03.540 --> 00:10:06.840 align:middle line:84% If we follow around and go look at the Peru margin sites. 00:10:06.840 --> 00:10:09.740 align:middle line:84% So that's a site where there are terrigenous sediments 00:10:09.740 --> 00:10:12.020 align:middle line:90% in a highly productive region. 00:10:12.020 --> 00:10:17.600 align:middle line:84% And here, again, you have the simple increasing profiles. 00:10:17.600 --> 00:10:22.100 align:middle line:84% But the asymptotic values range from about 500 to 800 micromoles 00:10:22.100 --> 00:10:22.800 align:middle line:90% per liter. 00:10:22.800 --> 00:10:24.300 align:middle line:84% And if you go to the Southern Ocean, 00:10:24.300 --> 00:10:25.900 align:middle line:84% you, see exactly the same pattern 00:10:25.900 --> 00:10:28.620 align:middle line:84% of silicic acid concentration, increasing with depth 00:10:28.620 --> 00:10:30.740 align:middle line:84% below the sediment water interface. 00:10:30.740 --> 00:10:35.020 align:middle line:84% And here, the asymptotic silicic acid concentration 00:10:35.020 --> 00:10:39.140 align:middle line:84% varies from about 500 to 750 micromoles per liter. 00:10:39.140 --> 00:10:43.340 align:middle line:84% So if you summarize these results, 00:10:43.340 --> 00:10:47.620 align:middle line:84% here are the different sites that I showed you on the x-axis. 00:10:47.620 --> 00:10:50.600 align:middle line:84% On the y-axis is the asymptotic silicic acid concentration 00:10:50.600 --> 00:10:52.160 align:middle line:90% reached in pore waters. 00:10:52.160 --> 00:10:55.640 align:middle line:84% And you see that in some locations, 00:10:55.640 --> 00:10:58.660 align:middle line:84% it's very, very far below equilibrium with opal. 00:10:58.660 --> 00:11:00.938 align:middle line:84% In other cases, such as the Southern Ocean-- in fact, 00:11:00.938 --> 00:11:02.980 align:middle line:84% if I included more sites from the Southern Ocean, 00:11:02.980 --> 00:11:05.313 align:middle line:84% there would be some that even reach apparent equilibrium 00:11:05.313 --> 00:11:06.320 align:middle line:90% with opal. 00:11:06.320 --> 00:11:10.620 align:middle line:84% But in all these cases, there is a distinct asymptotic value. 00:11:10.620 --> 00:11:14.120 align:middle line:84% But it's not at equilibrium with respect to opal. 00:11:14.120 --> 00:11:17.060 align:middle line:84% So the question you might ask is, why is that? 00:11:17.060 --> 00:11:20.400 align:middle line:84% And what actually is determining the behavior 00:11:20.400 --> 00:11:22.420 align:middle line:90% of opal in sediments? 00:11:22.420 --> 00:11:26.740 align:middle line:90% 00:11:26.740 --> 00:11:33.760 align:middle line:84% So Van Cappellen, Philippe Van Cappellen, 00:11:33.760 --> 00:11:38.000 align:middle line:84% did a series of studies using sediments 00:11:38.000 --> 00:11:39.100 align:middle line:90% from the Southern Ocean. 00:11:39.100 --> 00:11:43.820 align:middle line:84% And what they did, they collected sediment cores 00:11:43.820 --> 00:11:46.420 align:middle line:84% from a range of latitudes starting 00:11:46.420 --> 00:11:49.300 align:middle line:84% south of the Antarctic convergence, 00:11:49.300 --> 00:11:52.980 align:middle line:84% at this site that I'm showing with my arrow right here, 00:11:52.980 --> 00:11:55.020 align:middle line:84% and moving north to sites that are north 00:11:55.020 --> 00:11:57.700 align:middle line:84% of the Antarctic convergence in the Indian sector 00:11:57.700 --> 00:11:59.260 align:middle line:90% of the Southern Ocean. 00:11:59.260 --> 00:12:02.673 align:middle line:84% And the first thing they did was measure sediment composition. 00:12:02.673 --> 00:12:04.340 align:middle line:84% And what they found was, of course, what 00:12:04.340 --> 00:12:07.620 align:middle line:84% they expected to find was that the southernmost sites-- 00:12:07.620 --> 00:12:10.300 align:middle line:84% and here, I've followed it along with this arrow 00:12:10.300 --> 00:12:13.700 align:middle line:84% to a sediment composition plot below-- 00:12:13.700 --> 00:12:18.520 align:middle line:84% the southernmost sites are pretty close to being pure opal. 00:12:18.520 --> 00:12:21.700 align:middle line:84% So it's a very high opal concentration in the sediments. 00:12:21.700 --> 00:12:24.300 align:middle line:84% But as you move to the more northerly sites, 00:12:24.300 --> 00:12:29.080 align:middle line:84% there's a steady increase in this dark-shaded bars, 00:12:29.080 --> 00:12:32.160 align:middle line:84% which are the detrital component of the sediments. 00:12:32.160 --> 00:12:35.840 align:middle line:84% And there's a steady decrease in the unfilled bars, 00:12:35.840 --> 00:12:37.930 align:middle line:84% which are the opal concentration of the sediments. 00:12:37.930 --> 00:12:39.680 align:middle line:84% So they go from the southernmost stations, 00:12:39.680 --> 00:12:43.170 align:middle line:84% which are nearly pure opal, to the northernmost stations, which 00:12:43.170 --> 00:12:46.210 align:middle line:84% have a small opal concentration and a large detrital 00:12:46.210 --> 00:12:47.470 align:middle line:90% concentration. 00:12:47.470 --> 00:12:50.140 align:middle line:90% 00:12:50.140 --> 00:12:54.270 align:middle line:84% So they didn't stop at just measuring sediment composition. 00:12:54.270 --> 00:12:57.570 align:middle line:84% They also measured pore waters, pore water concentrations 00:12:57.570 --> 00:13:00.490 align:middle line:84% of some components in these same sediments. 00:13:00.490 --> 00:13:04.290 align:middle line:84% And in the top up here are silicic acid concentration 00:13:04.290 --> 00:13:06.530 align:middle line:90% profiles from those sediments. 00:13:06.530 --> 00:13:09.210 align:middle line:90% And you have the expected shape. 00:13:09.210 --> 00:13:12.410 align:middle line:84% But the southernmost sites, which 00:13:12.410 --> 00:13:16.050 align:middle line:84% have very high opal contents in the sediments, 00:13:16.050 --> 00:13:19.650 align:middle line:84% have asymptotic concentrations of silicic acid, which 00:13:19.650 --> 00:13:21.630 align:middle line:90% are very near opal solubility. 00:13:21.630 --> 00:13:24.650 align:middle line:84% But as they go further north and the detrital component 00:13:24.650 --> 00:13:27.250 align:middle line:84% in the sediments gets larger and larger, 00:13:27.250 --> 00:13:29.690 align:middle line:84% this asymptotic silicic acid concentration 00:13:29.690 --> 00:13:31.650 align:middle line:90% gets lower and lower. 00:13:31.650 --> 00:13:34.550 align:middle line:84% And that's summarized in the plot over here, 00:13:34.550 --> 00:13:40.510 align:middle line:84% which shows asymptotic silicic acid concentration versus weight 00:13:40.510 --> 00:13:43.550 align:middle line:84% percent detrital divided by weight percent opal. 00:13:43.550 --> 00:13:47.550 align:middle line:84% So increasing detrital concentration in the sediments 00:13:47.550 --> 00:13:49.710 align:middle line:84% is associated with a lower asymptotic 00:13:49.710 --> 00:13:53.470 align:middle line:90% silicic acid concentration. 00:13:53.470 --> 00:13:56.270 align:middle line:84% Of course, that detrital material-- whereas opal 00:13:56.270 --> 00:13:58.310 align:middle line:90% is essentially SiO2. 00:13:58.310 --> 00:14:00.350 align:middle line:90% It's silica. 00:14:00.350 --> 00:14:03.330 align:middle line:84% Detrital minerals are aluminosilicates, 00:14:03.330 --> 00:14:06.430 align:middle line:84% so in addition to SiO2 they also contain 00:14:06.430 --> 00:14:08.390 align:middle line:90% aluminum oxides, aluminum. 00:14:08.390 --> 00:14:12.430 align:middle line:84% And so it's reasonable to expect that as detrital component 00:14:12.430 --> 00:14:14.390 align:middle line:84% of the sediments increases, you'd 00:14:14.390 --> 00:14:17.110 align:middle line:84% expect a difference in the aluminum concentrations 00:14:17.110 --> 00:14:20.250 align:middle line:84% that are found in the pore waters of those sediments. 00:14:20.250 --> 00:14:23.610 align:middle line:84% So what's shown in the lower half of this picture? 00:14:23.610 --> 00:14:29.470 align:middle line:84% Here on the left are plots of the aluminum to silicon ratio 00:14:29.470 --> 00:14:30.810 align:middle line:90% in pore waters. 00:14:30.810 --> 00:14:33.270 align:middle line:84% And at those two southernmost stations, 00:14:33.270 --> 00:14:37.050 align:middle line:84% you can see there's essentially no structure to the profiles. 00:14:37.050 --> 00:14:39.090 align:middle line:90% And the values are very low. 00:14:39.090 --> 00:14:41.490 align:middle line:84% If you look at the northernmost stations 00:14:41.490 --> 00:14:46.130 align:middle line:84% with a large detrital component in the sediments, 00:14:46.130 --> 00:14:48.810 align:middle line:84% you get a very different picture, where this poor water 00:14:48.810 --> 00:14:52.650 align:middle line:84% aluminum silicon ratio has a distinct maximum 00:14:52.650 --> 00:14:54.850 align:middle line:84% near the sediment water interface. 00:14:54.850 --> 00:14:57.930 align:middle line:84% And the value the highest values are 00:14:57.930 --> 00:14:59.930 align:middle line:84% something like 10 times the aluminum 00:14:59.930 --> 00:15:04.050 align:middle line:84% to silicon ratio in the nearly pure opal sediments. 00:15:04.050 --> 00:15:06.090 align:middle line:84% Now, those findings are summarized 00:15:06.090 --> 00:15:08.930 align:middle line:84% on the right-hand side, which is a plot of the maximum aluminum 00:15:08.930 --> 00:15:12.330 align:middle line:84% concentration in the pore waters in nanomolar 00:15:12.330 --> 00:15:14.130 align:middle line:90% versus weight percent detrital. 00:15:14.130 --> 00:15:18.770 align:middle line:84% The higher the detrital content of the sediments, 00:15:18.770 --> 00:15:22.210 align:middle line:84% the higher the maximum aluminum concentration 00:15:22.210 --> 00:15:24.670 align:middle line:90% in the sediment pore waters. 00:15:24.670 --> 00:15:27.170 align:middle line:84% So they used these results, plus a whole series 00:15:27.170 --> 00:15:31.387 align:middle line:84% of laboratory experiments, to show a couple of things. 00:15:31.387 --> 00:15:33.470 align:middle line:84% And I don't have time to show the lab experiments. 00:15:33.470 --> 00:15:36.470 align:middle line:84% But I'll just describe the general findings. 00:15:36.470 --> 00:15:39.790 align:middle line:84% First, they found-- they and others have found as well, 00:15:39.790 --> 00:15:44.150 align:middle line:84% that as the aluminum concentration in biogenic silica 00:15:44.150 --> 00:15:47.590 align:middle line:84% increases, its solubility decreases. 00:15:47.590 --> 00:15:49.830 align:middle line:84% The second thing they found from their experiments 00:15:49.830 --> 00:15:54.310 align:middle line:84% was that if dissolved aluminum and silicic acid 00:15:54.310 --> 00:15:58.270 align:middle line:84% are present, along with solids mixed in, what happens 00:15:58.270 --> 00:16:02.110 align:middle line:84% is that they react together to form authigenic aluminosilicate 00:16:02.110 --> 00:16:03.110 align:middle line:90% minerals. 00:16:03.110 --> 00:16:09.510 align:middle line:84% And so they summarized their field observations 00:16:09.510 --> 00:16:12.830 align:middle line:84% and experimental findings in a picture 00:16:12.830 --> 00:16:15.870 align:middle line:84% of opal diagenesis in sediments that look something 00:16:15.870 --> 00:16:20.910 align:middle line:84% like this, both biogenic silica and detrital minerals 00:16:20.910 --> 00:16:22.590 align:middle line:90% fall to the sea floor. 00:16:22.590 --> 00:16:23.630 align:middle line:90% Two things happen. 00:16:23.630 --> 00:16:26.830 align:middle line:84% The biogenic silica dissolves because the pore waters 00:16:26.830 --> 00:16:29.790 align:middle line:84% are undersaturated with respect to biogenic silica. 00:16:29.790 --> 00:16:33.270 align:middle line:84% And the detrital minerals also dissolve to some extent. 00:16:33.270 --> 00:16:36.370 align:middle line:84% The result of those is-- those two processes 00:16:36.370 --> 00:16:39.450 align:middle line:84% are that silicic acid is released into the pore waters, 00:16:39.450 --> 00:16:43.410 align:middle line:84% and aluminum 3 is released into the pore waters. 00:16:43.410 --> 00:16:46.790 align:middle line:84% This aluminum can be included in biogenic silica. 00:16:46.790 --> 00:16:49.030 align:middle line:84% It can be added to the biogenic silica matrix, 00:16:49.030 --> 00:16:51.490 align:middle line:90% making it less soluble. 00:16:51.490 --> 00:16:53.810 align:middle line:84% But that change in solubility by itself 00:16:53.810 --> 00:16:56.850 align:middle line:84% isn't enough to explain this huge change 00:16:56.850 --> 00:16:59.210 align:middle line:84% in the asymptotic silicic acid concentrations 00:16:59.210 --> 00:17:00.070 align:middle line:90% that they observe. 00:17:00.070 --> 00:17:01.730 align:middle line:90% So something else is going on. 00:17:01.730 --> 00:17:05.490 align:middle line:84% And their experiments showed that the dissolved aluminum 00:17:05.490 --> 00:17:08.770 align:middle line:84% can react with the dissolved silicic acid 00:17:08.770 --> 00:17:13.170 align:middle line:84% to form authigenic aluminosilicate minerals. 00:17:13.170 --> 00:17:16.770 align:middle line:84% And so they concluded with a hypothesis 00:17:16.770 --> 00:17:21.290 align:middle line:84% about what could cause this variation 00:17:21.290 --> 00:17:24.810 align:middle line:84% in the asymptotic silicic acid concentration that 00:17:24.810 --> 00:17:27.650 align:middle line:84% are observed in sediments throughout the oceans. 00:17:27.650 --> 00:17:33.830 align:middle line:84% And their hypothesis is that it's not an equilibrium value, 00:17:33.830 --> 00:17:41.110 align:middle line:84% but rather it's a kinetically balanced value, where 00:17:41.110 --> 00:17:44.150 align:middle line:84% in a sediment that's pure opal, there's 00:17:44.150 --> 00:17:47.750 align:middle line:84% very little aluminosilicate-- authigenic aluminosilicate 00:17:47.750 --> 00:17:49.150 align:middle line:90% mineral formation. 00:17:49.150 --> 00:17:51.630 align:middle line:84% And so the asymptotic silicic acid concentration 00:17:51.630 --> 00:17:54.150 align:middle line:84% is very close to opal solubility. 00:17:54.150 --> 00:17:57.670 align:middle line:84% As the detrital content of the sediments increases, 00:17:57.670 --> 00:17:59.790 align:middle line:84% there's more and more rapid formation 00:17:59.790 --> 00:18:02.790 align:middle line:84% of this alumino-- authigenic aluminosilicate. 00:18:02.790 --> 00:18:06.950 align:middle line:84% And so the asymptotic silicic acid concentration 00:18:06.950 --> 00:18:10.830 align:middle line:84% is a balance between the rate of dissolution 00:18:10.830 --> 00:18:13.270 align:middle line:84% of opal in the sediments and the rate of formation 00:18:13.270 --> 00:18:16.770 align:middle line:84% of authigenic mineral, the authigenic aluminosilicate. 00:18:16.770 --> 00:18:21.350 align:middle line:84% So the more rapid the authigenic aluminosilicate formation is, 00:18:21.350 --> 00:18:24.990 align:middle line:84% the lower this asymptotic silicic acid concentration is. 00:18:24.990 --> 00:18:25.870 align:middle line:90% OK. 00:18:25.870 --> 00:18:27.870 align:middle line:84% AUDIENCE: Can you explain what authigenic means? 00:18:27.870 --> 00:18:31.980 align:middle line:84% PROFESSOR: Authigenic means formed in place. 00:18:31.980 --> 00:18:32.825 align:middle line:90% Is that the-- 00:18:32.825 --> 00:18:34.200 align:middle line:84% I mean, that's how I think of it. 00:18:34.200 --> 00:18:36.600 align:middle line:84% I don't know what is "authi" mean. 00:18:36.600 --> 00:18:37.566 align:middle line:90% I don't know. 00:18:37.566 --> 00:18:45.985 align:middle line:90% 00:18:45.985 --> 00:18:47.860 align:middle line:84% AUDIENCE: So it seems almost counterintuitive 00:18:47.860 --> 00:18:52.420 align:middle line:84% then, because as your solubility decreases, 00:18:52.420 --> 00:18:55.040 align:middle line:84% you're actually still ending up with less opal, right? 00:18:55.040 --> 00:19:00.760 align:middle line:84% It's just getting incorporated in different ways? 00:19:00.760 --> 00:19:01.760 align:middle line:90% PROFESSOR: That's right. 00:19:01.760 --> 00:19:05.640 align:middle line:84% Well, you have two different mineral phases. 00:19:05.640 --> 00:19:09.740 align:middle line:84% Of course, the solution can't be in equilibrium with two phases. 00:19:09.740 --> 00:19:13.460 align:middle line:84% One is a very insoluble aluminosilicate phase. 00:19:13.460 --> 00:19:16.180 align:middle line:84% And the other is a much more soluble opal phase. 00:19:16.180 --> 00:19:22.340 align:middle line:84% And what's observed is an apparent equilibrium that's 00:19:22.340 --> 00:19:24.020 align:middle line:90% somewhere in between those. 00:19:24.020 --> 00:19:27.900 align:middle line:84% The more rapid the formation of the authigenic aluminosilicate 00:19:27.900 --> 00:19:31.760 align:middle line:90% is, the lower this asymptotic-- 00:19:31.760 --> 00:19:34.480 align:middle line:84% the closer the solution comes to equilibrium with respect 00:19:34.480 --> 00:19:35.840 align:middle line:90% to that phase. 00:19:35.840 --> 00:19:37.095 align:middle line:90% Does that make sense to you? 00:19:37.095 --> 00:19:37.720 align:middle line:90% AUDIENCE: Yeah. 00:19:37.720 --> 00:19:40.440 align:middle line:90% 00:19:40.440 --> 00:19:42.520 align:middle line:90% So then-- right. 00:19:42.520 --> 00:19:46.503 align:middle line:84% So then you're actually getting less-- and in those cases-- 00:19:46.503 --> 00:19:47.920 align:middle line:84% or rather, is there a relationship 00:19:47.920 --> 00:19:50.140 align:middle line:84% between opal preservation and the clays? 00:19:50.140 --> 00:19:53.760 align:middle line:84% Is the more clays, the less opal you end up with? 00:19:53.760 --> 00:19:56.720 align:middle line:90% Or is it not so simple? 00:19:56.720 --> 00:19:59.740 align:middle line:84% PROFESSOR: I don't think anyone has answered that question. 00:19:59.740 --> 00:20:02.680 align:middle line:84% I'm going to show you some studies of opal, 00:20:02.680 --> 00:20:06.680 align:middle line:84% some mass balance studies of opal preservation. 00:20:06.680 --> 00:20:11.880 align:middle line:84% But I don't know that anyone has answered that specific question. 00:20:11.880 --> 00:20:13.960 align:middle line:84% Well, I can point you to somewhere 00:20:13.960 --> 00:20:16.620 align:middle line:84% to look, which I'll do in just a second. 00:20:16.620 --> 00:20:18.700 align:middle line:90% OK. 00:20:18.700 --> 00:20:22.520 align:middle line:84% We'll end up here with a little summary 00:20:22.520 --> 00:20:25.680 align:middle line:84% of some studies of the preservation rate of opal 00:20:25.680 --> 00:20:28.460 align:middle line:90% in deep sea sediments. 00:20:28.460 --> 00:20:32.340 align:middle line:84% In order to carry out these studies, what the investigators 00:20:32.340 --> 00:20:35.080 align:middle line:84% have measured is first the rain rate of opal to the sea floor. 00:20:35.080 --> 00:20:37.900 align:middle line:84% And they've done that using time series sediment traps. 00:20:37.900 --> 00:20:40.860 align:middle line:84% Second, they measure benthic remineralization rates, so 00:20:40.860 --> 00:20:44.880 align:middle line:84% the rate of dissolution of the opal in the pore waters. 00:20:44.880 --> 00:20:47.240 align:middle line:90% And they do that in two ways. 00:20:47.240 --> 00:20:50.060 align:middle line:84% One is to directly measure the flux of silicate 00:20:50.060 --> 00:20:53.460 align:middle line:84% across the sediment water interface using flux chambers. 00:20:53.460 --> 00:20:56.420 align:middle line:84% And the second is to calculate fluxes across the sediment water 00:20:56.420 --> 00:21:00.180 align:middle line:84% interface using pore water profiles of silicic acid. 00:21:00.180 --> 00:21:02.380 align:middle line:84% And the third component of these studies 00:21:02.380 --> 00:21:05.115 align:middle line:84% is the burial rate of opal in the sediments. 00:21:05.115 --> 00:21:06.740 align:middle line:84% And they've done that using solid phase 00:21:06.740 --> 00:21:09.020 align:middle line:84% measurements of opal concentrations 00:21:09.020 --> 00:21:11.380 align:middle line:90% and sediment accumulation rates. 00:21:11.380 --> 00:21:14.140 align:middle line:90% OK. 00:21:14.140 --> 00:21:18.380 align:middle line:84% Here's a set of results of a bunch of these studies. 00:21:18.380 --> 00:21:24.260 align:middle line:84% And each set of three bars here represents 00:21:24.260 --> 00:21:26.880 align:middle line:90% a study at one location. 00:21:26.880 --> 00:21:29.120 align:middle line:84% And at each location, what was measured 00:21:29.120 --> 00:21:33.280 align:middle line:84% was the rain rate of opal to the seafloor, the opal dissolution 00:21:33.280 --> 00:21:37.360 align:middle line:84% rate through the flux of dissolved silicate 00:21:37.360 --> 00:21:40.360 align:middle line:84% across the sediment water interface, and the opal burial 00:21:40.360 --> 00:21:41.640 align:middle line:90% rate. 00:21:41.640 --> 00:21:43.460 align:middle line:84% For internal consistency, in the study, 00:21:43.460 --> 00:21:46.043 align:middle line:84% you want to be able to add the burial rate and the dissolution 00:21:46.043 --> 00:21:47.740 align:middle line:84% rate and have it equal the rain rate. 00:21:47.740 --> 00:21:50.940 align:middle line:84% And that actually was very close to true in all of these studies. 00:21:50.940 --> 00:21:53.760 align:middle line:84% So they're internally consistent. 00:21:53.760 --> 00:21:57.840 align:middle line:84% And using these results, you can measure the preservation 00:21:57.840 --> 00:22:02.080 align:middle line:84% efficiency for opal by dividing the measured burial 00:22:02.080 --> 00:22:06.473 align:middle line:84% rate by the measured rain rate to the sea floor. 00:22:06.473 --> 00:22:08.640 align:middle line:84% I suppose I should tell you where all these are from 00:22:08.640 --> 00:22:13.600 align:middle line:84% before I show the final result. The left-hand most two 00:22:13.600 --> 00:22:16.600 align:middle line:84% are from the North Atlantic Ocean, Portuguese abyssal plain 00:22:16.600 --> 00:22:18.600 align:middle line:90% and the Bermuda site. 00:22:18.600 --> 00:22:23.040 align:middle line:84% The third one there is from the Indian Ocean. 00:22:23.040 --> 00:22:25.680 align:middle line:84% The fourth one is from the equatorial Pacific. 00:22:25.680 --> 00:22:28.760 align:middle line:84% And the remaining, 1, 2, 3, 4, are all from the Southern Ocean. 00:22:28.760 --> 00:22:32.660 align:middle line:84% And here are the burial efficiency results, 00:22:32.660 --> 00:22:34.740 align:middle line:84% summarized on the right-hand side. 00:22:34.740 --> 00:22:37.860 align:middle line:84% And you can see that all the results fall between about 5% 00:22:37.860 --> 00:22:41.060 align:middle line:84% and 15% preservation of opal, which 00:22:41.060 --> 00:22:44.860 align:middle line:84% is very near the mean that's found in essentially all 00:22:44.860 --> 00:22:47.340 align:middle line:84% the studies of opal preservation in sediments, 00:22:47.340 --> 00:22:51.540 align:middle line:90% somewhere between 5% and 15%. 00:22:51.540 --> 00:22:56.540 align:middle line:84% And as I said at the beginning, despite these low preservation 00:22:56.540 --> 00:23:00.220 align:middle line:84% efficiencies, there continues to be considerable interest 00:23:00.220 --> 00:23:04.540 align:middle line:84% in measuring opal and the opal-to-organic carbon ratio 00:23:04.540 --> 00:23:07.460 align:middle line:84% as indicators of diatom productivity 00:23:07.460 --> 00:23:09.000 align:middle line:90% changes in the past. 00:23:09.000 --> 00:23:12.820 align:middle line:84% And I'm not going to try to make the case for this. 00:23:12.820 --> 00:23:15.620 align:middle line:84% But if you read the Nelson paper, if you're interested 00:23:15.620 --> 00:23:17.620 align:middle line:84% and you read this Nelson paper, he 00:23:17.620 --> 00:23:20.540 align:middle line:84% makes the case using measurements 00:23:20.540 --> 00:23:22.260 align:middle line:90% from the Southern Ocean. 00:23:22.260 --> 00:23:26.390 align:middle line:84% And unfortunately, I don't have it referred to directly. 00:23:26.390 --> 00:23:28.530 align:middle line:84% But there's another Ragueneau paper 00:23:28.530 --> 00:23:33.730 align:middle line:84% that's a very, very long review paper covering essentially 00:23:33.730 --> 00:23:36.450 align:middle line:84% everything that was known as of about three or four 00:23:36.450 --> 00:23:39.290 align:middle line:90% years ago on opal in sediments. 00:23:39.290 --> 00:23:43.170 align:middle line:84% And it will answer your question if anybody has answered it. 00:23:43.170 --> 00:23:44.930 align:middle line:84% And it tries very hard to make the case 00:23:44.930 --> 00:23:47.830 align:middle line:84% for using opal as a paleo productivity indicator. 00:23:47.830 --> 00:23:50.450 align:middle line:90% 00:23:50.450 --> 00:23:57.130 align:middle line:84% One last thing before we leave the subject is a map from 00:23:57.130 --> 00:24:00.330 align:middle line:84% Sarmiento and Gruber-- not Gruber and Sarmiento-- 00:24:00.330 --> 00:24:03.158 align:middle line:90% of opal-- hmm? 00:24:03.158 --> 00:24:05.075 align:middle line:84% AUDIENCE: Here's what the publisher wrote down 00:24:05.075 --> 00:24:05.617 align:middle line:90% in the book-- 00:24:05.617 --> 00:24:06.370 align:middle line:90% PROFESSOR: Oh, OK. 00:24:06.370 --> 00:24:07.170 align:middle line:90% Yeah. 00:24:07.170 --> 00:24:08.690 align:middle line:90% OK. 00:24:08.690 --> 00:24:12.090 align:middle line:84% A map of opal concentrations in sediments. 00:24:12.090 --> 00:24:14.530 align:middle line:84% And for those of you who have Tracers 00:24:14.530 --> 00:24:17.690 align:middle line:84% in the Sea, the old Broecker and Peng book, 00:24:17.690 --> 00:24:20.450 align:middle line:84% throw away the opal concentration map in that book 00:24:20.450 --> 00:24:23.630 align:middle line:84% and use this one because what they don't tell you 00:24:23.630 --> 00:24:27.590 align:middle line:84% is they measure carbonate free opal concentrations. 00:24:27.590 --> 00:24:30.030 align:middle line:84% And the result is very high values 00:24:30.030 --> 00:24:31.590 align:middle line:90% in the equatorial Pacific. 00:24:31.590 --> 00:24:34.150 align:middle line:84% Throw away that map and look at this one. 00:24:34.150 --> 00:24:39.350 align:middle line:84% And the features of this map are, first, 00:24:39.350 --> 00:24:41.550 align:middle line:84% the highest opal concentrations in sediments 00:24:41.550 --> 00:24:45.070 align:middle line:84% are reached in the Southern Ocean, where you get sediments-- 00:24:45.070 --> 00:24:47.970 align:middle line:84% because of the high diatom productivity in the region, 00:24:47.970 --> 00:24:51.070 align:middle line:84% you get sediments that are almost pure opal. 00:24:51.070 --> 00:24:53.470 align:middle line:84% Second obvious feature in this map 00:24:53.470 --> 00:24:58.190 align:middle line:84% is that there's a belt of high opal concentrations in sediments 00:24:58.190 --> 00:25:00.190 align:middle line:90% around the equator. 00:25:00.190 --> 00:25:04.990 align:middle line:84% And it's especially prominent in the equatorial Pacific. 00:25:04.990 --> 00:25:07.730 align:middle line:84% The third feature is in the Northwest Pacific. 00:25:07.730 --> 00:25:11.230 align:middle line:84% There's also a region of fairly high opal concentrations 00:25:11.230 --> 00:25:12.310 align:middle line:90% in the sediments. 00:25:12.310 --> 00:25:16.630 align:middle line:84% But you can see it's a major phase in sediments 00:25:16.630 --> 00:25:20.690 align:middle line:84% where in the equatorial Pacific it's up to about 30% opal. 00:25:20.690 --> 00:25:24.290 align:middle line:84% And the Southern Ocean, it can be the predominant component 00:25:24.290 --> 00:25:25.290 align:middle line:90% of sediments. 00:25:25.290 --> 00:25:34.950 align:middle line:90% 00:25:34.950 --> 00:25:38.530 align:middle line:84% Now moving on to calcium carbonate-- 00:25:38.530 --> 00:25:42.930 align:middle line:84% so calcium carbonate is the predominant biogenic component 00:25:42.930 --> 00:25:45.550 align:middle line:84% of sediments over much of the sea floor. 00:25:45.550 --> 00:25:48.190 align:middle line:84% And in fact, in big areas of the sea floor, 00:25:48.190 --> 00:25:50.190 align:middle line:84% it's the predominant component of sediments, 00:25:50.190 --> 00:25:53.210 align:middle line:90% as we'll see in a little while. 00:25:53.210 --> 00:25:55.670 align:middle line:84% Why study calcium carbonate in sediments? 00:25:55.670 --> 00:25:59.010 align:middle line:90% 00:25:59.010 --> 00:26:03.170 align:middle line:84% It's important, and I hope that you'll talk about it later, 00:26:03.170 --> 00:26:05.570 align:middle line:84% talking about carbon cycle in the oceans, 00:26:05.570 --> 00:26:08.170 align:middle line:84% that changes in the marine alkalinity budget 00:26:08.170 --> 00:26:10.410 align:middle line:84% have been hypothesized as drivers 00:26:10.410 --> 00:26:14.050 align:middle line:90% of changes in atmospheric PCO2. 00:26:14.050 --> 00:26:16.430 align:middle line:84% And you'll go through those mechanisms, I hope. 00:26:16.430 --> 00:26:17.090 align:middle line:90% Yeah. 00:26:17.090 --> 00:26:20.810 align:middle line:84% And in addition to driving changes in atmospheric PCO2, 00:26:20.810 --> 00:26:22.590 align:middle line:84% those changes can involve changes 00:26:22.590 --> 00:26:26.230 align:middle line:84% in the accumulation rate of calcium carbonate in sediments. 00:26:26.230 --> 00:26:28.150 align:middle line:84% And the combination of those two things 00:26:28.150 --> 00:26:30.870 align:middle line:84% means that you may be able to use variations 00:26:30.870 --> 00:26:33.110 align:middle line:84% in the accumulation rate of calcium carbonate 00:26:33.110 --> 00:26:40.750 align:middle line:84% in sediments in the past to study how changes in the carbon 00:26:40.750 --> 00:26:44.590 align:middle line:84% cycle in the oceans have driven changes in atmospheric PCO2 00:26:44.590 --> 00:26:46.350 align:middle line:90% over time. 00:26:46.350 --> 00:26:49.150 align:middle line:84% So it's important to climate studies. 00:26:49.150 --> 00:26:52.950 align:middle line:84% The second thing is sediments represent-- 00:26:52.950 --> 00:26:55.610 align:middle line:84% surface sediments represent a pretty large, significant-- 00:26:55.610 --> 00:26:58.270 align:middle line:84% pretty large reservoir of calcium carbonate. 00:26:58.270 --> 00:27:03.550 align:middle line:84% It's the amount of carbon in surface sediments 00:27:03.550 --> 00:27:06.790 align:middle line:84% in the oceans is roughly the same as the amount of carbon 00:27:06.790 --> 00:27:08.830 align:middle line:90% in terrestrial reservoirs. 00:27:08.830 --> 00:27:11.510 align:middle line:84% And it's larger than the amount of carbon in the surface ocean 00:27:11.510 --> 00:27:12.910 align:middle line:90% by a good bit. 00:27:12.910 --> 00:27:16.010 align:middle line:84% And larger than the amount in the atmosphere by a good bit. 00:27:16.010 --> 00:27:18.710 align:middle line:84% But it's a smaller reservoir than the deep ocean. 00:27:18.710 --> 00:27:24.530 align:middle line:84% But anyway, it's a significant reservoir of calcium carbonate. 00:27:24.530 --> 00:27:28.650 align:middle line:84% And so for that reason, it may play a long-term role 00:27:28.650 --> 00:27:31.290 align:middle line:84% in buffering the oceans as acid is 00:27:31.290 --> 00:27:35.210 align:middle line:84% added to the oceans in the form of CO2. 00:27:35.210 --> 00:27:38.050 align:middle line:84% But long term is emphasized here, 00:27:38.050 --> 00:27:43.370 align:middle line:84% where the time scale for reaction 00:27:43.370 --> 00:27:46.770 align:middle line:84% of acids added to the ocean with calcium carbonate 00:27:46.770 --> 00:27:49.210 align:middle line:84% in marine sediments is thousands of years. 00:27:49.210 --> 00:27:55.330 align:middle line:90% 00:27:55.330 --> 00:27:57.730 align:middle line:84% So calcium carbonate is a mineral. 00:27:57.730 --> 00:28:02.363 align:middle line:84% And what happens in sediments is that calcium carbonate 00:28:02.363 --> 00:28:04.530 align:middle line:84% will dissolve in pore waters that are undersaturated 00:28:04.530 --> 00:28:06.650 align:middle line:90% with respect to the mineral. 00:28:06.650 --> 00:28:11.230 align:middle line:84% And unlike opal, there's not just one mineral to worry about. 00:28:11.230 --> 00:28:14.930 align:middle line:84% But there are-- the calcium carbonate minerals that 00:28:14.930 --> 00:28:19.080 align:middle line:84% are important in the oceans are essentially 00:28:19.080 --> 00:28:21.000 align:middle line:90% aragonite and calcite. 00:28:21.000 --> 00:28:24.920 align:middle line:84% Aragonite is formed by pteropods, gastropods, 00:28:24.920 --> 00:28:26.560 align:middle line:90% and corals. 00:28:26.560 --> 00:28:31.160 align:middle line:84% And calcite is formed by foraminifera and coccoliths. 00:28:31.160 --> 00:28:35.200 align:middle line:84% Aragonite is about 50% more soluble than calcite. 00:28:35.200 --> 00:28:37.840 align:middle line:84% But there's another complicating factor 00:28:37.840 --> 00:28:41.120 align:middle line:84% that since seawater contains a lot of magnesium, 00:28:41.120 --> 00:28:43.260 align:middle line:84% when calcite is precipitated in the oceans, 00:28:43.260 --> 00:28:45.640 align:middle line:84% it contains varying amounts of magnesium. 00:28:45.640 --> 00:28:51.720 align:middle line:84% And calcites that contain magnesium are somewhat to a lot 00:28:51.720 --> 00:28:55.040 align:middle line:90% more soluble than pure calcite. 00:28:55.040 --> 00:28:57.400 align:middle line:84% So in the order of solubility, aragonite 00:28:57.400 --> 00:29:01.440 align:middle line:84% is the most soluble of these calcium carbonate minerals. 00:29:01.440 --> 00:29:04.720 align:middle line:84% High magnesium calcites are intermediate in solubility 00:29:04.720 --> 00:29:07.340 align:middle line:90% between aragonite and calcite. 00:29:07.340 --> 00:29:10.200 align:middle line:84% And calcite is the least soluble of the minerals. 00:29:10.200 --> 00:29:12.220 align:middle line:84% Because of this solubility trend, 00:29:12.220 --> 00:29:17.220 align:middle line:84% calcite is the predominant mineral in deep sea sediments. 00:29:17.220 --> 00:29:19.280 align:middle line:84% So how do we quantify its solubility? 00:29:19.280 --> 00:29:22.060 align:middle line:90% 00:29:22.060 --> 00:29:25.100 align:middle line:84% Solubility has been determined in laboratory experiments 00:29:25.100 --> 00:29:26.960 align:middle line:84% as the conditional solubility product. 00:29:26.960 --> 00:29:28.460 align:middle line:84% If you remember when we were talking 00:29:28.460 --> 00:29:30.860 align:middle line:84% about conditional equilibrium constants, 00:29:30.860 --> 00:29:34.820 align:middle line:84% these are measured in a solution that has the major ion 00:29:34.820 --> 00:29:37.180 align:middle line:90% composition of seawater. 00:29:37.180 --> 00:29:41.140 align:middle line:84% So the solubility product for calcite or aragonite 00:29:41.140 --> 00:29:44.880 align:middle line:84% in the oceans is determined in seawater. 00:29:44.880 --> 00:29:48.540 align:middle line:90% And it applies to seawater-- 00:29:48.540 --> 00:29:52.620 align:middle line:84% to solutions with seawater major ion composition. 00:29:52.620 --> 00:29:56.900 align:middle line:84% The second thing to point out is that this conditional solubility 00:29:56.900 --> 00:30:00.740 align:middle line:84% product is the product of the concentrations of calcium 00:30:00.740 --> 00:30:04.820 align:middle line:90% ion and carbonate ion. 00:30:04.820 --> 00:30:08.620 align:middle line:84% One thing, you may, if you have a really good memory, 00:30:08.620 --> 00:30:10.760 align:middle line:84% remember about the carbonate ion in seawater. 00:30:10.760 --> 00:30:13.180 align:middle line:84% There's actually very little free carbonate ion. 00:30:13.180 --> 00:30:18.080 align:middle line:84% It's mostly complex, forming ion pairs with cations in solution. 00:30:18.080 --> 00:30:20.400 align:middle line:84% And so this carbonate ion concentration 00:30:20.400 --> 00:30:23.200 align:middle line:84% that appears in the calcite solubility product 00:30:23.200 --> 00:30:26.240 align:middle line:84% is the total carbonate ion concentration. 00:30:26.240 --> 00:30:30.680 align:middle line:84% So it's free carbonate plus ion pairs involving carbonate. 00:30:30.680 --> 00:30:34.000 align:middle line:90% 00:30:34.000 --> 00:30:36.720 align:middle line:84% Unless somebody has done some very recent work, 00:30:36.720 --> 00:30:40.200 align:middle line:84% the two determinations of the calcite solubility product 00:30:40.200 --> 00:30:43.520 align:middle line:84% in seawater that I know of were done a long time 00:30:43.520 --> 00:30:49.080 align:middle line:84% ago by Engel and Mucci The Engel constant is approximately 10% 00:30:49.080 --> 00:30:51.720 align:middle line:90% higher than the Mucci constant. 00:30:51.720 --> 00:30:54.545 align:middle line:84% So you can say that if you take these at-- if you give them 00:30:54.545 --> 00:30:55.920 align:middle line:84% equal weight, you can say there's 00:30:55.920 --> 00:30:58.200 align:middle line:84% an uncertainty in the calcite solubility of about 00:30:58.200 --> 00:30:59.920 align:middle line:90% plus or minus 5%. 00:30:59.920 --> 00:31:06.680 align:middle line:84% These days, the Engel constant has fallen out of favor 00:31:06.680 --> 00:31:09.020 align:middle line:84% for reasons that have never been clear to me. 00:31:09.020 --> 00:31:13.740 align:middle line:84% And the Mucci constant is the one that's 00:31:13.740 --> 00:31:17.900 align:middle line:90% essentially the accepted value. 00:31:17.900 --> 00:31:20.725 align:middle line:84% Being a cynic, I'd have to say that Mucci has stayed 00:31:20.725 --> 00:31:22.600 align:middle line:84% in the business, and Engel, as far as I know, 00:31:22.600 --> 00:31:24.220 align:middle line:90% is not in the business anymore. 00:31:24.220 --> 00:31:26.580 align:middle line:84% So he can promote himself, and she can't. 00:31:26.580 --> 00:31:29.100 align:middle line:90% So his constant rules. 00:31:29.100 --> 00:31:31.220 align:middle line:90% Anyway. 00:31:31.220 --> 00:31:32.660 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:31:32.660 --> 00:31:39.940 align:middle line:84% PROFESSOR: [LAUGHS] But anyway, there's about a 5% plus 00:31:39.940 --> 00:31:42.540 align:middle line:84% or minus 5% uncertainty in the calcite equilibrium 00:31:42.540 --> 00:31:44.040 align:middle line:90% constant in seawater. 00:31:44.040 --> 00:31:46.540 align:middle line:84% And aragonite is about 50% more soluble 00:31:46.540 --> 00:31:48.400 align:middle line:90% than calcite in the oceans. 00:31:48.400 --> 00:31:51.660 align:middle line:90% 00:31:51.660 --> 00:31:54.580 align:middle line:84% The second important determinations 00:31:54.580 --> 00:31:58.700 align:middle line:84% was when these guys measured the solubility of calcite 00:31:58.700 --> 00:32:02.360 align:middle line:84% in seawater, they also measured its temperature dependence. 00:32:02.360 --> 00:32:05.220 align:middle line:84% And they found that as temperature decreases, 00:32:05.220 --> 00:32:09.520 align:middle line:90% calcite solubility increases. 00:32:09.520 --> 00:32:11.840 align:middle line:84% Engel did experimental determinations 00:32:11.840 --> 00:32:13.680 align:middle line:90% of calcite solubility. 00:32:13.680 --> 00:32:17.040 align:middle line:84% And Millero has done thermodynamic calculations 00:32:17.040 --> 00:32:19.740 align:middle line:84% of calcite solubility as a function of pressure. 00:32:19.740 --> 00:32:22.360 align:middle line:90% 00:32:22.360 --> 00:32:24.560 align:middle line:84% The two studies are in pretty good agreement. 00:32:24.560 --> 00:32:27.060 align:middle line:84% And what was found that as pressure increases, 00:32:27.060 --> 00:32:30.460 align:middle line:84% calcite solubility increases in seawater. 00:32:30.460 --> 00:32:32.120 align:middle line:84% The result of both of these-- remember, 00:32:32.120 --> 00:32:34.540 align:middle line:84% that as you go from the surface ocean to the deep ocean, 00:32:34.540 --> 00:32:35.700 align:middle line:90% two things change. 00:32:35.700 --> 00:32:38.960 align:middle line:84% Temperature decreases and pressure increases. 00:32:38.960 --> 00:32:41.200 align:middle line:84% So that as you go from the surface ocean 00:32:41.200 --> 00:32:45.240 align:middle line:84% deeper into the ocean, the solubility of calcite increases. 00:32:45.240 --> 00:32:48.900 align:middle line:84% Calcite is more soluble with increasing depth in the ocean. 00:32:48.900 --> 00:32:54.320 align:middle line:90% 00:32:54.320 --> 00:32:56.360 align:middle line:84% The solubility that I've shown you up to here 00:32:56.360 --> 00:32:58.260 align:middle line:84% were determined in the laboratory. 00:32:58.260 --> 00:33:00.980 align:middle line:84% And it's always important, because conditions in sediments 00:33:00.980 --> 00:33:02.980 align:middle line:84% are different from conditions in the laboratory, 00:33:02.980 --> 00:33:08.030 align:middle line:84% to try to verify those constants using measurements in sediments. 00:33:08.030 --> 00:33:10.130 align:middle line:84% The reasons, of course, are, first, 00:33:10.130 --> 00:33:13.390 align:middle line:84% that the minerals that you clean up and use in the lab 00:33:13.390 --> 00:33:16.790 align:middle line:84% may not be exactly the same as what's present in sediments. 00:33:16.790 --> 00:33:20.130 align:middle line:84% And the second difference of importance is time. 00:33:20.130 --> 00:33:22.930 align:middle line:84% Whereas laboratory experiments are necessarily pretty short, 00:33:22.930 --> 00:33:25.190 align:middle line:84% limited to a few months at the most, 00:33:25.190 --> 00:33:28.590 align:middle line:84% whereas calcite sits in the sediments for hundreds 00:33:28.590 --> 00:33:32.373 align:middle line:84% or thousands of years before it-- well, 00:33:32.373 --> 00:33:33.790 align:middle line:84% has hundreds or thousands of years 00:33:33.790 --> 00:33:36.290 align:middle line:84% to come into equilibrium with the pore waters. 00:33:36.290 --> 00:33:39.830 align:middle line:84% So there have been studies-- well, 00:33:39.830 --> 00:33:42.990 align:middle line:84% a couple of studies done of poor water-based estimates 00:33:42.990 --> 00:33:44.830 align:middle line:90% of calcite solubility. 00:33:44.830 --> 00:33:48.490 align:middle line:84% And what these-- this is one, an old one by Sayles, 00:33:48.490 --> 00:33:50.990 align:middle line:84% and it may still be the only one. 00:33:50.990 --> 00:33:54.630 align:middle line:84% What's done is you measure alkalinity, total CO2, 00:33:54.630 --> 00:33:57.010 align:middle line:84% and calcium in sedimentary pore waters. 00:33:57.010 --> 00:33:59.990 align:middle line:90% 00:33:59.990 --> 00:34:03.430 align:middle line:84% Once you have those pore water data, 00:34:03.430 --> 00:34:05.570 align:middle line:84% from the alkalinity and total CO2 data, 00:34:05.570 --> 00:34:07.930 align:middle line:84% you can calculate the carbonate ion concentration 00:34:07.930 --> 00:34:09.250 align:middle line:90% in the pore waters. 00:34:09.250 --> 00:34:10.969 align:middle line:84% Then you can calculate the product 00:34:10.969 --> 00:34:15.770 align:middle line:84% of the measured carbonate ion and calcium ion concentrations. 00:34:15.770 --> 00:34:18.330 align:middle line:90% And that gives if-- 00:34:18.330 --> 00:34:20.610 align:middle line:84% the pore waters are in equilibrium with respect 00:34:20.610 --> 00:34:22.409 align:middle line:84% to calcite, that gives you an estimate 00:34:22.409 --> 00:34:26.632 align:middle line:84% of the solubility product of calcite in actual sediments. 00:34:26.632 --> 00:34:28.090 align:middle line:84% Before we look at the results, just 00:34:28.090 --> 00:34:33.449 align:middle line:84% look at what the data, what data of this sort looked like. 00:34:33.449 --> 00:34:35.969 align:middle line:84% And it's worth showing because these are actually 00:34:35.969 --> 00:34:39.610 align:middle line:90% very difficult data to obtain. 00:34:39.610 --> 00:34:42.610 align:middle line:84% If you picture yourself collecting a sediment core, 00:34:42.610 --> 00:34:45.750 align:middle line:84% what happens when you say you collect a sediment core at 3,000 00:34:45.750 --> 00:34:50.330 align:middle line:84% meters depth and bring it up to the ship in order to extract 00:34:50.330 --> 00:34:51.150 align:middle line:90% the pore waters? 00:34:51.150 --> 00:34:55.941 align:middle line:84% What do you think is going to happen inside that core? 00:34:55.941 --> 00:34:58.690 align:middle line:84% AUDIENCE: Changes in pressure and expansion-- 00:34:58.690 --> 00:35:01.050 align:middle line:84% PROFESSOR: Yeah, the pressure decreases. 00:35:01.050 --> 00:35:03.950 align:middle line:84% Well, expansion isn't the important thing. 00:35:03.950 --> 00:35:07.310 align:middle line:84% What's important is calcite solubility decreases 00:35:07.310 --> 00:35:09.710 align:middle line:84% because the pressure has decreased a lot. 00:35:09.710 --> 00:35:12.390 align:middle line:84% And so what that means is that calcite precipitates out 00:35:12.390 --> 00:35:15.830 align:middle line:84% of these sediment cores so you can no longer measure 00:35:15.830 --> 00:35:18.110 align:middle line:84% concentrations of carbonate system components 00:35:18.110 --> 00:35:19.910 align:middle line:90% in the pore waters. 00:35:19.910 --> 00:35:23.310 align:middle line:84% And so what you need in order to make these measurements 00:35:23.310 --> 00:35:25.630 align:middle line:84% is a machine that actually separates the pore waters 00:35:25.630 --> 00:35:29.310 align:middle line:84% from the sediments while it's sitting on the sea floor. 00:35:29.310 --> 00:35:31.350 align:middle line:90% It is a difficult thing to do. 00:35:31.350 --> 00:35:35.830 align:middle line:84% And the illustrious Fred Sayles, our emeritus scientist, 00:35:35.830 --> 00:35:38.150 align:middle line:84% is really the only one who's had a machine that's 00:35:38.150 --> 00:35:39.870 align:middle line:90% done it very successfully. 00:35:39.870 --> 00:35:42.550 align:middle line:84% And he's the one that collected these data. 00:35:42.550 --> 00:35:44.190 align:middle line:84% So they're very difficult to collect, 00:35:44.190 --> 00:35:46.390 align:middle line:84% and there aren't a whole lot of them. 00:35:46.390 --> 00:35:49.510 align:middle line:84% But this is what alkalinity, total CO2, and calcium profiles 00:35:49.510 --> 00:35:50.130 align:middle line:90% look like. 00:35:50.130 --> 00:35:53.430 align:middle line:84% At least this was a site in the western equatorial Atlantic. 00:35:53.430 --> 00:35:56.710 align:middle line:84% What you find is that you use alkalinity and total 00:35:56.710 --> 00:35:59.870 align:middle line:84% CO2 to calculate the carbonate ion concentration. 00:35:59.870 --> 00:36:02.830 align:middle line:84% You'll find that below a depth of a couple centimeters, 00:36:02.830 --> 00:36:05.610 align:middle line:90% it's constant. 00:36:05.610 --> 00:36:08.890 align:middle line:84% And you'll see that these calcium concentrations 00:36:08.890 --> 00:36:10.430 align:middle line:90% don't vary very much. 00:36:10.430 --> 00:36:14.610 align:middle line:84% They don't change very much, so that the product of calcium 00:36:14.610 --> 00:36:18.330 align:middle line:84% times carbonate ion is constant below a couple centimeters, 00:36:18.330 --> 00:36:20.170 align:middle line:84% below the sediment water interface. 00:36:20.170 --> 00:36:21.810 align:middle line:84% So a set of data like this gives you 00:36:21.810 --> 00:36:26.850 align:middle line:84% one estimate of calcite solubility at one depth. 00:36:26.850 --> 00:36:28.830 align:middle line:84% That is one temperature, pressure, 00:36:28.830 --> 00:36:30.190 align:middle line:90% and salinity in the oceans. 00:36:30.190 --> 00:36:32.690 align:middle line:84% And you can do this at a whole bunch of different locations, 00:36:32.690 --> 00:36:35.410 align:middle line:84% at different water depths, and derive a picture 00:36:35.410 --> 00:36:39.850 align:middle line:84% like this, where what's plotted on the vertical axis 00:36:39.850 --> 00:36:42.770 align:middle line:84% is the water depth, so the depth of the sediments 00:36:42.770 --> 00:36:45.250 align:middle line:84% in which pore water profiles were determined. 00:36:45.250 --> 00:36:48.450 align:middle line:84% And what's plotted on the bottom is the ion concentration 00:36:48.450 --> 00:36:52.290 align:middle line:84% product, the product of calcium and carbonate ion concentrations 00:36:52.290 --> 00:36:54.450 align:middle line:90% found in the pore waters. 00:36:54.450 --> 00:36:58.870 align:middle line:84% And the two curves that are through these-- 00:36:58.870 --> 00:37:02.630 align:middle line:84% well, I should say first, if you extrapolate these back 00:37:02.630 --> 00:37:06.990 align:middle line:84% to a depth of 0, they're in pretty good equilibrium, 00:37:06.990 --> 00:37:09.670 align:middle line:84% within the not insignificant error 00:37:09.670 --> 00:37:13.810 align:middle line:84% of Engel's equilibrium constant and also of Mucci's constant. 00:37:13.810 --> 00:37:16.070 align:middle line:84% You can't tell the difference in a study like this. 00:37:16.070 --> 00:37:23.150 align:middle line:84% And the dashed line is the theoretical pressure change 00:37:23.150 --> 00:37:26.230 align:middle line:84% in calcite solubility determined by Millero. 00:37:26.230 --> 00:37:29.770 align:middle line:84% And the solid line is the fit to the pore water data. 00:37:29.770 --> 00:37:32.590 align:middle line:84% And you can see that within the uncertainty in the data, 00:37:32.590 --> 00:37:35.070 align:middle line:84% the in situ determined solubility of calcite 00:37:35.070 --> 00:37:37.670 align:middle line:84% are the same as the laboratory determined 00:37:37.670 --> 00:37:41.030 align:middle line:84% and the laboratory determined values 00:37:41.030 --> 00:37:45.830 align:middle line:84% and the theoretical pressure change in its solubility. 00:37:45.830 --> 00:37:48.430 align:middle line:84% So laboratory and field measurements 00:37:48.430 --> 00:37:51.430 align:middle line:90% are in good agreement. 00:37:51.430 --> 00:37:56.330 align:middle line:84% So what's all this stuff good for? 00:37:56.330 --> 00:38:01.260 align:middle line:90% 00:38:01.260 --> 00:38:04.380 align:middle line:84% Before we get to that, I'll just mention the two ways 00:38:04.380 --> 00:38:07.220 align:middle line:84% of describing the saturation state of pore water and seawater 00:38:07.220 --> 00:38:08.688 align:middle line:90% with respect to calcite. 00:38:08.688 --> 00:38:10.480 align:middle line:84% It could also be with respect to aragonite. 00:38:10.480 --> 00:38:12.540 align:middle line:84% But we're thinking about calcite here. 00:38:12.540 --> 00:38:17.020 align:middle line:84% First is the degree of saturation, which 00:38:17.020 --> 00:38:19.260 align:middle line:90% is denoted by the symbol omega. 00:38:19.260 --> 00:38:22.340 align:middle line:84% And it's equal to the product of calcium and carbonate 00:38:22.340 --> 00:38:25.020 align:middle line:84% ion measured in the solution divided 00:38:25.020 --> 00:38:30.620 align:middle line:84% by the equilibrium constant for calcite, the solubility product. 00:38:30.620 --> 00:38:35.500 align:middle line:84% And it's often simplified as the ratio of the carbonate ion 00:38:35.500 --> 00:38:40.180 align:middle line:84% concentration calculated from measurements in seawater divided 00:38:40.180 --> 00:38:42.580 align:middle line:84% by the carbonate ion concentration 00:38:42.580 --> 00:38:45.660 align:middle line:84% that would be at equilibrium with at that temperature 00:38:45.660 --> 00:38:46.580 align:middle line:90% and pressure. 00:38:46.580 --> 00:38:50.380 align:middle line:84% And the reason for this simplification 00:38:50.380 --> 00:38:52.820 align:middle line:84% is that calcium is conserved in seawater 00:38:52.820 --> 00:38:55.940 align:middle line:84% and its concentration doesn't vary very much. 00:38:55.940 --> 00:38:58.340 align:middle line:84% So in supersaturated saturated seawater, 00:38:58.340 --> 00:39:00.000 align:middle line:90% omega is greater than 1. 00:39:00.000 --> 00:39:03.640 align:middle line:84% In undersaturated seawater, it's less than 1. 00:39:03.640 --> 00:39:08.200 align:middle line:84% The second way in which saturation state of seawater 00:39:08.200 --> 00:39:10.040 align:middle line:84% with respect to calcite is described 00:39:10.040 --> 00:39:12.000 align:middle line:84% is what's called delta carbonate. 00:39:12.000 --> 00:39:18.340 align:middle line:84% And this is just the carbonate ion concentration in water, 00:39:18.340 --> 00:39:22.280 align:middle line:84% in seawater calculated from measured quantities 00:39:22.280 --> 00:39:25.400 align:middle line:84% minus the equilibrium carbonate ion 00:39:25.400 --> 00:39:27.280 align:middle line:84% concentration, the concentration that 00:39:27.280 --> 00:39:29.240 align:middle line:84% would be there if the solution were 00:39:29.240 --> 00:39:32.920 align:middle line:90% in equilibrium with calcite. 00:39:32.920 --> 00:39:35.140 align:middle line:84% So if delta carbonate is greater than 0, 00:39:35.140 --> 00:39:38.240 align:middle line:84% the solution is supersaturated with respect to calcite. 00:39:38.240 --> 00:39:42.670 align:middle line:84% If it's less than 0, it's undersaturated. 00:39:42.670 --> 00:39:47.800 align:middle line:84% So how does calcite saturation-- well, one more thing, 00:39:47.800 --> 00:39:50.360 align:middle line:84% how the carbonate ion concentration varies 00:39:50.360 --> 00:39:52.760 align:middle line:90% with depth in the oceans. 00:39:52.760 --> 00:39:55.360 align:middle line:84% And this is another one of those plots 00:39:55.360 --> 00:39:59.220 align:middle line:84% that I pull out of my ancient files that people like Scott 00:39:59.220 --> 00:40:00.980 align:middle line:90% look at and gag. 00:40:00.980 --> 00:40:05.880 align:middle line:84% But these are very old data from the GEOSECS expedition, 00:40:05.880 --> 00:40:10.060 align:middle line:90% which took place 30 years ago? 00:40:10.060 --> 00:40:10.662 align:middle line:90% 30 years ago. 00:40:10.662 --> 00:40:11.620 align:middle line:90% AUDIENCE: More than 30. 00:40:11.620 --> 00:40:13.100 align:middle line:84% PROFESSOR: More than 30 years ago. 00:40:13.100 --> 00:40:15.960 align:middle line:84% But the picture you get is still the same. 00:40:15.960 --> 00:40:20.500 align:middle line:84% They measured alkalinity and total CO2 00:40:20.500 --> 00:40:23.020 align:middle line:84% and calculated carbonate ion concentration. 00:40:23.020 --> 00:40:26.460 align:middle line:84% On the top, are all their data from the North Atlantic. 00:40:26.460 --> 00:40:30.220 align:middle line:84% On the bottom in the dots are all their data 00:40:30.220 --> 00:40:32.440 align:middle line:90% from the North Pacific. 00:40:32.440 --> 00:40:34.280 align:middle line:84% And these are carbonate ion concentrations. 00:40:34.280 --> 00:40:37.820 align:middle line:84% You can see that carbonate ion concentration decreases 00:40:37.820 --> 00:40:41.100 align:middle line:84% from the surface ocean, going down into the water column. 00:40:41.100 --> 00:40:45.980 align:middle line:84% It has a minimum coincident with the oxygen minimum, 00:40:45.980 --> 00:40:47.700 align:middle line:84% and then increases slightly to the value 00:40:47.700 --> 00:40:49.900 align:middle line:84% in bottom water, which is still very low 00:40:49.900 --> 00:40:52.460 align:middle line:84% compared to the value in surface seawater. 00:40:52.460 --> 00:40:56.400 align:middle line:84% The second thing that you'll notice, in the bottom plot, 00:40:56.400 --> 00:41:01.200 align:middle line:84% the scatter plot is carbonate ion concentrations calculated 00:41:01.200 --> 00:41:03.000 align:middle line:90% for North Pacific data. 00:41:03.000 --> 00:41:06.040 align:middle line:84% And I've also drawn on that the envelope of points 00:41:06.040 --> 00:41:07.480 align:middle line:90% from the North Atlantic. 00:41:07.480 --> 00:41:10.960 align:middle line:84% And the point of that is to show that carbonate ion concentration 00:41:10.960 --> 00:41:13.600 align:middle line:84% in the deep Pacific is lower than the carbonate ion 00:41:13.600 --> 00:41:16.520 align:middle line:84% concentration in the deep Atlantic. 00:41:16.520 --> 00:41:24.400 align:middle line:84% And I know that you can all tell me why that is, or at least one 00:41:24.400 --> 00:41:31.400 align:middle line:84% of you can tell me why that is, or not. 00:41:31.400 --> 00:41:32.400 align:middle line:90% I know somebody-- 00:41:32.400 --> 00:41:34.240 align:middle line:84% AUDIENCE: --cancel class on Thursday. 00:41:34.240 --> 00:41:36.725 align:middle line:90% [LAUGHTER] 00:41:36.725 --> 00:41:38.100 align:middle line:84% PROFESSOR: I know somebody knows. 00:41:38.100 --> 00:41:39.392 align:middle line:90% You just don't want to tell me. 00:41:39.392 --> 00:41:42.320 align:middle line:90% 00:41:42.320 --> 00:41:50.260 align:middle line:84% OK, which seawater has been away from the surface longer, 00:41:50.260 --> 00:41:53.025 align:middle line:84% seawater in the deep Atlantic or in the deep Pacific? 00:41:53.025 --> 00:41:53.900 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:41:53.900 --> 00:41:54.483 align:middle line:90% PROFESSOR: OK. 00:41:54.483 --> 00:41:56.060 align:middle line:90% And as-- 00:41:56.060 --> 00:41:58.562 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:41:58.562 --> 00:41:59.520 align:middle line:90% PROFESSOR: That's true. 00:41:59.520 --> 00:42:02.280 align:middle line:84% That's basically saying the same thing as I've showed you here. 00:42:02.280 --> 00:42:03.600 align:middle line:90% But why is it lower? 00:42:03.600 --> 00:42:07.100 align:middle line:90% 00:42:07.100 --> 00:42:10.220 align:middle line:84% So as the water ages on its trip from the North Atlantic 00:42:10.220 --> 00:42:13.600 align:middle line:84% to the North Pacific, what happens in the water? 00:42:13.600 --> 00:42:16.580 align:middle line:84% AUDIENCE: It's continually dissolving 00:42:16.580 --> 00:42:20.980 align:middle line:90% calcium carbonate [INAUDIBLE] 00:42:20.980 --> 00:42:23.060 align:middle line:90% PROFESSOR: OK, I'll ask you-- 00:42:23.060 --> 00:42:24.713 align:middle line:84% I'll give you credit for answering. 00:42:24.713 --> 00:42:25.880 align:middle line:90% But I'll ask you a question. 00:42:25.880 --> 00:42:28.580 align:middle line:84% If you dissolve calcium carbonate-- if all that you do 00:42:28.580 --> 00:42:30.820 align:middle line:84% is dissolve calcium carbonate in the water, 00:42:30.820 --> 00:42:33.340 align:middle line:84% will the carbonate ion concentration in that water 00:42:33.340 --> 00:42:35.140 align:middle line:90% go up or go down. 00:42:35.140 --> 00:42:39.700 align:middle line:84% AUDIENCE: [INAUDIBLE] I'll have to draw it all out 00:42:39.700 --> 00:42:41.060 align:middle line:90% to Figure it out. 00:42:41.060 --> 00:42:42.500 align:middle line:90% PROFESSOR: OK, it'll go up. 00:42:42.500 --> 00:42:46.820 align:middle line:84% But what do you do in a water body 00:42:46.820 --> 00:42:49.320 align:middle line:84% to make the carbonate ion concentration go down? 00:42:49.320 --> 00:42:53.258 align:middle line:90% 00:42:53.258 --> 00:42:54.800 align:middle line:84% AUDIENCE: You oxidize organic matter. 00:42:54.800 --> 00:42:56.680 align:middle line:84% PROFESSOR: You oxidize organic matter, which 00:42:56.680 --> 00:42:59.280 align:middle line:90% releases acid into the water. 00:42:59.280 --> 00:43:02.560 align:middle line:84% And that causes the carbonate ion concentration to decrease. 00:43:02.560 --> 00:43:06.160 align:middle line:84% So going from the North Atlantic to the North Pacific, 00:43:06.160 --> 00:43:10.240 align:middle line:84% both carbonate dissolution and organic matter oxidation 00:43:10.240 --> 00:43:12.500 align:middle line:84% occur, but more organic matter oxidation. 00:43:12.500 --> 00:43:14.240 align:middle line:84% So the carbonate ion concentration 00:43:14.240 --> 00:43:16.200 align:middle line:84% is lower than the North Pacific than it 00:43:16.200 --> 00:43:17.800 align:middle line:90% is in the North Atlantic. 00:43:17.800 --> 00:43:20.160 align:middle line:90% OK. 00:43:20.160 --> 00:43:20.700 align:middle line:90% OK. 00:43:20.700 --> 00:43:24.360 align:middle line:90% 00:43:24.360 --> 00:43:26.360 align:middle line:84% Now, I'll put together the things 00:43:26.360 --> 00:43:30.040 align:middle line:84% I've talked about in the last couple of slides. 00:43:30.040 --> 00:43:32.040 align:middle line:84% And what's plotted here is another thing 00:43:32.040 --> 00:43:34.720 align:middle line:90% from Sarmiento and Gruber. 00:43:34.720 --> 00:43:39.280 align:middle line:84% There's two plots of carbonate ion concentration calculated 00:43:39.280 --> 00:43:41.280 align:middle line:84% from carbonate system measurements. 00:43:41.280 --> 00:43:43.360 align:middle line:90% And those are shown by the dots. 00:43:43.360 --> 00:43:45.500 align:middle line:84% The one on the left is in the South Atlantic. 00:43:45.500 --> 00:43:47.640 align:middle line:84% The one on the right is in the North Pacific. 00:43:47.640 --> 00:43:51.290 align:middle line:84% What's also plotted, here are the dashed line, 00:43:51.290 --> 00:43:53.470 align:middle line:84% that's the carbonate ion concentration that 00:43:53.470 --> 00:43:57.110 align:middle line:84% would be in equilibrium with aragonite 00:43:57.110 --> 00:43:59.850 align:middle line:84% at these depths in the water column. 00:43:59.850 --> 00:44:02.710 align:middle line:84% And the solid line is the carbonate ion concentration 00:44:02.710 --> 00:44:06.150 align:middle line:84% that would be in equilibrium with calcite. 00:44:06.150 --> 00:44:08.590 align:middle line:84% So if you look at the North Atlantic one, 00:44:08.590 --> 00:44:11.790 align:middle line:84% you can see that these dots are at higher carbonate ion 00:44:11.790 --> 00:44:14.590 align:middle line:84% concentrations than the calcite curve. 00:44:14.590 --> 00:44:16.150 align:middle line:84% The calcite equilibrium curve down 00:44:16.150 --> 00:44:19.310 align:middle line:90% to a depth of about 4200 meters. 00:44:19.310 --> 00:44:21.950 align:middle line:84% So the water column is super saturated with respect 00:44:21.950 --> 00:44:26.270 align:middle line:84% to calcite below about 4,200 meters and is undersaturated 00:44:26.270 --> 00:44:31.830 align:middle line:84% with respect to calcite below 4,200 meters. 00:44:31.830 --> 00:44:34.930 align:middle line:84% So I'll ask you to answer one question. 00:44:34.930 --> 00:44:38.150 align:middle line:84% If I were to plot omega versus depth, 00:44:38.150 --> 00:44:41.670 align:middle line:84% so the degree of supersaturation with respect to calcite, 00:44:41.670 --> 00:44:46.950 align:middle line:84% roughly speaking, where would the values be greater than 1? 00:44:46.950 --> 00:44:51.404 align:middle line:84% And where would they be less than 1 in this Atlantic plot? 00:44:51.404 --> 00:44:56.763 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:44:56.763 --> 00:44:57.430 align:middle line:90% PROFESSOR: Yeah. 00:44:57.430 --> 00:45:00.250 align:middle line:84% So they'd be greater than 1 above 4,200 meters and less than 00:45:00.250 --> 00:45:01.310 align:middle line:90% 1 below that. 00:45:01.310 --> 00:45:02.990 align:middle line:84% If I were to plot delta carbonate? 00:45:02.990 --> 00:45:06.210 align:middle line:90% 00:45:06.210 --> 00:45:08.930 align:middle line:84% Remember, delta carbonate can be positive or negative. 00:45:08.930 --> 00:45:12.123 align:middle line:90% AUDIENCE: Greater than 0 above. 00:45:12.123 --> 00:45:12.790 align:middle line:90% PROFESSOR: Yeah. 00:45:12.790 --> 00:45:14.090 align:middle line:90% And less than 0 below that. 00:45:14.090 --> 00:45:15.130 align:middle line:90% OK. 00:45:15.130 --> 00:45:19.370 align:middle line:84% So now going from the South Atlantic to the North Pacific, 00:45:19.370 --> 00:45:22.410 align:middle line:84% the picture is in some ways the same, but in some ways 00:45:22.410 --> 00:45:23.430 align:middle line:90% quite different. 00:45:23.430 --> 00:45:26.770 align:middle line:84% The carbonate ion concentration in these dots 00:45:26.770 --> 00:45:30.490 align:middle line:84% shows a pretty similarly shaped profile. 00:45:30.490 --> 00:45:34.650 align:middle line:84% But now, when you compare it to the carbonate ion concentration 00:45:34.650 --> 00:45:37.290 align:middle line:84% that would be in equilibrium with respect to calcite, 00:45:37.290 --> 00:45:40.530 align:middle line:84% you see that in the North Pacific, 00:45:40.530 --> 00:45:44.150 align:middle line:84% the water is only supersaturated with respect to calcite 00:45:44.150 --> 00:45:47.030 align:middle line:90% above about 600 meters. 00:45:47.030 --> 00:45:51.670 align:middle line:84% And then it's below saturation, but close to saturation for long 00:45:51.670 --> 00:45:54.650 align:middle line:84% depth interval down to about 4,000 meters. 00:45:54.650 --> 00:45:57.230 align:middle line:84% And then it becomes more distinctly undersaturated. 00:45:57.230 --> 00:46:01.430 align:middle line:84% But the important difference is that the South Atlantic water 00:46:01.430 --> 00:46:05.110 align:middle line:84% column, down to a depth of 4,200 meters or so, is supersaturated 00:46:05.110 --> 00:46:06.910 align:middle line:90% with respect to calcite. 00:46:06.910 --> 00:46:09.070 align:middle line:84% But the North Pacific water column 00:46:09.070 --> 00:46:10.950 align:middle line:84% is only supersaturated with respect 00:46:10.950 --> 00:46:14.990 align:middle line:84% to calcite, down to a depth of about 600 meters. 00:46:14.990 --> 00:46:18.510 align:middle line:84% I have one more of these plots to show you. 00:46:18.510 --> 00:46:21.830 align:middle line:84% And that's because the North Pacific is the extreme end 00:46:21.830 --> 00:46:26.550 align:middle line:84% member with the shallowest calcite saturation horizon. 00:46:26.550 --> 00:46:28.230 align:middle line:84% This depth in the water column, I 00:46:28.230 --> 00:46:31.950 align:middle line:84% forgot to tell you that, the depth in the water column, 00:46:31.950 --> 00:46:34.350 align:middle line:84% where there's this crossover between the equilibrium 00:46:34.350 --> 00:46:37.510 align:middle line:84% curve and the carbonate ion concentration curve, 00:46:37.510 --> 00:46:40.010 align:middle line:84% is called the calcite saturation horizon. 00:46:40.010 --> 00:46:42.990 align:middle line:84% That's where the water column is just at equilibrium with respect 00:46:42.990 --> 00:46:45.410 align:middle line:90% to calcite. 00:46:45.410 --> 00:46:48.330 align:middle line:90% OK, these are data-- 00:46:48.330 --> 00:46:49.670 align:middle line:90% these are WOCE data. 00:46:49.670 --> 00:46:51.670 align:middle line:84% So they're much more recent from-- 00:46:51.670 --> 00:46:54.530 align:middle line:90% 00:46:54.530 --> 00:46:57.330 align:middle line:84% they're WOCE and GEOSECS data, excuse me, 00:46:57.330 --> 00:46:59.490 align:middle line:84% from the western equatorial Pacific. 00:46:59.490 --> 00:47:02.570 align:middle line:84% And they show a curve that's actually typical of much 00:47:02.570 --> 00:47:05.730 align:middle line:84% of the Pacific Ocean, south of the equator anyway, 00:47:05.730 --> 00:47:11.010 align:middle line:84% where delta carbonate now shows you that the water column is 00:47:11.010 --> 00:47:13.070 align:middle line:84% super saturated with respect to calcite, 00:47:13.070 --> 00:47:16.130 align:middle line:84% down to a depth of about 2,000 to 2,500 meters 00:47:16.130 --> 00:47:17.910 align:middle line:90% and undersaturated below that. 00:47:17.910 --> 00:47:20.753 align:middle line:90% 00:47:20.753 --> 00:47:22.170 align:middle line:84% Now I'm going to show you one more 00:47:22.170 --> 00:47:26.170 align:middle line:84% thing on saturation state of the water column 00:47:26.170 --> 00:47:27.970 align:middle line:90% before we leave the subject. 00:47:27.970 --> 00:47:31.490 align:middle line:84% And these are data from the northwest Atlantic Ocean. 00:47:31.490 --> 00:47:35.710 align:middle line:84% On the left are the water column profile from a depth of 2,000 00:47:35.710 --> 00:47:39.710 align:middle line:84% meters to about 5,500 meters for dissolved inorganic carbon, 00:47:39.710 --> 00:47:41.730 align:middle line:90% total CO2 and alkalinity. 00:47:41.730 --> 00:47:45.430 align:middle line:84% And on the right, using those data to calculate carbonate ion 00:47:45.430 --> 00:47:49.210 align:middle line:84% concentration and presenting it in terms of omega, 00:47:49.210 --> 00:47:52.630 align:middle line:84% so degree of saturation of the water with respect to calcite. 00:47:52.630 --> 00:47:55.230 align:middle line:84% So a value of 1 is just at equilibrium with respect 00:47:55.230 --> 00:47:55.830 align:middle line:90% to calcite. 00:47:55.830 --> 00:47:57.690 align:middle line:84% Greater than 1 is supersaturated. 00:47:57.690 --> 00:48:00.550 align:middle line:90% Less than 1 is undersaturated. 00:48:00.550 --> 00:48:04.870 align:middle line:84% As for the plot I showed you for the South Atlantic, 00:48:04.870 --> 00:48:08.070 align:middle line:84% the North Atlantic water column is super saturated with respect 00:48:08.070 --> 00:48:12.930 align:middle line:84% to calcite, down to a depth here of something like 4,500 meters. 00:48:12.930 --> 00:48:15.470 align:middle line:84% But the important thing that I'd like to show you 00:48:15.470 --> 00:48:18.910 align:middle line:84% is there are two solid curves here. 00:48:18.910 --> 00:48:24.370 align:middle line:84% Those are both omega calcite-- well, 00:48:24.370 --> 00:48:28.950 align:middle line:84% they're both the saturation level 00:48:28.950 --> 00:48:36.930 align:middle line:84% for equilibrium of seawater with respect to calcite. 00:48:36.930 --> 00:48:40.150 align:middle line:84% One is calculated for the Mucci solubility and one 00:48:40.150 --> 00:48:41.610 align:middle line:90% for the Engel solubility. 00:48:41.610 --> 00:48:44.860 align:middle line:84% So these are the two different determinations 00:48:44.860 --> 00:48:47.460 align:middle line:84% of calcite solubility in seawater. 00:48:47.460 --> 00:48:52.380 align:middle line:84% The one that intersects one at the shallower level 00:48:52.380 --> 00:48:53.480 align:middle line:90% is the Engel curve. 00:48:53.480 --> 00:48:56.000 align:middle line:84% The one that intersects deeper is the Mucci curve. 00:48:56.000 --> 00:48:59.100 align:middle line:84% And you can see that this plus or minus 5% uncertainty 00:48:59.100 --> 00:49:02.100 align:middle line:84% in the solubility product for calcite translates 00:49:02.100 --> 00:49:05.460 align:middle line:84% into an uncertainty of about 1,000 meters of the depth 00:49:05.460 --> 00:49:07.660 align:middle line:84% at which the water goes from being supersaturated 00:49:07.660 --> 00:49:09.900 align:middle line:90% to undersaturated. 00:49:09.900 --> 00:49:13.360 align:middle line:84% So even this small change leads to significant uncertainties. 00:49:13.360 --> 00:49:19.940 align:middle line:90% 00:49:19.940 --> 00:49:22.020 align:middle line:84% Calcite is a mineral that dissolves 00:49:22.020 --> 00:49:23.720 align:middle line:90% in undersaturated water. 00:49:23.720 --> 00:49:26.900 align:middle line:84% And it doesn't dissolve when the water is supersaturated. 00:49:26.900 --> 00:49:30.060 align:middle line:84% And that gives us a really quite simple overall picture 00:49:30.060 --> 00:49:35.960 align:middle line:84% of the calcium carbonate concentration in seawater. 00:49:35.960 --> 00:49:39.260 align:middle line:84% And it's pretty easy to understand the basics. 00:49:39.260 --> 00:49:44.080 align:middle line:84% This is another Sarmiento and Gruber plot, 00:49:44.080 --> 00:49:48.040 align:middle line:84% where the dark regions are regions 00:49:48.040 --> 00:49:51.080 align:middle line:84% with high calcium carbonate content in the sediments. 00:49:51.080 --> 00:49:56.220 align:middle line:84% And the black regions are spots with more than 80%-- 00:49:56.220 --> 00:49:59.040 align:middle line:84% the sediments are more than 80% calcium carbonate. 00:49:59.040 --> 00:50:03.160 align:middle line:84% The dark gray are more than 50% calcium carbonate. 00:50:03.160 --> 00:50:08.480 align:middle line:84% And it's pretty easy to see that the high values 00:50:08.480 --> 00:50:10.960 align:middle line:84% for calcite concentration in the sediments 00:50:10.960 --> 00:50:14.080 align:middle line:84% follow the mid-ocean ridge system. 00:50:14.080 --> 00:50:17.720 align:middle line:84% So those are at relatively shallow water depths. 00:50:17.720 --> 00:50:21.520 align:middle line:84% So since calcite solubility increases 00:50:21.520 --> 00:50:26.440 align:middle line:84% with depth in the ocean, it's present in shallow-- 00:50:26.440 --> 00:50:30.720 align:middle line:84% it's at high concentrations in relatively shallow sediments 00:50:30.720 --> 00:50:33.720 align:middle line:84% and at lower concentrations in deep sediments. 00:50:33.720 --> 00:50:35.360 align:middle line:84% The other thing that really stands out 00:50:35.360 --> 00:50:37.920 align:middle line:84% here is the North Pacific, which is extremely 00:50:37.920 --> 00:50:41.060 align:middle line:90% low in calcium carbonate. 00:50:41.060 --> 00:50:46.440 align:middle line:84% And as we've seen, there are two reasons for that. 00:50:46.440 --> 00:50:49.740 align:middle line:84% First is that the water column becomes 00:50:49.740 --> 00:50:53.500 align:middle line:84% undersaturated with respect to calcite at a very shallow depth 00:50:53.500 --> 00:50:54.780 align:middle line:90% in the North Pacific. 00:50:54.780 --> 00:50:57.340 align:middle line:84% The second is that the North Pacific is very deep. 00:50:57.340 --> 00:50:59.020 align:middle line:84% And the combination of those two things 00:50:59.020 --> 00:51:03.740 align:middle line:84% means that the North Pacific is essentially devoid of calcite. 00:51:03.740 --> 00:51:08.020 align:middle line:84% So those are the main features of the calcium carbonate 00:51:08.020 --> 00:51:11.180 align:middle line:84% distribution in marine sediments. 00:51:11.180 --> 00:51:13.380 align:middle line:84% And they're pretty easy to explain. 00:51:13.380 --> 00:51:19.900 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:51:19.900 --> 00:51:20.560 align:middle line:90% PROFESSOR: OK. 00:51:20.560 --> 00:51:23.540 align:middle line:90% 00:51:23.540 --> 00:51:25.220 align:middle line:84% AUDIENCE: I'm not sure what Scott said. 00:51:25.220 --> 00:51:26.792 align:middle line:90% We couldn't hear him. 00:51:26.792 --> 00:51:28.000 align:middle line:90% PROFESSOR: Did you hear that? 00:51:28.000 --> 00:51:29.700 align:middle line:84% AUDIENCE: Yeah, she couldn't hear me. 00:51:29.700 --> 00:51:31.772 align:middle line:90% Ironically, I hear her saying-- 00:51:31.772 --> 00:51:32.980 align:middle line:90% PROFESSOR: That she couldn't. 00:51:32.980 --> 00:51:35.020 align:middle line:84% So what Scott was saying that you 00:51:35.020 --> 00:51:39.800 align:middle line:84% will get to explain that distribution in your homework. 00:51:39.800 --> 00:51:40.580 align:middle line:90% Lucky you. 00:51:40.580 --> 00:51:46.372 align:middle line:90% 00:51:46.372 --> 00:51:52.120 align:middle line:84% So I've said that changes in the marine carbon cycle 00:51:52.120 --> 00:51:54.760 align:middle line:84% can lead to changes in the calcium carbonate 00:51:54.760 --> 00:51:58.000 align:middle line:90% concentration in sediments. 00:51:58.000 --> 00:52:04.880 align:middle line:84% And so it's of interest to know how you might determine changes 00:52:04.880 --> 00:52:06.560 align:middle line:84% in calcium carbonate accumulation rate 00:52:06.560 --> 00:52:07.695 align:middle line:90% in sediments over time. 00:52:07.695 --> 00:52:09.320 align:middle line:84% And of course, the obvious way to do it 00:52:09.320 --> 00:52:12.158 align:middle line:84% is to measure percent calcium carbonate in the sediments. 00:52:12.158 --> 00:52:14.200 align:middle line:84% It's an easy thing to measure, and it does indeed 00:52:14.200 --> 00:52:17.720 align:middle line:90% vary at a given place over time. 00:52:17.720 --> 00:52:21.800 align:middle line:84% So you might ask-- well, first, there 00:52:21.800 --> 00:52:25.080 align:middle line:84% are two definitions that are important in studying percent 00:52:25.080 --> 00:52:29.400 align:middle line:90% calcium carbonate in sediments. 00:52:29.400 --> 00:52:31.440 align:middle line:84% And they're shown in-- these pictures 00:52:31.440 --> 00:52:34.720 align:middle line:84% here, what these pictures are of, 00:52:34.720 --> 00:52:37.540 align:middle line:84% and these are examples from the North Atlantic Ocean, 00:52:37.540 --> 00:52:41.580 align:middle line:84% are measurements of core top percent calcium carbonate 00:52:41.580 --> 00:52:46.060 align:middle line:84% in sediments taken from a range of water depths in the ocean. 00:52:46.060 --> 00:52:49.660 align:middle line:84% So each one of these dots represents a sample 00:52:49.660 --> 00:52:53.060 align:middle line:84% taken from a core that was taken at the water depth that's 00:52:53.060 --> 00:52:55.040 align:middle line:90% shown on the y-axis. 00:52:55.040 --> 00:52:59.660 align:middle line:84% And what you see is the details of these curves 00:52:59.660 --> 00:53:01.360 align:middle line:84% vary in different places in the ocean. 00:53:01.360 --> 00:53:03.500 align:middle line:84% But the general shape is the same. 00:53:03.500 --> 00:53:08.620 align:middle line:84% Where there's-- from shallow going to deep, 00:53:08.620 --> 00:53:12.500 align:middle line:84% there's a region where the concentration is either pretty 00:53:12.500 --> 00:53:17.300 align:middle line:84% constant or scattered with a slight decreasing trend as you 00:53:17.300 --> 00:53:18.560 align:middle line:90% go deeper in the water column. 00:53:18.560 --> 00:53:21.100 align:middle line:84% Then there's a kink in the curve. 00:53:21.100 --> 00:53:24.260 align:middle line:84% And from that kink, the calcium carbonate concentration 00:53:24.260 --> 00:53:27.740 align:middle line:84% drops off pretty rapidly to a water depth, 00:53:27.740 --> 00:53:31.340 align:middle line:84% where core top calcium carbonate concentration is 0. 00:53:31.340 --> 00:53:35.730 align:middle line:84% So there's two definitions that arise from this curve shape. 00:53:35.730 --> 00:53:38.650 align:middle line:84% The one is the location of this kink. 00:53:38.650 --> 00:53:41.730 align:middle line:84% And that's called the sedimentary lysocline. 00:53:41.730 --> 00:53:44.610 align:middle line:84% And that's the depth at which there's 00:53:44.610 --> 00:53:46.770 align:middle line:84% sedimentary evidence of dissolution 00:53:46.770 --> 00:53:48.370 align:middle line:90% of calcium carbonate. 00:53:48.370 --> 00:53:50.570 align:middle line:84% And in this picture that sedimentary evidence 00:53:50.570 --> 00:53:53.370 align:middle line:84% comes in the form of a decrease in the calcium carbonate 00:53:53.370 --> 00:53:56.270 align:middle line:84% concentration in core top sediments. 00:53:56.270 --> 00:54:00.770 align:middle line:84% So the lysocline is where the depth at which 00:54:00.770 --> 00:54:02.930 align:middle line:90% there's evidence of dissolution. 00:54:02.930 --> 00:54:05.290 align:middle line:84% AUDIENCE: Sorry, the lysocline is the-- 00:54:05.290 --> 00:54:07.890 align:middle line:84% PROFESSOR: It's basically the kink, 00:54:07.890 --> 00:54:10.810 align:middle line:84% the depth of the kink in that curve, 00:54:10.810 --> 00:54:12.370 align:middle line:84% where you can see they've simplified 00:54:12.370 --> 00:54:14.050 align:middle line:90% lines through the data. 00:54:14.050 --> 00:54:18.890 align:middle line:84% And the two lines look like this, 00:54:18.890 --> 00:54:21.050 align:middle line:84% where there is a very decreasing-- 00:54:21.050 --> 00:54:23.250 align:middle line:84% a very slowly decreasing part and a much more 00:54:23.250 --> 00:54:27.110 align:middle line:84% rapidly decreasing part below that where those two lines meet. 00:54:27.110 --> 00:54:29.730 align:middle line:90% The depth is the lysocline. 00:54:29.730 --> 00:54:34.510 align:middle line:84% The other definition, which is a much simpler one, 00:54:34.510 --> 00:54:36.990 align:middle line:84% is called the calcite compensation depth. 00:54:36.990 --> 00:54:39.790 align:middle line:84% This is the depth at which core top calcium carbonate 00:54:39.790 --> 00:54:42.550 align:middle line:90% concentration drops to 0. 00:54:42.550 --> 00:54:45.390 align:middle line:84% And it's called the compensation depth, because just 00:54:45.390 --> 00:54:47.630 align:middle line:84% by mass balance arguments, that's 00:54:47.630 --> 00:54:49.830 align:middle line:84% the depth at which the rate of dissolution 00:54:49.830 --> 00:54:53.670 align:middle line:84% of calcium carbonate in these sediments 00:54:53.670 --> 00:54:55.730 align:middle line:84% is exactly equal to the rain rate. 00:54:55.730 --> 00:54:57.930 align:middle line:84% And the result is a calcium carbonate concentration 00:54:57.930 --> 00:54:58.810 align:middle line:90% that's 0. 00:54:58.810 --> 00:55:03.790 align:middle line:90% 00:55:03.790 --> 00:55:05.890 align:middle line:84% So the question you might ask yourself here, 00:55:05.890 --> 00:55:10.310 align:middle line:84% if you're going to use these definitions 00:55:10.310 --> 00:55:13.390 align:middle line:84% and say you're going to use measurements of percent calcium 00:55:13.390 --> 00:55:17.190 align:middle line:84% carbonate over time in sediments as an indicator 00:55:17.190 --> 00:55:19.510 align:middle line:84% of the accumulation rate of calcium carbonate 00:55:19.510 --> 00:55:21.910 align:middle line:84% in the sediments, how does that relate 00:55:21.910 --> 00:55:24.750 align:middle line:84% to the rain rate of calcium carbonate to the sea floor? 00:55:24.750 --> 00:55:28.590 align:middle line:84% Or worded in another way, is percent calcium carbonate 00:55:28.590 --> 00:55:31.290 align:middle line:84% in the sediments a sensitive indicator 00:55:31.290 --> 00:55:33.730 align:middle line:84% of the rate of dissolution of calcium 00:55:33.730 --> 00:55:36.290 align:middle line:90% carbonate in those sediments? 00:55:36.290 --> 00:55:40.290 align:middle line:84% And the important point to consider here 00:55:40.290 --> 00:55:43.450 align:middle line:84% is what I showed you in the distribution of calcium 00:55:43.450 --> 00:55:45.330 align:middle line:84% carbonate sediments in the ocean. 00:55:45.330 --> 00:55:48.350 align:middle line:84% And what that showed you is that at these shallow water depths, 00:55:48.350 --> 00:55:50.550 align:middle line:84% well above the calcite saturation horizon, 00:55:50.550 --> 00:55:56.610 align:middle line:84% the sediments are 80% to 90% calcium carbonate. 00:55:56.610 --> 00:56:01.890 align:middle line:84% And that's because in these pelagic environments, away 00:56:01.890 --> 00:56:06.010 align:middle line:84% from the continents, the material to the sea floor 00:56:06.010 --> 00:56:08.170 align:middle line:84% is in many places in the ocean made up mostly 00:56:08.170 --> 00:56:09.970 align:middle line:90% of calcium carbonate. 00:56:09.970 --> 00:56:13.250 align:middle line:84% So that gives rise to this example here, 00:56:13.250 --> 00:56:18.530 align:middle line:84% where this curved line here represents the sea floor. 00:56:18.530 --> 00:56:22.610 align:middle line:84% So this would be a rise in the ocean, where 00:56:22.610 --> 00:56:25.090 align:middle line:84% at the top of the rise, the sea floor is well 00:56:25.090 --> 00:56:29.990 align:middle line:84% above the calcite lysocline, which 00:56:29.990 --> 00:56:31.590 align:middle line:90% is shown by the dashed line. 00:56:31.590 --> 00:56:34.670 align:middle line:84% And at the deeper depths on the rise, 00:56:34.670 --> 00:56:38.910 align:middle line:84% the sediment water interface is below the lysocline. 00:56:38.910 --> 00:56:41.950 align:middle line:84% So suppose this is one of those regions in the ocean where 00:56:41.950 --> 00:56:44.990 align:middle line:84% the rain rate, the material to the sea floor is about 90% 00:56:44.990 --> 00:56:48.550 align:middle line:84% calcium carbonate, which is not an unusual occurrence. 00:56:48.550 --> 00:56:53.390 align:middle line:84% So above the lysocline, where no calcium carbonate is dissolving, 00:56:53.390 --> 00:56:57.790 align:middle line:84% the inputs are reflected in sediment composition. 00:56:57.790 --> 00:56:59.950 align:middle line:90% So if the rain rate-- 00:56:59.950 --> 00:57:02.550 align:middle line:84% the material rain to the sea floor is 9%-- 00:57:02.550 --> 00:57:07.870 align:middle line:84% is 9 parts calcite to one part other stuff, 00:57:07.870 --> 00:57:10.790 align:middle line:84% other non-reactive stuff, then the sediments that are 00:57:10.790 --> 00:57:13.590 align:middle line:84% accumulating when there's no dissolution of the calcite are 00:57:13.590 --> 00:57:15.670 align:middle line:90% about 90% calcite. 00:57:15.670 --> 00:57:18.870 align:middle line:84% Now say we go below the saturation horizon, a little bit 00:57:18.870 --> 00:57:21.030 align:middle line:84% further down this rise, the rain rate 00:57:21.030 --> 00:57:23.670 align:middle line:84% of material to the sea floor is still 9 parts calcium 00:57:23.670 --> 00:57:25.670 align:middle line:90% carbonate and 1%-- 00:57:25.670 --> 00:57:28.850 align:middle line:90% 1 part other non-reactive stuff. 00:57:28.850 --> 00:57:31.508 align:middle line:84% But now we're below the calcite lysocline. 00:57:31.508 --> 00:57:33.050 align:middle line:84% And suppose, for the sake of argument 00:57:33.050 --> 00:57:36.530 align:middle line:84% here, that 5 units of this calcium carbonate 00:57:36.530 --> 00:57:39.210 align:middle line:84% rain to the sea floor are dissolving. 00:57:39.210 --> 00:57:43.410 align:middle line:84% So now what's accumulating is 4 parts calcium carbonate 00:57:43.410 --> 00:57:47.050 align:middle line:84% and 1 part other non-reactive stuff. 00:57:47.050 --> 00:57:51.210 align:middle line:84% So that means in this location the sediments are 80% calcite. 00:57:51.210 --> 00:57:54.090 align:middle line:84% So more than half of the carbonate calcite rained 00:57:54.090 --> 00:57:55.690 align:middle line:90% to the sea floor is dissolving. 00:57:55.690 --> 00:57:59.770 align:middle line:84% But the sediments are still 80% calcite. 00:57:59.770 --> 00:58:03.250 align:middle line:84% And the upshot is that because calcium carbonate is 00:58:03.250 --> 00:58:05.990 align:middle line:84% such a large component of the rain to the sea floor, 00:58:05.990 --> 00:58:07.690 align:middle line:84% percent calcium carbonate in sediments 00:58:07.690 --> 00:58:11.290 align:middle line:84% is a very insensitive indicator of the dissolution of calcium 00:58:11.290 --> 00:58:12.070 align:middle line:90% carbonate. 00:58:12.070 --> 00:58:18.000 align:middle line:90% 00:58:18.000 --> 00:58:19.830 align:middle line:90% So why go through that argument? 00:58:19.830 --> 00:58:25.810 align:middle line:84% Well, it's because-- just because bottom waters that 00:58:25.810 --> 00:58:28.740 align:middle line:84% are lying above the sediment water interface 00:58:28.740 --> 00:58:31.840 align:middle line:84% are supersaturated with respect to calcite 00:58:31.840 --> 00:58:35.480 align:middle line:84% doesn't necessarily mean that the pore waters in the sediments 00:58:35.480 --> 00:58:37.280 align:middle line:84% just below the sediment water interface 00:58:37.280 --> 00:58:40.320 align:middle line:84% are also supersaturated with respect to calcite. 00:58:40.320 --> 00:58:43.400 align:middle line:84% And to see why that is look at here 00:58:43.400 --> 00:58:48.040 align:middle line:84% at what happens when organic matter is oxidized using oxygen 00:58:48.040 --> 00:58:50.360 align:middle line:90% as an electron acceptor. 00:58:50.360 --> 00:58:56.080 align:middle line:84% When that happens, the organic carbon 00:58:56.080 --> 00:59:00.360 align:middle line:84% is released to the pore waters as CO2, which is an acid. 00:59:00.360 --> 00:59:03.280 align:middle line:84% And in addition, when the organic nitrogen 00:59:03.280 --> 00:59:07.800 align:middle line:84% is oxidized to nitrate, that releases acid into the solution. 00:59:07.800 --> 00:59:10.200 align:middle line:84% So oxic decomposition of organic matter 00:59:10.200 --> 00:59:13.480 align:middle line:84% releases acids into sediment pore waters. 00:59:13.480 --> 00:59:16.840 align:middle line:84% So the question is, what happens to those acids? 00:59:16.840 --> 00:59:19.720 align:middle line:84% Well, they're going to be neutralized because it's 00:59:19.720 --> 00:59:20.580 align:middle line:90% a buffered solution. 00:59:20.580 --> 00:59:22.920 align:middle line:84% And they can be neutralized in a couple of ways. 00:59:22.920 --> 00:59:28.100 align:middle line:84% First, they can react with carbonate ion 00:59:28.100 --> 00:59:29.780 align:middle line:90% to form bicarbonate. 00:59:29.780 --> 00:59:32.980 align:middle line:84% Second, they can react with borate ion 00:59:32.980 --> 00:59:35.140 align:middle line:90% to produce bicarbonate. 00:59:35.140 --> 00:59:39.500 align:middle line:84% Or third, they can react with solid calcium carbonate 00:59:39.500 --> 00:59:42.020 align:middle line:90% to produce bicarbonate. 00:59:42.020 --> 00:59:47.100 align:middle line:84% So if you picture what happens as, for instance, CO2 00:59:47.100 --> 00:59:49.740 align:middle line:84% is added to the pore waters, the CO2 concentration 00:59:49.740 --> 00:59:53.060 align:middle line:84% will increase in the pore waters and the carbonate ion 00:59:53.060 --> 00:59:56.020 align:middle line:84% concentration will decrease in the pore waters. 00:59:56.020 --> 00:59:57.500 align:middle line:84% What that means is carbonate ions 00:59:57.500 --> 01:00:01.620 align:middle line:84% can diffuse into the pore waters from bottom water. 01:00:01.620 --> 01:00:04.820 align:middle line:84% CO2 can diffuse out of the pore waters into bottom water. 01:00:04.820 --> 01:00:07.100 align:middle line:84% And when both of those things occur, 01:00:07.100 --> 01:00:10.580 align:middle line:84% they tend to cause neutralization 01:00:10.580 --> 01:00:12.860 align:middle line:84% of this acid that's added to the pore waters 01:00:12.860 --> 01:00:17.380 align:middle line:84% by carbonate ion that's in the water to produce bicarbonate. 01:00:17.380 --> 01:00:21.780 align:middle line:84% On the other hand, the competing reaction 01:00:21.780 --> 01:00:24.480 align:middle line:84% is the reaction of these acids with solid phase calcium 01:00:24.480 --> 01:00:25.220 align:middle line:90% carbonate. 01:00:25.220 --> 01:00:28.000 align:middle line:84% And to the extent that the CO2 that's 01:00:28.000 --> 01:00:30.620 align:middle line:84% released into the pore water reacts with calcium carbonate, 01:00:30.620 --> 01:00:33.360 align:middle line:84% that obviously causes dissolution of calcium carbonate 01:00:33.360 --> 01:00:34.840 align:middle line:90% in the sediments. 01:00:34.840 --> 01:00:37.600 align:middle line:84% So in many places in the ocean, pore waters 01:00:37.600 --> 01:00:39.760 align:middle line:84% can have enough acid added to them 01:00:39.760 --> 01:00:41.960 align:middle line:84% by oxic decomposition of organic matter 01:00:41.960 --> 01:00:45.040 align:middle line:84% to become undersaturated with respect to calcite. 01:00:45.040 --> 01:00:47.000 align:middle line:84% And then these metabolically produced 01:00:47.000 --> 01:00:50.360 align:middle line:84% acids will cause dissolution of calcium carbonate 01:00:50.360 --> 01:00:53.040 align:middle line:90% in the pore waters. 01:00:53.040 --> 01:00:58.920 align:middle line:84% And that can happen both below the calcite saturation horizon 01:00:58.920 --> 01:01:02.680 align:middle line:84% and above it, because now the pore waters, because 01:01:02.680 --> 01:01:06.380 align:middle line:84% of this addition of acids from the oxidation of organic matter, 01:01:06.380 --> 01:01:08.760 align:middle line:84% the pore waters can be at a lower saturation 01:01:08.760 --> 01:01:11.640 align:middle line:84% state than the overlying bottom water. 01:01:11.640 --> 01:01:14.880 align:middle line:84% So calcium carbonate can dissolve in sediments 01:01:14.880 --> 01:01:18.800 align:middle line:84% even if the sediments lie above the calcite saturation 01:01:18.800 --> 01:01:20.560 align:middle line:90% horizon in the water. 01:01:20.560 --> 01:01:25.280 align:middle line:84% And you wouldn't see that in curves, 01:01:25.280 --> 01:01:30.660 align:middle line:84% in plots like I just showed you of percent calcium 01:01:30.660 --> 01:01:33.700 align:middle line:84% carbonate in core tops versus water column depth 01:01:33.700 --> 01:01:37.480 align:middle line:84% because they're so insensitive to dissolution. 01:01:37.480 --> 01:01:40.855 align:middle line:90% 01:01:40.855 --> 01:01:45.440 align:middle line:84% So the question is, this process can certainly occur in theory, 01:01:45.440 --> 01:01:47.820 align:middle line:90% does it actually happen? 01:01:47.820 --> 01:01:54.900 align:middle line:84% And, OK, here's one effort to show that it actually 01:01:54.900 --> 01:01:56.180 align:middle line:90% does happen. 01:01:56.180 --> 01:01:58.180 align:middle line:84% You can tell that this was in the old days, back 01:01:58.180 --> 01:02:00.560 align:middle line:84% before they had lots of computer graphics, 01:02:00.560 --> 01:02:03.460 align:middle line:84% because the graphics in this paper are awful. 01:02:03.460 --> 01:02:06.700 align:middle line:84% But anyway, this is a Burke Hales paper. 01:02:06.700 --> 01:02:07.580 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:02:07.580 --> 01:02:08.330 align:middle line:90% PROFESSOR: Pardon? 01:02:08.330 --> 01:02:10.757 align:middle line:84% AUDIENCE: [INAUDIBLE] not talking stencils. 01:02:10.757 --> 01:02:11.340 align:middle line:90% PROFESSOR: No. 01:02:11.340 --> 01:02:12.215 align:middle line:90% AUDIENCE: --stencils. 01:02:12.215 --> 01:02:17.900 align:middle line:84% PROFESSOR: This is primitive computer graphics days and also 01:02:17.900 --> 01:02:23.680 align:middle line:84% Hales who wasn't into pretty graphics. 01:02:23.680 --> 01:02:27.320 align:middle line:84% But anyway, what he did was he used 01:02:27.320 --> 01:02:31.880 align:middle line:84% a procedure called in situ microelectrode profiling. 01:02:31.880 --> 01:02:35.480 align:middle line:84% And I think I described this for oxygen profiles in sediments 01:02:35.480 --> 01:02:39.520 align:middle line:84% before, he had an instrument that landed on the sea floor 01:02:39.520 --> 01:02:43.200 align:middle line:84% and on a plate that was lowered down towards the sediment water 01:02:43.200 --> 01:02:43.700 align:middle line:90% interface. 01:02:43.700 --> 01:02:45.240 align:middle line:84% There were both oxygen electrodes 01:02:45.240 --> 01:02:47.400 align:middle line:90% and pH sensitive electrodes. 01:02:47.400 --> 01:02:49.400 align:middle line:84% And so he could measure concentrations 01:02:49.400 --> 01:02:52.960 align:middle line:84% of oxygen in the pore waters and the pH of the pore waters 01:02:52.960 --> 01:02:55.320 align:middle line:84% on a very fine depth scale, close to the sediment water 01:02:55.320 --> 01:02:56.480 align:middle line:90% interface. 01:02:56.480 --> 01:03:00.100 align:middle line:90% And here, these are results. 01:03:00.100 --> 01:03:03.200 align:middle line:84% And they're plotted as for pH in the pore waters. 01:03:03.200 --> 01:03:04.660 align:middle line:90% They're plotted as delta pH. 01:03:04.660 --> 01:03:08.120 align:middle line:84% So that's the difference in the pH of the pore waters compared 01:03:08.120 --> 01:03:09.480 align:middle line:90% to the overlying water. 01:03:09.480 --> 01:03:17.680 align:middle line:84% And these dots, which I point out if my arrow would reappear, 01:03:17.680 --> 01:03:20.430 align:middle line:84% these dots are the measurements of pH 01:03:20.430 --> 01:03:22.190 align:middle line:84% in the pore waters as a function of depth 01:03:22.190 --> 01:03:23.990 align:middle line:84% below the sediment water interface. 01:03:23.990 --> 01:03:28.230 align:middle line:90% And the depth is in centimeters. 01:03:28.230 --> 01:03:32.350 align:middle line:84% So that's 1 centimeter and 2 centimeters. 01:03:32.350 --> 01:03:34.770 align:middle line:84% So on the upper-- this much of the sediment column, 01:03:34.770 --> 01:03:37.273 align:middle line:84% that's a profile of the pH of the pore waters. 01:03:37.273 --> 01:03:39.190 align:middle line:84% And you can see that the pH in the pore waters 01:03:39.190 --> 01:03:42.710 align:middle line:84% is lower than the pH in overlying water. 01:03:42.710 --> 01:03:44.830 align:middle line:84% That's because of the dissolution-- 01:03:44.830 --> 01:03:47.110 align:middle line:84% because of the decomposition of organic matter 01:03:47.110 --> 01:03:49.470 align:middle line:90% in the pore waters. 01:03:49.470 --> 01:03:54.230 align:middle line:84% What he did then was he built this model 01:03:54.230 --> 01:03:57.950 align:middle line:84% of combined oxidation of organic matter 01:03:57.950 --> 01:04:01.710 align:middle line:84% by oxygen and dissolution of calcium carbonate. 01:04:01.710 --> 01:04:05.770 align:middle line:84% And he did several different runs. 01:04:05.770 --> 01:04:09.270 align:middle line:84% And what I've outlined in red in this plot here and in 01:04:09.270 --> 01:04:13.470 align:middle line:84% this plot here are what his model predicted-- 01:04:13.470 --> 01:04:17.370 align:middle line:84% the pore water pH would be if no calcium carbonate dissolution 01:04:17.370 --> 01:04:20.690 align:middle line:84% were occurring as a result of the addition of these acids 01:04:20.690 --> 01:04:23.050 align:middle line:90% to pore waters. 01:04:23.050 --> 01:04:25.450 align:middle line:84% And you can see that those are significantly 01:04:25.450 --> 01:04:28.930 align:middle line:84% lower than the pHs he actually measured in the pore waters. 01:04:28.930 --> 01:04:32.690 align:middle line:84% And based on his model, his conclusion 01:04:32.690 --> 01:04:35.170 align:middle line:84% was that the reason for this difference 01:04:35.170 --> 01:04:38.090 align:middle line:84% is that calcium carbonate is being dissolved. 01:04:38.090 --> 01:04:41.090 align:middle line:84% And therefore, the decrease in pH of the pore waters 01:04:41.090 --> 01:04:43.850 align:middle line:84% is less than would occur if no calcium carbonate were 01:04:43.850 --> 01:04:46.690 align:middle line:90% dissolving in the pore waters. 01:04:46.690 --> 01:04:49.970 align:middle line:84% So this was taken as evidence that as a result 01:04:49.970 --> 01:04:52.850 align:middle line:84% of oxic respiration and the acid that's 01:04:52.850 --> 01:04:54.870 align:middle line:84% released to pore waters because of it, 01:04:54.870 --> 01:04:57.890 align:middle line:84% calcium carbonate was dissolving in these sediments. 01:04:57.890 --> 01:05:03.090 align:middle line:84% Now, the two stations where I've drawn circles 01:05:03.090 --> 01:05:06.530 align:middle line:84% around their numbers, station 5 and station 7, 01:05:06.530 --> 01:05:08.490 align:middle line:84% are at depths in the water column, which 01:05:08.490 --> 01:05:12.190 align:middle line:84% are at or above the calcite compensation depth, 01:05:12.190 --> 01:05:14.190 align:middle line:90% the calcite saturation horizon. 01:05:14.190 --> 01:05:18.670 align:middle line:84% So if there were no metabolic acids added to the pore waters 01:05:18.670 --> 01:05:22.190 align:middle line:84% by oxidative respiration, there would be no calcite dissolution 01:05:22.190 --> 01:05:23.550 align:middle line:90% in these sediments. 01:05:23.550 --> 01:05:29.330 align:middle line:84% You can see that by his calculations in these sediments, 01:05:29.330 --> 01:05:31.310 align:middle line:84% as a result of respiration, something 01:05:31.310 --> 01:05:33.270 align:middle line:84% like half of the calcium carbonate 01:05:33.270 --> 01:05:35.950 align:middle line:84% that was falling to the sea floor was dissolving. 01:05:35.950 --> 01:05:37.630 align:middle line:84% So this metabolic dissolution can 01:05:37.630 --> 01:05:41.430 align:middle line:84% be a very important process in the calcium carbonate 01:05:41.430 --> 01:05:43.370 align:middle line:90% cycle in sediments. 01:05:43.370 --> 01:05:46.270 align:middle line:90% 01:05:46.270 --> 01:05:50.950 align:middle line:84% Just to show you one other set of data, it could be argued, 01:05:50.950 --> 01:05:54.190 align:middle line:84% and it has been argued, that they 01:05:54.190 --> 01:05:56.950 align:middle line:84% have to apply a carbonate speciation model to their pH 01:05:56.950 --> 01:06:00.430 align:middle line:84% data in order to calculate the saturation state with respect 01:06:00.430 --> 01:06:01.870 align:middle line:90% to calcite. 01:06:01.870 --> 01:06:05.510 align:middle line:84% And therefore, their conclusion that metabolic acids 01:06:05.510 --> 01:06:07.270 align:middle line:84% drive calcite dissolution depends 01:06:07.270 --> 01:06:11.730 align:middle line:84% on their speciation model for carbonate species in seawater. 01:06:11.730 --> 01:06:15.210 align:middle line:84% And if that's wrong, then their result is wrong. 01:06:15.210 --> 01:06:20.410 align:middle line:84% Another approach determines is the measurement 01:06:20.410 --> 01:06:23.210 align:middle line:84% of alkalinity profiles in the pore waters. 01:06:23.210 --> 01:06:27.890 align:middle line:84% And here, remember that oxic decomposition of organic matter 01:06:27.890 --> 01:06:31.350 align:middle line:84% essentially doesn't change the alkalinity of the pore waters. 01:06:31.350 --> 01:06:34.130 align:middle line:84% In fact, it decreases it slightly. 01:06:34.130 --> 01:06:35.850 align:middle line:84% But calcium carbonate dissolution 01:06:35.850 --> 01:06:37.770 align:middle line:84% increases the alkalinity of the pore waters 01:06:37.770 --> 01:06:39.510 align:middle line:90% by addition of carbonate ion. 01:06:39.510 --> 01:06:43.050 align:middle line:84% So if you measure alkalinity concentrations in pore waters 01:06:43.050 --> 01:06:45.850 align:middle line:84% and you find that the alkalinity increases, 01:06:45.850 --> 01:06:49.570 align:middle line:84% then that must be because calcium carbonate is dissolving 01:06:49.570 --> 01:06:51.850 align:middle line:90% in the sedimentary pore waters. 01:06:51.850 --> 01:06:55.490 align:middle line:84% And that's what was found here at a site 01:06:55.490 --> 01:06:58.010 align:middle line:84% on the Cape Verde plateau, which is well 01:06:58.010 --> 01:07:00.690 align:middle line:84% above the calcite saturation horizon, 01:07:00.690 --> 01:07:04.890 align:middle line:84% where delta carbonate in the bottom waters is about plus 20. 01:07:04.890 --> 01:07:06.930 align:middle line:84% So since it lies, since the site lies well 01:07:06.930 --> 01:07:09.588 align:middle line:84% above the calcite saturation horizon, 01:07:09.588 --> 01:07:11.130 align:middle line:84% then the only way for the pore waters 01:07:11.130 --> 01:07:13.890 align:middle line:84% to become undersaturated with respect to calcite, 01:07:13.890 --> 01:07:15.670 align:middle line:84% leading to calcite dissolution is 01:07:15.670 --> 01:07:19.150 align:middle line:84% formed from the addition of acids, from oxic respiration 01:07:19.150 --> 01:07:22.270 align:middle line:90% in the pore waters. 01:07:22.270 --> 01:07:25.070 align:middle line:84% So this was more pore water-based evidence 01:07:25.070 --> 01:07:28.710 align:middle line:84% that calcium carbonate does dissolve 01:07:28.710 --> 01:07:32.030 align:middle line:84% in sediments that lie above the calcite saturation horizon 01:07:32.030 --> 01:07:34.050 align:middle line:90% because of oxic metabolism. 01:07:34.050 --> 01:07:36.870 align:middle line:90% 01:07:36.870 --> 01:07:39.430 align:middle line:84% But people measuring concentrations 01:07:39.430 --> 01:07:41.510 align:middle line:84% of pH and alkalinity in pore waters 01:07:41.510 --> 01:07:43.110 align:middle line:84% aren't the only people that have tried 01:07:43.110 --> 01:07:47.190 align:middle line:84% to determine the occurrence of metabolic dissolution of calcium 01:07:47.190 --> 01:07:51.790 align:middle line:84% carbonate in sediments lying above the saturation horizon. 01:07:51.790 --> 01:07:54.950 align:middle line:84% And what's shown here is evidence 01:07:54.950 --> 01:07:58.190 align:middle line:84% based on direct measurements of alkalinity and calcium 01:07:58.190 --> 01:08:00.550 align:middle line:90% fluxes out of the sediments. 01:08:00.550 --> 01:08:03.490 align:middle line:84% So if calcium carbonate is dissolving in the sediments, 01:08:03.490 --> 01:08:06.710 align:middle line:84% then there must be a flux of calcium out of the sediments 01:08:06.710 --> 01:08:09.810 align:middle line:84% and a flux of alkalinity out of the sediments. 01:08:09.810 --> 01:08:13.090 align:middle line:84% So these fluxes were calculated-- 01:08:13.090 --> 01:08:16.090 align:middle line:84% were measured directly using chambers sitting on the sea 01:08:16.090 --> 01:08:20.050 align:middle line:84% floor rather than calculated from pore water data. 01:08:20.050 --> 01:08:29.130 align:middle line:84% And what's shown here is the alkalinity flux divided by 2. 01:08:29.130 --> 01:08:31.050 align:middle line:84% So the alkalinity flux divided by 2 01:08:31.050 --> 01:08:37.170 align:middle line:84% should be equal to the rate of dissolution of calcite. 01:08:37.170 --> 01:08:41.470 align:middle line:84% Remember that every time a calcium carbonate dissolves, 01:08:41.470 --> 01:08:44.649 align:middle line:84% it releases carbonate ion into the water. 01:08:44.649 --> 01:08:49.810 align:middle line:84% And that represents an increase of 2 units in the alkalinity. 01:08:49.810 --> 01:08:54.510 align:middle line:84% And what's plotted here are alkalinity flux divided by 2, 01:08:54.510 --> 01:08:57.689 align:middle line:84% so that's calcite remineralization flux, 01:08:57.689 --> 01:08:59.970 align:middle line:84% determined with benthic flux chambers 01:08:59.970 --> 01:09:04.890 align:middle line:84% for, on the left-hand side of this plot, stations 01:09:04.890 --> 01:09:06.689 align:middle line:84% where the water is undersaturated 01:09:06.689 --> 01:09:08.109 align:middle line:90% with respect to calcite. 01:09:08.109 --> 01:09:10.600 align:middle line:84% So that's where the bottom where the sediments lie 01:09:10.600 --> 01:09:13.040 align:middle line:84% below the calcite saturation horizon. 01:09:13.040 --> 01:09:19.080 align:middle line:84% And not surprisingly, you see that there's 01:09:19.080 --> 01:09:21.560 align:middle line:84% a flux of alkalinity and of calcium out 01:09:21.560 --> 01:09:23.399 align:middle line:90% of the sedimentary pore waters. 01:09:23.399 --> 01:09:24.960 align:middle line:84% What's plotted on the right-hand side 01:09:24.960 --> 01:09:28.479 align:middle line:84% here in these dark gray boxes, are 01:09:28.479 --> 01:09:30.399 align:middle line:84% sites where the same measurements 01:09:30.399 --> 01:09:34.840 align:middle line:84% were made at stations where the sediments have 01:09:34.840 --> 01:09:37.580 align:middle line:84% a low concentration of calcium carbonate in them. 01:09:37.580 --> 01:09:42.720 align:middle line:84% And their definition of low is 50% calcium carbonate. 01:09:42.720 --> 01:09:45.720 align:middle line:84% But what's plotted in the black here 01:09:45.720 --> 01:09:48.760 align:middle line:84% are sites that lie-- where the sediments lie 01:09:48.760 --> 01:09:52.120 align:middle line:84% above the calcite saturation horizon, 01:09:52.120 --> 01:09:54.160 align:middle line:84% but where the concentration of calcium 01:09:54.160 --> 01:09:57.480 align:middle line:84% carbonate in the sediments is high, above 50%. 01:09:57.480 --> 01:10:01.240 align:middle line:84% And what they saw in these cases was essentially 01:10:01.240 --> 01:10:05.400 align:middle line:84% no dissolution of calcium carbonate in the sediments. 01:10:05.400 --> 01:10:08.300 align:middle line:84% And an important point is that I showed 01:10:08.300 --> 01:10:11.300 align:middle line:84% you pore water data from one spot that 01:10:11.300 --> 01:10:14.140 align:middle line:84% showed a clear increase in total alkalinity 01:10:14.140 --> 01:10:15.880 align:middle line:90% with depth in the pore waters. 01:10:15.880 --> 01:10:18.260 align:middle line:84% And that actually is one of these spots where 01:10:18.260 --> 01:10:21.500 align:middle line:84% they found an alkalinity flux into the sediments, 01:10:21.500 --> 01:10:23.940 align:middle line:90% instead out of the sediments. 01:10:23.940 --> 01:10:27.020 align:middle line:84% And these sites also include a couple 01:10:27.020 --> 01:10:30.580 align:middle line:84% of places where pH data led to the prediction 01:10:30.580 --> 01:10:32.340 align:middle line:84% that calcium carbonate was actually 01:10:32.340 --> 01:10:33.840 align:middle line:90% dissolving in the sediments. 01:10:33.840 --> 01:10:37.540 align:middle line:84% So we have two different types of measurements directly 01:10:37.540 --> 01:10:39.580 align:middle line:84% contradicting each other in these high calcium 01:10:39.580 --> 01:10:41.700 align:middle line:90% carbonate sediments. 01:10:41.700 --> 01:10:45.220 align:middle line:84% These guys have made two arguments. 01:10:45.220 --> 01:10:47.940 align:middle line:90% And both of them relate to-- 01:10:47.940 --> 01:10:51.180 align:middle line:84% well, one of them relates to the potential importance 01:10:51.180 --> 01:10:55.740 align:middle line:84% of complexation reactions with the surface of calcium carbonate 01:10:55.740 --> 01:10:59.900 align:middle line:84% minerals, saying that these calcium carbonate minerals may 01:10:59.900 --> 01:11:03.120 align:middle line:90% buffer the solution. 01:11:03.120 --> 01:11:08.320 align:middle line:84% So that, for instance, the speciation model that's used-- 01:11:08.320 --> 01:11:11.560 align:middle line:84% that's applied to the pH profiles is just wrong. 01:11:11.560 --> 01:11:14.500 align:middle line:84% And because that speciation model is wrong, 01:11:14.500 --> 01:11:16.172 align:middle line:84% it leads them to an incorrect prediction 01:11:16.172 --> 01:11:17.880 align:middle line:84% that there will be dissolution of calcium 01:11:17.880 --> 01:11:20.600 align:middle line:90% carbonate in the sediments. 01:11:20.600 --> 01:11:25.045 align:middle line:84% That, of course, doesn't answer the disagreement 01:11:25.045 --> 01:11:26.920 align:middle line:84% between their measurements and the pore water 01:11:26.920 --> 01:11:29.840 align:middle line:90% alkalinity profiles. 01:11:29.840 --> 01:11:32.280 align:middle line:84% To try to reconcile those two different measurements, what 01:11:32.280 --> 01:11:35.360 align:middle line:84% they've said is that, well, those alkalinity profiles 01:11:35.360 --> 01:11:38.160 align:middle line:84% have a pretty high resolution, but it's not high enough 01:11:38.160 --> 01:11:40.200 align:middle line:84% because it's alkalinity is diffusing up 01:11:40.200 --> 01:11:42.600 align:middle line:84% very close to the sediment water interface. 01:11:42.600 --> 01:11:46.200 align:middle line:84% And there it's precipitating as calcium carbonate. 01:11:46.200 --> 01:11:50.920 align:middle line:84% And so that there's no net dissolution. 01:11:50.920 --> 01:11:54.000 align:middle line:84% Now, that argument was actually discounted a long, long time 01:11:54.000 --> 01:11:58.380 align:middle line:84% ago, where people said, well, if that were the case, 01:11:58.380 --> 01:12:00.960 align:middle line:84% then marine sediments would just be 01:12:00.960 --> 01:12:03.680 align:middle line:84% covered with a blanket of inorganically precipitated 01:12:03.680 --> 01:12:06.140 align:middle line:84% calcium carbonate because so many of the sediments 01:12:06.140 --> 01:12:08.560 align:middle line:84% lie above the calcite saturation horizon. 01:12:08.560 --> 01:12:11.420 align:middle line:90% 01:12:11.420 --> 01:12:14.120 align:middle line:84% These guys counter that argument by saying, 01:12:14.120 --> 01:12:16.820 align:middle line:84% well, the process is very, very slow, 01:12:16.820 --> 01:12:22.140 align:middle line:84% and it's actually recycled inorganic calcium carbonate. 01:12:22.140 --> 01:12:26.480 align:middle line:90% So this alkalinity diffuses up. 01:12:26.480 --> 01:12:29.380 align:middle line:84% Inorganic calcium carbonate precipitates very slowly 01:12:29.380 --> 01:12:30.640 align:middle line:90% and is mixed down. 01:12:30.640 --> 01:12:32.780 align:middle line:84% And it's actually this inorganically precipitated 01:12:32.780 --> 01:12:35.220 align:middle line:84% calcium carbonate that's dissolving 01:12:35.220 --> 01:12:38.860 align:middle line:84% and causing alkalinity profiles to show an increase with depth. 01:12:38.860 --> 01:12:44.180 align:middle line:84% Then it's being mixed back to the surface. 01:12:44.180 --> 01:12:47.480 align:middle line:84% So it's dissolving-- alkalinity that appears as a result, 01:12:47.480 --> 01:12:49.780 align:middle line:84% diffuses back to the surface and just causes 01:12:49.780 --> 01:12:51.700 align:middle line:84% a cycle of precipitation of a very 01:12:51.700 --> 01:12:55.920 align:middle line:84% small amount of inorganically precipitated calcium carbonate. 01:12:55.920 --> 01:13:00.220 align:middle line:90% 01:13:00.220 --> 01:13:07.742 align:middle line:84% So one person, me, said, well, if we 01:13:07.742 --> 01:13:10.200 align:middle line:84% can't measure this in solution, what if we try measuring it 01:13:10.200 --> 01:13:11.480 align:middle line:90% in the solid phase? 01:13:11.480 --> 01:13:14.480 align:middle line:84% And the way this approach worked was 01:13:14.480 --> 01:13:16.800 align:middle line:84% it used the fact-- have you talked about this? 01:13:16.800 --> 01:13:18.245 align:middle line:90% That the thorium-230 flux-- 01:13:18.245 --> 01:13:19.120 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:13:19.120 --> 01:13:19.703 align:middle line:90% PROFESSOR: OK. 01:13:19.703 --> 01:13:21.440 align:middle line:84% The thorium-230 flux to the sea floor 01:13:21.440 --> 01:13:24.160 align:middle line:84% is essentially constant at a given location, 01:13:24.160 --> 01:13:28.160 align:middle line:84% because thorium-230 is produced by decay of uranium-234, which 01:13:28.160 --> 01:13:29.880 align:middle line:90% is conservative in seawater. 01:13:29.880 --> 01:13:33.240 align:middle line:84% And thorium-230 is essentially all removed by scavenging 01:13:33.240 --> 01:13:34.720 align:middle line:90% and falls to the sea floor. 01:13:34.720 --> 01:13:37.580 align:middle line:84% So at a given location, there's a constant accumulation rate, 01:13:37.580 --> 01:13:42.080 align:middle line:84% a constant flux of thorium-230 to the sea floor. 01:13:42.080 --> 01:13:46.160 align:middle line:84% What happens if there's a constant flux of thorium-230 01:13:46.160 --> 01:13:48.800 align:middle line:90% to the sea floor, it'll have-- 01:13:48.800 --> 01:13:52.160 align:middle line:84% and the mass accumulation rate of the sediment 01:13:52.160 --> 01:13:55.600 align:middle line:84% is decreasing as you go deeper from the sediment water 01:13:55.600 --> 01:13:57.840 align:middle line:84% interface down into the sediments, 01:13:57.840 --> 01:14:00.720 align:middle line:84% because the flux of thorium-230 is constant 01:14:00.720 --> 01:14:03.230 align:middle line:84% and the mass accumulation, the total mass accumulation 01:14:03.230 --> 01:14:04.910 align:middle line:84% is decreasing, then the concentration 01:14:04.910 --> 01:14:07.510 align:middle line:84% of thorium-230 in these circumstances 01:14:07.510 --> 01:14:10.030 align:middle line:84% will increase because its concentration is 01:14:10.030 --> 01:14:14.790 align:middle line:84% the ratio of its flux to the mass accumulation rate. 01:14:14.790 --> 01:14:20.110 align:middle line:84% So you can imagine a case where the mass accumulation 01:14:20.110 --> 01:14:23.550 align:middle line:84% rate is decreasing because calcium carbonate is dissolving. 01:14:23.550 --> 01:14:26.390 align:middle line:84% If that's happening, then what you would see 01:14:26.390 --> 01:14:28.070 align:middle line:84% is an increase in the concentration 01:14:28.070 --> 01:14:30.910 align:middle line:84% of thorium-230 in the sediments going downward 01:14:30.910 --> 01:14:33.190 align:middle line:84% from the sediment water interface. 01:14:33.190 --> 01:14:36.730 align:middle line:84% If calcium carbonate is a major component of the sediments, 01:14:36.730 --> 01:14:39.270 align:middle line:84% then its dissolution will cause a significant decrease 01:14:39.270 --> 01:14:41.350 align:middle line:84% in the total mass accumulation rate. 01:14:41.350 --> 01:14:43.430 align:middle line:84% And you'd expect to see the concentration 01:14:43.430 --> 01:14:48.550 align:middle line:84% of thorium-230 increase with depth in the sediments. 01:14:48.550 --> 01:14:51.550 align:middle line:84% Here's a spot from the western equatorial 01:14:51.550 --> 01:14:54.310 align:middle line:84% Pacific, lying above the calcite saturation 01:14:54.310 --> 01:14:58.470 align:middle line:84% horizon, where excess thorium-230 profiles were 01:14:58.470 --> 01:14:58.970 align:middle line:90% measured. 01:14:58.970 --> 01:15:01.370 align:middle line:84% And you can see looking at the upper left, 01:15:01.370 --> 01:15:04.570 align:middle line:84% that the excess thorium-230 concentration in the sediments 01:15:04.570 --> 01:15:09.010 align:middle line:90% increases by about 25%. 01:15:09.010 --> 01:15:12.730 align:middle line:84% And you can use that, plus the calcium carbonate concentration 01:15:12.730 --> 01:15:15.090 align:middle line:84% profile with depth in the sediments, 01:15:15.090 --> 01:15:17.290 align:middle line:84% to calculate a calcium carbonate accumulation 01:15:17.290 --> 01:15:18.870 align:middle line:90% rate in these sediments. 01:15:18.870 --> 01:15:23.850 align:middle line:84% And you can see that it decreases by 25% or so, 01:15:23.850 --> 01:15:27.490 align:middle line:84% I think, 25% or 30% in the upper 1.5 centimeters 01:15:27.490 --> 01:15:30.650 align:middle line:84% of the sediment column, which is just the region where you'd 01:15:30.650 --> 01:15:32.450 align:middle line:84% expect calcium carbonate to be dissolving 01:15:32.450 --> 01:15:35.510 align:middle line:84% as a result of metabolic dissolution. 01:15:35.510 --> 01:15:40.730 align:middle line:84% So here's solid phase evidence that calcium carbonate 01:15:40.730 --> 01:15:44.090 align:middle line:84% is dissolving in the sediment pore waters 01:15:44.090 --> 01:15:47.050 align:middle line:84% at a site that lies above the calcium carbonate saturation 01:15:47.050 --> 01:15:49.930 align:middle line:90% horizon. 01:15:49.930 --> 01:15:53.610 align:middle line:84% When I showed these data to Jahncke, 01:15:53.610 --> 01:15:55.710 align:middle line:84% the one who says that process doesn't happen, 01:15:55.710 --> 01:15:58.430 align:middle line:84% he says, well, you're just not measuring on a fine enough depth 01:15:58.430 --> 01:15:58.930 align:middle line:90% scale. 01:15:58.930 --> 01:16:02.150 align:middle line:90% And actually this thorium-230-- 01:16:02.150 --> 01:16:04.310 align:middle line:84% actually, the calcium carbonate, you're 01:16:04.310 --> 01:16:06.930 align:middle line:84% not seeing any net dissolution of calcium carbonate. 01:16:06.930 --> 01:16:09.110 align:middle line:84% It's actually just precipitating again very close 01:16:09.110 --> 01:16:11.207 align:middle line:90% to the sediment water interface. 01:16:11.207 --> 01:16:12.290 align:middle line:90% But there's a catch there. 01:16:12.290 --> 01:16:14.270 align:middle line:84% And the catch is that this is about 30% 01:16:14.270 --> 01:16:20.550 align:middle line:84% of the calcium carbonate flux, so that the inorganically 01:16:20.550 --> 01:16:22.270 align:middle line:84% precipitated calcium carbonate, which 01:16:22.270 --> 01:16:25.410 align:middle line:84% is easily distinguishable from biogenic calcium carbonate, 01:16:25.410 --> 01:16:26.990 align:middle line:90% would be clearly visible. 01:16:26.990 --> 01:16:28.310 align:middle line:90% But it's not. 01:16:28.310 --> 01:16:29.952 align:middle line:84% And so this is a counterargument, 01:16:29.952 --> 01:16:31.910 align:middle line:84% saying that, yes, indeed, metabolic dissolution 01:16:31.910 --> 01:16:34.230 align:middle line:84% does occur, even though measurements 01:16:34.230 --> 01:16:37.270 align:middle line:84% of dissolved phases in the-- dissolved species 01:16:37.270 --> 01:16:41.230 align:middle line:84% in the sediments lead to contradictory conclusions. 01:16:41.230 --> 01:16:46.810 align:middle line:84% And as far as I know, there has been no real resolution of this. 01:16:46.810 --> 01:16:49.850 align:middle line:90% And so there are two camps. 01:16:49.850 --> 01:16:52.390 align:middle line:84% One says metabolic dissolution certainly 01:16:52.390 --> 01:16:55.250 align:middle line:84% does occur in sediments and is an important process. 01:16:55.250 --> 01:16:58.250 align:middle line:84% And the other camp that says, no, it doesn't. 01:16:58.250 --> 01:17:01.130 align:middle line:84% And included in that second camp are 01:17:01.130 --> 01:17:02.770 align:middle line:84% a lot of paleo oceanographers who 01:17:02.770 --> 01:17:05.650 align:middle line:84% don't want it as a complicating factor in interpretation 01:17:05.650 --> 01:17:07.770 align:middle line:90% of their results. 01:17:07.770 --> 01:17:11.930 align:middle line:84% So conclusions about calcium carbonate-- dissolution 01:17:11.930 --> 01:17:15.930 align:middle line:84% obviously is driven by undersaturation in the water. 01:17:15.930 --> 01:17:18.490 align:middle line:84% Calcite is the most important carbonate mineral 01:17:18.490 --> 01:17:21.330 align:middle line:90% in the deep sea. 01:17:21.330 --> 01:17:23.930 align:middle line:84% Calcite solubility, plus the action 01:17:23.930 --> 01:17:27.390 align:middle line:84% of biogeochemical cycles in the ocean, 01:17:27.390 --> 01:17:29.850 align:middle line:84% mean that the degree of saturation of seawater 01:17:29.850 --> 01:17:31.850 align:middle line:84% with respect to calcite decreases 01:17:31.850 --> 01:17:34.330 align:middle line:84% with increasing depth at any given spot 01:17:34.330 --> 01:17:36.570 align:middle line:84% and decreases going from the deep Atlantic 01:17:36.570 --> 01:17:38.330 align:middle line:90% to the deep Pacific. 01:17:38.330 --> 01:17:41.090 align:middle line:84% And that, of course, results in the simple distribution 01:17:41.090 --> 01:17:42.810 align:middle line:84% of calcium carbonate that I showed 01:17:42.810 --> 01:17:46.490 align:middle line:84% in that ocean-wide plot of calcium carbonate concentrations 01:17:46.490 --> 01:17:49.250 align:middle line:90% in sediments. 01:17:49.250 --> 01:17:52.890 align:middle line:84% But oxic metabolism can drive dissolution of calcite 01:17:52.890 --> 01:17:56.390 align:middle line:84% in sediments lying above the calcite saturation horizon. 01:17:56.390 --> 01:17:59.670 align:middle line:84% This metabolic dissolution may play an important role 01:17:59.670 --> 01:18:01.830 align:middle line:90% in the marine carbonate cycle. 01:18:01.830 --> 01:18:05.030 align:middle line:90% But its occurrence is debated. 01:18:05.030 --> 01:18:08.710 align:middle line:84% Just to bring these points home-- 01:18:08.710 --> 01:18:11.517 align:middle line:84% of the calcium carbonate that dissolves in the oceans 01:18:11.517 --> 01:18:13.350 align:middle line:84% after it's formed in the surface, about half 01:18:13.350 --> 01:18:15.870 align:middle line:84% of the dissolution occurs in the sediments. 01:18:15.870 --> 01:18:19.510 align:middle line:84% And up to about half of that dissolution 01:18:19.510 --> 01:18:22.670 align:middle line:84% occurring in the sediments may be metabolically driven. 01:18:22.670 --> 01:18:27.590 align:middle line:84% So it's an important process that one day we 01:18:27.590 --> 01:18:29.330 align:middle line:90% should figure out. 01:18:29.330 --> 01:18:29.830 align:middle line:90% OK. 01:18:29.830 --> 01:18:33.732 align:middle line:90% 01:18:33.732 --> 01:18:34.710 align:middle line:90% AUDIENCE: Professor? 01:18:34.710 --> 01:18:35.430 align:middle line:90% PROFESSOR: Yeah. 01:18:35.430 --> 01:18:37.590 align:middle line:84% AUDIENCE: How [INAUDIBLE] do you think 01:18:37.590 --> 01:18:39.070 align:middle line:84% it's the percentage of carbonates 01:18:39.070 --> 01:18:41.950 align:middle line:84% to estimate water depths in sediments [INAUDIBLE]. 01:18:41.950 --> 01:18:44.270 align:middle line:84% If you look at a core of sediments 01:18:44.270 --> 01:18:47.430 align:middle line:84% and you see the amount of carbonate, how it fluctuates, 01:18:47.430 --> 01:18:51.930 align:middle line:84% could you reconstruct that-- is that saying, OK, there's less, 01:18:51.930 --> 01:18:55.300 align:middle line:84% so maybe it was deeper and it was under the [INAUDIBLE], 01:18:55.300 --> 01:18:59.660 align:middle line:84% or it was uplifted or something like that? 01:18:59.660 --> 01:19:03.660 align:middle line:90% PROFESSOR: Now, if water-- 01:19:03.660 --> 01:19:08.260 align:middle line:84% I think the more likely cause of changes in calcium carbonate 01:19:08.260 --> 01:19:12.660 align:middle line:84% concentration is a shift in the depth of the calcite saturation 01:19:12.660 --> 01:19:16.020 align:middle line:84% horizon, rather than a change in water depth. 01:19:16.020 --> 01:19:19.580 align:middle line:84% So I think that's what you'd be measuring. 01:19:19.580 --> 01:19:20.997 align:middle line:90% Do you understand-- 01:19:20.997 --> 01:19:21.580 align:middle line:90% AUDIENCE: Yes. 01:19:21.580 --> 01:19:25.998 align:middle line:90% 01:19:25.998 --> 01:19:27.040 align:middle line:90% PROFESSOR: Anything else? 01:19:27.040 --> 01:19:29.990 align:middle line:90% OK. 01:19:29.990 --> 01:19:32.300 align:middle line:84% AUDIENCE: We have a question down here. 01:19:32.300 --> 01:19:35.045 align:middle line:90% PROFESSOR: OK. 01:19:35.045 --> 01:19:38.000 align:middle line:84% AUDIENCE: Does all of this also apply to aragonite, 01:19:38.000 --> 01:19:41.220 align:middle line:84% or is that just a lesser component of the total 01:19:41.220 --> 01:19:44.300 align:middle line:90% carbonate? 01:19:44.300 --> 01:19:46.720 align:middle line:84% PROFESSOR: Aragonite is important in shallow sediments. 01:19:46.720 --> 01:19:50.860 align:middle line:90% 01:19:50.860 --> 01:19:53.240 align:middle line:84% And in fact, aragonite and high magnesium calcite 01:19:53.240 --> 01:19:57.160 align:middle line:84% are very important on continental margin sediments. 01:19:57.160 --> 01:20:02.460 align:middle line:84% Now, you can apply all these arguments to that, 01:20:02.460 --> 01:20:04.635 align:middle line:84% but I don't know that it's been done-- 01:20:04.635 --> 01:20:05.260 align:middle line:90% AUDIENCE: Yeah. 01:20:05.260 --> 01:20:07.960 align:middle line:90% 01:20:07.960 --> 01:20:11.680 align:middle line:84% Because aragonite is more soluble, very little of it 01:20:11.680 --> 01:20:13.700 align:middle line:84% makes it into the deep sea sediments. 01:20:13.700 --> 01:20:15.420 align:middle line:84% It just doesn't make it to the seafloor. 01:20:15.420 --> 01:20:18.400 align:middle line:84% And there's some data now that suggests 01:20:18.400 --> 01:20:22.400 align:middle line:84% when you look at alkalinity distributions 01:20:22.400 --> 01:20:25.760 align:middle line:84% in the upper ocean, in the thermocline, that you actually 01:20:25.760 --> 01:20:31.840 align:middle line:84% can see regions where alkalinity is being added 01:20:31.840 --> 01:20:35.200 align:middle line:84% that correlate quite nicely with where the aragonite saturation 01:20:35.200 --> 01:20:36.060 align:middle line:90% horizon is. 01:20:36.060 --> 01:20:37.600 align:middle line:84% And the argument is that things that 01:20:37.600 --> 01:20:41.160 align:middle line:84% are falling that are made out of aragonite, like pteropod shells, 01:20:41.160 --> 01:20:45.183 align:middle line:84% get remineralized in the water column much shallower 01:20:45.183 --> 01:20:47.100 align:middle line:84% before they actually make it to the sediments, 01:20:47.100 --> 01:20:48.600 align:middle line:90% because they're more soluble. 01:20:48.600 --> 01:20:51.970 align:middle line:84% So that's some work that Dick Feely's been doing.