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 --> 00:00:18.560 align:middle line:90% at ocw.mit.edu. 00:00:18.560 --> 00:00:21.440 align:middle line:90% 00:00:21.440 --> 00:00:24.160 align:middle line:84% SCOTT DONEY: So what we've been talking about 00:00:24.160 --> 00:00:30.400 align:middle line:84% is looking at the formation of, in the upper ocean, 00:00:30.400 --> 00:00:35.480 align:middle line:84% of particulate organic matter that would then gravitationally 00:00:35.480 --> 00:00:44.000 align:middle line:84% sink, and some fraction of it would be respired at depth. 00:00:44.000 --> 00:00:48.440 align:middle line:84% And then we talked some on Tuesday 00:00:48.440 --> 00:00:53.780 align:middle line:84% also about dissolved organic matter that would be advected, 00:00:53.780 --> 00:00:56.320 align:middle line:84% and some of it would end up in the deep ocean 00:00:56.320 --> 00:01:01.050 align:middle line:90% where it would be respired. 00:01:01.050 --> 00:01:04.730 align:middle line:84% We're going to finish up this section of the class 00:01:04.730 --> 00:01:07.650 align:middle line:90% by looking at-- 00:01:07.650 --> 00:01:10.570 align:middle line:84% rather than looking at the particulate organic matter 00:01:10.570 --> 00:01:13.510 align:middle line:84% and the dissolved organic matter itself, 00:01:13.510 --> 00:01:19.422 align:middle line:84% we're going to look at the geochemical signatures 00:01:19.422 --> 00:01:23.250 align:middle line:84% of the respiration and remineralization 00:01:23.250 --> 00:01:29.050 align:middle line:84% of this organic matter on carbon nutrients, and a little bit 00:01:29.050 --> 00:01:31.290 align:middle line:90% at the end on calcium-- 00:01:31.290 --> 00:01:36.290 align:middle line:84% the effect of remineralization of calcium carbonate and opal. 00:01:36.290 --> 00:01:44.170 align:middle line:84% And in fact, a lot of the early information on respiration 00:01:44.170 --> 00:01:49.090 align:middle line:84% and remineralization pathways were not actually discovered 00:01:49.090 --> 00:01:51.330 align:middle line:84% based on putting out sediment traps 00:01:51.330 --> 00:01:53.410 align:middle line:84% and measuring particulate organic matter 00:01:53.410 --> 00:01:56.010 align:middle line:84% or looking at dissolved organic matter, 00:01:56.010 --> 00:01:59.860 align:middle line:84% they were based on looking at the oxygen gradients, 00:01:59.860 --> 00:02:03.840 align:middle line:84% the nutrient gradients, and more recently, 00:02:03.840 --> 00:02:09.479 align:middle line:84% the DIC and alkalinity distributions in the deep ocean. 00:02:09.479 --> 00:02:14.380 align:middle line:90% 00:02:14.380 --> 00:02:17.140 align:middle line:84% And in particular, what we'll be looking at today 00:02:17.140 --> 00:02:20.740 align:middle line:84% is the geochemical signatures, and then also 00:02:20.740 --> 00:02:28.320 align:middle line:84% estimates of the respiration and remineralization rates. 00:02:28.320 --> 00:02:35.900 align:middle line:90% 00:02:35.900 --> 00:02:41.060 align:middle line:84% So simply looking at the geochemical signatures 00:02:41.060 --> 00:02:43.800 align:middle line:84% gives you a sense of where things might be occurring, 00:02:43.800 --> 00:02:46.660 align:middle line:84% but it doesn't tell you how fast it's occurring. 00:02:46.660 --> 00:02:49.300 align:middle line:84% And so we need to combine information 00:02:49.300 --> 00:02:52.980 align:middle line:84% on the special distributions of oxygen nutrients, DIC, 00:02:52.980 --> 00:02:57.310 align:middle line:84% and alkalinity with, for example, transient tracers 00:02:57.310 --> 00:03:04.870 align:middle line:84% to actually get at the rates that you could then compare 00:03:04.870 --> 00:03:08.630 align:middle line:84% with, say, your estimates of the divergence of sinking 00:03:08.630 --> 00:03:12.210 align:middle line:84% particulate organic matter and see if everything held together, 00:03:12.210 --> 00:03:15.630 align:middle line:84% see if attacking the problem from two different directions 00:03:15.630 --> 00:03:19.350 align:middle line:90% gave you a consistent story. 00:03:19.350 --> 00:03:24.070 align:middle line:84% So, just as a reminder, we'll be talking-- 00:03:24.070 --> 00:03:28.430 align:middle line:84% to convert back and forth again between the different elemental 00:03:28.430 --> 00:03:30.150 align:middle line:90% currencies. 00:03:30.150 --> 00:03:35.110 align:middle line:84% We'll be using the Redfield elemental stoichiometries, 00:03:35.110 --> 00:03:40.070 align:middle line:84% and we'll talk a little bit about how those were actually 00:03:40.070 --> 00:03:42.470 align:middle line:84% determined, and in part, that was 00:03:42.470 --> 00:03:46.070 align:middle line:84% based on these geochemical signatures. 00:03:46.070 --> 00:03:48.610 align:middle line:84% There's lots of different variants on this, 00:03:48.610 --> 00:03:53.470 align:middle line:84% and we've talked about that, but most of the ones 00:03:53.470 --> 00:04:00.320 align:middle line:84% that I was able to find from a survey of the literature-- 00:04:00.320 --> 00:04:05.320 align:middle line:84% I should say an unscientific, informal survey, 00:04:05.320 --> 00:04:08.760 align:middle line:84% if you looked at the ratio of carbon organic 00:04:08.760 --> 00:04:13.480 align:middle line:84% to nitrogen to phosphorus to the oxygen 00:04:13.480 --> 00:04:17.079 align:middle line:84% demand for the respiration of this organic matter, 00:04:17.079 --> 00:04:19.640 align:middle line:90% the ratio was something like-- 00:04:19.640 --> 00:04:37.160 align:middle line:84% 117 plus or minus 14, 16 plus or minus 1, 1 00:04:37.160 --> 00:04:39.560 align:middle line:84% because everything's been ratioed to phosphorus, 00:04:39.560 --> 00:04:48.600 align:middle line:84% so that's invariant, and then minus 170, probably 00:04:48.600 --> 00:04:50.460 align:middle line:90% close to plus or minus 30. 00:04:50.460 --> 00:04:52.450 align:middle line:84% There are some wide-ranging values 00:04:52.450 --> 00:04:56.690 align:middle line:90% of oxygen in the literature. 00:04:56.690 --> 00:04:59.570 align:middle line:84% And there's still some open debate. 00:04:59.570 --> 00:05:03.290 align:middle line:84% In part, there's such variation because 00:05:03.290 --> 00:05:16.210 align:middle line:84% the elemental stoichiometries are not constant in nature. 00:05:16.210 --> 00:05:19.090 align:middle line:84% And so this is an approximation or a fiction 00:05:19.090 --> 00:05:22.250 align:middle line:84% that we use for convenience, but there's good reason 00:05:22.250 --> 00:05:24.710 align:middle line:84% to think that they're not constant, 00:05:24.710 --> 00:05:29.930 align:middle line:84% both because of changes in production-- 00:05:29.930 --> 00:05:34.090 align:middle line:84% so the actual organic matter that's being produced 00:05:34.090 --> 00:05:35.330 align:middle line:90% will differ. 00:05:35.330 --> 00:05:40.890 align:middle line:84% So for example, late in a bloom, when 00:05:40.890 --> 00:05:47.050 align:middle line:84% you have nutrient limitation, you'll 00:05:47.050 --> 00:05:51.280 align:middle line:84% tend to get high carbon-to-nitrogen ratios. 00:05:51.280 --> 00:05:55.060 align:middle line:90% 00:05:55.060 --> 00:05:58.940 align:middle line:84% Basically, the organisms are under nutrient stress 00:05:58.940 --> 00:06:03.300 align:middle line:84% and there's not a lot of available nitrogen, 00:06:03.300 --> 00:06:07.540 align:middle line:84% so they'll still attempt to grow and conserve as much 00:06:07.540 --> 00:06:10.940 align:middle line:84% of the nitrogen they have, and what results is a high carbon 00:06:10.940 --> 00:06:13.340 align:middle line:90% and nitrogen in the biomass. 00:06:13.340 --> 00:06:16.060 align:middle line:84% Similarly, when we start to talk about nitrogen-fixing 00:06:16.060 --> 00:06:17.080 align:middle line:90% organisms-- 00:06:17.080 --> 00:06:20.100 align:middle line:90% 00:06:20.100 --> 00:06:23.260 align:middle line:84% and Karen will be coming back-- [? Karen ?] [? Calciadi ?] will 00:06:23.260 --> 00:06:27.860 align:middle line:84% be coming back and giving a fairly detailed set of lectures 00:06:27.860 --> 00:06:31.820 align:middle line:84% later-- or a fairly detailed lecture on the nitrogen cycle, 00:06:31.820 --> 00:06:34.580 align:middle line:84% but when you have nitrogen-fixing organisms, 00:06:34.580 --> 00:06:38.860 align:middle line:84% they tend to be phosphorus-limited, 00:06:38.860 --> 00:06:44.220 align:middle line:84% and so they can generate some very high N-to-P ratios 00:06:44.220 --> 00:06:48.770 align:middle line:90% and carbon-to-phosphorus ratios. 00:06:48.770 --> 00:06:52.230 align:middle line:90% 00:06:52.230 --> 00:07:01.310 align:middle line:84% Also, there can be differentiable remineralization. 00:07:01.310 --> 00:07:04.710 align:middle line:84% There can be differentiable remineralization. 00:07:04.710 --> 00:07:09.750 align:middle line:84% And this hasn't been completely resolved in a general case, 00:07:09.750 --> 00:07:14.110 align:middle line:90% but typically we think of-- 00:07:14.110 --> 00:07:21.630 align:middle line:84% if you were to look at the length scale of sinking 00:07:21.630 --> 00:07:23.830 align:middle line:90% organic matter-- 00:07:23.830 --> 00:07:28.030 align:middle line:84% So here's the base of the euphotic zone, say, 00:07:28.030 --> 00:07:34.550 align:middle line:84% and you had some flux as a function of depth, 00:07:34.550 --> 00:07:39.942 align:middle line:90% the length scale would be-- 00:07:39.942 --> 00:07:42.670 align:middle line:84% flux would-- you could approximate flux 00:07:42.670 --> 00:07:52.620 align:middle line:84% is equal to some flux near the surface e to the minus z over. 00:07:52.620 --> 00:07:56.200 align:middle line:90% 00:07:56.200 --> 00:08:00.740 align:middle line:84% z remin where z remin would be some length scale. 00:08:00.740 --> 00:08:05.600 align:middle line:84% So you could approximate the loss of organic matter 00:08:05.600 --> 00:08:10.040 align:middle line:84% as an exponential, at least in the upper water column. 00:08:10.040 --> 00:08:18.680 align:middle line:84% And the z remin tends to be shallowest for phosphorus. 00:08:18.680 --> 00:08:21.120 align:middle line:84% So organic compounds that are rich in phosphorus 00:08:21.120 --> 00:08:27.880 align:middle line:84% tend to get remineralized early, earlier than those with nitrogen 00:08:27.880 --> 00:08:30.800 align:middle line:84% and earlier than those with carbon. 00:08:30.800 --> 00:08:35.159 align:middle line:84% Now it's a rule of thumb, but different systems 00:08:35.159 --> 00:08:37.880 align:middle line:84% will respond somewhat differently, 00:08:37.880 --> 00:08:41.559 align:middle line:84% but there is the idea that the Redfield ratio assumes 00:08:41.559 --> 00:08:45.610 align:middle line:84% that all the organic matter is the same, it's one bulk pool, 00:08:45.610 --> 00:08:48.370 align:middle line:84% but in reality, some of this organic matter 00:08:48.370 --> 00:08:53.370 align:middle line:84% is either tastier to the bacteria or is more labile 00:08:53.370 --> 00:08:55.570 align:middle line:84% or is in a particulate form that is more 00:08:55.570 --> 00:08:58.010 align:middle line:84% likely to be remineralized with depth, 00:08:58.010 --> 00:09:00.170 align:middle line:84% and that will lead to separation. 00:09:00.170 --> 00:09:03.970 align:middle line:84% So you would see, as you move down the water column, 00:09:03.970 --> 00:09:05.650 align:middle line:84% the remineralization signal would 00:09:05.650 --> 00:09:08.050 align:middle line:84% change because you're actually remineralizing 00:09:08.050 --> 00:09:09.150 align:middle line:90% different material. 00:09:09.150 --> 00:09:13.570 align:middle line:90% 00:09:13.570 --> 00:09:15.710 align:middle line:90% Now-- sure. 00:09:15.710 --> 00:09:18.010 align:middle line:84% STUDENT: So is this exponential graph 00:09:18.010 --> 00:09:21.670 align:middle line:84% the amount of organic matter or the degree of remineralization? 00:09:21.670 --> 00:09:24.970 align:middle line:84% SCOTT DONEY: This is just flux for some component. 00:09:24.970 --> 00:09:27.090 align:middle line:84% STUDENT: Flux of organic matter into the water-- 00:09:27.090 --> 00:09:28.130 align:middle line:90% SCOTT DONEY: Yeah. 00:09:28.130 --> 00:09:31.410 align:middle line:84% And the derivative of that would be the remineralization. 00:09:31.410 --> 00:09:33.610 align:middle line:84% So of course, the derivative of an exponential 00:09:33.610 --> 00:09:38.650 align:middle line:84% is an exponential, so if I were to plot the remineralization, 00:09:38.650 --> 00:09:40.590 align:middle line:90% it would look the same. 00:09:40.590 --> 00:09:41.590 align:middle line:90% It would scale the same. 00:09:41.590 --> 00:09:43.298 align:middle line:84% Of course, it would have different units. 00:09:43.298 --> 00:09:45.900 align:middle line:90% 00:09:45.900 --> 00:09:47.200 align:middle line:90% Questions on that? 00:09:47.200 --> 00:09:49.860 align:middle line:90% 00:09:49.860 --> 00:09:51.360 align:middle line:90% OK. 00:09:51.360 --> 00:09:54.489 align:middle line:90% 00:09:54.489 --> 00:09:57.720 align:middle line:84% Usually the way-- I'm going to switch pages. 00:09:57.720 --> 00:10:07.220 align:middle line:90% 00:10:07.220 --> 00:10:13.740 align:middle line:84% One of the ways that the Redfield ratios were originally 00:10:13.740 --> 00:10:19.780 align:middle line:90% defined was to follow-- 00:10:19.780 --> 00:10:23.140 align:middle line:90% so here's our ocean surface. 00:10:23.140 --> 00:10:28.300 align:middle line:84% If you were to plot an isopycnal surface-- 00:10:28.300 --> 00:10:37.020 align:middle line:84% so a density surface, one of the early ideas 00:10:37.020 --> 00:10:40.220 align:middle line:90% in chemical oceanography-- 00:10:40.220 --> 00:10:42.350 align:middle line:84% one of the early ideas in chemical 00:10:42.350 --> 00:10:46.790 align:middle line:84% oceanography was that if you knew where a water mass formed 00:10:46.790 --> 00:10:48.750 align:middle line:90% with some set of properties-- 00:10:48.750 --> 00:10:53.630 align:middle line:84% so this would be the surface properties 00:10:53.630 --> 00:10:59.350 align:middle line:84% of any particular chemical species-- 00:10:59.350 --> 00:11:05.510 align:middle line:84% so you could, say, define that as this would be x0. 00:11:05.510 --> 00:11:16.310 align:middle line:84% That if you were to trace along the flow path and look at x, 00:11:16.310 --> 00:11:19.750 align:middle line:84% it would be defined by some initial state 00:11:19.750 --> 00:11:33.070 align:middle line:84% x0 plus the biogeochemical sources and sinks. 00:11:33.070 --> 00:11:35.230 align:middle line:90% So in this case, this would be-- 00:11:35.230 --> 00:11:38.390 align:middle line:84% for example, if it was carbon, the remineralization 00:11:38.390 --> 00:11:43.740 align:middle line:84% of organic matter would add carbon with time, 00:11:43.740 --> 00:11:50.240 align:middle line:84% and so you would expect, if you were to plot, say, distance 00:11:50.240 --> 00:11:59.040 align:middle line:84% from the outcrop, you would expect x0 00:11:59.040 --> 00:12:03.720 align:middle line:84% to increase with distance from the outcrop because 00:12:03.720 --> 00:12:06.240 align:middle line:84% of remineralization of organic matter, respiration 00:12:06.240 --> 00:12:08.440 align:middle line:90% of organic matter. 00:12:08.440 --> 00:12:15.440 align:middle line:84% One of the ways we will discuss that is to use what's called 00:12:15.440 --> 00:12:25.200 align:middle line:84% preformed nutrients or preformed carbon. 00:12:25.200 --> 00:12:27.960 align:middle line:84% And preformed is just a term that's 00:12:27.960 --> 00:12:30.320 align:middle line:84% propped up in the literature, and it basically 00:12:30.320 --> 00:12:33.600 align:middle line:84% just means the initial conditions at the surface. 00:12:33.600 --> 00:12:36.490 align:middle line:84% So in this case, x0 would be your are 00:12:36.490 --> 00:12:40.090 align:middle line:90% preformed nutrients or carbon. 00:12:40.090 --> 00:12:43.490 align:middle line:84% And so what you might see, for example, 00:12:43.490 --> 00:12:46.670 align:middle line:84% is you might see a derivation where someone says, 00:12:46.670 --> 00:12:52.970 align:middle line:84% OK, I have an observed DIC concentration 00:12:52.970 --> 00:12:56.170 align:middle line:84% and I'm going to partition that and say, well, some of that 00:12:56.170 --> 00:13:02.730 align:middle line:84% was due to my preformed DIC at the surface, 00:13:02.730 --> 00:13:06.470 align:middle line:84% and then there's been an addition 00:13:06.470 --> 00:13:15.770 align:middle line:84% due to organic matter, respiration, and then 00:13:15.770 --> 00:13:23.570 align:middle line:84% an addition due to calcium carbonate remineralization. 00:13:23.570 --> 00:13:25.510 align:middle line:84% And I could do the same thing for nutrients. 00:13:25.510 --> 00:13:28.250 align:middle line:84% I could write out the observed nitrate 00:13:28.250 --> 00:13:33.530 align:middle line:84% would be equal to the preformed nitrate plus the organic matter 00:13:33.530 --> 00:13:37.120 align:middle line:84% nitrate similarly for alkalinity, but in that case, 00:13:37.120 --> 00:13:40.360 align:middle line:84% it would just be due to the calcium carbonate, 00:13:40.360 --> 00:13:42.780 align:middle line:84% and of course, it would be twice the calcium carbonate 00:13:42.780 --> 00:13:45.380 align:middle line:84% remineralization because alkalinity, the carbonate 00:13:45.380 --> 00:13:48.740 align:middle line:90% comes in and adds two. 00:13:48.740 --> 00:13:57.420 align:middle line:84% For oxygen, typically what is done is a little different. 00:13:57.420 --> 00:14:02.100 align:middle line:84% You don't always hear about preformed oxygen. Instead-- 00:14:02.100 --> 00:14:03.580 align:middle line:90% hey. 00:14:03.580 --> 00:14:11.820 align:middle line:84% For oxygen, often we use what's called an apparent oxygen 00:14:11.820 --> 00:14:14.340 align:middle line:90% utilization rate-- utilization. 00:14:14.340 --> 00:14:17.340 align:middle line:90% Utilization. 00:14:17.340 --> 00:14:30.300 align:middle line:84% And so AOU is just defined as what 00:14:30.300 --> 00:14:34.390 align:middle line:84% you would expect for equilibrium saturation 00:14:34.390 --> 00:14:38.070 align:middle line:84% at your potential temperature and salinity 00:14:38.070 --> 00:14:42.850 align:middle line:90% minus your observed value. 00:14:42.850 --> 00:14:46.070 align:middle line:90% 00:14:46.070 --> 00:14:56.430 align:middle line:84% And AOU is basically a measure of the delta oxygen 00:14:56.430 --> 00:15:02.390 align:middle line:84% due to organic matter respiration 00:15:02.390 --> 00:15:04.430 align:middle line:84% because that's what you-- as an expectation 00:15:04.430 --> 00:15:06.750 align:middle line:84% you would expect the preform oxygen that 00:15:06.750 --> 00:15:11.310 align:middle line:84% leaves the surface would be saturated, 00:15:11.310 --> 00:15:14.510 align:middle line:84% and then minus some organic matter respiration 00:15:14.510 --> 00:15:16.910 align:middle line:84% term because of-- the negative sign 00:15:16.910 --> 00:15:19.150 align:middle line:84% is because you're using up oxygen. 00:15:19.150 --> 00:15:21.190 align:middle line:84% We just typically switch it around so 00:15:21.190 --> 00:15:23.890 align:middle line:84% that we actually are talking about positive values 00:15:23.890 --> 00:15:25.430 align:middle line:90% in the case of AOU. 00:15:25.430 --> 00:15:30.030 align:middle line:84% So AOU is-- almost everywhere AOU will be positive, 00:15:30.030 --> 00:15:33.000 align:middle line:84% but that means that there's been oxygen loss just because 00:15:33.000 --> 00:15:35.960 align:middle line:84% of the sign convention of the definition. 00:15:35.960 --> 00:15:38.520 align:middle line:84% Does that make sense to everybody? 00:15:38.520 --> 00:15:41.382 align:middle line:84% This is one of those things you have to just-- 00:15:41.382 --> 00:15:42.840 align:middle line:84% you have to memorize because you're 00:15:42.840 --> 00:15:47.320 align:middle line:84% going to see it all the time, is people will talk about AOU. 00:15:47.320 --> 00:15:56.600 align:middle line:84% This assumes equilibrium of surface waters. 00:15:56.600 --> 00:15:58.520 align:middle line:84% The surface waters that are getting 00:15:58.520 --> 00:16:00.500 align:middle line:90% advected down this isopycnal. 00:16:00.500 --> 00:16:04.840 align:middle line:90% 00:16:04.840 --> 00:16:10.480 align:middle line:84% So let's look at a couple of figures. 00:16:10.480 --> 00:16:13.000 align:middle line:84% And I'm just going to show you some of the color plates out 00:16:13.000 --> 00:16:16.240 align:middle line:90% of Sarmiento and Gruber. 00:16:16.240 --> 00:16:19.880 align:middle line:84% And they're nice because they've kind of lumped together 00:16:19.880 --> 00:16:22.320 align:middle line:90% a whole bunch of sections. 00:16:22.320 --> 00:16:28.120 align:middle line:84% And Nicky has this odd convention where he plots-- 00:16:28.120 --> 00:16:35.210 align:middle line:84% he starts in the North Atlantic and ends in the North Pacific, 00:16:35.210 --> 00:16:37.450 align:middle line:84% and he puts it on one section where it goes 00:16:37.450 --> 00:16:39.570 align:middle line:90% through the Southern Ocean. 00:16:39.570 --> 00:16:41.930 align:middle line:84% So it basically goes-- it'll be a section that 00:16:41.930 --> 00:16:44.910 align:middle line:84% goes down the North Atlantic, through the Southern Ocean, 00:16:44.910 --> 00:16:46.690 align:middle line:90% and then up the North Pacific. 00:16:46.690 --> 00:16:50.070 align:middle line:84% So it's along the pathway of the global conveyor belt, 00:16:50.070 --> 00:17:01.330 align:middle line:84% so you would expect-- so this would be the global thermohaline 00:17:01.330 --> 00:17:03.410 align:middle line:84% circulation where you have inputs 00:17:03.410 --> 00:17:08.369 align:middle line:84% of new deep water in the North Atlantic and the Southern Ocean, 00:17:08.369 --> 00:17:12.690 align:middle line:84% and then you have generalized upwelling in the North Pacific. 00:17:12.690 --> 00:17:15.710 align:middle line:84% Of course, as we talked about early in the semester, 00:17:15.710 --> 00:17:20.347 align:middle line:84% it's much more complicated than that in any detail. 00:17:20.347 --> 00:17:22.430 align:middle line:84% So if you want, I'll just show you a few of these, 00:17:22.430 --> 00:17:27.170 align:middle line:84% and then you can always go back and stare at these ad nauseam 00:17:27.170 --> 00:17:29.020 align:middle line:90% if you either-- 00:17:29.020 --> 00:17:31.540 align:middle line:90% while these are on reserve. 00:17:31.540 --> 00:17:40.820 align:middle line:84% So let's-- so some of this should be very familiar 00:17:40.820 --> 00:17:44.060 align:middle line:84% because we've talked about the nutrient distributions, but-- 00:17:44.060 --> 00:17:47.140 align:middle line:84% So for example, this is nitrate where 00:17:47.140 --> 00:17:50.420 align:middle line:84% you see low values entering in the northern North 00:17:50.420 --> 00:17:59.940 align:middle line:84% Atlantic and low values of nitrate coming in in the North 00:17:59.940 --> 00:18:03.660 align:middle line:84% Atlantic deep water, nitrate continuing to build up 00:18:03.660 --> 00:18:08.180 align:middle line:84% as you move into the North Pacific, 00:18:08.180 --> 00:18:10.700 align:middle line:84% and then higher values at mid-depth because that's 00:18:10.700 --> 00:18:13.600 align:middle line:84% where most of the remineralization is occurring. 00:18:13.600 --> 00:18:16.220 align:middle line:90% 00:18:16.220 --> 00:18:23.200 align:middle line:84% Now, one of the tricks of this preformed nutrients-- 00:18:23.200 --> 00:18:23.700 align:middle line:90% oops. 00:18:23.700 --> 00:18:32.430 align:middle line:90% 00:18:32.430 --> 00:18:37.190 align:middle line:84% The original model we showed up here 00:18:37.190 --> 00:18:40.950 align:middle line:84% had only a single spot where deep water-- or water was 00:18:40.950 --> 00:18:44.830 align:middle line:84% forming, and then it was advecting down in isopycnal. 00:18:44.830 --> 00:18:48.030 align:middle line:84% The reality is, in large parts of the ocean, 00:18:48.030 --> 00:18:51.550 align:middle line:84% you'll have sources that come from multiple places. 00:18:51.550 --> 00:18:53.510 align:middle line:90% So you might have-- 00:18:53.510 --> 00:18:59.390 align:middle line:84% along the same isopycnal, you'll have a northern source 00:18:59.390 --> 00:19:00.890 align:middle line:90% and a southern source. 00:19:00.890 --> 00:19:03.750 align:middle line:90% 00:19:03.750 --> 00:19:05.710 align:middle line:90% Source. 00:19:05.710 --> 00:19:15.150 align:middle line:84% And those might have different preformed nutrient 00:19:15.150 --> 00:19:16.410 align:middle line:90% concentrations. 00:19:16.410 --> 00:19:19.550 align:middle line:90% 00:19:19.550 --> 00:19:27.120 align:middle line:84% And so what's often done is people will use salinity 00:19:27.120 --> 00:19:32.580 align:middle line:90% to correct for source water. 00:19:32.580 --> 00:19:48.600 align:middle line:90% 00:19:48.600 --> 00:19:54.520 align:middle line:84% And it's pretty straightforward in the case of a single-- 00:19:54.520 --> 00:19:58.720 align:middle line:84% if you have either a single endmember or two endmembers. 00:19:58.720 --> 00:20:01.520 align:middle line:84% It looks, in some ways, like what 00:20:01.520 --> 00:20:05.200 align:middle line:90% we were looking at for rivers. 00:20:05.200 --> 00:20:14.000 align:middle line:84% So let's say we have a two-endmember case. 00:20:14.000 --> 00:20:17.880 align:middle line:84% So you have one surface water, you have another surface water, 00:20:17.880 --> 00:20:24.750 align:middle line:84% and you have water that is a mixture of these two endmembers. 00:20:24.750 --> 00:20:32.050 align:middle line:90% 00:20:32.050 --> 00:20:33.270 align:middle line:90% I have to switch pages. 00:20:33.270 --> 00:20:34.630 align:middle line:90% You guys, everybody ready? 00:20:34.630 --> 00:20:52.690 align:middle line:90% 00:20:52.690 --> 00:20:56.410 align:middle line:84% So again, the two-endmember case where 00:20:56.410 --> 00:21:00.050 align:middle line:84% you've got water, that's a mixture of two endmembers. 00:21:00.050 --> 00:21:06.690 align:middle line:84% Well, if I were to plot against a conservative tracer-- 00:21:06.690 --> 00:21:09.930 align:middle line:84% in this case, we'll use salinity, 00:21:09.930 --> 00:21:14.190 align:middle line:84% so we're going to treat salinity as a conservative tracer. 00:21:14.190 --> 00:21:17.010 align:middle line:90% 00:21:17.010 --> 00:21:19.290 align:middle line:84% Along the isopycnal surface, which is a pretty good 00:21:19.290 --> 00:21:23.540 align:middle line:84% assumption as you're far enough away from the surface 00:21:23.540 --> 00:21:25.780 align:middle line:84% and there's not a lot of diapycnal mixing 00:21:25.780 --> 00:21:27.880 align:middle line:84% and you're not in a region of salt fingering. 00:21:27.880 --> 00:21:30.500 align:middle line:90% 00:21:30.500 --> 00:21:36.740 align:middle line:84% If I plotted whatever tracer I wanted, say C0-- 00:21:36.740 --> 00:21:41.900 align:middle line:84% or C, in this case, I'd have these two endmembers. 00:21:41.900 --> 00:21:43.960 align:middle line:84% So these are my surface endmembers. 00:21:43.960 --> 00:21:49.300 align:middle line:90% 00:21:49.300 --> 00:21:50.780 align:middle line:90% And so what's going to happen-- 00:21:50.780 --> 00:21:53.180 align:middle line:84% let's say there's no biogeochemistry, there's just 00:21:53.180 --> 00:21:55.920 align:middle line:84% mixing, what's that plot going to look like? 00:21:55.920 --> 00:21:59.098 align:middle line:90% 00:21:59.098 --> 00:22:00.140 align:middle line:90% STUDENT: A straight line? 00:22:00.140 --> 00:22:02.540 align:middle line:84% SCOTT DONEY: It'll be a straight line right. 00:22:02.540 --> 00:22:09.060 align:middle line:84% So this would be no biogeochemistry. 00:22:09.060 --> 00:22:12.660 align:middle line:84% It would just be conservative, you'd have conservative mixing. 00:22:12.660 --> 00:22:16.060 align:middle line:84% So what you would-- just like in the river in the estuarine case, 00:22:16.060 --> 00:22:23.670 align:middle line:84% what you'd be looking for are deviations from linearity 00:22:23.670 --> 00:22:25.210 align:middle line:90% from a salinity curve. 00:22:25.210 --> 00:22:28.350 align:middle line:90% 00:22:28.350 --> 00:22:33.350 align:middle line:84% So it's very analogous to what's done in estuaries. 00:22:33.350 --> 00:22:38.550 align:middle line:84% And this-- at any particular point, 00:22:38.550 --> 00:22:48.390 align:middle line:84% this would be the delta C net, would be-- the net contribution 00:22:48.390 --> 00:22:52.110 align:middle line:84% of biogeochemistry would be the deviation 00:22:52.110 --> 00:22:54.630 align:middle line:90% from that straight line. 00:22:54.630 --> 00:22:57.002 align:middle line:84% STUDENT: What is the y-axis in here? 00:22:57.002 --> 00:22:58.710 align:middle line:84% SCOTT DONEY: It would be any tracer, just 00:22:58.710 --> 00:23:02.010 align:middle line:90% C. Any arbitrary tracer. 00:23:02.010 --> 00:23:04.950 align:middle line:84% It could be oxygen, it could be nitrate, silicate. 00:23:04.950 --> 00:23:10.270 align:middle line:90% 00:23:10.270 --> 00:23:15.430 align:middle line:84% And so, in fact, one of the ways that the people go out and make 00:23:15.430 --> 00:23:22.240 align:middle line:84% Redfield ratio measurements, or estimates, 00:23:22.240 --> 00:23:26.960 align:middle line:84% is they go out and they map tracer distributions along 00:23:26.960 --> 00:23:32.120 align:middle line:84% isopycnal surfaces and then correct for the mixing from 00:23:32.120 --> 00:23:35.400 align:middle line:84% the different endmembers, these different pre-formed endmembers, 00:23:35.400 --> 00:23:38.200 align:middle line:90% and then compare-- 00:23:38.200 --> 00:23:47.240 align:middle line:84% so they would compare the delta phosphorus net 00:23:47.240 --> 00:23:55.640 align:middle line:84% to delta nitrogen net to delta carbon net 00:23:55.640 --> 00:23:57.660 align:middle line:90% along a series of isopycnals. 00:23:57.660 --> 00:24:01.320 align:middle line:90% 00:24:01.320 --> 00:24:07.200 align:middle line:84% And you have to find a place where the biogeochemical signal 00:24:07.200 --> 00:24:12.200 align:middle line:84% is large relative to all the advection and mixing signals 00:24:12.200 --> 00:24:16.230 align:middle line:84% because if there's not a large contribution of organic matter 00:24:16.230 --> 00:24:19.570 align:middle line:84% or you're not looking at a big enough distance, what you get 00:24:19.570 --> 00:24:22.050 align:middle line:84% is most of these things end up being linear, 00:24:22.050 --> 00:24:25.130 align:middle line:84% and it's just the advection and the mixing 00:24:25.130 --> 00:24:31.050 align:middle line:84% between two different endmembers rather than the biogeochemistry. 00:24:31.050 --> 00:24:34.050 align:middle line:84% So you can't just look at-- you can't just plot-- 00:24:34.050 --> 00:24:43.650 align:middle line:84% don't simply plot, say, phosphate concentration over DIC 00:24:43.650 --> 00:24:48.450 align:middle line:84% and expect to get a Redfield ratio because a lot 00:24:48.450 --> 00:24:53.050 align:middle line:84% of what's there actually has to do with equilibrium, 00:24:53.050 --> 00:24:54.970 align:middle line:90% the preformed equilibrium-- 00:24:54.970 --> 00:24:56.930 align:middle line:84% or the equilibrium concentrations 00:24:56.930 --> 00:24:59.290 align:middle line:84% of the preformed surface concentrations 00:24:59.290 --> 00:25:01.950 align:middle line:84% that the water gets advected into the interior with. 00:25:01.950 --> 00:25:19.540 align:middle line:90% 00:25:19.540 --> 00:25:29.980 align:middle line:84% There's one complication, which is the fact that a lot of times, 00:25:29.980 --> 00:25:34.340 align:middle line:84% you don't have just a simple two-endmember case. 00:25:34.340 --> 00:25:39.300 align:middle line:84% You might have three or more endmembers. 00:25:39.300 --> 00:25:46.260 align:middle line:84% And it gets rather messy, and you don't always have a good-- 00:25:46.260 --> 00:25:51.900 align:middle line:84% you don't always have a good set of conservative tracers 00:25:51.900 --> 00:25:55.700 align:middle line:84% because once you get away from-- remember, along an isopycnal, 00:25:55.700 --> 00:25:57.840 align:middle line:84% temperature and salinity are correlated, 00:25:57.840 --> 00:26:00.840 align:middle line:84% so you can only use one or the other, 00:26:00.840 --> 00:26:04.100 align:middle line:84% you can't use both, because as you change salinity 00:26:04.100 --> 00:26:05.660 align:middle line:84% along an isopycnal, you have to have 00:26:05.660 --> 00:26:08.220 align:middle line:84% a corresponding change in temperature 00:26:08.220 --> 00:26:10.620 align:middle line:84% for you to stay along the isopycnal. 00:26:10.620 --> 00:26:12.750 align:middle line:84% So there are a variety of techniques 00:26:12.750 --> 00:26:18.830 align:middle line:84% that people have tried to use for these multiple endmember 00:26:18.830 --> 00:26:27.350 align:middle line:84% cases, and there's basically a series of mixing models where 00:26:27.350 --> 00:26:30.430 align:middle line:84% people try to set up mathematical techniques where 00:26:30.430 --> 00:26:34.070 align:middle line:84% they can pull out and resolve from the data 00:26:34.070 --> 00:26:40.790 align:middle line:84% the surface endmembers that go into the mixing model. 00:26:40.790 --> 00:26:44.510 align:middle line:84% The problem is, sometimes it's difficult. 00:26:44.510 --> 00:26:49.330 align:middle line:84% We don't always know the surface endmembers. 00:26:49.330 --> 00:26:53.190 align:middle line:90% 00:26:53.190 --> 00:26:59.030 align:middle line:84% And it's sometimes difficult to figure out the fractions 00:26:59.030 --> 00:27:02.350 align:middle line:90% of the different sources. 00:27:02.350 --> 00:27:03.950 align:middle line:84% And so it is a complication, and you 00:27:03.950 --> 00:27:05.830 align:middle line:84% should be aware of that a lot of this work 00:27:05.830 --> 00:27:09.250 align:middle line:84% depends upon how well you can do this separation. 00:27:09.250 --> 00:27:11.840 align:middle line:90% 00:27:11.840 --> 00:27:14.500 align:middle line:84% Fraction of sources or mixing ratios. 00:27:14.500 --> 00:27:18.600 align:middle line:90% 00:27:18.600 --> 00:27:22.800 align:middle line:84% I do just want to say one thing about this surface endmembers. 00:27:22.800 --> 00:27:24.640 align:middle line:84% Part of the reason why we don't know 00:27:24.640 --> 00:27:29.000 align:middle line:84% this is the surface endmembers should really 00:27:29.000 --> 00:27:32.520 align:middle line:90% be the winter water-- 00:27:32.520 --> 00:27:34.960 align:middle line:90% waters. 00:27:34.960 --> 00:27:36.940 align:middle line:90% The water at the out-- 00:27:36.940 --> 00:27:40.440 align:middle line:84% so what you want is the water that 00:27:40.440 --> 00:27:48.160 align:middle line:84% forms at the actual outcrop of an isopycnal surface in winter. 00:27:48.160 --> 00:27:53.640 align:middle line:84% And so if I were to plot my density surface, 00:27:53.640 --> 00:27:55.920 align:middle line:84% what you'll see in the thermocline 00:27:55.920 --> 00:28:03.160 align:middle line:84% is, during the summer, this isopycnal-- so this is, 00:28:03.160 --> 00:28:07.520 align:middle line:84% say, a particular isopycnal with a certain water mass. 00:28:07.520 --> 00:28:11.570 align:middle line:84% And during the winter, it might outcrop like that. 00:28:11.570 --> 00:28:15.370 align:middle line:90% And so this would be winter. 00:28:15.370 --> 00:28:20.170 align:middle line:84% But during the summer, you'll get much warmer water there. 00:28:20.170 --> 00:28:23.650 align:middle line:90% So let's say this is cold. 00:28:23.650 --> 00:28:27.010 align:middle line:84% And so during the summer, this will 00:28:27.010 --> 00:28:31.690 align:middle line:90% outcrop much further poleward. 00:28:31.690 --> 00:28:33.270 align:middle line:90% So this would be poleward. 00:28:33.270 --> 00:28:37.450 align:middle line:90% 00:28:37.450 --> 00:28:46.010 align:middle line:84% But all of this water-- this is all in the seasonal thermocline, 00:28:46.010 --> 00:28:48.930 align:middle line:84% and that's not the water we want to look at. 00:28:48.930 --> 00:28:51.610 align:middle line:84% And so unfortunately, a lot of our geochemical surveys 00:28:51.610 --> 00:28:55.490 align:middle line:84% are done during the summer, particularly in high latitudes, 00:28:55.490 --> 00:28:57.610 align:middle line:90% because of weather constraints. 00:28:57.610 --> 00:28:59.730 align:middle line:84% And sometimes you have to actually try 00:28:59.730 --> 00:29:04.170 align:middle line:84% to interpolate back to what the conditions were in winter so 00:29:04.170 --> 00:29:07.180 align:middle line:84% that you know what the winter source waters are because it's 00:29:07.180 --> 00:29:12.180 align:middle line:84% actually the winter water that gets advected down for the most 00:29:12.180 --> 00:29:16.460 align:middle line:84% part onto the isopycnals, not what's happening on the summer. 00:29:16.460 --> 00:29:19.140 align:middle line:84% Does that make sense to everybody? 00:29:19.140 --> 00:29:22.300 align:middle line:90% 00:29:22.300 --> 00:29:23.320 align:middle line:90% Is that OK with you? 00:29:23.320 --> 00:29:24.600 align:middle line:90% Everybody up at MIT? 00:29:24.600 --> 00:29:28.460 align:middle line:90% 00:29:28.460 --> 00:29:32.340 align:middle line:84% STUDENT: What's the scale on that diagram? 00:29:32.340 --> 00:29:34.060 align:middle line:90% Like latitude-wise. 00:29:34.060 --> 00:29:37.140 align:middle line:84% SCOTT DONEY: It could be 10 or 15 degrees of latitude. 00:29:37.140 --> 00:29:40.400 align:middle line:84% STUDENT: So you just-- like you said, survey up there? 00:29:40.400 --> 00:29:41.780 align:middle line:90% You can just go down there-- 00:29:41.780 --> 00:29:42.280 align:middle line:90% [COUGHING] 00:29:42.280 --> 00:29:43.460 align:middle line:90% --deeper? 00:29:43.460 --> 00:29:48.340 align:middle line:84% SCOTT DONEY: Well, one approach is to look along the isopycnals 00:29:48.340 --> 00:29:51.580 align:middle line:84% and see where you think winter water was formed the last time 00:29:51.580 --> 00:29:54.980 align:middle line:84% winter water was formed, but stuff has already-- there's 00:29:54.980 --> 00:29:57.940 align:middle line:84% been biogeochemical changes already to that water, 00:29:57.940 --> 00:30:03.140 align:middle line:84% and so people have attempted to build tools to correct back 00:30:03.140 --> 00:30:08.190 align:middle line:84% to what they think winter concentrations might be. 00:30:08.190 --> 00:30:10.070 align:middle line:84% The best way, of course, is to figure out 00:30:10.070 --> 00:30:13.430 align:middle line:84% a way to make your measurements remotely on some autonomous 00:30:13.430 --> 00:30:14.032 align:middle line:90% platform. 00:30:14.032 --> 00:30:15.990 align:middle line:84% Throw the thing out there and let it bob around 00:30:15.990 --> 00:30:19.350 align:middle line:84% in the winter making measurements rather than being 00:30:19.350 --> 00:30:23.070 align:middle line:84% out there at sea during the winter. 00:30:23.070 --> 00:30:27.670 align:middle line:90% But just a caveat to that. 00:30:27.670 --> 00:30:29.890 align:middle line:90% So I showed you the-- 00:30:29.890 --> 00:30:32.510 align:middle line:90% 00:30:32.510 --> 00:30:34.410 align:middle line:90% remember, this was the nitrate. 00:30:34.410 --> 00:30:39.870 align:middle line:90% 00:30:39.870 --> 00:30:43.050 align:middle line:84% So this was the nitrate distributions, North Atlantic 00:30:43.050 --> 00:30:45.790 align:middle line:90% deep water, North Pacific. 00:30:45.790 --> 00:30:47.910 align:middle line:84% If we were to look at an estimate, then, 00:30:47.910 --> 00:30:50.350 align:middle line:90% of the preformed nitrate-- 00:30:50.350 --> 00:30:53.090 align:middle line:84% this is the nitrate that's coming in, 00:30:53.090 --> 00:30:56.870 align:middle line:84% what you see is that the gradients are much smaller. 00:30:56.870 --> 00:31:03.400 align:middle line:84% There's a fairly-- there are differences 00:31:03.400 --> 00:31:05.983 align:middle line:84% between the preformed nitrogen that's 00:31:05.983 --> 00:31:07.400 align:middle line:84% being formed in the northern North 00:31:07.400 --> 00:31:10.000 align:middle line:84% Atlantic versus the Southern Ocean, 00:31:10.000 --> 00:31:12.480 align:middle line:84% but they are considerably smaller. 00:31:12.480 --> 00:31:15.480 align:middle line:90% For phosphate, that's not true. 00:31:15.480 --> 00:31:19.440 align:middle line:84% If we look at the preformed concentration of phosphate, 00:31:19.440 --> 00:31:22.840 align:middle line:84% there's actually quite large gradients 00:31:22.840 --> 00:31:25.880 align:middle line:84% between what's formed in the Northern Hemisphere 00:31:25.880 --> 00:31:28.080 align:middle line:84% and what's formed in the Southern Hemisphere. 00:31:28.080 --> 00:31:31.560 align:middle line:84% And this difference between the preformed phosphate 00:31:31.560 --> 00:31:34.000 align:middle line:84% between the northern and the southern waters 00:31:34.000 --> 00:31:37.380 align:middle line:84% is actually used a lot to help to partition that out. 00:31:37.380 --> 00:31:39.280 align:middle line:84% There are various techniques that have 00:31:39.280 --> 00:31:43.200 align:middle line:90% been developed to try to tool. 00:31:43.200 --> 00:31:46.000 align:middle line:84% But what you need to do is, for example, if you're 00:31:46.000 --> 00:31:48.400 align:middle line:84% interested in the remineralization, 00:31:48.400 --> 00:31:51.340 align:middle line:84% you would take, say, for nitrate, 00:31:51.340 --> 00:31:55.200 align:middle line:90% you'd take the nitrate field-- 00:31:55.200 --> 00:31:57.700 align:middle line:84% let's do it for phosphate because it's a bigger signal. 00:31:57.700 --> 00:32:02.770 align:middle line:84% You take the phosphate field, you then subtract off 00:32:02.770 --> 00:32:05.690 align:middle line:84% the preformed phosphate, and then you 00:32:05.690 --> 00:32:09.233 align:middle line:84% get the remineralization phosphate term, 00:32:09.233 --> 00:32:11.650 align:middle line:84% which is what you're actually interested in because that's 00:32:11.650 --> 00:32:12.990 align:middle line:90% the biogeochemistry. 00:32:12.990 --> 00:32:20.890 align:middle line:90% 00:32:20.890 --> 00:32:22.990 align:middle line:90% And the other thing-- 00:32:22.990 --> 00:32:26.410 align:middle line:84% I'll either put-- these figures are probably 00:32:26.410 --> 00:32:30.590 align:middle line:84% just a couple of sections on the homework, the upcoming homework, 00:32:30.590 --> 00:32:33.130 align:middle line:84% so you guys can get spend a little more time looking 00:32:33.130 --> 00:32:36.210 align:middle line:84% at them, but we had talked about AOU. 00:32:36.210 --> 00:32:39.450 align:middle line:84% Remember, by the way the definition's set up, 00:32:39.450 --> 00:32:41.530 align:middle line:90% AOU is always positive. 00:32:41.530 --> 00:32:43.690 align:middle line:84% So for example, the North Atlantic deep waters 00:32:43.690 --> 00:32:47.890 align:middle line:84% will have low AOUs, and as you move into the North Pacific, 00:32:47.890 --> 00:32:57.770 align:middle line:84% you get higher and higher AOUs because you're using up oxygen. 00:32:57.770 --> 00:33:03.580 align:middle line:84% OK, so that's the large-scale distribution of nutrients. 00:33:03.580 --> 00:33:07.640 align:middle line:84% The next question is, can we get some rate estimates out of that? 00:33:07.640 --> 00:33:10.580 align:middle line:90% 00:33:10.580 --> 00:33:12.460 align:middle line:84% And there's a couple of techniques that have 00:33:12.460 --> 00:33:15.580 align:middle line:90% been developed to try to get-- 00:33:15.580 --> 00:33:22.900 align:middle line:84% oops-- to get geochemical rate estimates that you could then 00:33:22.900 --> 00:33:26.780 align:middle line:84% compare with some of the other more 00:33:26.780 --> 00:33:31.540 align:middle line:84% biological techniques or more process oriented techniques. 00:33:31.540 --> 00:33:33.720 align:middle line:84% Remember, we talked a lot in the upper ocean 00:33:33.720 --> 00:33:36.740 align:middle line:84% how one of the advantages of geochemical techniques 00:33:36.740 --> 00:33:39.660 align:middle line:84% is that you're not actually manipulating the system, 00:33:39.660 --> 00:33:45.060 align:middle line:84% you're not doing incubations, you're not trying to grow bugs. 00:33:45.060 --> 00:33:50.460 align:middle line:84% So the geochemical techniques have that advantage, that they-- 00:33:50.460 --> 00:33:52.640 align:middle line:84% no direct manipulation of the system. 00:33:52.640 --> 00:33:56.910 align:middle line:90% 00:33:56.910 --> 00:34:00.190 align:middle line:84% They also have the advantage and disadvantage 00:34:00.190 --> 00:34:09.170 align:middle line:84% that they integrate over large time and space scales. 00:34:09.170 --> 00:34:12.270 align:middle line:90% 00:34:12.270 --> 00:34:16.830 align:middle line:84% So if I'm going to use the geochemical gradients of nitrate 00:34:16.830 --> 00:34:18.630 align:middle line:90% through the North Atlantic-- 00:34:18.630 --> 00:34:22.190 align:middle line:84% or through the Atlantic to try to estimate remineralization 00:34:22.190 --> 00:34:25.909 align:middle line:84% or respiration, I'm inherently making 00:34:25.909 --> 00:34:27.949 align:middle line:84% a basin-wide average right, because I'm 00:34:27.949 --> 00:34:30.469 align:middle line:90% using basin-wide gradients. 00:34:30.469 --> 00:34:34.469 align:middle line:84% I can't distinguish between what's going on here and a point 00:34:34.469 --> 00:34:36.790 align:middle line:84% 100 kilometers away, but I might be 00:34:36.790 --> 00:34:39.909 align:middle line:84% able to give you some average value over the North Atlantic 00:34:39.909 --> 00:34:43.230 align:middle line:84% over the ventilation timescales of that water. 00:34:43.230 --> 00:34:47.590 align:middle line:84% So it might be 1,000 kilometers or multi-thousand kilometers 00:34:47.590 --> 00:34:49.909 align:middle line:90% and decades. 00:34:49.909 --> 00:34:52.790 align:middle line:84% But maybe that's a better estimate 00:34:52.790 --> 00:34:55.880 align:middle line:84% than somebody going out 1 February and making 00:34:55.880 --> 00:34:59.623 align:middle line:84% one measurement at one on one particular day because that 00:34:59.623 --> 00:35:02.040 align:middle line:84% might have been-- there might have been a storm or a bloom 00:35:02.040 --> 00:35:03.920 align:middle line:84% event, or they might just have had 00:35:03.920 --> 00:35:07.480 align:middle line:84% some big piece of marine snow that 00:35:07.480 --> 00:35:10.680 align:middle line:84% got stuck in their incubation bottle 00:35:10.680 --> 00:35:13.140 align:middle line:90% and might alias their results. 00:35:13.140 --> 00:35:17.360 align:middle line:90% 00:35:17.360 --> 00:35:21.560 align:middle line:84% This has been-- a lot of the focus 00:35:21.560 --> 00:35:27.360 align:middle line:84% has been on oxygen and AOU, in part because of the larger 00:35:27.360 --> 00:35:30.520 align:middle line:84% database and the longer historical database 00:35:30.520 --> 00:35:33.040 align:middle line:90% for oxygen and AOU. 00:35:33.040 --> 00:35:38.840 align:middle line:84% So one of the terms you'll see is the OUR, 00:35:38.840 --> 00:35:48.160 align:middle line:84% which is the Oxygen Utilization Rate, so it's now 00:35:48.160 --> 00:35:51.440 align:middle line:90% being turned into a time rate. 00:35:51.440 --> 00:36:02.260 align:middle line:84% And that's simply the rate of change of AOU with time, 00:36:02.260 --> 00:36:13.020 align:middle line:84% and that can be approximated as delta AOU over delta age 00:36:13.020 --> 00:36:18.060 align:middle line:84% where age is some age derived from some tracer. 00:36:18.060 --> 00:36:20.060 align:middle line:84% And usually the assumption is that we 00:36:20.060 --> 00:36:23.220 align:middle line:84% start at the surface with zero AOU 00:36:23.220 --> 00:36:25.560 align:middle line:84% and then we just look at the age, 00:36:25.560 --> 00:36:31.440 align:middle line:84% though, people have done more localized estimates as well. 00:36:31.440 --> 00:36:35.335 align:middle line:90% 00:36:35.335 --> 00:36:37.420 align:middle line:84% STUDENT: Why [? would ?] [? the ?] AOU to be zero 00:36:37.420 --> 00:36:38.553 align:middle line:90% unchanged. 00:36:38.553 --> 00:36:39.220 align:middle line:90% SCOTT DONEY: Ah. 00:36:39.220 --> 00:36:40.845 align:middle line:84% Let's go back to the definition of AOU. 00:36:40.845 --> 00:36:44.060 align:middle line:90% 00:36:44.060 --> 00:36:47.440 align:middle line:84% So this was the definition of AOU at the surface. 00:36:47.440 --> 00:36:52.190 align:middle line:84% It was the thermodynamic gas exchange-- or the gas exchange 00:36:52.190 --> 00:36:56.230 align:middle line:84% equilibrium that you'd expect at that temperature and salinity 00:36:56.230 --> 00:36:59.790 align:middle line:90% minus the observed oxygen. 00:36:59.790 --> 00:37:02.830 align:middle line:84% So to first order, you expect surface waters 00:37:02.830 --> 00:37:07.190 align:middle line:84% to be in equilibrium with the atmosphere. 00:37:07.190 --> 00:37:09.790 align:middle line:84% That's not always true, but during the winter, 00:37:09.790 --> 00:37:11.630 align:middle line:84% that's a reasonable assumption unless you 00:37:11.630 --> 00:37:13.950 align:middle line:90% have very deep mixing. 00:37:13.950 --> 00:37:16.072 align:middle line:84% In some places, like the Labrador Sea, 00:37:16.072 --> 00:37:18.030 align:middle line:84% where you have a lot of deep convective mixing, 00:37:18.030 --> 00:37:19.270 align:middle line:90% you bring up-- 00:37:19.270 --> 00:37:24.230 align:middle line:84% you rapidly bring up deep water that has a positive AOU, 00:37:24.230 --> 00:37:26.590 align:middle line:84% the gas exchange just can't keep up, 00:37:26.590 --> 00:37:30.190 align:middle line:84% and so the convection collapses and the water sinks back down 00:37:30.190 --> 00:37:32.430 align:middle line:90% and still has an AOU signature. 00:37:32.430 --> 00:37:35.750 align:middle line:84% But in places with relatively shallow mixing, 00:37:35.750 --> 00:37:38.450 align:middle line:84% the gas exchange will equilibrate oxygen. Remember, 00:37:38.450 --> 00:37:41.910 align:middle line:84% oxygen has a time scale of about a month-- or a few weeks 00:37:41.910 --> 00:37:43.350 align:middle line:90% to a month. 00:37:43.350 --> 00:37:46.550 align:middle line:84% And so as long as you have enough time for gas exchange 00:37:46.550 --> 00:37:50.600 align:middle line:90% to occur, this should be true. 00:37:50.600 --> 00:37:53.680 align:middle line:84% The same is not true for DIC, and that's 00:37:53.680 --> 00:37:59.960 align:middle line:84% why you don't see people writing CO2 saturation minus-- 00:37:59.960 --> 00:38:05.080 align:middle line:84% DIC saturation minus DIC observed as frequently 00:38:05.080 --> 00:38:08.060 align:middle line:84% because the DIC doesn't have enough time. 00:38:08.060 --> 00:38:10.520 align:middle line:84% Remember, DIC has a much longer gas exchange time 00:38:10.520 --> 00:38:16.280 align:middle line:84% scale, so it doesn't have enough time to equilibrate. 00:38:16.280 --> 00:38:22.500 align:middle line:84% We'll come back to that I guess the last two lectures. 00:38:22.500 --> 00:38:24.920 align:middle line:84% We do anthropogenic CO2, and we'll talk about some 00:38:24.920 --> 00:38:27.000 align:middle line:84% of the techniques that people use 00:38:27.000 --> 00:38:28.800 align:middle line:84% to estimate anthropogenic CO2, and we'll 00:38:28.800 --> 00:38:31.700 align:middle line:90% get into CO2 disequilibrium. 00:38:31.700 --> 00:38:35.600 align:middle line:90% 00:38:35.600 --> 00:38:37.100 align:middle line:90% OK. 00:38:37.100 --> 00:38:42.280 align:middle line:84% So, what we were talking about was oxygen utilization rate, 00:38:42.280 --> 00:38:47.080 align:middle line:84% where it's the AOU divided by age. 00:38:47.080 --> 00:38:49.110 align:middle line:84% So where do you get the age model from? 00:38:49.110 --> 00:38:50.590 align:middle line:84% Where do you get the age estimates? 00:38:50.590 --> 00:38:56.610 align:middle line:90% 00:38:56.610 --> 00:39:01.330 align:middle line:84% One of the first papers on this was by-- 00:39:01.330 --> 00:39:15.330 align:middle line:90% estimates by Riley in 1951. 00:39:15.330 --> 00:39:21.090 align:middle line:84% And he tried to combine geostrophic velocities 00:39:21.090 --> 00:39:40.850 align:middle line:84% with oxygen gradients to try to get it at OUR. 00:39:40.850 --> 00:39:42.970 align:middle line:90% So essentially, if you had a-- 00:39:42.970 --> 00:39:46.340 align:middle line:84% let's say you had flow along an isopycnal, 00:39:46.340 --> 00:39:50.980 align:middle line:84% and if you knew what the velocity was, and you knew 00:39:50.980 --> 00:39:58.140 align:middle line:84% that you had an oxygen value at some point 1 00:39:58.140 --> 00:40:02.100 align:middle line:84% and you had an oxygen value at some point 2, 00:40:02.100 --> 00:40:07.220 align:middle line:84% you could difference these to get your AOU, 00:40:07.220 --> 00:40:09.680 align:middle line:84% and then you can estimate the time scale 00:40:09.680 --> 00:40:12.940 align:middle line:84% if you know the velocity of how long it takes to get from one 00:40:12.940 --> 00:40:14.360 align:middle line:90% point to the next point. 00:40:14.360 --> 00:40:19.340 align:middle line:90% 00:40:19.340 --> 00:40:24.100 align:middle line:84% Probably the next big paper in this realm 00:40:24.100 --> 00:40:29.500 align:middle line:84% was actually by Bill Jenkins, who's down here at WHOI. 00:40:29.500 --> 00:40:32.460 align:middle line:84% He had a whole series of papers, but it 00:40:32.460 --> 00:40:41.220 align:middle line:84% started with a paper in 1980 where he used the tritium/helium 00:40:41.220 --> 00:40:43.420 align:middle line:90% age method. 00:40:43.420 --> 00:40:46.950 align:middle line:90% And we went over this-- 00:40:46.950 --> 00:40:50.530 align:middle line:84% we went over this some when we were doing ocean physics, 00:40:50.530 --> 00:40:52.990 align:middle line:90% but I'll just review it briefly. 00:40:52.990 --> 00:40:57.470 align:middle line:84% Remember that tritium is an isotope of water. 00:40:57.470 --> 00:41:02.310 align:middle line:84% Decays to helium-3, which is a gas. 00:41:02.310 --> 00:41:04.950 align:middle line:90% So this is part of H2O-- 00:41:04.950 --> 00:41:08.290 align:middle line:90% or actually, H3HO. 00:41:08.290 --> 00:41:11.350 align:middle line:90% 00:41:11.350 --> 00:41:17.530 align:middle line:84% And it decays with a half-life of about 12 and 1/2 years. 00:41:17.530 --> 00:41:24.350 align:middle line:90% 00:41:24.350 --> 00:41:29.870 align:middle line:84% The assumption is that when you take a parcel of water 00:41:29.870 --> 00:41:36.310 align:middle line:84% from the ocean surface and it sinks down, 00:41:36.310 --> 00:41:40.370 align:middle line:84% the assumption is that in this surface box, 00:41:40.370 --> 00:41:47.320 align:middle line:84% there was no excess helium-3 where that would 00:41:47.320 --> 00:41:49.580 align:middle line:90% be excess above saturation. 00:41:49.580 --> 00:41:54.600 align:middle line:90% 00:41:54.600 --> 00:41:56.920 align:middle line:84% And so usually when you see-- almost always when 00:41:56.920 --> 00:41:59.560 align:middle line:84% you see helium-3 measurements, they're 00:41:59.560 --> 00:42:06.080 align:middle line:84% actually reporting the excess above thermodynamic saturation 00:42:06.080 --> 00:42:08.960 align:middle line:90% with the atmosphere. 00:42:08.960 --> 00:42:11.100 align:middle line:84% And so if you were to write out the-- 00:42:11.100 --> 00:42:14.560 align:middle line:90% 00:42:14.560 --> 00:42:18.960 align:middle line:84% assuming no mixing of that parcel, if you were to write out 00:42:18.960 --> 00:42:23.360 align:middle line:84% the time derivative of tritium, it's 00:42:23.360 --> 00:42:34.280 align:middle line:84% going to be equal to minus lambda times tritium. 00:42:34.280 --> 00:42:40.120 align:middle line:84% Similarly for helium-3, and this is excess helium-3, 00:42:40.120 --> 00:42:47.710 align:middle line:84% it's going to be equal to plus lambda times the tritium. 00:42:47.710 --> 00:42:50.450 align:middle line:90% 00:42:50.450 --> 00:42:54.810 align:middle line:90% And so usually the-- 00:42:54.810 --> 00:43:01.650 align:middle line:84% you can solve this since we know that any excess helium was 00:43:01.650 --> 00:43:04.050 align:middle line:84% originally-- is just dead tritium, it's 00:43:04.050 --> 00:43:09.250 align:middle line:84% tritium that decayed, we can think of the tritium 00:43:09.250 --> 00:43:14.570 align:middle line:84% concentration in the box as equaling 00:43:14.570 --> 00:43:23.010 align:middle line:84% the initial tritium when it was at the surface times 00:43:23.010 --> 00:43:26.570 align:middle line:84% some decay, exponential decay rate. 00:43:26.570 --> 00:43:32.010 align:middle line:84% And then that initial tritium can actually 00:43:32.010 --> 00:43:38.860 align:middle line:84% be set as equal to the tritium that you have at present plus 00:43:38.860 --> 00:43:42.060 align:middle line:84% all the excess helium-3, assuming that you 00:43:42.060 --> 00:43:44.740 align:middle line:90% haven't had any gas exchange. 00:43:44.740 --> 00:43:48.100 align:middle line:84% So as long as this parcel followed a path where 00:43:48.100 --> 00:43:50.660 align:middle line:84% it was at the surface and then sank down 00:43:50.660 --> 00:43:53.180 align:middle line:84% and has been isolated from the atmosphere, 00:43:53.180 --> 00:43:58.260 align:middle line:84% this helium-3 will grow in and basically gives you a clock 00:43:58.260 --> 00:44:03.900 align:middle line:84% because you can then solve this equation. 00:44:03.900 --> 00:44:08.660 align:middle line:84% You measure you measure this, that's the same thing. 00:44:08.660 --> 00:44:17.140 align:middle line:84% You measure that, and then you lambda, so you can solve for t. 00:44:17.140 --> 00:44:28.100 align:middle line:90% And this is the H. 00:44:28.100 --> 00:44:36.355 align:middle line:84% So if you do this-- and this was originally done off of Bermuda 00:44:36.355 --> 00:44:38.470 align:middle line:84% because it's a convenient spot in the ocean 00:44:38.470 --> 00:44:41.030 align:middle line:84% to go to because it has an island and a biostation 00:44:41.030 --> 00:44:41.690 align:middle line:90% and a ship. 00:44:41.690 --> 00:44:49.710 align:middle line:90% 00:44:49.710 --> 00:44:56.510 align:middle line:84% Bill and colleagues looked, as a function of depth, at the age. 00:44:56.510 --> 00:45:00.930 align:middle line:84% So this would be the tritium/helium-3 age. 00:45:00.930 --> 00:45:05.830 align:middle line:90% 00:45:05.830 --> 00:45:11.750 align:middle line:84% And it tended to be close to 0 at the surface and somewhere 00:45:11.750 --> 00:45:16.430 align:middle line:90% around 30 years at depth. 00:45:16.430 --> 00:45:19.530 align:middle line:84% And this was-- the depth was the thermocline, 00:45:19.530 --> 00:45:23.750 align:middle line:90% so this would be 1,000 meters. 00:45:23.750 --> 00:45:26.930 align:middle line:84% You could then plug this-- if you had the AOU measurements-- 00:45:26.930 --> 00:45:32.590 align:middle line:84% so you have an AOU profile, they could then 00:45:32.590 --> 00:45:36.440 align:middle line:84% combine the age estimate with the AOU profile 00:45:36.440 --> 00:45:45.800 align:middle line:84% and come up with a profile of OUR. 00:45:45.800 --> 00:45:50.560 align:middle line:84% So a profile of the oxygen utilization rate. 00:45:50.560 --> 00:45:54.320 align:middle line:84% And what they found was that at the peak, which 00:45:54.320 --> 00:45:57.760 align:middle line:84% was pretty shallow, right below the euphotic zone, 00:45:57.760 --> 00:46:08.200 align:middle line:84% in the shallow aphotic zone, AOU was 10 to 30 micromoles 00:46:08.200 --> 00:46:10.640 align:middle line:90% per kilogram per year-- 00:46:10.640 --> 00:46:12.920 align:middle line:90% or per year. 00:46:12.920 --> 00:46:17.680 align:middle line:84% And by the time you got down to the base of the thermocline, 00:46:17.680 --> 00:46:21.720 align:middle line:84% it was more like 1 to 3 micromoles 00:46:21.720 --> 00:46:23.580 align:middle line:90% per kilogram per year. 00:46:23.580 --> 00:46:27.040 align:middle line:90% 00:46:27.040 --> 00:46:29.960 align:middle line:84% And this was actually occurring about the same time 00:46:29.960 --> 00:46:32.800 align:middle line:84% that people were putting out sediment traps, 00:46:32.800 --> 00:46:36.690 align:middle line:84% and the sediment trap data was showing that the particle 00:46:36.690 --> 00:46:39.950 align:middle line:84% flux was dropping off fairly dramatically in the upper ocean, 00:46:39.950 --> 00:46:42.730 align:middle line:84% and then not so much in the deep ocean. 00:46:42.730 --> 00:46:47.170 align:middle line:84% And so to first order, these appeared to be consistent, 00:46:47.170 --> 00:46:50.967 align:middle line:84% that most of the remineralization/respiration's 00:46:50.967 --> 00:46:53.050 align:middle line:84% occurring in the upper couple hundred meters right 00:46:53.050 --> 00:46:55.610 align:middle line:84% below the base of the euphotic zone, 00:46:55.610 --> 00:47:01.690 align:middle line:84% and then tends to drop to much lower rates in the deep water. 00:47:01.690 --> 00:47:05.090 align:middle line:90% If you integrate that-- 00:47:05.090 --> 00:47:10.770 align:middle line:84% so if I do a vertical integral of OUR with depth, 00:47:10.770 --> 00:47:13.970 align:middle line:90% I get the water column demand. 00:47:13.970 --> 00:47:18.790 align:middle line:84% And that tends to be fairly large. 00:47:18.790 --> 00:47:20.270 align:middle line:84% They were getting values of, say, 00:47:20.270 --> 00:47:30.050 align:middle line:84% 3 to 6 moles of oxygen per meter squared per year. 00:47:30.050 --> 00:47:32.360 align:middle line:84% Which at the time, when Bill first published, 00:47:32.360 --> 00:47:38.300 align:middle line:84% this was a strikingly large number because people thought 00:47:38.300 --> 00:47:41.580 align:middle line:84% that the product-- if you converted this to carbon units, 00:47:41.580 --> 00:47:44.940 align:middle line:84% people thought at the time, that all the primary productivity 00:47:44.940 --> 00:47:46.100 align:middle line:90% at Bermuda-- 00:47:46.100 --> 00:47:48.720 align:middle line:84% the total primary productivity, not export production, 00:47:48.720 --> 00:47:51.400 align:middle line:84% but primary productivity, was in about the same-- 00:47:51.400 --> 00:47:53.860 align:middle line:90% was about the same values. 00:47:53.860 --> 00:47:57.300 align:middle line:84% And so there was a big ongoing controversy 00:47:57.300 --> 00:48:01.900 align:middle line:84% of a lot of the people doing incubation studies arguing 00:48:01.900 --> 00:48:03.900 align:middle line:84% that the geochemical techniques had to be wrong 00:48:03.900 --> 00:48:06.280 align:middle line:84% and the geochemists scratching their heads saying, 00:48:06.280 --> 00:48:09.660 align:middle line:84% well, we can't figure out why it's wrong. 00:48:09.660 --> 00:48:13.060 align:middle line:84% These numbers have held up over time, 00:48:13.060 --> 00:48:15.220 align:middle line:84% and estimates of the primary productivity 00:48:15.220 --> 00:48:16.980 align:middle line:90% have been reevaluated. 00:48:16.980 --> 00:48:20.200 align:middle line:84% And it appears that, as [INAUDIBLE] cycle, 00:48:20.200 --> 00:48:22.140 align:middle line:90% you miss a lot of events. 00:48:22.140 --> 00:48:27.340 align:middle line:84% And so if you're not careful, you're totally biasing-- 00:48:27.340 --> 00:48:30.190 align:middle line:84% you totally bias your estimate, and you 00:48:30.190 --> 00:48:34.430 align:middle line:84% tend to bias it low because the events tend to be very episodic. 00:48:34.430 --> 00:48:37.490 align:middle line:84% And so the primary production might go along like this 00:48:37.490 --> 00:48:40.590 align:middle line:84% and then have some big event and another big spike. 00:48:40.590 --> 00:48:43.910 align:middle line:84% And so because it's so skewed, you 00:48:43.910 --> 00:48:47.310 align:middle line:84% tend to always-- you tend to estimate low values. 00:48:47.310 --> 00:48:49.950 align:middle line:84% STUDENT: When did they come fix the trace metal 00:48:49.950 --> 00:48:50.990 align:middle line:90% issues with the-- 00:48:50.990 --> 00:48:54.690 align:middle line:84% SCOTT DONEY: The trace metal issues were early to mid '70s. 00:48:54.690 --> 00:48:56.030 align:middle line:90% So this was post-- 00:48:56.030 --> 00:49:00.350 align:middle line:84% a lot of the data that went into Epley and Peterson in '79 00:49:00.350 --> 00:49:04.190 align:middle line:84% is just still horribly trace metal-contaminated. 00:49:04.190 --> 00:49:07.430 align:middle line:84% And so there was some of that in this argument, 00:49:07.430 --> 00:49:10.030 align:middle line:84% that there were trace metal issues as well, so it 00:49:10.030 --> 00:49:11.650 align:middle line:84% is a combination-- yeah, you're right, 00:49:11.650 --> 00:49:13.233 align:middle line:84% it is a combination of the trace metal 00:49:13.233 --> 00:49:15.430 align:middle line:84% effects and the primary production 00:49:15.430 --> 00:49:18.570 align:middle line:84% and some really, really pig-headed people. 00:49:18.570 --> 00:49:22.590 align:middle line:90% 00:49:22.590 --> 00:49:27.940 align:middle line:84% There are some caveats and biases with the-- 00:49:27.940 --> 00:49:30.720 align:middle line:90% 00:49:30.720 --> 00:49:35.280 align:middle line:84% mixing can tend to screw up tracer ages. 00:49:35.280 --> 00:49:37.460 align:middle line:84% And I won't go into detail about that, 00:49:37.460 --> 00:49:42.480 align:middle line:84% but there's been a lot of work on that, of how you-- 00:49:42.480 --> 00:49:46.240 align:middle line:84% remember, the assumption was that the water left the surface 00:49:46.240 --> 00:49:49.800 align:middle line:84% and went down as a nice Lagrangian parcel, 00:49:49.800 --> 00:49:56.280 align:middle line:84% but if there's mixing, it can tend to alias the tracer ages. 00:49:56.280 --> 00:49:57.880 align:middle line:84% And one of the ways is people have 00:49:57.880 --> 00:50:00.180 align:middle line:90% looked at is multiple tracers. 00:50:00.180 --> 00:50:05.120 align:middle line:84% So they'll look at CFCs, they'll look at radium-228, 00:50:05.120 --> 00:50:09.840 align:middle line:84% which tends to have a continental source-- 00:50:09.840 --> 00:50:12.960 align:middle line:84% continental shelf source that comes out in the surface water 00:50:12.960 --> 00:50:17.040 align:middle line:84% and then gets advected into depth. 00:50:17.040 --> 00:50:21.200 align:middle line:84% And then they'll also use the tritium/helium. 00:50:21.200 --> 00:50:25.600 align:middle line:84% And by combining tracers, you can resolve 00:50:25.600 --> 00:50:28.110 align:middle line:90% some of these mixing issues. 00:50:28.110 --> 00:50:34.250 align:middle line:90% 00:50:34.250 --> 00:50:38.230 align:middle line:84% OK, I might actually get all caught up today. 00:50:38.230 --> 00:50:41.370 align:middle line:90% 00:50:41.370 --> 00:50:45.290 align:middle line:84% So these have-- the tritium/helium/freon 00:50:45.290 --> 00:50:49.290 align:middle line:84% approaches have been used quite a bit in the upper ocean 00:50:49.290 --> 00:50:52.730 align:middle line:84% and are now an accepted technique 00:50:52.730 --> 00:50:56.750 align:middle line:84% for backing out oxygen utilization rates. 00:50:56.750 --> 00:50:59.610 align:middle line:84% They've now been applied to dissolved 00:50:59.610 --> 00:51:03.530 align:middle line:84% inorganic carbon, alkalinity, nutrients, 00:51:03.530 --> 00:51:05.190 align:middle line:90% you name it, as a tracer. 00:51:05.190 --> 00:51:07.090 align:middle line:84% People have tried to use these age 00:51:07.090 --> 00:51:10.090 align:middle line:84% approaches to get at the production 00:51:10.090 --> 00:51:12.890 align:middle line:84% rates, the geochemical production rates 00:51:12.890 --> 00:51:15.970 align:middle line:90% in the thermocline. 00:51:15.970 --> 00:51:21.770 align:middle line:84% In the deep water, there's just not enough tritium or CFCs 00:51:21.770 --> 00:51:26.460 align:middle line:84% to be particularly confident about your age model 00:51:26.460 --> 00:51:29.920 align:middle line:84% because the water is, for the most part, pretty old, 00:51:29.920 --> 00:51:33.340 align:middle line:84% and you have a little bit of a signal coming in 00:51:33.340 --> 00:51:35.340 align:middle line:90% from these transient tracers. 00:51:35.340 --> 00:51:40.540 align:middle line:84% And so what's typically done for the older waters 00:51:40.540 --> 00:51:46.740 align:middle line:84% is to rely on radiocarbon to get at those slow remineralization 00:51:46.740 --> 00:51:47.440 align:middle line:90% rates. 00:51:47.440 --> 00:51:54.580 align:middle line:90% 00:51:54.580 --> 00:51:56.960 align:middle line:84% And some of this was work-- well, 00:51:56.960 --> 00:52:01.580 align:middle line:84% this has been pioneered by a number of people. 00:52:01.580 --> 00:52:06.860 align:middle line:84% Harmon Craig, Walter Monk, Wally Broecker starting 00:52:06.860 --> 00:52:10.300 align:middle line:84% in the '60s and in the '70s when they started to make the first 00:52:10.300 --> 00:52:12.740 align:middle line:90% radiocarbon measurements. 00:52:12.740 --> 00:52:26.590 align:middle line:84% And so what was done was to use radiocarbon for your age model. 00:52:26.590 --> 00:52:33.110 align:middle line:84% And you had to make corrections because of endmember mixing. 00:52:33.110 --> 00:52:38.050 align:middle line:84% So this would be age model for deepwater. 00:52:38.050 --> 00:52:41.790 align:middle line:90% 00:52:41.790 --> 00:52:52.670 align:middle line:84% And so often, what you would see is that the radiocarbon 00:52:52.670 --> 00:52:55.230 align:middle line:84% would have different endmembers, just like we were 00:52:55.230 --> 00:52:57.870 align:middle line:90% talking about for preformed. 00:52:57.870 --> 00:53:01.370 align:middle line:84% And some of the assumptions were very crude. 00:53:01.370 --> 00:53:02.810 align:middle line:90% They didn't even use salinity. 00:53:02.810 --> 00:53:07.230 align:middle line:84% Sometimes they used salinity, sometimes they just 00:53:07.230 --> 00:53:11.705 align:middle line:84% used latitude and treated the whole deepwater as one box 00:53:11.705 --> 00:53:13.330 align:middle line:84% because there were so few measurements. 00:53:13.330 --> 00:53:16.230 align:middle line:84% This was back before the AMS techniques, 00:53:16.230 --> 00:53:19.690 align:middle line:84% so you needed very large volume samples to measure radiocarbon, 00:53:19.690 --> 00:53:25.960 align:middle line:84% and it was rather tedious to collect. 00:53:25.960 --> 00:53:33.040 align:middle line:84% And so what people would do would be to just fit-- 00:53:33.040 --> 00:53:37.160 align:middle line:84% look at the observations relative to a mixing curve, 00:53:37.160 --> 00:53:43.960 align:middle line:84% estimated delta for delta tracer relative to the approximation 00:53:43.960 --> 00:53:46.120 align:middle line:90% of a mixing curve. 00:53:46.120 --> 00:53:53.640 align:middle line:84% And so for example, you could plot delta oxygen or minus delta 00:53:53.640 --> 00:53:54.880 align:middle line:90% oxygen-- 00:53:54.880 --> 00:53:56.760 align:middle line:90% nope, sorry. 00:53:56.760 --> 00:53:58.400 align:middle line:84% Plus delta oxygen because I stole this 00:53:58.400 --> 00:54:00.320 align:middle line:90% from a different place. 00:54:00.320 --> 00:54:12.280 align:middle line:84% Or AOU versus delta delta 14C where the second delta is just 00:54:12.280 --> 00:54:16.220 align:middle line:84% the fact that you've corrected it for the mixing model. 00:54:16.220 --> 00:54:18.770 align:middle line:90% 00:54:18.770 --> 00:54:23.130 align:middle line:84% And when you do that, you get a plot 00:54:23.130 --> 00:54:28.730 align:middle line:90% that looks something like this. 00:54:28.730 --> 00:54:30.810 align:middle line:84% Waters that have high radiocarbon 00:54:30.810 --> 00:54:38.870 align:middle line:84% tend to have high oxygen. And as you get to older waters, 00:54:38.870 --> 00:54:42.256 align:middle line:90% they tend to have lower-- 00:54:42.256 --> 00:54:43.690 align:middle line:90% strike that, hang on. 00:54:43.690 --> 00:54:46.930 align:middle line:84% I'm having sign convention problems here. 00:54:46.930 --> 00:54:54.770 align:middle line:84% So this would be delta AOU or delta minus oxygen. 00:54:54.770 --> 00:55:00.810 align:middle line:84% So where you have high delta AOU-- 00:55:00.810 --> 00:55:02.810 align:middle line:90% hang on. 00:55:02.810 --> 00:55:05.970 align:middle line:90% Did that backwards again. 00:55:05.970 --> 00:55:14.970 align:middle line:84% Where you had-- well, where you have younger waters, 00:55:14.970 --> 00:55:18.380 align:middle line:84% you don't have nearly as much change in oxygen, 00:55:18.380 --> 00:55:20.300 align:middle line:84% and where you have older waters, you 00:55:20.300 --> 00:55:23.020 align:middle line:84% have a large change in oxygen. And they 00:55:23.020 --> 00:55:28.860 align:middle line:84% got rates of approximately 1 micromole or less 00:55:28.860 --> 00:55:33.820 align:middle line:90% per kilogram per mL. 00:55:33.820 --> 00:55:36.180 align:middle line:84% That's what the slope would look like. 00:55:36.180 --> 00:55:39.480 align:middle line:84% And that gives you your OUR estimate. 00:55:39.480 --> 00:55:42.540 align:middle line:90% 00:55:42.540 --> 00:55:45.460 align:middle line:90% And if you convert that into-- 00:55:45.460 --> 00:55:51.380 align:middle line:84% if you convert this into age for the Atlantic and Pacific, 00:55:51.380 --> 00:55:56.420 align:middle line:84% it's something like 10 to 15 micromoles 00:55:56.420 --> 00:56:00.500 align:middle line:90% per kilogram per century. 00:56:00.500 --> 00:56:02.780 align:middle line:84% Remember before, we were talking about at the base 00:56:02.780 --> 00:56:05.060 align:middle line:84% of the thermocline, it was 1 to 3 micromoles 00:56:05.060 --> 00:56:06.900 align:middle line:90% per kilogram per year. 00:56:06.900 --> 00:56:08.860 align:middle line:84% We're now talking about something 00:56:08.860 --> 00:56:13.620 align:middle line:84% that's about a factor of 10 less because it's 10 to 15 00:56:13.620 --> 00:56:16.590 align:middle line:90% over 100 years. 00:56:16.590 --> 00:56:19.070 align:middle line:84% And those are probably still the best estimates 00:56:19.070 --> 00:56:24.590 align:middle line:84% we have for remineralization rates in the deepwater 00:56:24.590 --> 00:56:29.950 align:middle line:84% because a lot of the biochemical assays, 00:56:29.950 --> 00:56:32.830 align:middle line:84% say, for electron transport in bacteria 00:56:32.830 --> 00:56:35.190 align:middle line:84% or for differencing sediment traps are just 00:56:35.190 --> 00:56:40.270 align:middle line:84% too imprecise to really get a long-term average value that we 00:56:40.270 --> 00:56:41.170 align:middle line:90% would trust. 00:56:41.170 --> 00:56:46.098 align:middle line:90% 00:56:46.098 --> 00:56:48.430 align:middle line:84% STUDENT: Is that in the deep Pacific? 00:56:48.430 --> 00:56:50.110 align:middle line:90% SCOTT DONEY: What? 00:56:50.110 --> 00:56:52.057 align:middle line:84% STUDENT: That's in the deep Pacific, is it? 00:56:52.057 --> 00:56:54.390 align:middle line:84% SCOTT DONEY: Those are values for both the deep Atlantic 00:56:54.390 --> 00:56:55.770 align:middle line:90% and the deep Pacific. 00:56:55.770 --> 00:56:58.910 align:middle line:90% 00:56:58.910 --> 00:57:02.510 align:middle line:84% I took a compilation-- this was a plot that 00:57:02.510 --> 00:57:07.470 align:middle line:84% was a compilation of both, where they plotted everything together 00:57:07.470 --> 00:57:09.090 align:middle line:90% and calculated one slope. 00:57:09.090 --> 00:57:13.080 align:middle line:90% 00:57:13.080 --> 00:57:14.560 align:middle line:90% OK. 00:57:14.560 --> 00:57:19.760 align:middle line:84% Bill's going to talk a lot more about calcium carbonate. 00:57:19.760 --> 00:57:23.120 align:middle line:90% I think he will. 00:57:23.120 --> 00:57:26.580 align:middle line:84% So I was going to move out of organic matter. 00:57:26.580 --> 00:57:29.640 align:middle line:84% So this was the organic matter remineralization. 00:57:29.640 --> 00:57:38.760 align:middle line:84% I did want to talk a little bit about opal, or silica, 00:57:38.760 --> 00:57:41.080 align:middle line:84% before we conclude, and then we'll wrap up a little 00:57:41.080 --> 00:57:44.120 align:middle line:90% early today. 00:57:44.120 --> 00:57:49.960 align:middle line:84% So opal, we talked about you form 00:57:49.960 --> 00:57:55.880 align:middle line:84% a lot of silicate-- or silica in the surface water 00:57:55.880 --> 00:57:57.380 align:middle line:90% due to diatoms. 00:57:57.380 --> 00:58:03.000 align:middle line:90% 00:58:03.000 --> 00:58:11.490 align:middle line:84% About half of that is remineralized 00:58:11.490 --> 00:58:12.790 align:middle line:90% in the euphotic zone. 00:58:12.790 --> 00:58:18.130 align:middle line:90% 00:58:18.130 --> 00:58:21.490 align:middle line:84% The other half sinks down to depth 00:58:21.490 --> 00:58:24.930 align:middle line:84% and is remineralized either in the water column 00:58:24.930 --> 00:58:32.130 align:middle line:84% or at the sea floor with some fraction of it getting buried. 00:58:32.130 --> 00:58:39.110 align:middle line:84% The reaction we're looking at is something like this. 00:58:39.110 --> 00:58:44.930 align:middle line:90% It's a hydrated silica. 00:58:44.930 --> 00:58:48.410 align:middle line:84% So the biogenic opal is not pure. 00:58:48.410 --> 00:58:53.210 align:middle line:84% It's not pure silica, but it's actually a hydrated amorphous 00:58:53.210 --> 00:58:57.150 align:middle line:90% silica combining with water. 00:58:57.150 --> 00:59:00.050 align:middle line:84% And the n just means that there's a variable number 00:59:00.050 --> 00:59:02.570 align:middle line:90% of waters in the hydration. 00:59:02.570 --> 00:59:05.846 align:middle line:90% 00:59:05.846 --> 00:59:11.360 align:middle line:90% To go back to silicious acid. 00:59:11.360 --> 00:59:14.060 align:middle line:90% 00:59:14.060 --> 00:59:22.300 align:middle line:84% And then that silicious acid can lose protons. 00:59:22.300 --> 00:59:26.420 align:middle line:84% That would be two protons because you add a proton there. 00:59:26.420 --> 00:59:27.580 align:middle line:90% I will give you a-- 00:59:27.580 --> 00:59:31.460 align:middle line:84% I'll give you an important secret. 00:59:31.460 --> 00:59:34.340 align:middle line:84% For reasons that escape me, in the chemical oceanography 00:59:34.340 --> 00:59:39.220 align:middle line:84% literature, people often write silicate as SiO3, 00:59:39.220 --> 00:59:41.860 align:middle line:84% which annoys some of the chemists, particularly 00:59:41.860 --> 00:59:44.460 align:middle line:84% some of the chemists at MIT, to no end. 00:59:44.460 --> 00:59:48.700 align:middle line:84% So if you're doing general exams and you're in chemistry, 00:59:48.700 --> 00:59:54.580 align:middle line:84% save yourself some grief by writing it out as siliceous acid 00:59:54.580 --> 00:59:56.460 align:middle line:90% and taking the protons off. 00:59:56.460 --> 00:59:58.300 align:middle line:84% STUDENT: But you will see it in all three 00:59:58.300 --> 01:00:00.382 align:middle line:90% of those different forms? 01:00:00.382 --> 01:00:02.340 align:middle line:84% SCOTT DONEY: You'll see it written as all three 01:00:02.340 --> 01:00:06.740 align:middle line:84% of those forms, but just giving you a hint because I've 01:00:06.740 --> 01:00:09.030 align:middle line:90% seen it enough. 01:00:09.030 --> 01:00:10.910 align:middle line:84% It'll always come up, why did you-- 01:00:10.910 --> 01:00:16.933 align:middle line:84% what's the valence of silicon and siliceous acid? 01:00:16.933 --> 01:00:18.850 align:middle line:84% And they make you walk all the way through it, 01:00:18.850 --> 01:00:22.130 align:middle line:84% and it's just easier to save yourself the grief. 01:00:22.130 --> 01:00:25.710 align:middle line:90% 01:00:25.710 --> 01:00:26.650 align:middle line:90% STUDENT: It's painful. 01:00:26.650 --> 01:00:28.225 align:middle line:90% SCOTT DONEY: Yeah. 01:00:28.225 --> 01:00:29.850 align:middle line:84% I can't remember if you had to do that. 01:00:29.850 --> 01:00:30.650 align:middle line:90% STUDENT: Wasn't me. 01:00:30.650 --> 01:00:34.430 align:middle line:90% I got warned, I think. 01:00:34.430 --> 01:00:39.230 align:middle line:84% Because when we were studying, we were like, wait a second. 01:00:39.230 --> 01:00:40.963 align:middle line:90% No consistency. 01:00:40.963 --> 01:00:41.630 align:middle line:90% SCOTT DONEY: No. 01:00:41.630 --> 01:00:43.880 align:middle line:84% That's because chemical oceanographers aren't actually 01:00:43.880 --> 01:00:44.550 align:middle line:90% chemists. 01:00:44.550 --> 01:00:46.883 align:middle line:84% STUDENT: Well, when you're looking for the Ksp's and you 01:00:46.883 --> 01:00:50.390 align:middle line:84% start freaking out, in which case. 01:00:50.390 --> 01:00:53.390 align:middle line:84% SCOTT DONEY: And these are the k-values for these acid-base-- 01:00:53.390 --> 01:00:55.770 align:middle line:90% for the acid-base reactions. 01:00:55.770 --> 01:01:00.750 align:middle line:90% 01:01:00.750 --> 01:01:07.440 align:middle line:84% As you increase the pH of seawater, 01:01:07.440 --> 01:01:14.800 align:middle line:84% you will tend to decrease the amount of siliceous acid. 01:01:14.800 --> 01:01:17.240 align:middle line:84% And it's actually the siliceous acid 01:01:17.240 --> 01:01:23.520 align:middle line:84% that's involved in the dissolution solubility reactions 01:01:23.520 --> 01:01:26.500 align:middle line:84% with the amorphous hydrated silica. 01:01:26.500 --> 01:01:29.080 align:middle line:90% 01:01:29.080 --> 01:01:32.240 align:middle line:84% And so as you increase the pH of the water, 01:01:32.240 --> 01:01:34.540 align:middle line:84% you actually increase the solubility. 01:01:34.540 --> 01:01:37.240 align:middle line:84% And so one of the things that affects 01:01:37.240 --> 01:01:41.760 align:middle line:84% siliceous acid dissolution is the pH, 01:01:41.760 --> 01:01:44.920 align:middle line:84% and so there's a coupling between the CO2 system 01:01:44.920 --> 01:01:47.540 align:middle line:90% and the siliceous acid system. 01:01:47.540 --> 01:01:56.100 align:middle line:90% 01:01:56.100 --> 01:01:57.740 align:middle line:90% And there's a-- 01:01:57.740 --> 01:02:02.720 align:middle line:84% I'm trying to remember if it's in-- is it in Pilsen? 01:02:02.720 --> 01:02:11.210 align:middle line:84% There's a rather entertaining discussion of how you would-- 01:02:11.210 --> 01:02:21.890 align:middle line:84% how you go about computing the Ksp for silica where 01:02:21.890 --> 01:02:27.930 align:middle line:84% he walks you through some of the issues of what you would-- 01:02:27.930 --> 01:02:33.290 align:middle line:84% if you were a geochemist and you just went to a geological table, 01:02:33.290 --> 01:02:34.970 align:middle line:84% and what you'd get out versus what 01:02:34.970 --> 01:02:37.610 align:middle line:84% you actually get in the real ocean. 01:02:37.610 --> 01:02:40.090 align:middle line:84% And the way this was done was people 01:02:40.090 --> 01:02:43.130 align:middle line:84% would go out into the deep ocean and-- they 01:02:43.130 --> 01:02:48.650 align:middle line:84% did this both for calcite and aragonite and for silica, 01:02:48.650 --> 01:02:54.290 align:middle line:84% is they would take, say, diatom shells or benthic forams, 01:02:54.290 --> 01:02:58.190 align:middle line:84% put them out in the water at depth for a period of time, 01:02:58.190 --> 01:03:00.810 align:middle line:84% and then see how much they'd remineralized. 01:03:00.810 --> 01:03:03.980 align:middle line:90% Or see what the-- 01:03:03.980 --> 01:03:06.940 align:middle line:84% if you put enough out, see what they would come to equilibrium 01:03:06.940 --> 01:03:08.260 align:middle line:90% with in the lab. 01:03:08.260 --> 01:03:09.860 align:middle line:84% So they would do kinetic experiments 01:03:09.860 --> 01:03:13.140 align:middle line:84% where they'd put these things out and watch them dissolve 01:03:13.140 --> 01:03:17.300 align:middle line:84% with time, or they would put these in the lab in seawater 01:03:17.300 --> 01:03:19.260 align:middle line:84% and basically allow it to dissolve 01:03:19.260 --> 01:03:21.420 align:middle line:84% until it came into equilibrium with the water. 01:03:21.420 --> 01:03:25.500 align:middle line:84% Because these are not pure mineral phases, and particularly 01:03:25.500 --> 01:03:29.460 align:middle line:84% because of this amorphous hydration, a lot of the kinetics 01:03:29.460 --> 01:03:32.540 align:middle line:84% you would estimate from pure phases 01:03:32.540 --> 01:03:36.220 align:middle line:84% and the solubility product you'd estimate from pure phases 01:03:36.220 --> 01:03:37.780 align:middle line:90% are unrealistic. 01:03:37.780 --> 01:03:44.180 align:middle line:84% They're not going to be very useful for environmental work. 01:03:44.180 --> 01:03:50.820 align:middle line:84% The Ksp for silica, for the amorphous silica in the ocean, 01:03:50.820 --> 01:03:58.340 align:middle line:84% is something like 1,000 micromoles per kilogram at 3 01:03:58.340 --> 01:03:59.120 align:middle line:90% degrees. 01:03:59.120 --> 01:04:02.430 align:middle line:90% 01:04:02.430 --> 01:04:08.570 align:middle line:84% It's much larger than what you would get, say, for quartz. 01:04:08.570 --> 01:04:14.350 align:middle line:90% 01:04:14.350 --> 01:04:16.370 align:middle line:90% And it's basically the-- 01:04:16.370 --> 01:04:19.870 align:middle line:90% 01:04:19.870 --> 01:04:24.350 align:middle line:84% it has a much more imperfect crystalline structure, 01:04:24.350 --> 01:04:26.770 align:middle line:84% and it's much easier for it to solubilize, 01:04:26.770 --> 01:04:31.110 align:middle line:84% and you need a higher silicate concentration to balance that. 01:04:31.110 --> 01:04:33.470 align:middle line:84% It'll come into equilibrium with a much higher silicate 01:04:33.470 --> 01:04:35.590 align:middle line:90% concentration. 01:04:35.590 --> 01:04:41.070 align:middle line:84% Now, 1,000 is actually the highest values you see 01:04:41.070 --> 01:04:43.470 align:middle line:90% in the ocean. 01:04:43.470 --> 01:04:45.710 align:middle line:90% I think you can see the-- 01:04:45.710 --> 01:04:49.670 align:middle line:84% I don't know if you can see the color bar here. 01:04:49.670 --> 01:04:53.430 align:middle line:84% This is the silicate distribution along that section 01:04:53.430 --> 01:04:54.550 align:middle line:90% that we've been-- 01:04:54.550 --> 01:04:56.670 align:middle line:90% I've been showing you. 01:04:56.670 --> 01:04:59.360 align:middle line:84% Values are quite low in the surface ocean, 01:04:59.360 --> 01:05:02.280 align:middle line:84% less than 10, and in fact, a lot of the North Atlantic 01:05:02.280 --> 01:05:04.720 align:middle line:90% has values less than 10. 01:05:04.720 --> 01:05:06.880 align:middle line:84% The highest values you see in the North Pacific 01:05:06.880 --> 01:05:10.360 align:middle line:90% are up somewhere around 200. 01:05:10.360 --> 01:05:17.480 align:middle line:84% So essentially, everywhere in the water column, 01:05:17.480 --> 01:05:24.160 align:middle line:84% everywhere in the water column, it's 01:05:24.160 --> 01:05:30.860 align:middle line:84% undersaturated with respect to this amorphous silica. 01:05:30.860 --> 01:05:33.800 align:middle line:90% 01:05:33.800 --> 01:05:36.320 align:middle line:84% The reason why the particles actually 01:05:36.320 --> 01:05:39.400 align:middle line:84% can make it to the sea floor and not dissolve 01:05:39.400 --> 01:05:44.380 align:middle line:84% has to do with the kinetics and the sinking velocity. 01:05:44.380 --> 01:05:46.920 align:middle line:90% 01:05:46.920 --> 01:05:50.520 align:middle line:84% So the kinetics have to be slow enough such 01:05:50.520 --> 01:05:54.620 align:middle line:84% that we know silica reaches the bottom. 01:05:54.620 --> 01:05:57.570 align:middle line:84% In fact, there are large parts of the oceans that the sediments 01:05:57.570 --> 01:06:05.210 align:middle line:84% are made up of diatom frustules which says that the kinetics 01:06:05.210 --> 01:06:06.770 align:middle line:84% must be slow enough sinking through-- 01:06:06.770 --> 01:06:09.370 align:middle line:90% that the sinking velocity is-- 01:06:09.370 --> 01:06:12.490 align:middle line:84% can get them to the bottom before they dissolve. 01:06:12.490 --> 01:06:20.570 align:middle line:84% Or they're somehow protected by some organic coating 01:06:20.570 --> 01:06:24.930 align:middle line:90% or some other factor. 01:06:24.930 --> 01:06:27.510 align:middle line:84% Well, usually the organic-- the organic coating. 01:06:27.510 --> 01:06:33.490 align:middle line:90% 01:06:33.490 --> 01:06:39.090 align:middle line:84% The fact that silica isn't dissolving in the water column 01:06:39.090 --> 01:06:41.210 align:middle line:84% once you get out of the surface layer, 01:06:41.210 --> 01:06:43.650 align:middle line:84% there's a fair amount of remineralization in the surface 01:06:43.650 --> 01:06:46.890 align:middle line:84% layer because you have processing by organisms. 01:06:46.890 --> 01:06:49.550 align:middle line:84% You have copepods that are eating diatoms, 01:06:49.550 --> 01:06:55.475 align:middle line:84% and that gets into-- you're processing through guts. 01:06:55.475 --> 01:06:59.980 align:middle line:90% Lots of biological processing. 01:06:59.980 --> 01:07:03.140 align:middle line:84% Once you get below about 1,000 meters, 01:07:03.140 --> 01:07:05.740 align:middle line:84% it looks like most of the silica actually makes it down 01:07:05.740 --> 01:07:09.260 align:middle line:84% to the bottom, and then some of it dissolves there because it 01:07:09.260 --> 01:07:11.820 align:middle line:84% then it has a long time, it's sitting on the bottom exposed 01:07:11.820 --> 01:07:16.140 align:middle line:84% to deep waters, and some of that remineralizes. 01:07:16.140 --> 01:07:27.020 align:middle line:84% And so if you make a plot with depth of H for SiO-- 01:07:27.020 --> 01:07:29.860 align:middle line:90% 01:07:29.860 --> 01:07:38.060 align:middle line:84% or, excuse me-- H2SiO4-, what you see is that you have low 01:07:38.060 --> 01:07:40.900 align:middle line:90% values at the surface-- 01:07:40.900 --> 01:07:42.720 align:middle line:84% oh, I didn't draw that very well. 01:07:42.720 --> 01:07:44.220 align:middle line:84% What I was trying to do was show you 01:07:44.220 --> 01:07:47.580 align:middle line:84% that it tends to build up deep in the water column. 01:07:47.580 --> 01:07:50.340 align:middle line:84% So if I were to show you nitrate-- 01:07:50.340 --> 01:07:53.910 align:middle line:84% so this would be, say, nitrate versus silicate. 01:07:53.910 --> 01:07:57.790 align:middle line:84% That silicate tends to build up deeper in the water column 01:07:57.790 --> 01:08:01.750 align:middle line:84% and be more bottom-intensified than nitrate. 01:08:01.750 --> 01:08:07.150 align:middle line:84% And I can show you that in these plots. 01:08:07.150 --> 01:08:08.730 align:middle line:90% So this is nitrate. 01:08:08.730 --> 01:08:13.270 align:middle line:84% You get a big subsurface maximum around 1,000 meters, 01:08:13.270 --> 01:08:17.050 align:middle line:84% where silicate, the maximum is either at the bottom 01:08:17.050 --> 01:08:19.310 align:middle line:90% or in the deep waters. 01:08:19.310 --> 01:08:22.790 align:middle line:84% And that's just a function of where the remineralization is 01:08:22.790 --> 01:08:23.830 align:middle line:90% occurring. 01:08:23.830 --> 01:08:26.390 align:middle line:90% Nitrate is coming mostly from-- 01:08:26.390 --> 01:08:28.175 align:middle line:84% coming all from the organic matter. 01:08:28.175 --> 01:08:30.550 align:middle line:84% The organic matter, a lot of it's going to be mineralized 01:08:30.550 --> 01:08:36.550 align:middle line:84% in the upper 1,000 meters, where the silicate, 01:08:36.550 --> 01:08:39.790 align:middle line:84% most of it that escapes the euphotic zone is making it 01:08:39.790 --> 01:08:42.770 align:middle line:84% to the bottom and it's getting remineralized there, 01:08:42.770 --> 01:08:45.529 align:middle line:84% and so it has more of a bottom signature. 01:08:45.529 --> 01:08:50.779 align:middle line:90% 01:08:50.779 --> 01:08:53.560 align:middle line:90% STUDENT: [INAUDIBLE] 01:08:53.560 --> 01:08:56.450 align:middle line:90% 01:08:56.450 --> 01:08:57.200 align:middle line:90% SCOTT DONEY: Sure. 01:08:57.200 --> 01:09:04.160 align:middle line:90% 01:09:04.160 --> 01:09:08.279 align:middle line:90% Any questions on silica cycling? 01:09:08.279 --> 01:09:08.960 align:middle line:90% Sure. 01:09:08.960 --> 01:09:10.899 align:middle line:84% STUDENT: In that mineral layer at the bottom, 01:09:10.899 --> 01:09:13.433 align:middle line:84% is silica saturated in the water? 01:09:13.433 --> 01:09:14.100 align:middle line:90% SCOTT DONEY: No. 01:09:14.100 --> 01:09:18.359 align:middle line:84% It's still not-- it's never saturated in the water column. 01:09:18.359 --> 01:09:21.500 align:middle line:84% It does get saturated if you move down the sediment core, 01:09:21.500 --> 01:09:25.319 align:middle line:84% and Bill will talk about that some next week. 01:09:25.319 --> 01:09:26.080 align:middle line:90% Yeah? 01:09:26.080 --> 01:09:28.359 align:middle line:84% STUDENT: For the silica plot that you should-- 01:09:28.359 --> 01:09:31.920 align:middle line:84% there's a build-up-- like, it's higher concentrations 01:09:31.920 --> 01:09:34.460 align:middle line:84% along the bottom, but then when you get to the Pacific, 01:09:34.460 --> 01:09:36.859 align:middle line:84% there's higher concentrations also in the water column. 01:09:36.859 --> 01:09:37.651 align:middle line:90% SCOTT DONEY: Right. 01:09:37.651 --> 01:09:39.240 align:middle line:90% STUDENT: Is that from upwelling? 01:09:39.240 --> 01:09:40.140 align:middle line:90% Or what is that? 01:09:40.140 --> 01:09:44.120 align:middle line:84% SCOTT DONEY: The geochemical signature 01:09:44.120 --> 01:09:46.399 align:middle line:84% is going to be a combination of the remineralization 01:09:46.399 --> 01:09:48.640 align:middle line:90% rates plus the ventilation. 01:09:48.640 --> 01:09:51.609 align:middle line:84% And so in the North Pacific, the younger waters 01:09:51.609 --> 01:09:53.770 align:middle line:84% are actually coming in along the bottom. 01:09:53.770 --> 01:09:56.630 align:middle line:84% They're really cold Antarctic bottom water, 01:09:56.630 --> 01:09:59.130 align:middle line:84% and so that's penetrating right along the bottom, 01:09:59.130 --> 01:10:03.050 align:middle line:84% and then the water is above it are actually are somewhat older. 01:10:03.050 --> 01:10:05.730 align:middle line:84% And so that water has just had more time 01:10:05.730 --> 01:10:11.730 align:middle line:84% and seen more remineralization over time. 01:10:11.730 --> 01:10:13.230 align:middle line:90% OK. 01:10:13.230 --> 01:10:16.370 align:middle line:84% There are lots of other-- we could do-- 01:10:16.370 --> 01:10:19.910 align:middle line:84% I've shown you nutrients and silicate. 01:10:19.910 --> 01:10:25.130 align:middle line:84% We could look at-- there are other biologically influenced 01:10:25.130 --> 01:10:26.170 align:middle line:90% elements. 01:10:26.170 --> 01:10:28.410 align:middle line:90% Things like barium. 01:10:28.410 --> 01:10:33.010 align:middle line:84% Even to a degree, strontium is slightly 01:10:33.010 --> 01:10:37.370 align:middle line:90% biologically influenced. 01:10:37.370 --> 01:10:41.110 align:middle line:84% Those are tangential to the main emphasis of the course. 01:10:41.110 --> 01:10:43.810 align:middle line:84% So I think what we'll do is we'll stop there. 01:10:43.810 --> 01:10:49.160 align:middle line:84% Bill will pick up and talk about sediments for two lectures.