WEBVTT 00:00:00.000 --> 00:00:02.640 align:middle line:84% The following content is provided under a Creative 00:00:02.640 --> 00:00:04.000 align:middle line:90% Commons license. 00:00:04.000 --> 00:00:06.400 align:middle line:84% Your support will help MIT OpenCourseWare 00:00:06.400 --> 00:00:10.080 align:middle line:84% continue to offer high-quality educational resources for free. 00:00:10.080 --> 00:00:13.320 align:middle line:84% To make a donation or view additional materials 00:00:13.320 --> 00:00:17.280 align:middle line:84% from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:17.280 --> 00:00:20.200 align:middle line:90% at ocw.mit.edu. 00:00:20.200 --> 00:00:23.960 align:middle line:84% SCOTT DONEY: So where we were on Tuesday 00:00:23.960 --> 00:00:28.240 align:middle line:84% is we were talking about the sinking of organic matter 00:00:28.240 --> 00:00:31.520 align:middle line:84% and the remineralization in the water column. 00:00:31.520 --> 00:00:35.080 align:middle line:84% So if you remember, if we plotted versus depth 00:00:35.080 --> 00:00:39.000 align:middle line:84% and plotted flux of organic matter, 00:00:39.000 --> 00:00:43.440 align:middle line:84% it tended to drop off fairly steeply in the thermocline 00:00:43.440 --> 00:00:50.080 align:middle line:84% and then was rather flat through the deep water. 00:00:50.080 --> 00:00:55.080 align:middle line:84% And we talked about putting out sediment traps in the water 00:00:55.080 --> 00:01:02.315 align:middle line:84% column, these would be little sediment traps, 00:01:02.315 --> 00:01:04.230 align:middle line:90% to collect material. 00:01:04.230 --> 00:01:07.390 align:middle line:84% And we were talking about a couple of different hypotheses 00:01:07.390 --> 00:01:10.430 align:middle line:84% that people had for what controlled 00:01:10.430 --> 00:01:15.510 align:middle line:84% some of the processes involved in this decrease in flux 00:01:15.510 --> 00:01:17.670 align:middle line:84% as you went down the water column. 00:01:17.670 --> 00:01:25.550 align:middle line:84% And so in order to support a change in flux with depth, 00:01:25.550 --> 00:01:29.910 align:middle line:84% you have to have either transformation 00:01:29.910 --> 00:01:35.470 align:middle line:84% of large particles, large sinking particles, 00:01:35.470 --> 00:01:45.590 align:middle line:84% into small particles that would then get respired. 00:01:45.590 --> 00:01:48.950 align:middle line:84% Or you needed some process that would just respire 00:01:48.950 --> 00:01:52.670 align:middle line:90% those particles directly. 00:01:52.670 --> 00:01:55.610 align:middle line:84% Now, we talked a little bit about what those particles are. 00:01:55.610 --> 00:01:59.310 align:middle line:84% And I don't know if I ever listed them down specifically. 00:01:59.310 --> 00:02:08.070 align:middle line:84% But for example, this flux of particles, because of Stokes law 00:02:08.070 --> 00:02:11.017 align:middle line:84% and the viscosity of water, the flux 00:02:11.017 --> 00:02:13.350 align:middle line:84% of particles, the particles themselves that are sinking, 00:02:13.350 --> 00:02:14.970 align:middle line:90% have to be relatively large. 00:02:14.970 --> 00:02:16.770 align:middle line:84% Remember, when we talked about Stokes law, 00:02:16.770 --> 00:02:18.145 align:middle line:84% we were talking about things that 00:02:18.145 --> 00:02:24.070 align:middle line:84% were greater than 100 microns or somewhere in that range. 00:02:24.070 --> 00:02:33.950 align:middle line:84% And so it could be large, dead cells. 00:02:33.950 --> 00:02:38.310 align:middle line:84% It could be aggregates of cells, "aggregates." 00:02:38.310 --> 00:02:42.550 align:middle line:90% 00:02:42.550 --> 00:02:46.490 align:middle line:84% And in fact, it's not uncommon for diatom blooms. 00:02:46.490 --> 00:02:49.350 align:middle line:90% 00:02:49.350 --> 00:02:53.230 align:middle line:84% At the end of the bloom, at the termination of the bloom, 00:02:53.230 --> 00:03:01.420 align:middle line:84% often there'll be a aggregation of the dead and sometimes living 00:03:01.420 --> 00:03:05.220 align:middle line:84% cells, and then they will sink out as an algal mat. 00:03:05.220 --> 00:03:10.020 align:middle line:84% And this can be observed in the deep sea by collecting, 00:03:10.020 --> 00:03:13.940 align:middle line:84% basically, the organic mush on the bottom 00:03:13.940 --> 00:03:14.880 align:middle line:90% of the sediment layer. 00:03:14.880 --> 00:03:18.220 align:middle line:84% And it will have a high chlorophyll content, suggesting 00:03:18.220 --> 00:03:21.180 align:middle line:84% that it's very fresh, recent material that has recently 00:03:21.180 --> 00:03:24.500 align:middle line:90% rained down onto the surface. 00:03:24.500 --> 00:03:27.120 align:middle line:84% It can also be zooplankton carcasses-- 00:03:27.120 --> 00:03:30.660 align:middle line:90% 00:03:30.660 --> 00:03:33.140 align:middle line:84% I'm not going to get "carcass" right, but carcass. 00:03:33.140 --> 00:03:37.060 align:middle line:84% I'll end up spelling Caracas or something-- 00:03:37.060 --> 00:03:39.280 align:middle line:90% or zooplankton fecal pellets. 00:03:39.280 --> 00:03:43.660 align:middle line:90% 00:03:43.660 --> 00:03:46.780 align:middle line:84% And these are things like, for the zooplankton that 00:03:46.780 --> 00:03:56.060 align:middle line:84% form fecal pellets, things like copepods and then salps, 00:03:56.060 --> 00:04:00.300 align:middle line:84% which are gelatinous filter feeders that tend to filter out 00:04:00.300 --> 00:04:03.740 align:middle line:84% a lot of the small particles and mid-sized particles out 00:04:03.740 --> 00:04:05.660 align:middle line:90% of the water column. 00:04:05.660 --> 00:04:08.500 align:middle line:84% And then they end up producing these very tightly 00:04:08.500 --> 00:04:15.180 align:middle line:84% compacted fecal pellets that tend to sink very rapidly. 00:04:15.180 --> 00:04:25.300 align:middle line:84% The last large component of the sinking flux 00:04:25.300 --> 00:04:27.440 align:middle line:84% is what's often called marine snow. 00:04:27.440 --> 00:04:31.500 align:middle line:90% 00:04:31.500 --> 00:04:40.420 align:middle line:84% And these are just organic aggregates 00:04:40.420 --> 00:04:43.340 align:middle line:84% that come from a variety of different sources. 00:04:43.340 --> 00:04:50.040 align:middle line:84% One of them could be, some organisms give off mucus. 00:04:50.040 --> 00:04:52.810 align:middle line:90% 00:04:52.810 --> 00:04:58.530 align:middle line:84% There are a variety of organisms that have organic feeding 00:04:58.530 --> 00:05:13.730 align:middle line:84% structures, for example, the appendicularians, which are 00:05:13.730 --> 00:05:15.390 align:middle line:90% another form of filter feeder. 00:05:15.390 --> 00:05:20.170 align:middle line:90% 00:05:20.170 --> 00:05:24.950 align:middle line:84% And they have this organic web that they use for filtering, 00:05:24.950 --> 00:05:29.050 align:middle line:84% and they shed that on a periodic basis. 00:05:29.050 --> 00:05:32.290 align:middle line:84% And then that can go into these marine snow 00:05:32.290 --> 00:05:35.210 align:middle line:90% or marine aggregates. 00:05:35.210 --> 00:05:37.970 align:middle line:84% And then there's also the possibility 00:05:37.970 --> 00:05:47.650 align:middle line:84% for a spontaneous formation, where 00:05:47.650 --> 00:05:55.490 align:middle line:84% you have either particle-particle interactions 00:05:55.490 --> 00:06:01.730 align:middle line:84% or DOM-particle interactions that 00:06:01.730 --> 00:06:07.530 align:middle line:84% could lead to the formation of an organic aggregate. 00:06:07.530 --> 00:06:11.290 align:middle line:84% As these aggregates grow, so let's 00:06:11.290 --> 00:06:17.890 align:middle line:84% say you've got some marine aggregate here. 00:06:17.890 --> 00:06:23.410 align:middle line:84% As they grow, remember increased size leads 00:06:23.410 --> 00:06:26.110 align:middle line:90% to increased sinking speed. 00:06:26.110 --> 00:06:30.610 align:middle line:90% 00:06:30.610 --> 00:06:33.570 align:middle line:84% And one of the things these aggregates can do 00:06:33.570 --> 00:06:39.810 align:middle line:84% is actually collect smaller particles by essentially just, 00:06:39.810 --> 00:06:43.090 align:middle line:84% as the aggregate's sinking, it catches up 00:06:43.090 --> 00:06:46.890 align:middle line:84% with suspended material or smaller material 00:06:46.890 --> 00:06:49.080 align:middle line:90% that isn't sinking as fast. 00:06:49.080 --> 00:06:58.080 align:middle line:84% And these can be organic or inorganic, for example, 00:06:58.080 --> 00:07:09.400 align:middle line:90% coccoliths or dust, et cetera. 00:07:09.400 --> 00:07:11.720 align:middle line:84% And so these marine aggregates, they 00:07:11.720 --> 00:07:16.160 align:middle line:84% may start off from a particular source, for example, a feeding 00:07:16.160 --> 00:07:19.080 align:middle line:84% web from an appendicularian, but then it 00:07:19.080 --> 00:07:22.480 align:middle line:84% will accumulate other material as it sinks down 00:07:22.480 --> 00:07:25.840 align:middle line:84% the water column because it basically just sweeps out 00:07:25.840 --> 00:07:32.575 align:middle line:84% in front of it, many of the smaller particles 00:07:32.575 --> 00:07:33.980 align:middle line:90% that it falls through. 00:07:33.980 --> 00:07:45.040 align:middle line:90% 00:07:45.040 --> 00:07:48.360 align:middle line:84% Now, in addition to sediment traps, 00:07:48.360 --> 00:07:57.500 align:middle line:90% sediment traps give you flux. 00:07:57.500 --> 00:08:07.720 align:middle line:84% And that's usually organic carbon, maybe inorganic carbon, 00:08:07.720 --> 00:08:21.640 align:middle line:84% silica or opal, and a terrigenous component. 00:08:21.640 --> 00:08:27.240 align:middle line:90% 00:08:27.240 --> 00:08:31.760 align:middle line:84% And it gives you some sense of the composition. 00:08:31.760 --> 00:08:34.780 align:middle line:84% So you can look in the sediment traps and look at the material, 00:08:34.780 --> 00:08:36.940 align:middle line:84% and you can pick out individual carcasses, 00:08:36.940 --> 00:08:41.640 align:middle line:84% you can pick out individual fecal pellets 00:08:41.640 --> 00:08:44.560 align:middle line:84% and sometimes dead cells, forams, 00:08:44.560 --> 00:08:46.390 align:middle line:90% things that are large enough. 00:08:46.390 --> 00:08:55.070 align:middle line:84% But often there's a lot of just organic mush. 00:08:55.070 --> 00:08:59.510 align:middle line:84% And the thought is that a lot of that is this marine snow. 00:08:59.510 --> 00:09:03.150 align:middle line:84% People have gone out and actually gone into the water 00:09:03.150 --> 00:09:08.910 align:middle line:84% column doing scuba diving in the upper water column, 00:09:08.910 --> 00:09:12.790 align:middle line:84% and then also submarine work, to try to look at these aggregates 00:09:12.790 --> 00:09:14.470 align:middle line:90% as they're actually sinking. 00:09:14.470 --> 00:09:16.870 align:middle line:84% Because once they get into the sediment traps, 00:09:16.870 --> 00:09:19.690 align:middle line:84% they just become this organic mush at the bottom. 00:09:19.690 --> 00:09:22.150 align:middle line:84% And it's very hard to actually figure out 00:09:22.150 --> 00:09:26.510 align:middle line:84% what their original size was or anything characteristic 00:09:26.510 --> 00:09:30.410 align:middle line:84% about them, other than their bulk composition. 00:09:30.410 --> 00:09:32.990 align:middle line:90% 00:09:32.990 --> 00:09:36.350 align:middle line:84% But one of the problems with sediment traps 00:09:36.350 --> 00:09:49.990 align:middle line:84% is all the sediment trap tells you is net d flux dz, at best, 00:09:49.990 --> 00:09:52.290 align:middle line:84% the net change in flux with depth. 00:09:52.290 --> 00:09:55.510 align:middle line:84% But there could be a lot of interactions along the way 00:09:55.510 --> 00:09:58.430 align:middle line:84% where you could have, say, large particles going 00:09:58.430 --> 00:10:01.350 align:middle line:84% to small particles, going back to small particles being 00:10:01.350 --> 00:10:06.510 align:middle line:84% mediated by either some of these zooplankton. 00:10:06.510 --> 00:10:08.990 align:middle line:84% And so we need some other way of looking 00:10:08.990 --> 00:10:13.190 align:middle line:84% at the dynamics in the subsurface of these particles. 00:10:13.190 --> 00:10:17.230 align:middle line:84% And we're going to turn back to radionuclides 00:10:17.230 --> 00:10:20.210 align:middle line:84% for two reasons-- one is they're quite useful and, two, 00:10:20.210 --> 00:10:21.410 align:middle line:90% it's chemistry class. 00:10:21.410 --> 00:10:27.470 align:middle line:90% 00:10:27.470 --> 00:10:33.190 align:middle line:84% So this will look very similar at the outset to what we 00:10:33.190 --> 00:10:39.270 align:middle line:84% were discussing with thorium-234 for the upper water column, 00:10:39.270 --> 00:10:43.580 align:middle line:84% except we're going to choose different isotopes of thorium 00:10:43.580 --> 00:10:47.060 align:middle line:84% that have timescales that are more relevant to the thermocline 00:10:47.060 --> 00:10:48.700 align:middle line:90% in deep ocean. 00:10:48.700 --> 00:10:51.500 align:middle line:84% So in particular, if you remember, 00:10:51.500 --> 00:10:55.060 align:middle line:84% when we were doing thorium-234, we 00:10:55.060 --> 00:11:02.300 align:middle line:84% had uranium-238 decayed to thorium-234. 00:11:02.300 --> 00:11:12.020 align:middle line:84% But that thorium-234 will decay to uranium-234, which 00:11:12.020 --> 00:11:18.140 align:middle line:90% then decays to thorium-230. 00:11:18.140 --> 00:11:25.420 align:middle line:84% So we were using this in the upper water column 00:11:25.420 --> 00:11:28.320 align:middle line:84% because thorium-234's half-life is short. 00:11:28.320 --> 00:11:30.540 align:middle line:84% It's approximately-- Do you guys remember? 00:11:30.540 --> 00:11:32.180 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:11:32.180 --> 00:11:33.520 align:middle line:90% SCOTT DONEY: About 24 days? 00:11:33.520 --> 00:11:37.500 align:middle line:90% 00:11:37.500 --> 00:11:51.690 align:middle line:84% The half-life of thorium-230 is more like 75,000 years. 00:11:51.690 --> 00:12:00.490 align:middle line:84% And where what we saw before was that below the euphotic zone, 00:12:00.490 --> 00:12:07.770 align:middle line:84% thorium-234 was in secular equilibrium with uranium-238, 00:12:07.770 --> 00:12:10.730 align:middle line:84% we're now going to look at deviations. 00:12:10.730 --> 00:12:13.250 align:middle line:84% And in the surface waters, thorium-234 00:12:13.250 --> 00:12:16.170 align:middle line:84% had a deficit with respect to uranium-238. 00:12:16.170 --> 00:12:18.170 align:middle line:84% We're going to do the same thing where we're now 00:12:18.170 --> 00:12:22.250 align:middle line:84% going to look at the deficit of thorium-230 with respect 00:12:22.250 --> 00:12:28.610 align:middle line:90% to uranium-234. 00:12:28.610 --> 00:12:30.710 align:middle line:84% We talked about this for the upper water column. 00:12:30.710 --> 00:12:35.370 align:middle line:84% Uranium has a fairly long residence time in the ocean 00:12:35.370 --> 00:12:39.050 align:middle line:84% because there aren't a lot of large sources and sinks. 00:12:39.050 --> 00:12:41.930 align:middle line:84% So we can approximate the uranium concentration 00:12:41.930 --> 00:12:44.250 align:middle line:90% as a function of salinity. 00:12:44.250 --> 00:12:48.650 align:middle line:90% And we did that for uranium-238. 00:12:48.650 --> 00:13:01.610 align:middle line:84% The activity of 234-uranium is slightly higher than 00:13:01.610 --> 00:13:07.810 align:middle line:90% the activity of 238-uranium. 00:13:07.810 --> 00:13:10.170 align:middle line:84% And Meg mentioned this when she was doing 00:13:10.170 --> 00:13:12.570 align:middle line:90% her lectures on weathering. 00:13:12.570 --> 00:13:16.330 align:middle line:84% Does anybody remember the cause of that? 00:13:16.330 --> 00:13:17.970 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:13:17.970 --> 00:13:18.930 align:middle line:90% SCOTT DONEY: Yeah. 00:13:18.930 --> 00:13:23.610 align:middle line:84% In this process of the decay of uranium-238, 00:13:23.610 --> 00:13:26.530 align:middle line:84% it's releasing alpha particles and it's actually 00:13:26.530 --> 00:13:31.810 align:middle line:84% breaking up and damaging the mineral lattice, the crystal 00:13:31.810 --> 00:13:35.290 align:middle line:84% lattice that the uranium was originally in. 00:13:35.290 --> 00:13:38.400 align:middle line:84% And so you get a little bit more leaching out 00:13:38.400 --> 00:13:43.280 align:middle line:84% of the uranium-234 because of that damage. 00:13:43.280 --> 00:13:53.080 align:middle line:84% And so there's, in seawater, a slight elevation of uranium-234. 00:13:53.080 --> 00:13:57.660 align:middle line:84% So some of the original work on thorium-230, 00:13:57.660 --> 00:13:59.880 align:middle line:84% or one of the seminal papers, is a paper 00:13:59.880 --> 00:14:06.800 align:middle line:90% by Bacon and Anderson, 1982. 00:14:06.800 --> 00:14:11.240 align:middle line:84% And we'll see if we can track the reference down for you. 00:14:11.240 --> 00:14:14.320 align:middle line:84% So what you have is something very similar to what 00:14:14.320 --> 00:14:20.360 align:middle line:84% we saw before, which is, you have uranium-234, 00:14:20.360 --> 00:14:27.400 align:middle line:84% that's going to decay into thorium-230, 00:14:27.400 --> 00:14:29.180 align:middle line:90% into a dissolved phase. 00:14:29.180 --> 00:14:33.160 align:middle line:90% 00:14:33.160 --> 00:14:35.920 align:middle line:84% That dissolved phase has two paths. 00:14:35.920 --> 00:14:41.960 align:middle line:84% It can either decay itself through radioactive decay, 00:14:41.960 --> 00:14:50.160 align:middle line:84% or it can get absorbed onto a particulate phase that 00:14:50.160 --> 00:14:55.280 align:middle line:84% would then sink out of the system, 00:14:55.280 --> 00:14:57.780 align:middle line:84% being carried with the particles themselves. 00:14:57.780 --> 00:15:03.480 align:middle line:90% 00:15:03.480 --> 00:15:06.280 align:middle line:84% So the first variant of this we're going to look at 00:15:06.280 --> 00:15:12.900 align:middle line:84% is an irreversible scavenging model. 00:15:12.900 --> 00:15:18.400 align:middle line:90% 00:15:18.400 --> 00:15:26.000 align:middle line:84% So in irreversible scavenging, there's one kinetic constant. 00:15:26.000 --> 00:15:29.680 align:middle line:84% And that's the forward scavenging of dissolved thorium 00:15:29.680 --> 00:15:31.360 align:middle line:90% under particulate thorium. 00:15:31.360 --> 00:15:34.730 align:middle line:84% And once it gets on there, it never comes back off. 00:15:34.730 --> 00:15:37.670 align:middle line:90% 00:15:37.670 --> 00:15:41.510 align:middle line:84% So the equations you would write out 00:15:41.510 --> 00:15:46.950 align:middle line:84% would be an equation for dissolved thorium, 00:15:46.950 --> 00:15:51.630 align:middle line:84% so the rate of change of dissolved thorium with time. 00:15:51.630 --> 00:15:55.070 align:middle line:84% And we're going to be interested in the steady-state solutions 00:15:55.070 --> 00:15:56.590 align:middle line:90% of that. 00:15:56.590 --> 00:16:03.990 align:middle line:84% And so that's going to equal the activity of U-234. 00:16:03.990 --> 00:16:06.990 align:middle line:90% So that's the production term. 00:16:06.990 --> 00:16:12.110 align:middle line:84% And then we would have these two loss terms, one due to decay 00:16:12.110 --> 00:16:16.070 align:middle line:90% and one due to scavenging. 00:16:16.070 --> 00:16:19.990 align:middle line:84% And they would be multiplied by the dissolved thorium 00:16:19.990 --> 00:16:22.910 align:middle line:90% concentration. 00:16:22.910 --> 00:16:25.270 align:middle line:84% Similarly, you would have an equation 00:16:25.270 --> 00:16:29.270 align:middle line:84% for the particulate thorium and its steady state. 00:16:29.270 --> 00:16:32.710 align:middle line:84% The source term would be coming from the scavenging 00:16:32.710 --> 00:16:35.070 align:middle line:90% of the dissolved phase. 00:16:35.070 --> 00:16:41.550 align:middle line:84% There would be decay of the particulate thorium. 00:16:41.550 --> 00:16:46.110 align:middle line:84% And then there'd be a loss term, where the loss term 00:16:46.110 --> 00:16:57.190 align:middle line:84% is going to be the vertical gradient, the vertical gradient 00:16:57.190 --> 00:17:03.470 align:middle line:84% of the particulate thorium and the sinking velocity. 00:17:03.470 --> 00:17:06.710 align:middle line:84% So this would be something that would look like this. 00:17:06.710 --> 00:17:12.990 align:middle line:90% 00:17:12.990 --> 00:17:19.589 align:middle line:84% Let's say you had a layer, a layer bounded 00:17:19.589 --> 00:17:23.150 align:middle line:90% by say two surfaces. 00:17:23.150 --> 00:17:28.980 align:middle line:84% So inside the layer, you have formation 00:17:28.980 --> 00:17:34.540 align:middle line:90% of particulate thorium. 00:17:34.540 --> 00:17:39.940 align:middle line:84% And you would have two fluxes of particulate thorium, 00:17:39.940 --> 00:17:41.460 align:middle line:84% one at the top of the interface, one 00:17:41.460 --> 00:17:43.040 align:middle line:90% at the bottom of the interface. 00:17:43.040 --> 00:17:45.580 align:middle line:90% 00:17:45.580 --> 00:17:49.740 align:middle line:84% Flux is going to equal w, the sinking 00:17:49.740 --> 00:17:55.780 align:middle line:84% velocity, times the thorium concentration. 00:17:55.780 --> 00:18:06.860 align:middle line:84% And so you would have a flux in and a flux out. 00:18:06.860 --> 00:18:15.220 align:middle line:84% And so if you have net formation of particulate thorium 00:18:15.220 --> 00:18:20.680 align:middle line:84% in that box, the flux out has to exceed the flux in. 00:18:20.680 --> 00:18:25.620 align:middle line:84% So if the formation is greater than zero, 00:18:25.620 --> 00:18:33.460 align:middle line:84% then flux out has to be greater than flux in. 00:18:33.460 --> 00:18:43.340 align:middle line:84% And the way of looking at that, the way of thinking about that 00:18:43.340 --> 00:18:46.740 align:middle line:84% is by looking at the gradient, you're actually 00:18:46.740 --> 00:18:50.220 align:middle line:84% taking this depth range, delta z, 00:18:50.220 --> 00:18:53.980 align:middle line:84% and looking at the rate of change of that flux, 00:18:53.980 --> 00:19:00.100 align:middle line:84% assuming that w is constant over that small interval. 00:19:00.100 --> 00:19:02.140 align:middle line:84% And so this term basically gives you 00:19:02.140 --> 00:19:08.380 align:middle line:84% the divergence of the flux in and flux out term. 00:19:08.380 --> 00:19:12.100 align:middle line:84% And at steady state, if there was just production, 00:19:12.100 --> 00:19:15.785 align:middle line:84% those two would have to equal each other. 00:19:15.785 --> 00:19:17.160 align:middle line:84% Does that make sense to you guys? 00:19:17.160 --> 00:19:22.765 align:middle line:90% 00:19:22.765 --> 00:19:23.640 align:middle line:90% Does that make sense? 00:19:23.640 --> 00:19:26.450 align:middle line:90% Yeah, question? 00:19:26.450 --> 00:19:30.450 align:middle line:84% AUDIENCE: If flux out is greater than flux in, 00:19:30.450 --> 00:19:33.050 align:middle line:84% I would think that the particulate thorium 00:19:33.050 --> 00:19:34.790 align:middle line:84% concentration would be going down. 00:19:34.790 --> 00:19:38.010 align:middle line:90% 00:19:38.010 --> 00:19:39.790 align:middle line:84% And you have formation greater than 0, 00:19:39.790 --> 00:19:41.250 align:middle line:90% I'm not sure what that means. 00:19:41.250 --> 00:19:45.450 align:middle line:84% SCOTT DONEY: OK, so let's look back at this equation. 00:19:45.450 --> 00:19:49.890 align:middle line:84% Let's ignore for the moment the decay 00:19:49.890 --> 00:19:53.930 align:middle line:90% of the particulate thorium. 00:19:53.930 --> 00:19:59.890 align:middle line:84% This would say that the formation term, K1, thorium 00:19:59.890 --> 00:20:01.490 align:middle line:90% dissolved. 00:20:01.490 --> 00:20:05.870 align:middle line:84% So that would be formation of the particulate thorium. 00:20:05.870 --> 00:20:09.450 align:middle line:84% That's the adsorption of the dissolved thorium 00:20:09.450 --> 00:20:11.650 align:middle line:90% onto the particles. 00:20:11.650 --> 00:20:25.090 align:middle line:84% It has to equal the gradient times the sinking velocity. 00:20:25.090 --> 00:20:37.410 align:middle line:84% And so let's say we start off with a flux at some point 00:20:37.410 --> 00:20:41.290 align:middle line:84% and then you're producing more particulate thorium, 00:20:41.290 --> 00:20:44.730 align:middle line:84% the next point should have a larger flux. 00:20:44.730 --> 00:20:47.310 align:middle line:84% And then again, the next point should have a larger flux, 00:20:47.310 --> 00:20:51.810 align:middle line:84% the next point should have a larger flux. 00:20:51.810 --> 00:21:02.530 align:middle line:84% If I looked at the gradient of flux, which would just 00:21:02.530 --> 00:21:12.090 align:middle line:84% be d dz of W thorium P, and then we're going to approximate that 00:21:12.090 --> 00:21:19.070 align:middle line:84% by saying W is constant, you get that term. 00:21:19.070 --> 00:21:23.160 align:middle line:84% And so this is actually positive because it's 00:21:23.160 --> 00:21:26.720 align:middle line:90% increasing as you increase z. 00:21:26.720 --> 00:21:34.320 align:middle line:84% And that's just balancing the formation. 00:21:34.320 --> 00:21:37.780 align:middle line:84% So if you're forming it in the water column, 00:21:37.780 --> 00:21:40.600 align:middle line:90% it has to go someplace. 00:21:40.600 --> 00:21:42.400 align:middle line:84% The decay term isn't going to be big enough 00:21:42.400 --> 00:21:44.525 align:middle line:84% because it's not going to hang around for very long 00:21:44.525 --> 00:21:47.640 align:middle line:84% because it's going to be scavenged onto particles that 00:21:47.640 --> 00:21:49.080 align:middle line:90% sink out. 00:21:49.080 --> 00:21:53.960 align:middle line:84% And if it's scavenging onto sinking particles, 00:21:53.960 --> 00:21:59.360 align:middle line:84% then the sinking flux of thorium has to actually increase 00:21:59.360 --> 00:22:04.080 align:middle line:84% with depth to basically balance the formation in the water 00:22:04.080 --> 00:22:04.700 align:middle line:90% column. 00:22:04.700 --> 00:22:09.800 align:middle line:90% 00:22:09.800 --> 00:22:11.880 align:middle line:90% Does that answer that? 00:22:11.880 --> 00:22:16.360 align:middle line:90% 00:22:16.360 --> 00:22:18.560 align:middle line:90% Does that answer your question? 00:22:18.560 --> 00:22:21.730 align:middle line:84% AUDIENCE: Scott, you're forming particulate thorium 00:22:21.730 --> 00:22:24.870 align:middle line:84% at the same rate throughout the water column, correct? 00:22:24.870 --> 00:22:26.490 align:middle line:84% SCOTT DONEY: Yes, the assumption-- 00:22:26.490 --> 00:22:26.910 align:middle line:90% AUDIENCE: It falls out. 00:22:26.910 --> 00:22:28.562 align:middle line:84% So as you go down deeper, there's 00:22:28.562 --> 00:22:30.270 align:middle line:84% more and more falling out because there's 00:22:30.270 --> 00:22:31.285 align:middle line:90% more and more above you. 00:22:31.285 --> 00:22:31.910 align:middle line:90% AUDIENCE: Yeah. 00:22:31.910 --> 00:22:34.990 align:middle line:84% SCOTT DONEY: The assumption, if we went all the way back up 00:22:34.990 --> 00:22:42.230 align:middle line:84% here, was that you have a fairly uniform source 00:22:42.230 --> 00:22:44.290 align:middle line:90% due to uranium-234 decay. 00:22:44.290 --> 00:22:49.270 align:middle line:84% So you're forming thorium-230 throughout the water column 00:22:49.270 --> 00:22:52.550 align:middle line:84% at a fairly uniform rate, barring small deviations 00:22:52.550 --> 00:22:53.370 align:middle line:90% in salinity. 00:22:53.370 --> 00:22:54.570 align:middle line:90% But we'll ignore that. 00:22:54.570 --> 00:22:56.750 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:22:56.750 --> 00:22:59.190 align:middle line:84% SCOTT DONEY: And the thorium is getting scavenged fairly 00:22:59.190 --> 00:22:59.990 align:middle line:90% quickly-- 00:22:59.990 --> 00:23:01.030 align:middle line:90% AUDIENCE: Yeah. 00:23:01.030 --> 00:23:03.110 align:middle line:84% SCOTT DONEY: --onto the sinking particles. 00:23:03.110 --> 00:23:09.150 align:middle line:84% And so essentially, the depth is integrating all of the thorium-- 00:23:09.150 --> 00:23:11.630 align:middle line:84% the flux at any particular depth is 00:23:11.630 --> 00:23:13.950 align:middle line:84% integrating all of the production that 00:23:13.950 --> 00:23:15.850 align:middle line:90% has occurred above that depth. 00:23:15.850 --> 00:23:21.170 align:middle line:90% 00:23:21.170 --> 00:23:21.670 align:middle line:90% Let's see. 00:23:21.670 --> 00:23:30.739 align:middle line:90% 00:23:30.739 --> 00:23:34.150 align:middle line:84% I'm trying to see, I have another way that I explained it 00:23:34.150 --> 00:23:35.390 align:middle line:90% in the notes. 00:23:35.390 --> 00:23:41.870 align:middle line:84% Let me go back, and we'll try that and see if that helps. 00:23:41.870 --> 00:23:43.360 align:middle line:90% AUDIENCE: A quick clarification. 00:23:43.360 --> 00:23:44.110 align:middle line:90% SCOTT DONEY: Yeah? 00:23:44.110 --> 00:23:47.370 align:middle line:84% AUDIENCE: Let's say we have a formation form greater than 0. 00:23:47.370 --> 00:23:54.070 align:middle line:84% You're talking about formation of dissolved thorium? 00:23:54.070 --> 00:23:57.270 align:middle line:84% SCOTT DONEY: Well, here I was talking about the formation of-- 00:23:57.270 --> 00:23:58.850 align:middle line:90% There's two formations, right? 00:23:58.850 --> 00:24:03.470 align:middle line:90% 00:24:03.470 --> 00:24:07.470 align:middle line:84% There's the production of dissolved thorium 00:24:07.470 --> 00:24:11.470 align:middle line:90% from uranium decay. 00:24:11.470 --> 00:24:14.270 align:middle line:90% And that can go two pathways. 00:24:14.270 --> 00:24:19.660 align:middle line:84% It can either decay itself or it can get scavenged onto thorium. 00:24:19.660 --> 00:24:21.600 align:middle line:84% When it gets scavenged onto thorium, 00:24:21.600 --> 00:24:24.920 align:middle line:84% you can think of it as the formation of particulate. 00:24:24.920 --> 00:24:26.320 align:middle line:84% It gets scavenged onto particles, 00:24:26.320 --> 00:24:28.300 align:middle line:84% you can think of that as the formation 00:24:28.300 --> 00:24:31.060 align:middle line:90% of particulate thorium. 00:24:31.060 --> 00:24:33.400 align:middle line:84% And so once it gets scavenged onto there, 00:24:33.400 --> 00:24:37.180 align:middle line:84% you have this net flow of thorium onto the particles. 00:24:37.180 --> 00:24:39.660 align:middle line:84% So you have a net production term. 00:24:39.660 --> 00:24:42.560 align:middle line:84% At steady state, something has to balance that. 00:24:42.560 --> 00:24:46.240 align:middle line:84% And the sinking-out flux is balancing that. 00:24:46.240 --> 00:24:51.540 align:middle line:90% 00:24:51.540 --> 00:24:54.020 align:middle line:84% Maybe let me try showing this in equations 00:24:54.020 --> 00:24:58.620 align:middle line:90% and see if that makes sense. 00:24:58.620 --> 00:25:03.400 align:middle line:84% So if I take this equation up here and assume steady state, 00:25:03.400 --> 00:25:05.420 align:middle line:84% which I did, and then rearrange it, 00:25:05.420 --> 00:25:11.220 align:middle line:84% I would get that the activity of uranium-234 00:25:11.220 --> 00:25:24.540 align:middle line:84% would equal K1 thorium dissolved. 00:25:24.540 --> 00:25:29.200 align:middle line:84% So this is the scavenging term plus the decay term. 00:25:29.200 --> 00:25:50.780 align:middle line:90% 00:25:50.780 --> 00:25:59.540 align:middle line:84% We're going to make the assumption that for thorium-230, 00:25:59.540 --> 00:26:06.940 align:middle line:84% that K1 is much greater than lambda. 00:26:06.940 --> 00:26:10.620 align:middle line:84% Basically, the scavenging is going to dominate over 00:26:10.620 --> 00:26:14.010 align:middle line:90% decay in the water column. 00:26:14.010 --> 00:26:17.570 align:middle line:84% And this would then allow us to approximate. 00:26:17.570 --> 00:26:24.730 align:middle line:84% This can be written as thorium dissolved equals 00:26:24.730 --> 00:26:32.350 align:middle line:84% the activity of uranium-234 over K1 plus lambda. 00:26:32.350 --> 00:26:37.690 align:middle line:84% With this approximation, that collapses down. 00:26:37.690 --> 00:26:47.610 align:middle line:84% And we get that the thorium dissolved concentration 00:26:47.610 --> 00:26:53.210 align:middle line:84% is approximately equal to the activity of uranium-234 00:26:53.210 --> 00:26:55.430 align:middle line:90% over the scavenging coefficient. 00:26:55.430 --> 00:26:58.370 align:middle line:90% 00:26:58.370 --> 00:27:00.090 align:middle line:90% AUDIENCE: Question about that. 00:27:00.090 --> 00:27:02.130 align:middle line:90% SCOTT DONEY: Sure. 00:27:02.130 --> 00:27:04.490 align:middle line:84% AUDIENCE: So generally, we used to always pick 00:27:04.490 --> 00:27:06.290 align:middle line:84% things where the scavenging rate was 00:27:06.290 --> 00:27:11.250 align:middle line:84% on the order of your timescale of the life or whatever. 00:27:11.250 --> 00:27:17.668 align:middle line:84% Because otherwise, you're using a meter stick 00:27:17.668 --> 00:27:19.710 align:middle line:84% to measure the distance to the moon or something. 00:27:19.710 --> 00:27:20.585 align:middle line:90% You know what I mean. 00:27:20.585 --> 00:27:22.870 align:middle line:84% It's not an appropriate measurement. 00:27:22.870 --> 00:27:27.054 align:middle line:90% 00:27:27.054 --> 00:27:28.850 align:middle line:84% SCOTT DONEY: This isn't perhaps ideal. 00:27:28.850 --> 00:27:33.770 align:middle line:84% Because what you'll see is that the amount of thorium that's 00:27:33.770 --> 00:27:37.130 align:middle line:84% left in the water column is very small relative 00:27:37.130 --> 00:27:40.570 align:middle line:90% to the uranium activity. 00:27:40.570 --> 00:27:42.250 align:middle line:90% We'll show that in a minute. 00:27:42.250 --> 00:27:46.170 align:middle line:84% The problem is that there's not another set 00:27:46.170 --> 00:27:53.690 align:middle line:84% of isotope systematics that is better and easily measured. 00:27:53.690 --> 00:27:56.230 align:middle line:90% So you take what you got. 00:27:56.230 --> 00:27:58.970 align:middle line:84% And in this case, the radionuclides 00:27:58.970 --> 00:28:02.270 align:middle line:84% just don't line up as well with the processes. 00:28:02.270 --> 00:28:03.930 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:28:03.930 --> 00:28:05.810 align:middle line:84% SCOTT DONEY: That said, as long as you 00:28:05.810 --> 00:28:11.600 align:middle line:84% can make precise measurements of thorium, you can use this. 00:28:11.600 --> 00:28:14.200 align:middle line:84% But what you'll find is that thorium concentrations 00:28:14.200 --> 00:28:19.820 align:middle line:84% are orders of magnitude lower than the original parent. 00:28:19.820 --> 00:28:24.280 align:middle line:84% So it's quite a ways out of secular equilibrium. 00:28:24.280 --> 00:28:27.000 align:middle line:84% I don't know if I have a plot of that. 00:28:27.000 --> 00:28:31.560 align:middle line:84% Well, you'll see that in the homework, the homework that 00:28:31.560 --> 00:28:33.620 align:middle line:90% goes online tonight. 00:28:33.620 --> 00:28:38.320 align:middle line:90% 00:28:38.320 --> 00:28:41.120 align:middle line:90% So what does that say? 00:28:41.120 --> 00:28:57.040 align:middle line:90% 00:28:57.040 --> 00:28:59.760 align:middle line:84% If thorium dissolved is approximately 00:28:59.760 --> 00:29:03.320 align:middle line:84% equal to the uranium activity over K1, 00:29:03.320 --> 00:29:08.880 align:middle line:84% if we were to plot that with depth, 00:29:08.880 --> 00:29:12.720 align:middle line:84% we know that uranium is basically a conserved species 00:29:12.720 --> 00:29:13.660 align:middle line:90% in seawater. 00:29:13.660 --> 00:29:15.720 align:middle line:84% It's only going to vary with salinity. 00:29:15.720 --> 00:29:23.120 align:middle line:84% So the thorium dissolved concentration 00:29:23.120 --> 00:29:26.780 align:middle line:84% is going to be approximately a constant with depth, 00:29:26.780 --> 00:29:29.920 align:middle line:90% assuming that K1 is constant. 00:29:29.920 --> 00:29:34.680 align:middle line:84% And we'll say we're dealing with a homogeneous water 00:29:34.680 --> 00:29:35.900 align:middle line:90% column for the moment. 00:29:35.900 --> 00:29:39.680 align:middle line:90% 00:29:39.680 --> 00:29:43.600 align:middle line:84% Now, if we go back to the original equation 00:29:43.600 --> 00:29:52.500 align:middle line:84% for particulate thorium, we can write out similarly 00:29:52.500 --> 00:29:56.040 align:middle line:90% and solve this equation. 00:29:56.040 --> 00:29:59.100 align:middle line:84% Now, we now know what thorium dissolved is, right? 00:29:59.100 --> 00:30:00.660 align:middle line:84% We have an approximation for that. 00:30:00.660 --> 00:30:03.040 align:middle line:90% So I'm going to plug that in. 00:30:03.040 --> 00:30:12.630 align:middle line:84% And so from the d thorium particulate dt equation, 00:30:12.630 --> 00:30:19.830 align:middle line:84% we're going to get that the activity of U-234 00:30:19.830 --> 00:30:26.770 align:middle line:84% is going to equal lambda thorium particle. 00:30:26.770 --> 00:30:38.230 align:middle line:84% So this is now the formation of dissolved thorium 00:30:38.230 --> 00:30:40.110 align:middle line:90% from uranium-234. 00:30:40.110 --> 00:30:45.430 align:middle line:84% And remember, we approximated thorium dissolve times K1 is 00:30:45.430 --> 00:30:48.370 align:middle line:90% equal to the uranium activity. 00:30:48.370 --> 00:30:50.550 align:middle line:84% So we're going to plug that in there. 00:30:50.550 --> 00:30:54.030 align:middle line:84% We have gamma, we have the loss rate of particulate thorium 00:30:54.030 --> 00:30:56.030 align:middle line:90% by radioactive decay. 00:30:56.030 --> 00:31:01.890 align:middle line:84% And then we have wd thorium p dz. 00:31:01.890 --> 00:31:17.110 align:middle line:90% 00:31:17.110 --> 00:31:19.710 align:middle line:84% We're going to make the assumption now 00:31:19.710 --> 00:31:25.550 align:middle line:84% that the activity of uranium-234 is 00:31:25.550 --> 00:31:34.810 align:middle line:84% much greater than the activity of thorium on particles. 00:31:34.810 --> 00:31:40.550 align:middle line:84% Remember, gamma times thorium, the decay constant times 00:31:40.550 --> 00:31:45.430 align:middle line:84% the number of atoms just gives you the activity. 00:31:45.430 --> 00:31:52.236 align:middle line:84% And then that will allow us to solve for-- 00:31:52.236 --> 00:31:58.710 align:middle line:84% U-234 is approximately equal to-- 00:31:58.710 --> 00:32:02.140 align:middle line:90% this is an assumption-- 00:32:02.140 --> 00:32:12.860 align:middle line:84% solve for the vertical gradient of the particulate thorium. 00:32:12.860 --> 00:32:16.860 align:middle line:84% So the vertical gradient of the particulate thorium 00:32:16.860 --> 00:32:20.580 align:middle line:90% is equal to a constant times W. 00:32:20.580 --> 00:32:23.620 align:middle line:84% If we assume W is constant, that says 00:32:23.620 --> 00:32:29.460 align:middle line:84% that the thorium particulate concentration is just 00:32:29.460 --> 00:32:38.960 align:middle line:84% going to be equal to A U-234 over W times depth. 00:32:38.960 --> 00:32:44.340 align:middle line:90% 00:32:44.340 --> 00:32:52.540 align:middle line:84% So if we go back to our diagram up here, 00:32:52.540 --> 00:32:55.680 align:middle line:84% this would be the dissolve curve, 00:32:55.680 --> 00:32:59.940 align:middle line:84% this would be the particulate curve. 00:32:59.940 --> 00:33:03.330 align:middle line:84% So you'd expect to see uniform thorium 00:33:03.330 --> 00:33:09.170 align:middle line:84% concentration and increasing particle 00:33:09.170 --> 00:33:11.250 align:middle line:90% concentrations of thorium. 00:33:11.250 --> 00:33:19.210 align:middle line:84% And since flux equals W thorium P, the flux of thorium 00:33:19.210 --> 00:33:22.230 align:middle line:84% should also increase linearly with depth. 00:33:22.230 --> 00:33:31.250 align:middle line:90% 00:33:31.250 --> 00:33:34.830 align:middle line:90% Now, that's the prediction. 00:33:34.830 --> 00:33:37.410 align:middle line:90% 00:33:37.410 --> 00:33:42.370 align:middle line:84% This is the prediction for the irreversible model. 00:33:42.370 --> 00:33:45.530 align:middle line:84% The problem is that when people went out and actually 00:33:45.530 --> 00:33:53.950 align:middle line:84% made observations by either putting out sediment traps 00:33:53.950 --> 00:33:56.330 align:middle line:84% and measuring the flux, or pumping 00:33:56.330 --> 00:34:00.490 align:middle line:84% large volumes of seawater at depth to see how much thorium 00:34:00.490 --> 00:34:04.450 align:middle line:84% was on the particles, what they observed 00:34:04.450 --> 00:34:13.850 align:middle line:84% was that thorium dissolved actually increased with depth 00:34:13.850 --> 00:34:17.489 align:middle line:90% and so did thorium particulate. 00:34:17.489 --> 00:34:22.130 align:middle line:84% So there must be something wrong with the irreversible model 00:34:22.130 --> 00:34:25.570 align:middle line:90% because it didn't work. 00:34:25.570 --> 00:34:32.370 align:middle line:84% So Bacon and Anderson formulated a slightly more complicated 00:34:32.370 --> 00:34:38.170 align:middle line:84% model, one where they now allow reversible scavenging. 00:34:38.170 --> 00:34:42.409 align:middle line:84% So the uranium still goes to the dissolved. 00:34:42.409 --> 00:34:51.290 align:middle line:84% But then, you can have dissolved thorium 00:34:51.290 --> 00:34:54.889 align:middle line:84% going onto the particulate phase and then 00:34:54.889 --> 00:34:57.640 align:middle line:84% thorium coming off the particulate phase going back 00:34:57.640 --> 00:35:00.840 align:middle line:90% into the dissolved pool. 00:35:00.840 --> 00:35:05.160 align:middle line:90% And then you have sinking. 00:35:05.160 --> 00:35:09.040 align:middle line:90% And you also have decay. 00:35:09.040 --> 00:35:12.420 align:middle line:84% You also have decay of this, but it's very, very small. 00:35:12.420 --> 00:35:19.280 align:middle line:90% 00:35:19.280 --> 00:35:21.280 align:middle line:84% We can similarly then write out a set 00:35:21.280 --> 00:35:26.580 align:middle line:84% of equations, again, for thorium dissolved, assume steady state. 00:35:26.580 --> 00:35:29.240 align:middle line:90% 00:35:29.240 --> 00:35:31.740 align:middle line:84% It's going to look very similar to the last one, 00:35:31.740 --> 00:35:34.240 align:middle line:84% except for the addition of one term. 00:35:34.240 --> 00:35:37.520 align:middle line:84% So you have production by radioactive decay symbolized 00:35:37.520 --> 00:35:41.960 align:middle line:90% by the activity of U-234. 00:35:41.960 --> 00:35:47.640 align:middle line:84% You have a loss term of scavenging a dissolved thorium 00:35:47.640 --> 00:35:48.980 align:middle line:90% onto the particles. 00:35:48.980 --> 00:35:52.600 align:middle line:90% 00:35:52.600 --> 00:35:54.300 align:middle line:90% You have radioactive decay. 00:35:54.300 --> 00:35:58.440 align:middle line:90% 00:35:58.440 --> 00:36:05.120 align:middle line:84% And then you have a source term due to the release of thorium 00:36:05.120 --> 00:36:07.400 align:middle line:90% off of the particulate phase. 00:36:07.400 --> 00:36:08.660 align:middle line:90% That's a K minus 1. 00:36:08.660 --> 00:36:13.640 align:middle line:84% Often, you'll see rates written as K1 and K minus 1. 00:36:13.640 --> 00:36:18.080 align:middle line:84% So K1 is the forward reaction for 1, K minus 1 00:36:18.080 --> 00:36:23.260 align:middle line:84% is the backward reaction rate for equation 1. 00:36:23.260 --> 00:36:26.720 align:middle line:90% 00:36:26.720 --> 00:36:34.560 align:middle line:84% Then we'd have the same sort of thing for the particulate phase, 00:36:34.560 --> 00:36:46.030 align:middle line:84% where you'd have K1, thorium dissolve, plus w d thorium P dz 00:36:46.030 --> 00:36:58.590 align:middle line:84% minus K minus 1 thorium P minus lambda thorium P. 00:36:58.590 --> 00:37:07.150 align:middle line:84% And if we make the same assumptions as we made earlier, 00:37:07.150 --> 00:37:15.950 align:middle line:84% we'll say K1 and K minus 1 are much greater than lambda, 00:37:15.950 --> 00:37:22.590 align:middle line:84% then you get that thorium dissolved is approximately 00:37:22.590 --> 00:37:40.470 align:middle line:84% equal to A U-234 plus K minus 1 thorium particulate over K1. 00:37:40.470 --> 00:37:42.990 align:middle line:84% So before, we got a term, that it 00:37:42.990 --> 00:37:46.870 align:middle line:84% was the dissolved thorium depended solely 00:37:46.870 --> 00:37:49.510 align:middle line:90% on the activity of uranium-234. 00:37:49.510 --> 00:37:52.450 align:middle line:84% Now with reversible dissociation, 00:37:52.450 --> 00:37:56.810 align:middle line:84% we find that it also depends upon the flux. 00:37:56.810 --> 00:38:00.390 align:middle line:84% So this is a production term because it's radioactive decay. 00:38:00.390 --> 00:38:02.830 align:middle line:84% This is a production term for dissolved thorium 00:38:02.830 --> 00:38:05.270 align:middle line:84% because it's a release of thorium. 00:38:05.270 --> 00:38:09.730 align:middle line:84% And so we actually need to account for both of those. 00:38:09.730 --> 00:38:15.990 align:middle line:90% 00:38:15.990 --> 00:38:20.230 align:middle line:84% Let's see, the model for dissolved thorium 00:38:20.230 --> 00:38:27.690 align:middle line:84% is actually somewhat more complicated. 00:38:27.690 --> 00:38:30.150 align:middle line:90% But we can still approximate it. 00:38:30.150 --> 00:38:32.670 align:middle line:90% 00:38:32.670 --> 00:38:35.590 align:middle line:84% You write out a very long equation, 00:38:35.590 --> 00:38:38.310 align:middle line:84% but what you find when you actually scale things 00:38:38.310 --> 00:38:44.870 align:middle line:84% properly is that the particulate thorium still 00:38:44.870 --> 00:38:51.900 align:middle line:84% scales approximately as U-234 activity over the sinking 00:38:51.900 --> 00:38:56.760 align:middle line:84% velocity, which is what we found for the irreversible model. 00:38:56.760 --> 00:39:07.060 align:middle line:84% And that's simply because the end reaction or the main sink 00:39:07.060 --> 00:39:10.180 align:middle line:84% of thorium production from uranium decay 00:39:10.180 --> 00:39:14.580 align:middle line:84% still is the sinking thorium flux on the particles. 00:39:14.580 --> 00:39:17.480 align:middle line:90% So it has to get there somehow. 00:39:17.480 --> 00:39:21.180 align:middle line:84% And so it eventually all does still pileup in the particles 00:39:21.180 --> 00:39:22.200 align:middle line:90% and sink out. 00:39:22.200 --> 00:39:25.300 align:middle line:90% 00:39:25.300 --> 00:39:29.240 align:middle line:84% And the nice thing is, this now is much more consistent, 00:39:29.240 --> 00:39:34.380 align:middle line:84% this simple, reversible model is much more consistent 00:39:34.380 --> 00:39:39.780 align:middle line:84% with the observations in that you have dissolved thorium 00:39:39.780 --> 00:39:42.060 align:middle line:90% increasing linearly with depth. 00:39:42.060 --> 00:39:46.660 align:middle line:84% And that increase has to do with-- 00:39:46.660 --> 00:39:50.500 align:middle line:84% it's basically scaling because there's more particulate thorium 00:39:50.500 --> 00:39:54.700 align:middle line:84% at depth that's releasing more dissolved thorium at depth 00:39:54.700 --> 00:39:57.540 align:middle line:90% from this term. 00:39:57.540 --> 00:40:01.260 align:middle line:84% I had mentioned earlier this assumption 00:40:01.260 --> 00:40:07.500 align:middle line:84% that the scavenging rates were much greater than lambda. 00:40:07.500 --> 00:40:18.460 align:middle line:84% The half-life was in the order, 75,000 years. 00:40:18.460 --> 00:40:23.220 align:middle line:84% The particle residence time, based on estimates of thorium 00:40:23.220 --> 00:40:28.820 align:middle line:84% in the dissolved phase and in the particulate phase, 00:40:28.820 --> 00:40:31.160 align:middle line:90% is only about 40 years. 00:40:31.160 --> 00:40:34.780 align:middle line:90% 00:40:34.780 --> 00:40:37.220 align:middle line:84% And so very little of the thorium 00:40:37.220 --> 00:40:41.060 align:middle line:84% actually hangs around long enough to decay 00:40:41.060 --> 00:40:42.700 align:middle line:90% in the water column. 00:40:42.700 --> 00:40:45.380 align:middle line:84% Almost all of it is scavenged onto particles 00:40:45.380 --> 00:40:50.490 align:middle line:84% on some decadal timescale and then ends up 00:40:50.490 --> 00:40:53.330 align:middle line:90% sinking out to the bottom. 00:40:53.330 --> 00:40:59.010 align:middle line:84% Now, in addition to being useful for estimating scavenging 00:40:59.010 --> 00:41:03.190 align:middle line:84% rates-- so these are scavenging rates. 00:41:03.190 --> 00:41:06.410 align:middle line:90% 00:41:06.410 --> 00:41:08.490 align:middle line:84% They tell you a lot about, particularly, 00:41:08.490 --> 00:41:14.530 align:middle line:84% this whole solution that the irreversible model didn't work. 00:41:14.530 --> 00:41:18.410 align:middle line:84% There needs to be some way for exchanging thorium 00:41:18.410 --> 00:41:20.810 align:middle line:90% on and off of particles. 00:41:20.810 --> 00:41:24.890 align:middle line:84% Now, the next step would be, if you could actually 00:41:24.890 --> 00:41:31.530 align:middle line:84% start to differentiate between small particles 00:41:31.530 --> 00:41:42.650 align:middle line:84% and large particles, you might be 00:41:42.650 --> 00:41:47.050 align:middle line:84% able to begin to distinguish some of the models 00:41:47.050 --> 00:41:53.890 align:middle line:84% that we talked about earlier, between whether the flux 00:41:53.890 --> 00:41:58.290 align:middle line:84% of organic matter is simply sinking down and decaying, 00:41:58.290 --> 00:42:01.610 align:middle line:84% or whether that sinking flux might get turned into smaller 00:42:01.610 --> 00:42:07.150 align:middle line:84% particles and then aggregate back to larger particles, 00:42:07.150 --> 00:42:10.610 align:middle line:84% sink down, smaller particles, larger particles. 00:42:10.610 --> 00:42:14.330 align:middle line:84% This solution where you see this increase in thorium with depth, 00:42:14.330 --> 00:42:17.890 align:middle line:84% says that it's likely that at least thorium 00:42:17.890 --> 00:42:21.730 align:middle line:84% must be coming off of the large particles that are sinking down. 00:42:21.730 --> 00:42:25.210 align:middle line:84% Because that's what's transporting additional thorium 00:42:25.210 --> 00:42:26.850 align:middle line:90% down to depth. 00:42:26.850 --> 00:42:29.610 align:middle line:84% It's likely that organic matter is also 00:42:29.610 --> 00:42:32.930 align:middle line:84% coming off and on, as well, associated 00:42:32.930 --> 00:42:36.310 align:middle line:90% with these reversible processes. 00:42:36.310 --> 00:42:39.330 align:middle line:90% 00:42:39.330 --> 00:42:43.090 align:middle line:84% So the thorium data and some of the other radionuclides 00:42:43.090 --> 00:42:46.760 align:middle line:84% are starting to elucidate what the interactions are 00:42:46.760 --> 00:42:50.120 align:middle line:84% between the different pools of small particles 00:42:50.120 --> 00:42:51.920 align:middle line:90% and large particles. 00:42:51.920 --> 00:42:59.460 align:middle line:84% But the conclusions aren't yet robust enough 00:42:59.460 --> 00:43:01.650 align:middle line:84% to be completely convincing, in part 00:43:01.650 --> 00:43:03.400 align:middle line:84% because these measurements are fairly hard 00:43:03.400 --> 00:43:06.840 align:middle line:90% and there isn't enough data yet. 00:43:06.840 --> 00:43:09.520 align:middle line:84% Or at least, it's on the borderline 00:43:09.520 --> 00:43:15.240 align:middle line:84% of having enough data to really make a good case. 00:43:15.240 --> 00:43:22.400 align:middle line:84% There's one other thing I wanted to say about the thorium 00:43:22.400 --> 00:43:25.280 align:middle line:90% before we move to protactinium. 00:43:25.280 --> 00:43:27.560 align:middle line:90% Oh, that's right. 00:43:27.560 --> 00:43:35.840 align:middle line:84% The other thing is you can use thorium-230 as a deep sediment 00:43:35.840 --> 00:43:38.720 align:middle line:90% trap calibration. 00:43:38.720 --> 00:43:43.230 align:middle line:90% 00:43:43.230 --> 00:43:46.470 align:middle line:84% Because if we go back up, remember, 00:43:46.470 --> 00:43:57.190 align:middle line:84% actually, you can look here, that this particulate thorium 00:43:57.190 --> 00:44:01.490 align:middle line:84% flux is increasing as we go down the water column. 00:44:01.490 --> 00:44:04.790 align:middle line:90% 00:44:04.790 --> 00:44:13.190 align:middle line:84% And since you know the activity of uranium-234 00:44:13.190 --> 00:44:15.848 align:middle line:84% and you know the depth over which you're integrating-- 00:44:15.848 --> 00:44:17.390 align:middle line:84% sorry, I'd forgotten that Z, I think, 00:44:17.390 --> 00:44:19.830 align:middle line:90% when I first wrote this out. 00:44:19.830 --> 00:44:22.990 align:middle line:84% This is the same as the irreversible case where 00:44:22.990 --> 00:44:28.530 align:middle line:84% it's the integrated over Z. You might not know W specifically, 00:44:28.530 --> 00:44:31.350 align:middle line:84% you might not know the thorium particulate concentration 00:44:31.350 --> 00:44:36.510 align:middle line:84% specifically, but the flux is constrained 00:44:36.510 --> 00:44:46.150 align:middle line:84% by the total amount of activity of uranium-234 and the depth. 00:44:46.150 --> 00:44:49.950 align:middle line:84% And so if you have a sediment trap where you can measure how 00:44:49.950 --> 00:44:55.550 align:middle line:84% much thorium-230 ends up in your trap for a particular time-- 00:44:55.550 --> 00:45:00.830 align:middle line:84% that would be sinking-out flux of thorium-230-- 00:45:00.830 --> 00:45:06.955 align:middle line:84% you can compare that to what you expect 00:45:06.955 --> 00:45:10.990 align:middle line:84% based on the amount of decay of uranium-234 in the overlying 00:45:10.990 --> 00:45:12.230 align:middle line:90% water column. 00:45:12.230 --> 00:45:15.310 align:middle line:84% And if those don't match, then probably something's 00:45:15.310 --> 00:45:17.110 align:middle line:90% wrong with your sediment trap. 00:45:17.110 --> 00:45:19.310 align:middle line:84% There's been some hydrodynamic effect 00:45:19.310 --> 00:45:23.030 align:middle line:84% and you've either over collected or under collected. 00:45:23.030 --> 00:45:29.550 align:middle line:84% Similarly, this is often and commonly used in sediment work 00:45:29.550 --> 00:45:35.470 align:middle line:84% to look at the accumulation in the surface 00:45:35.470 --> 00:45:41.140 align:middle line:84% layer of thorium-230, basically, the flux of thorium-230 that's 00:45:41.140 --> 00:45:46.220 align:middle line:84% making it into the sediments, and comparing that 00:45:46.220 --> 00:45:49.380 align:middle line:84% with the overlying activity of uranium-234 00:45:49.380 --> 00:45:54.460 align:middle line:84% to see if there's winnowing or convergence of sediments. 00:45:54.460 --> 00:45:57.300 align:middle line:84% If you're in a spot where there's bottom currents that 00:45:57.300 --> 00:46:00.820 align:middle line:84% are resuspending sediments and then depositing it 00:46:00.820 --> 00:46:05.580 align:middle line:84% all in one spot-- it's like in a snowstorm, when 00:46:05.580 --> 00:46:07.300 align:middle line:84% the wind blows and then all the snow 00:46:07.300 --> 00:46:08.860 align:middle line:84% piles up on the side of your garage 00:46:08.860 --> 00:46:11.140 align:middle line:84% or the side of your house, and there's no snow out 00:46:11.140 --> 00:46:12.340 align:middle line:90% on the street. 00:46:12.340 --> 00:46:14.780 align:middle line:84% Well, the same thing happens with sediments. 00:46:14.780 --> 00:46:18.340 align:middle line:84% And this gives you an idea or a calibration point 00:46:18.340 --> 00:46:23.580 align:middle line:84% for saying whether there's been excess deposition of sediments 00:46:23.580 --> 00:46:25.600 align:middle line:90% or a deficit of sediments. 00:46:25.600 --> 00:46:29.740 align:middle line:84% You can use the thorium-230 flux estimates in the water column 00:46:29.740 --> 00:46:34.140 align:middle line:84% to get at that and calibrate your sediment data as well, 00:46:34.140 --> 00:46:35.880 align:middle line:90% so surface sediments. 00:46:35.880 --> 00:46:39.460 align:middle line:90% 00:46:39.460 --> 00:46:42.740 align:middle line:84% Thorium-230, that I want to go over, 00:46:42.740 --> 00:46:48.220 align:middle line:84% and that has to do with comparing thorium-230 00:46:48.220 --> 00:46:52.800 align:middle line:84% with another isotope system, namely protactinium. 00:46:52.800 --> 00:47:04.340 align:middle line:90% 00:47:04.340 --> 00:47:13.280 align:middle line:84% So thorium-230 has a half-life, we said, of about-- 00:47:13.280 --> 00:47:17.240 align:middle line:90% 00:47:17.240 --> 00:47:21.940 align:middle line:84% it's either 72 or 75, now I'm forgetting, 72,000. 00:47:21.940 --> 00:47:31.520 align:middle line:84% Protactinium-231 has a half-life of 32,000 years. 00:47:31.520 --> 00:47:35.770 align:middle line:84% So they both have fairly long, half-lives. 00:47:35.770 --> 00:47:45.930 align:middle line:84% And protactinium-231, the source of it is uranium-235. 00:47:45.930 --> 00:47:49.730 align:middle line:84% So all of what we've said about uranium 00:47:49.730 --> 00:47:53.810 align:middle line:84% being stable and conservative in seawater 00:47:53.810 --> 00:47:56.090 align:middle line:90% also holds for protactinium. 00:47:56.090 --> 00:47:59.970 align:middle line:84% So just as how you can use your uranium 00:47:59.970 --> 00:48:09.190 align:middle line:84% estimates to come up with a source flux of thorium, 00:48:09.190 --> 00:48:11.330 align:middle line:84% you can do the same thing for protactinium. 00:48:11.330 --> 00:48:20.530 align:middle line:84% So it has a known seawater source. 00:48:20.530 --> 00:48:33.210 align:middle line:84% Now, the difference is that thorium is more particle 00:48:33.210 --> 00:48:39.850 align:middle line:90% reactive than protactinium. 00:48:39.850 --> 00:48:46.010 align:middle line:84% And so the idea is that you know the production 00:48:46.010 --> 00:49:00.790 align:middle line:84% ratio of protactinium to thorium in the water column. 00:49:00.790 --> 00:49:09.994 align:middle line:90% 00:49:09.994 --> 00:49:21.370 align:middle line:84% The thorium is assumed to be removed locally. 00:49:21.370 --> 00:49:24.650 align:middle line:84% So all the thorium that forms in the water column 00:49:24.650 --> 00:49:29.530 align:middle line:84% sinks out right underneath where it's formed. 00:49:29.530 --> 00:49:34.300 align:middle line:84% The protactinium eventually gets scavenged, 00:49:34.300 --> 00:49:40.420 align:middle line:84% but it can move laterally before scavenging. 00:49:40.420 --> 00:49:48.040 align:middle line:90% 00:49:48.040 --> 00:49:51.940 align:middle line:84% So protactinium, it's not scavenged as quickly, 00:49:51.940 --> 00:49:54.460 align:middle line:90% so it might advect a bit. 00:49:54.460 --> 00:49:59.080 align:middle line:84% Remember, we were saying that the scavenging timescale 00:49:59.080 --> 00:50:01.940 align:middle line:90% for thorium was about 40 years. 00:50:01.940 --> 00:50:04.480 align:middle line:90% 00:50:04.480 --> 00:50:07.800 align:middle line:84% Protactinium might be more like a couple of hundred. 00:50:07.800 --> 00:50:13.680 align:middle line:84% And so it can actually in the deep water, it can move. 00:50:13.680 --> 00:50:21.300 align:middle line:84% And what you often see, so this is about 40 years, 00:50:21.300 --> 00:50:23.280 align:middle line:90% this scavenging time. 00:50:23.280 --> 00:50:31.840 align:middle line:84% This is more like 100 to 200 years of scavenging time. 00:50:31.840 --> 00:50:40.320 align:middle line:84% And if you were to plot, make a little schematic of your ocean, 00:50:40.320 --> 00:50:43.400 align:middle line:84% so you have thorium and protactinium being 00:50:43.400 --> 00:50:45.720 align:middle line:90% formed in the water column. 00:50:45.720 --> 00:50:51.000 align:middle line:84% Some of it's going to sink and be scavenged right there. 00:50:51.000 --> 00:50:55.520 align:middle line:84% And some of the protactinium is actually 00:50:55.520 --> 00:51:01.240 align:middle line:84% going to advect over and get scavenged someplace else. 00:51:01.240 --> 00:51:04.040 align:middle line:84% And in particular, it's going to get scavenged in regions 00:51:04.040 --> 00:51:12.360 align:middle line:84% like the margins where you have high productivity 00:51:12.360 --> 00:51:15.300 align:middle line:90% and high organic flux. 00:51:15.300 --> 00:51:18.000 align:middle line:90% 00:51:18.000 --> 00:51:21.360 align:middle line:84% So regions such as the margins, if there's just 00:51:21.360 --> 00:51:24.000 align:middle line:84% more organic flux sinking down, you 00:51:24.000 --> 00:51:27.310 align:middle line:84% have more surface area under which to scavenge. 00:51:27.310 --> 00:51:31.090 align:middle line:84% So this would be a region of high scavenging. 00:51:31.090 --> 00:51:33.910 align:middle line:90% 00:51:33.910 --> 00:51:36.290 align:middle line:84% And this would be low scavenging. 00:51:36.290 --> 00:51:39.430 align:middle line:90% 00:51:39.430 --> 00:51:41.070 align:middle line:84% So if you were to look in the sediments 00:51:41.070 --> 00:51:44.830 align:middle line:84% or in the sediment traps, what you'd find 00:51:44.830 --> 00:51:47.990 align:middle line:84% is that out here, you would have low protactinium-to-thorium 00:51:47.990 --> 00:51:50.190 align:middle line:90% ratios. 00:51:50.190 --> 00:51:54.270 align:middle line:84% And here you would have high protactinium-to-thorium ratios. 00:51:54.270 --> 00:51:58.734 align:middle line:90% 00:51:58.734 --> 00:52:00.950 align:middle line:84% With the argument that all the thorium 00:52:00.950 --> 00:52:03.910 align:middle line:84% is constant because it just depends upon water depth, 00:52:03.910 --> 00:52:08.470 align:middle line:84% but that some of the protactinium that was formed out 00:52:08.470 --> 00:52:10.590 align:middle line:84% here that it should have fallen here 00:52:10.590 --> 00:52:12.390 align:middle line:84% had a long enough time that it ended up 00:52:12.390 --> 00:52:14.290 align:middle line:84% in this high-scavenging environment, 00:52:14.290 --> 00:52:17.890 align:middle line:84% and it got trapped and removed in the margins. 00:52:17.890 --> 00:52:25.500 align:middle line:90% 00:52:25.500 --> 00:52:28.420 align:middle line:84% So there's been considerable work 00:52:28.420 --> 00:52:34.540 align:middle line:84% to understand this as a modern day proxy for scavenging. 00:52:34.540 --> 00:52:37.960 align:middle line:84% But you can also use it in a paleo sense, 00:52:37.960 --> 00:52:42.060 align:middle line:84% because these isotopes have a fairly long half-life. 00:52:42.060 --> 00:52:46.260 align:middle line:84% So you can go back through the glacial-interglacial cycles 00:52:46.260 --> 00:52:49.860 align:middle line:90% and see, did scavenging change? 00:52:49.860 --> 00:52:52.060 align:middle line:84% After you've decay corrected them, 00:52:52.060 --> 00:52:54.260 align:middle line:84% did my accumulation of protactinium 00:52:54.260 --> 00:52:57.320 align:middle line:84% to thorium in the open ocean or protactinium 00:52:57.320 --> 00:53:01.280 align:middle line:84% to thorium in the margins change during glacial cycles? 00:53:01.280 --> 00:53:03.500 align:middle line:84% Which might tell you something about how 00:53:03.500 --> 00:53:07.560 align:middle line:84% productivity changed there relative to modern conditions. 00:53:07.560 --> 00:53:22.500 align:middle line:90% 00:53:22.500 --> 00:53:31.118 align:middle line:84% So any questions on uranium isotopes, thorium isotopes? 00:53:31.118 --> 00:53:33.160 align:middle line:84% Because we're going to change gears a little bit. 00:53:33.160 --> 00:53:35.700 align:middle line:90% 00:53:35.700 --> 00:53:36.340 align:middle line:90% Nope? 00:53:36.340 --> 00:53:37.880 align:middle line:90% Any questions up at MIT? 00:53:37.880 --> 00:53:40.580 align:middle line:90% 00:53:40.580 --> 00:53:42.080 align:middle line:90% OK. 00:53:42.080 --> 00:53:46.486 align:middle line:90% 00:53:46.486 --> 00:53:55.620 align:middle line:84% So far we've been talking a lot about sinking particles. 00:53:55.620 --> 00:53:58.220 align:middle line:84% We're now going to go back, remember, so this is, 00:53:58.220 --> 00:53:59.800 align:middle line:90% say, the euphotic zone. 00:53:59.800 --> 00:54:03.540 align:middle line:90% 00:54:03.540 --> 00:54:06.660 align:middle line:84% And there were two pathways we talked about for export. 00:54:06.660 --> 00:54:11.540 align:middle line:84% There was the particulate organic matter pathway. 00:54:11.540 --> 00:54:17.260 align:middle line:84% But there's also this dissolved organic matter pathway. 00:54:17.260 --> 00:54:19.220 align:middle line:84% And so we want to go back and talk a little bit 00:54:19.220 --> 00:54:28.050 align:middle line:84% about dissolved organic matter and how that impacts export 00:54:28.050 --> 00:54:30.910 align:middle line:84% and what the fate of that dissolved organic matter is. 00:54:30.910 --> 00:54:33.450 align:middle line:90% 00:54:33.450 --> 00:54:37.950 align:middle line:84% So I'm just going to flip the page so I have a clean page. 00:54:37.950 --> 00:54:52.850 align:middle line:90% 00:54:52.850 --> 00:54:57.130 align:middle line:90% OK, so dissolved organic matter. 00:54:57.130 --> 00:55:01.490 align:middle line:84% We've talked a lot that we don't know 00:55:01.490 --> 00:55:05.250 align:middle line:84% the chemical characterization of DOM as well as we would like. 00:55:05.250 --> 00:55:16.290 align:middle line:84% It's a very heterogeneous set of organic compounds, many of which 00:55:16.290 --> 00:55:18.310 align:middle line:84% were not what was originally produced. 00:55:18.310 --> 00:55:24.590 align:middle line:84% The DOM has undergone chemical reactions and cross-linking. 00:55:24.590 --> 00:55:29.230 align:middle line:84% And so some fraction of it is a large geopolymer. 00:55:29.230 --> 00:55:31.890 align:middle line:90% 00:55:31.890 --> 00:55:34.930 align:middle line:84% And so it's hard to characterize, 00:55:34.930 --> 00:55:41.810 align:middle line:84% other than in some of the bulk characteristics, which 00:55:41.810 --> 00:55:44.890 align:middle line:84% we talked a little bit about and we'll talk a little bit more 00:55:44.890 --> 00:55:45.550 align:middle line:90% today. 00:55:45.550 --> 00:55:49.450 align:middle line:90% 00:55:49.450 --> 00:55:52.970 align:middle line:84% And we talked the other day about its measurement 00:55:52.970 --> 00:55:58.090 align:middle line:84% using things like UV oxidation or high-temperature combustion, 00:55:58.090 --> 00:56:00.970 align:middle line:84% where you're basically just burning or oxidizing 00:56:00.970 --> 00:56:03.610 align:middle line:84% the dissolved organic matter, turning it into CO2 00:56:03.610 --> 00:56:07.050 align:middle line:90% and then analyzing the CO2. 00:56:07.050 --> 00:56:11.010 align:middle line:84% If you go out in the water column and look in the upper 00:56:11.010 --> 00:56:15.610 align:middle line:84% thermocline, so this would be the upper 1,000 meters, 00:56:15.610 --> 00:56:19.440 align:middle line:84% and look at the dissolved organic matter as a function 00:56:19.440 --> 00:56:26.360 align:middle line:84% of depth, by the time you get down to about 1,000 meters, 00:56:26.360 --> 00:56:31.600 align:middle line:84% you're going to have a fairly uniform background concentration 00:56:31.600 --> 00:56:36.300 align:middle line:84% of about 40 micromoles per kilogram. 00:56:36.300 --> 00:56:40.480 align:middle line:90% 00:56:40.480 --> 00:56:45.480 align:middle line:84% And as you head up to the surface, in many places, 00:56:45.480 --> 00:56:52.400 align:middle line:84% you'll actually see increases, with surface values ranging 00:56:52.400 --> 00:57:00.020 align:middle line:84% between, say, 60 and maybe 90 micromoles per kilogram. 00:57:00.020 --> 00:57:03.040 align:middle line:90% 00:57:03.040 --> 00:57:04.900 align:middle line:84% And those are micromoles of carbon. 00:57:04.900 --> 00:57:08.440 align:middle line:90% 00:57:08.440 --> 00:57:15.440 align:middle line:84% These are in places like the subtropics and convergence 00:57:15.440 --> 00:57:16.680 align:middle line:90% zones. 00:57:16.680 --> 00:57:19.540 align:middle line:84% Subtopics are regions of convergence. 00:57:19.540 --> 00:57:23.800 align:middle line:84% Remember, the water is being upwelled in the subpolar 00:57:23.800 --> 00:57:25.440 align:middle line:90% and the equator. 00:57:25.440 --> 00:57:28.320 align:middle line:84% And so the water in the subtropics 00:57:28.320 --> 00:57:31.560 align:middle line:84% has been at the surface for a fair length of time. 00:57:31.560 --> 00:57:34.820 align:middle line:84% And if this organic matter pool is kind of building up, 00:57:34.820 --> 00:57:37.380 align:middle line:84% it would be transported into the middle of the subtropics. 00:57:37.380 --> 00:57:44.840 align:middle line:84% And so it's had some time to build up concentrations. 00:57:44.840 --> 00:57:52.440 align:middle line:84% If you look in the polar waters, particularly in the Southern 00:57:52.440 --> 00:58:07.240 align:middle line:84% Ocean, they tend to have lower dissolved organic carbon 00:58:07.240 --> 00:58:11.260 align:middle line:84% concentrations, not that much above this deep background. 00:58:11.260 --> 00:58:19.310 align:middle line:90% 00:58:19.310 --> 00:58:23.270 align:middle line:84% So that gives you some sense of the profile with depth. 00:58:23.270 --> 00:58:29.430 align:middle line:84% We could also look, for example, along the path of deep water 00:58:29.430 --> 00:58:31.230 align:middle line:90% formation. 00:58:31.230 --> 00:58:38.070 align:middle line:84% So if we started in the North Atlantic, Southern Ocean, North 00:58:38.070 --> 00:58:43.470 align:middle line:84% Pacific, now we expect the nutrient concentrations 00:58:43.470 --> 00:58:46.210 align:middle line:84% to do what, as you go from the deep North Atlantic? 00:58:46.210 --> 00:58:48.493 align:middle line:90% So this would be deep. 00:58:48.493 --> 00:58:49.910 align:middle line:84% To go from the deep North Atlantic 00:58:49.910 --> 00:58:52.190 align:middle line:84% to the deep North Pacific, nutrients will? 00:58:52.190 --> 00:58:53.270 align:middle line:90% AUDIENCE: Increase. 00:58:53.270 --> 00:58:54.350 align:middle line:90% SCOTT DONEY: Increase. 00:58:54.350 --> 00:58:55.838 align:middle line:90% Oxygen is going to? 00:58:55.838 --> 00:58:56.630 align:middle line:90% AUDIENCE: Decrease. 00:58:56.630 --> 00:58:59.670 align:middle line:90% SCOTT DONEY: Decrease. 00:58:59.670 --> 00:59:04.030 align:middle line:84% Some of the early data suggested that there was a slight decrease 00:59:04.030 --> 00:59:08.270 align:middle line:84% of dissolved organic matter, dissolved 00:59:08.270 --> 00:59:17.230 align:middle line:84% organic carbon, from something like maybe 48 to 34. 00:59:17.230 --> 00:59:23.070 align:middle line:84% The more recent data, it looks more like that. 00:59:23.070 --> 00:59:25.850 align:middle line:84% And so it's not completely clear that there is a gradient. 00:59:25.850 --> 00:59:29.130 align:middle line:84% There may be, but that still needs to be worked out. 00:59:29.130 --> 00:59:30.910 align:middle line:84% And in fact, right now they're trying 00:59:30.910 --> 00:59:34.350 align:middle line:84% to collect enough dissolved organic carbon samples on some 00:59:34.350 --> 00:59:38.630 align:middle line:84% of the new global transects to be able to really clarify 00:59:38.630 --> 00:59:42.470 align:middle line:84% what the deep water concentrations of DOC are 00:59:42.470 --> 00:59:45.950 align:middle line:84% and whether we can map them out as well as we can map out 00:59:45.950 --> 00:59:49.510 align:middle line:90% nutrients and things like that. 00:59:49.510 --> 00:59:55.270 align:middle line:84% Now, these concentrations are actually fairly high. 00:59:55.270 --> 00:59:58.450 align:middle line:84% They're not as high obviously, as dissolved inorganic carbon. 00:59:58.450 --> 01:00:02.710 align:middle line:84% Remember, dissolved inorganic carbon, DIC, 01:00:02.710 --> 01:00:08.910 align:middle line:84% was something like approximately 2,000 micromoles per kilogram. 01:00:08.910 --> 01:00:11.720 align:middle line:84% But 40 is a lot of organic matter. 01:00:11.720 --> 01:00:15.160 align:middle line:84% And 60 to 90 is a huge amount of organic matter, 01:00:15.160 --> 01:00:18.140 align:middle line:84% particularly in the subtropics, where 01:00:18.140 --> 01:00:21.500 align:middle line:84% the amount of biomass or the amount 01:00:21.500 --> 01:00:27.620 align:middle line:84% of particulate organic matter may be one or two 01:00:27.620 --> 01:00:30.980 align:middle line:90% orders of magnitude smaller. 01:00:30.980 --> 01:00:39.660 align:middle line:84% So there's a lot more DOC than there is POC in the ocean. 01:00:39.660 --> 01:00:44.060 align:middle line:84% And the global estimates are that there's 01:00:44.060 --> 01:00:52.260 align:middle line:84% something around 700 petagrams carbon as DOC, 01:00:52.260 --> 01:00:58.760 align:middle line:84% where there was only about, say, 3 petagrams carbon as POC. 01:00:58.760 --> 01:01:08.740 align:middle line:90% 01:01:08.740 --> 01:01:11.200 align:middle line:84% What else do we know about the dissolved organic pool? 01:01:11.200 --> 01:01:13.480 align:middle line:84% Well, we can make measurements of DON. 01:01:13.480 --> 01:01:18.420 align:middle line:90% 01:01:18.420 --> 01:01:25.380 align:middle line:84% DON is something like 4 to 6 micromoles per kilogram, 01:01:25.380 --> 01:01:32.580 align:middle line:84% which would give us C-to-N ratio of 10 to 15. 01:01:32.580 --> 01:01:35.660 align:middle line:84% Does anybody remember what typical Redfield ratio is 01:01:35.660 --> 01:01:37.840 align:middle line:90% for carbon-to-nitrogen ratios? 01:01:37.840 --> 01:01:40.623 align:middle line:90% AUDIENCE: [INAUDIBLE]. 01:01:40.623 --> 01:01:43.040 align:middle line:84% SCOTT DONEY: Carbon to nitrogen, not carbon to phosphorus. 01:01:43.040 --> 01:01:45.660 align:middle line:90% 01:01:45.660 --> 01:01:47.260 align:middle line:90% AUDIENCE: 106 to 16. 01:01:47.260 --> 01:01:51.980 align:middle line:90% SCOTT DONEY: 106 to 16, so 6.6. 01:01:51.980 --> 01:02:00.540 align:middle line:84% So by comparison, Redfield would be about 6.6. 01:02:00.540 --> 01:02:07.930 align:middle line:84% So the material is carbon-rich or nitrogen-poor. 01:02:07.930 --> 01:02:12.770 align:middle line:84% If you were to go down the list of carbohydrate, protein, DNA, 01:02:12.770 --> 01:02:18.130 align:middle line:84% lipid, you'd be looking at things like carbohydrates, 01:02:18.130 --> 01:02:21.250 align:middle line:90% just based on C-to-N ratios. 01:02:21.250 --> 01:02:24.690 align:middle line:84% You might think that, well, maybe the source 01:02:24.690 --> 01:02:28.650 align:middle line:84% material for DOC looks more like carbohydrates than it would, 01:02:28.650 --> 01:02:32.930 align:middle line:84% say, proteins, because the C-to-N ratio of carbohydrates 01:02:32.930 --> 01:02:35.730 align:middle line:84% is fairly high, it's carbon-rich material, 01:02:35.730 --> 01:02:40.463 align:middle line:84% where amino acids have a lot of nitrogen in them. 01:02:40.463 --> 01:02:41.630 align:middle line:90% AUDIENCE: I have a question. 01:02:41.630 --> 01:02:42.380 align:middle line:90% SCOTT DONEY: Sure. 01:02:42.380 --> 01:02:44.890 align:middle line:90% 01:02:44.890 --> 01:02:49.250 align:middle line:84% AUDIENCE: Backing up to the deep water graph. 01:02:49.250 --> 01:02:52.090 align:middle line:84% When you say DOC, are you meaning 01:02:52.090 --> 01:02:54.810 align:middle line:90% interchangeable with DOM? 01:02:54.810 --> 01:02:58.370 align:middle line:84% SCOTT DONEY: Yeah, this was in carbon. 01:02:58.370 --> 01:03:02.090 align:middle line:84% If I go back up to this plot, these concentrations, actually, 01:03:02.090 --> 01:03:09.720 align:middle line:84% these were the amount of carbon in DOM. 01:03:09.720 --> 01:03:13.640 align:middle line:84% So these measurements were based on carbon measurements. 01:03:13.640 --> 01:03:14.927 align:middle line:90% These are carbon measurements. 01:03:14.927 --> 01:03:17.260 align:middle line:84% And now I'm saying, well, what are the other components? 01:03:17.260 --> 01:03:19.560 align:middle line:84% How much nitrogen and how much phosphorus 01:03:19.560 --> 01:03:22.520 align:middle line:90% is in that organic matter? 01:03:22.520 --> 01:03:24.580 align:middle line:90% Is that OK? 01:03:24.580 --> 01:03:26.080 align:middle line:84% AUDIENCE: Yeah, along the trajectory 01:03:26.080 --> 01:03:28.610 align:middle line:84% from North Atlantic to North Pacific? 01:03:28.610 --> 01:03:29.360 align:middle line:90% SCOTT DONEY: Yeah. 01:03:29.360 --> 01:03:32.160 align:middle line:90% 01:03:32.160 --> 01:03:35.960 align:middle line:84% AUDIENCE: I guess I would think that more organic matter is 01:03:35.960 --> 01:03:40.420 align:middle line:84% either sinking or being dropped and it would increase. 01:03:40.420 --> 01:03:42.600 align:middle line:84% So why is it decreasing and not increasing? 01:03:42.600 --> 01:03:45.760 align:middle line:84% And then if DOC has a long residence time, 01:03:45.760 --> 01:03:47.960 align:middle line:90% it's pretty stable. 01:03:47.960 --> 01:03:51.120 align:middle line:84% So it's not being destroyed by other processes. 01:03:51.120 --> 01:03:54.360 align:middle line:84% Why would it decrease rather than increase? 01:03:54.360 --> 01:03:57.020 align:middle line:84% SCOTT DONEY: Well, that's a good question. 01:03:57.020 --> 01:04:01.240 align:middle line:84% If it's decreasing, that means there has to be a net loss. 01:04:01.240 --> 01:04:04.840 align:middle line:90% If it truly was decreasing-- 01:04:04.840 --> 01:04:09.520 align:middle line:84% and what it may be, from some of the data I've seen, 01:04:09.520 --> 01:04:12.640 align:middle line:84% what it might end up being once we have more data is, 01:04:12.640 --> 01:04:16.680 align:middle line:84% there was a sharp decrease in the North Atlantic. 01:04:16.680 --> 01:04:18.880 align:middle line:84% And then it might be relatively flat, 01:04:18.880 --> 01:04:22.720 align:middle line:84% and then a little bit of a decrease in the North Pacific. 01:04:22.720 --> 01:04:25.920 align:middle line:84% The original paper didn't have enough data 01:04:25.920 --> 01:04:27.920 align:middle line:90% to well describe this curve. 01:04:27.920 --> 01:04:31.800 align:middle line:84% And then there might be some scatter. 01:04:31.800 --> 01:04:34.600 align:middle line:90% So let's say there's a net loss. 01:04:34.600 --> 01:04:41.120 align:middle line:84% So you mentioned there might be a source due to decaying 01:04:41.120 --> 01:04:43.840 align:middle line:90% organic particles. 01:04:43.840 --> 01:04:49.200 align:middle line:84% So along this trajectory, particles are sinking down. 01:04:49.200 --> 01:04:52.560 align:middle line:84% And some of that, those particles 01:04:52.560 --> 01:04:57.960 align:middle line:84% are being destroyed and forming new dissolved organic matter. 01:04:57.960 --> 01:04:59.340 align:middle line:90% What would be a sink? 01:04:59.340 --> 01:05:02.950 align:middle line:90% 01:05:02.950 --> 01:05:05.710 align:middle line:84% AUDIENCE: Water upwelling out of the deep water? 01:05:05.710 --> 01:05:06.510 align:middle line:90% SCOTT DONEY: Yeah. 01:05:06.510 --> 01:05:12.270 align:middle line:84% Another one would be a few 100 years to 1,000 years' lifetime, 01:05:12.270 --> 01:05:16.070 align:middle line:84% maybe some of it's being removed by respiration. 01:05:16.070 --> 01:05:19.530 align:middle line:84% So based solely on this kind of plot, 01:05:19.530 --> 01:05:23.390 align:middle line:84% you would say that, well, there might be a source, 01:05:23.390 --> 01:05:25.430 align:middle line:84% but the net sink from respiration 01:05:25.430 --> 01:05:28.230 align:middle line:90% has to be larger than that. 01:05:28.230 --> 01:05:32.390 align:middle line:84% So as the water ages, you're losing dissolved organic matter 01:05:32.390 --> 01:05:35.110 align:middle line:90% in the deep water. 01:05:35.110 --> 01:05:38.990 align:middle line:84% That's certainly what you would think from this plot, where 01:05:38.990 --> 01:05:42.430 align:middle line:84% you've got, in some regions, more dissolved 01:05:42.430 --> 01:05:46.350 align:middle line:84% organic matter in the surface region and less at depth, 01:05:46.350 --> 01:05:48.870 align:middle line:84% that as you move down the water column, 01:05:48.870 --> 01:05:53.350 align:middle line:84% this dissolved organic matter has been respired. 01:05:53.350 --> 01:05:57.730 align:middle line:84% If this water, some of this gets mixed down or advected down, 01:05:57.730 --> 01:06:00.230 align:middle line:84% there must be some respiration going on 01:06:00.230 --> 01:06:03.870 align:middle line:84% to remove that dissolved organic matter. 01:06:03.870 --> 01:06:05.210 align:middle line:90% But that's a good question. 01:06:05.210 --> 01:06:07.630 align:middle line:84% In fact, one of the things people are looking at right 01:06:07.630 --> 01:06:12.230 align:middle line:84% now are one fraction or subset of this dissolved 01:06:12.230 --> 01:06:15.810 align:middle line:84% organic matter is the part that's colored, 01:06:15.810 --> 01:06:17.510 align:middle line:90% that's chromophoric. 01:06:17.510 --> 01:06:20.190 align:middle line:84% So remember we talked a little bit 01:06:20.190 --> 01:06:21.690 align:middle line:84% when we were talking about optics, 01:06:21.690 --> 01:06:26.590 align:middle line:84% how the junk, the gelbstoff, in the water 01:06:26.590 --> 01:06:28.630 align:middle line:84% absorbs a lot of photons, particularly 01:06:28.630 --> 01:06:30.990 align:middle line:90% down into the blue region. 01:06:30.990 --> 01:06:34.630 align:middle line:84% Well, you can actually measure that and look at it 01:06:34.630 --> 01:06:35.850 align:middle line:90% in the deep water. 01:06:35.850 --> 01:06:37.390 align:middle line:84% And one of the questions they still 01:06:37.390 --> 01:06:45.270 align:middle line:84% don't know in the deep water is, is respiration a bigger sink 01:06:45.270 --> 01:06:48.190 align:middle line:84% than release of, say, chromophoric 01:06:48.190 --> 01:06:51.870 align:middle line:84% dissolved organic matter from particles is a net source? 01:06:51.870 --> 01:06:54.990 align:middle line:84% And so they don't even know, if you were to take a parcel 01:06:54.990 --> 01:06:58.270 align:middle line:84% and age it, we think if you age a parcel, 01:06:58.270 --> 01:07:01.780 align:middle line:84% even in the deep water, that dissolved organic matter will 01:07:01.780 --> 01:07:05.900 align:middle line:84% go down, that there is a net sink over time. 01:07:05.900 --> 01:07:13.520 align:middle line:84% But for the chromophoric subset or subfraction 01:07:13.520 --> 01:07:16.140 align:middle line:84% we don't even know what the sign of that is yet. 01:07:16.140 --> 01:07:18.188 align:middle line:84% There's conflicting data on that. 01:07:18.188 --> 01:07:19.480 align:middle line:90% Yeah, that was a good question. 01:07:19.480 --> 01:07:22.280 align:middle line:84% Sorry, I had looked at this plot long enough, 01:07:22.280 --> 01:07:25.360 align:middle line:84% I didn't even think to put it in the right context. 01:07:25.360 --> 01:07:27.260 align:middle line:90% So that was a good question. 01:07:27.260 --> 01:07:30.700 align:middle line:84% Does that make sense to you guys? 01:07:30.700 --> 01:07:32.700 align:middle line:84% AUDIENCE: Another one I guess I don't understand 01:07:32.700 --> 01:07:37.020 align:middle line:84% is, you said earlier that the dissolved organic matter is 01:07:37.020 --> 01:07:40.880 align:middle line:84% resistant to being respired and has a long residence time. 01:07:40.880 --> 01:07:41.700 align:middle line:90% SCOTT DONEY: Yeah. 01:07:41.700 --> 01:07:43.367 align:middle line:84% AUDIENCE: You're making the case that it 01:07:43.367 --> 01:07:45.740 align:middle line:90% respires in the deep ocean. 01:07:45.740 --> 01:07:47.940 align:middle line:84% And you're simultaneously making the case 01:07:47.940 --> 01:07:50.900 align:middle line:84% that it's accumulating in the surface because 01:07:50.900 --> 01:07:52.580 align:middle line:90% of ocean circulation. 01:07:52.580 --> 01:07:54.500 align:middle line:90% SCOTT DONEY: Right. 01:07:54.500 --> 01:07:57.520 align:middle line:84% AUDIENCE: But they respire in both spots. 01:07:57.520 --> 01:08:00.340 align:middle line:90% 01:08:00.340 --> 01:08:03.560 align:middle line:84% SCOTT DONEY: The question is, in the surface, 01:08:03.560 --> 01:08:05.500 align:middle line:90% it's what's the net? 01:08:05.500 --> 01:08:08.640 align:middle line:84% In the surface, you have a lot of production. 01:08:08.640 --> 01:08:11.940 align:middle line:84% And so you have production of dissolved organic matter. 01:08:11.940 --> 01:08:17.899 align:middle line:84% And so the production, let's say we go back 01:08:17.899 --> 01:08:21.700 align:middle line:90% to our subtropical gyre. 01:08:21.700 --> 01:08:26.060 align:middle line:90% So these are isopycnal surfaces. 01:08:26.060 --> 01:08:30.120 align:middle line:84% So let's say you have upwelling of water and net convergence. 01:08:30.120 --> 01:08:34.340 align:middle line:90% 01:08:34.340 --> 01:08:39.420 align:middle line:84% And along the way you have production of DOM. 01:08:39.420 --> 01:08:42.859 align:middle line:84% You would also have some respiration. 01:08:42.859 --> 01:08:45.979 align:middle line:84% But let's say that respiration is slower than the production 01:08:45.979 --> 01:08:46.939 align:middle line:90% term. 01:08:46.939 --> 01:08:51.140 align:middle line:84% So as you moved along that axis, you 01:08:51.140 --> 01:08:55.689 align:middle line:84% might see DOM concentrations increasing 01:08:55.689 --> 01:08:59.890 align:middle line:84% as you got into the interior of the subtropical gyre. 01:08:59.890 --> 01:09:06.370 align:middle line:84% But once some of this water gets pumped down into the deep, 01:09:06.370 --> 01:09:10.090 align:middle line:90% there's a lot less production. 01:09:10.090 --> 01:09:14.149 align:middle line:84% And in fact, there might be very little production, 01:09:14.149 --> 01:09:16.970 align:middle line:84% depending upon what's going on with the particles. 01:09:16.970 --> 01:09:22.450 align:middle line:84% Then, respiration is going to take over 01:09:22.450 --> 01:09:25.529 align:middle line:84% because you've moved away from the production zone. 01:09:25.529 --> 01:09:28.330 align:middle line:90% And so you just have a net sink. 01:09:28.330 --> 01:09:33.290 align:middle line:84% And so the question is, what are the relevant timescales? 01:09:33.290 --> 01:09:35.810 align:middle line:84% The timescales in the deep water are 01:09:35.810 --> 01:09:38.790 align:middle line:90% hundreds to thousands of years. 01:09:38.790 --> 01:09:41.729 align:middle line:90% 01:09:41.729 --> 01:09:46.810 align:middle line:84% Up here, we're talking, 1 to maybe 10 01:09:46.810 --> 01:09:53.930 align:middle line:84% years for surface circulation, or tens, tens of years. 01:09:53.930 --> 01:09:58.850 align:middle line:84% And so let's say you had a slow respiration term, even if you 01:09:58.850 --> 01:10:02.010 align:middle line:84% had a slow respiration term that was working everywhere, 01:10:02.010 --> 01:10:04.450 align:middle line:84% you wouldn't see it up in the surface 01:10:04.450 --> 01:10:08.130 align:middle line:84% because it's only working over about 10 years. 01:10:08.130 --> 01:10:13.490 align:middle line:84% Where if you've got 1,000 years for that slow respiration term 01:10:13.490 --> 01:10:18.570 align:middle line:84% to work in the deep water, it could lead to a slow decline 01:10:18.570 --> 01:10:19.930 align:middle line:90% of DOC. 01:10:19.930 --> 01:10:22.910 align:middle line:84% So you just have to compare the timescales. 01:10:22.910 --> 01:10:27.034 align:middle line:90% 01:10:27.034 --> 01:10:28.550 align:middle line:90% How much time do we have? 01:10:28.550 --> 01:10:32.410 align:middle line:90% 01:10:32.410 --> 01:10:36.010 align:middle line:84% OK, I did want to get back and cover a couple of other things 01:10:36.010 --> 01:10:39.330 align:middle line:90% today. 01:10:39.330 --> 01:10:44.970 align:middle line:84% If memory serves, next Tuesday's lecture, 01:10:44.970 --> 01:10:47.170 align:middle line:84% we should be on time for finishing 01:10:47.170 --> 01:10:48.970 align:middle line:90% this block of lectures. 01:10:48.970 --> 01:10:53.138 align:middle line:84% And so after I'm done today, if there's still 01:10:53.138 --> 01:10:55.680 align:middle line:84% questions on dissolved organic matter, we can come back to it 01:10:55.680 --> 01:10:56.412 align:middle line:90% on Tuesday. 01:10:56.412 --> 01:10:58.120 align:middle line:84% Because I think I should have enough time 01:10:58.120 --> 01:11:01.160 align:middle line:90% to finish everything. 01:11:01.160 --> 01:11:03.080 align:middle line:84% And then next Thursday, Bill Martin 01:11:03.080 --> 01:11:07.620 align:middle line:84% will be talking about some of the sediment fluxes. 01:11:07.620 --> 01:11:20.160 align:middle line:90% 01:11:20.160 --> 01:11:22.760 align:middle line:90% So we talked about the DON. 01:11:22.760 --> 01:11:27.300 align:middle line:90% The DOP is relatively small. 01:11:27.300 --> 01:11:31.080 align:middle line:84% So this would be dissolved organic phosphorus. 01:11:31.080 --> 01:11:37.200 align:middle line:84% It's only maybe 0.1 to 0.4 micromolar. 01:11:37.200 --> 01:11:40.680 align:middle line:84% It's not nearly as well characterized. 01:11:40.680 --> 01:11:43.840 align:middle line:90% Most groups don't measure it. 01:11:43.840 --> 01:11:48.760 align:middle line:84% Most groups who do this measure DOC, some subset of that measure 01:11:48.760 --> 01:11:53.280 align:middle line:84% DON, and a smaller subset measure DOP. 01:11:53.280 --> 01:11:55.520 align:middle line:90% These would be on the surface. 01:11:55.520 --> 01:11:59.720 align:middle line:90% There's hardly any DOP at depth. 01:11:59.720 --> 01:12:05.360 align:middle line:84% And this gives C-to-P ratios of something like 200 to 600, 01:12:05.360 --> 01:12:10.520 align:middle line:84% which is quite a bit bigger than the canonical Redfield, which 01:12:10.520 --> 01:12:18.320 align:middle line:90% is, say, 116. 01:12:18.320 --> 01:12:23.380 align:middle line:84% Now, I've mentioned that DOM is heterogeneous. 01:12:23.380 --> 01:12:28.560 align:middle line:90% 01:12:28.560 --> 01:12:31.240 align:middle line:90% And it's actually a spectrum. 01:12:31.240 --> 01:12:36.680 align:middle line:84% There will be a whole spectrum of characteristics. 01:12:36.680 --> 01:12:43.360 align:middle line:84% But we try to lump it into a few pools. 01:12:43.360 --> 01:12:48.310 align:middle line:84% So you'll often see DOM fairly artificially 01:12:48.310 --> 01:12:58.850 align:middle line:84% partitioned by timescale into a labile component. 01:12:58.850 --> 01:13:01.470 align:middle line:90% 01:13:01.470 --> 01:13:08.990 align:middle line:84% And this would have timescales of, say, hours to days. 01:13:08.990 --> 01:13:10.650 align:middle line:90% And it's going to be small. 01:13:10.650 --> 01:13:15.510 align:middle line:84% It's going to be less than 5 micromoles per kilogram. 01:13:15.510 --> 01:13:18.850 align:middle line:84% And that's only going to be in the upper water column. 01:13:18.850 --> 01:13:21.090 align:middle line:84% And this is micromoles of carbon. 01:13:21.090 --> 01:13:24.430 align:middle line:90% 01:13:24.430 --> 01:13:28.010 align:middle line:90% There's a semi-labile. 01:13:28.010 --> 01:13:31.110 align:middle line:90% 01:13:31.110 --> 01:13:35.870 align:middle line:84% You can tell this is an exact term, semi-labile, of days 01:13:35.870 --> 01:13:37.250 align:middle line:90% to maybe months. 01:13:37.250 --> 01:13:41.110 align:middle line:90% 01:13:41.110 --> 01:13:48.820 align:middle line:84% And that's going to be anywhere from about 0 01:13:48.820 --> 01:13:56.580 align:middle line:84% up to maybe 30 micromoles carbon per kilogram. 01:13:56.580 --> 01:14:03.260 align:middle line:84% And this is, if we go back to that plot on the previous page, 01:14:03.260 --> 01:14:08.900 align:middle line:84% this bulge of DOC that you saw in the subtropics, that's 01:14:08.900 --> 01:14:11.200 align:middle line:90% this semi-labile stuff. 01:14:11.200 --> 01:14:15.460 align:middle line:84% It lasts long enough to build up in the surface. 01:14:15.460 --> 01:14:20.140 align:middle line:84% But it's not so refractory that it isn't respired 01:14:20.140 --> 01:14:24.060 align:middle line:84% before it gets into the deep water. 01:14:24.060 --> 01:14:31.220 align:middle line:84% And then there's a refractory component, 01:14:31.220 --> 01:14:39.780 align:middle line:84% which is hundreds to thousands of years. 01:14:39.780 --> 01:14:41.300 align:middle line:84% And that's this background that's 01:14:41.300 --> 01:14:44.760 align:middle line:84% about 40 micromoles per kilogram. 01:14:44.760 --> 01:14:50.020 align:middle line:90% 01:14:50.020 --> 01:14:55.220 align:middle line:84% Now, you do have a net production 01:14:55.220 --> 01:15:00.860 align:middle line:90% of DOC near the surface. 01:15:00.860 --> 01:15:06.220 align:middle line:84% And this net production gets exported mostly 01:15:06.220 --> 01:15:09.260 align:middle line:90% into the thermocline. 01:15:09.260 --> 01:15:13.520 align:middle line:84% And that's via mixing and advection. 01:15:13.520 --> 01:15:16.340 align:middle line:90% 01:15:16.340 --> 01:15:27.220 align:middle line:84% The current understanding is that in the upper thermocline, 01:15:27.220 --> 01:15:29.300 align:middle line:84% if you were to compare the respiration 01:15:29.300 --> 01:15:33.780 align:middle line:84% of this dissolved organic carbon versus the total respiration, 01:15:33.780 --> 01:15:39.840 align:middle line:84% it's going to be less than about half of total respiration. 01:15:39.840 --> 01:15:44.130 align:middle line:90% 01:15:44.130 --> 01:15:49.630 align:middle line:84% There were some hypotheses a decade or so ago-- oh, gosh, 01:15:49.630 --> 01:15:51.530 align:middle line:90% no longer than that-- 01:15:51.530 --> 01:15:54.810 align:middle line:84% going on two decades ago, that DOC 01:15:54.810 --> 01:15:58.530 align:middle line:84% could be the dominant source of organic matter for respiration 01:15:58.530 --> 01:16:00.050 align:middle line:90% in the upper thermocline. 01:16:00.050 --> 01:16:04.490 align:middle line:84% But a closer examination of the data and new data 01:16:04.490 --> 01:16:06.510 align:middle line:84% suggests that that's not actually true. 01:16:06.510 --> 01:16:11.970 align:middle line:90% 01:16:11.970 --> 01:16:18.330 align:middle line:84% In addition to DOC spatial gradients, 01:16:18.330 --> 01:16:22.250 align:middle line:84% we also know something about DOC dynamics 01:16:22.250 --> 01:16:26.230 align:middle line:90% via radiocarbon measurements. 01:16:26.230 --> 01:16:29.130 align:middle line:90% 01:16:29.130 --> 01:16:34.170 align:middle line:84% So you can measure the radiocarbon content 01:16:34.170 --> 01:16:37.250 align:middle line:90% of dissolved organic matter. 01:16:37.250 --> 01:16:39.890 align:middle line:84% And a fair amount of this work has been done, 01:16:39.890 --> 01:16:44.610 align:middle line:84% one of the leaders on this is Ellen Druffel who's 01:16:44.610 --> 01:16:48.170 align:middle line:90% out at UC Irvine, and Bauer. 01:16:48.170 --> 01:16:50.530 align:middle line:84% And I can't remember Bauer's first name 01:16:50.530 --> 01:16:51.830 align:middle line:90% off the top of my head. 01:16:51.830 --> 01:16:55.410 align:middle line:90% 01:16:55.410 --> 01:17:00.810 align:middle line:84% This is fairly new work in the last, oh, say, 10 or 15 years. 01:17:00.810 --> 01:17:04.830 align:middle line:84% But if you were to look as a function of depth-- 01:17:04.830 --> 01:17:09.470 align:middle line:84% And most of their samples have come from the North Pacific. 01:17:09.470 --> 01:17:14.370 align:middle line:90% 01:17:14.370 --> 01:17:18.730 align:middle line:84% So if you plot radiocarbon, so you have minus 1,000, 01:17:18.730 --> 01:17:32.630 align:middle line:84% which is essentially completely dead, minus 500, 0, 500. 01:17:32.630 --> 01:17:36.890 align:middle line:90% So this is delta C-14. 01:17:36.890 --> 01:17:40.360 align:middle line:84% And Bill went over quite a bit about radiocarbon 01:17:40.360 --> 01:17:41.880 align:middle line:90% before the midterm. 01:17:41.880 --> 01:17:46.120 align:middle line:84% Does anybody remember what the pre-industrial ocean was, say, 01:17:46.120 --> 01:17:49.780 align:middle line:84% for inorganic carbon, the surface levels? 01:17:49.780 --> 01:17:54.920 align:middle line:90% 01:17:54.920 --> 01:17:57.240 align:middle line:84% Well, what was the pre-industrial atmosphere 01:17:57.240 --> 01:17:58.280 align:middle line:90% for radiocarbon? 01:17:58.280 --> 01:18:01.480 align:middle line:90% 01:18:01.480 --> 01:18:04.840 align:middle line:90% What does delta C-14 mean? 01:18:04.840 --> 01:18:08.520 align:middle line:84% Oh, come on, we got three minutes left. 01:18:08.520 --> 01:18:10.500 align:middle line:84% AUDIENCE: You don't actually want the content. 01:18:10.500 --> 01:18:12.690 align:middle line:90% You just want 0. 01:18:12.690 --> 01:18:13.440 align:middle line:90% SCOTT DONEY: Yeah. 01:18:13.440 --> 01:18:14.520 align:middle line:90% AUDIENCE: OK. 01:18:14.520 --> 01:18:15.640 align:middle line:90% SCOTT DONEY: Why 0? 01:18:15.640 --> 01:18:17.307 align:middle line:84% AUDIENCE: Because that's the definition. 01:18:17.307 --> 01:18:26.200 align:middle line:84% SCOTT DONEY: Yeah the pre-industrial atmosphere 01:18:26.200 --> 01:18:32.640 align:middle line:84% had a delta C-14 of approximately 0. 01:18:32.640 --> 01:18:37.320 align:middle line:84% That was the standard that's used for delta C-14 was set up 01:18:37.320 --> 01:18:40.680 align:middle line:84% such that the pre-industrial atmosphere had about 0. 01:18:40.680 --> 01:18:46.320 align:middle line:84% And that meant that freshly produced biological material 01:18:46.320 --> 01:18:54.240 align:middle line:84% in the pre-industrial period had a delta, 14C of about 0. 01:18:54.240 --> 01:18:57.140 align:middle line:84% The surface ocean was somewhat depleted. 01:18:57.140 --> 01:19:00.680 align:middle line:84% The inorganic pool was somewhat depleted relative 01:19:00.680 --> 01:19:04.440 align:middle line:84% to the atmosphere because of reservoir effects. 01:19:04.440 --> 01:19:13.960 align:middle line:84% It just takes some time for radiocarbon-rich CO2 01:19:13.960 --> 01:19:16.600 align:middle line:90% to mix into the ocean. 01:19:16.600 --> 01:19:19.400 align:middle line:84% And during that time, you have the chance for decay. 01:19:19.400 --> 01:19:22.600 align:middle line:84% So if I were to look at the pre-industrial ocean, 01:19:22.600 --> 01:19:27.320 align:middle line:84% the dissolved inorganic carbon would have been slightly 01:19:27.320 --> 01:19:30.560 align:middle line:84% below 0 in the surface and then would 01:19:30.560 --> 01:19:33.780 align:middle line:90% have decreased some with depth. 01:19:33.780 --> 01:19:36.630 align:middle line:90% 01:19:36.630 --> 01:19:38.870 align:middle line:90% So this would be DIC. 01:19:38.870 --> 01:19:43.110 align:middle line:84% And it decreases with depth because deep water 01:19:43.110 --> 01:19:48.310 align:middle line:90% is older than surface water. 01:19:48.310 --> 01:19:51.790 align:middle line:84% And so you'd expect deep DIC pools 01:19:51.790 --> 01:19:56.650 align:middle line:84% to be somewhere around minus 200 per mil. 01:19:56.650 --> 01:20:01.070 align:middle line:90% 01:20:01.070 --> 01:20:08.190 align:middle line:84% Because of the release of C-14 into the atmosphere by weapons 01:20:08.190 --> 01:20:12.790 align:middle line:84% testing, if you actually go out and look at the actual values 01:20:12.790 --> 01:20:16.390 align:middle line:90% now they're above 0. 01:20:16.390 --> 01:20:19.910 align:middle line:84% And that's because you've had an invasion of bomb C-14 01:20:19.910 --> 01:20:21.250 align:middle line:90% into the surface water. 01:20:21.250 --> 01:20:27.030 align:middle line:90% 01:20:27.030 --> 01:20:45.710 align:middle line:84% Currently, particulate organic matter all has a bomb spike. 01:20:45.710 --> 01:20:50.950 align:middle line:84% If you go out and look at sediment trap material or even 01:20:50.950 --> 01:20:56.550 align:middle line:84% suspended particles, they tend to be elevated above the DIC 01:20:56.550 --> 01:20:58.950 align:middle line:84% concentrations and elevated above what we think 01:20:58.950 --> 01:21:01.150 align:middle line:90% pre-industrial levels were. 01:21:01.150 --> 01:21:04.790 align:middle line:84% This is one line of evidence that the suspended particles 01:21:04.790 --> 01:21:07.590 align:middle line:84% in the deep ocean have to be communicating 01:21:07.590 --> 01:21:09.590 align:middle line:84% with the large sinking particles. 01:21:09.590 --> 01:21:12.150 align:middle line:84% Because if you were to go out to the deep North Pacific 01:21:12.150 --> 01:21:14.590 align:middle line:84% and look at a suspended particle, 01:21:14.590 --> 01:21:17.230 align:middle line:84% if that had advected there from the North 01:21:17.230 --> 01:21:19.030 align:middle line:84% Atlantic or the Southern Ocean, it 01:21:19.030 --> 01:21:20.890 align:middle line:84% should be a couple hundred years old. 01:21:20.890 --> 01:21:26.150 align:middle line:84% It shouldn't have any bomb radiocarbon in it, but they do. 01:21:26.150 --> 01:21:27.790 align:middle line:84% And that's another line of evidence 01:21:27.790 --> 01:21:31.890 align:middle line:84% that in addition to the thorium isotopes, 01:21:31.890 --> 01:21:36.580 align:middle line:84% that there's exchange of material between the sinking 01:21:36.580 --> 01:21:39.780 align:middle line:84% particles and the suspended particles. 01:21:39.780 --> 01:21:43.300 align:middle line:84% If we go out and look at the dissolved organic matter, 01:21:43.300 --> 01:21:48.220 align:middle line:84% there's a surface increase, but the dissolved organic matter 01:21:48.220 --> 01:21:54.980 align:middle line:84% is really old relative to both the current source 01:21:54.980 --> 01:21:58.780 align:middle line:84% of particulate organic matter and the current dissolved 01:21:58.780 --> 01:22:02.540 align:middle line:90% inorganic carbon pool. 01:22:02.540 --> 01:22:09.020 align:middle line:84% So some fraction of the DOC must be really old. 01:22:09.020 --> 01:22:15.100 align:middle line:84% And in fact, if you just do calendar ages based 01:22:15.100 --> 01:22:21.380 align:middle line:84% on radiocarbon, you get that in the deep water DOC 01:22:21.380 --> 01:22:27.300 align:middle line:84% in the Atlantic is about 4,000 years old. 01:22:27.300 --> 01:22:34.420 align:middle line:84% And in the Pacific, it's about 6,000 years old. 01:22:34.420 --> 01:22:40.860 align:middle line:84% Now, that doesn't mean, remember, DOC is not 01:22:40.860 --> 01:22:44.560 align:middle line:90% a homogeneous organic material. 01:22:44.560 --> 01:22:46.020 align:middle line:90% It's heterogeneous. 01:22:46.020 --> 01:22:48.480 align:middle line:84% Some of the DOC might actually be dead. 01:22:48.480 --> 01:22:50.340 align:middle line:84% It might have no radiocarbon at all. 01:22:50.340 --> 01:22:52.460 align:middle line:90% It might be really refractory. 01:22:52.460 --> 01:22:57.240 align:middle line:84% Other components might have some recent, 01:22:57.240 --> 01:23:00.740 align:middle line:84% there might be some exchange between the POC and the DOC 01:23:00.740 --> 01:23:01.580 align:middle line:90% pool. 01:23:01.580 --> 01:23:04.380 align:middle line:84% And in fact, there is evidence for that in the radiocarbon 01:23:04.380 --> 01:23:05.220 align:middle line:90% data. 01:23:05.220 --> 01:23:09.160 align:middle line:84% But on the whole, DOC has a much lower radiocarbon. 01:23:09.160 --> 01:23:13.300 align:middle line:84% And that's one of the reasons why we talk about DOC 01:23:13.300 --> 01:23:16.540 align:middle line:90% as being quite refractory. 01:23:16.540 --> 01:23:18.180 align:middle line:90% So we'll stop there. 01:23:18.180 --> 01:23:23.460 align:middle line:84% And we'll pick it up with looking 01:23:23.460 --> 01:23:30.770 align:middle line:84% at Redfield ratios and preformed nutrients and respiration.