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 --> 00:00:06.440 align:middle line:84% Your support will help MIT OpenCourseWare 00:00:06.440 --> 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 --> 00:00:19.240 align:middle line:90% at ocw.mit.edu. 00:00:19.240 --> 00:00:24.680 align:middle line:84% WILLIAM MARTIN: Today is the last chapter 00:00:24.680 --> 00:00:29.380 align:middle line:84% in the major ion in seawater discussion, 00:00:29.380 --> 00:00:33.360 align:middle line:84% and the first chapter in the carbon in seawater discussion. 00:00:33.360 --> 00:00:36.360 align:middle line:84% And what I'm going to do is talk about the chemistry 00:00:36.360 --> 00:00:39.120 align:middle line:84% of inorganic carbon in the oceans, 00:00:39.120 --> 00:00:42.320 align:middle line:84% and specifically dissolved in the ocean. 00:00:42.320 --> 00:00:47.000 align:middle line:84% I'm going to start with a little introduction to set the stage. 00:00:47.000 --> 00:00:49.840 align:middle line:84% And I'm going to spend a good chunk of the lecture talking 00:00:49.840 --> 00:00:53.840 align:middle line:84% about inorganic carbon equilibria in the oceans, 00:00:53.840 --> 00:00:57.320 align:middle line:84% and the measurements we make and how we use those measurements. 00:00:57.320 --> 00:00:59.690 align:middle line:84% Then I'm going to finish up by talking 00:00:59.690 --> 00:01:03.370 align:middle line:84% about how we use those measurements to teach us 00:01:03.370 --> 00:01:08.130 align:middle line:84% something about biogeochemical processes in the ocean. 00:01:08.130 --> 00:01:13.830 align:middle line:84% So without further ado, why study carbonate in the ocean? 00:01:13.830 --> 00:01:16.992 align:middle line:90% Well, the first-- whoops. 00:01:16.992 --> 00:01:17.950 align:middle line:90% AUDIENCE: It's obvious. 00:01:17.950 --> 00:01:19.090 align:middle line:84% WILLIAM MARTIN: It was so obvious. 00:01:19.090 --> 00:01:21.090 align:middle line:84% [? It ?] [? was ?] [? kind ?] [? of ?] [? sudden. ?] The first 00:01:21.090 --> 00:01:23.950 align:middle line:84% and most obvious thing is that we're carbon-based life forms. 00:01:23.950 --> 00:01:26.310 align:middle line:84% And every life form on Earth is carbon-based. 00:01:26.310 --> 00:01:29.210 align:middle line:84% So we need to know something about carbon cycle on Earth 00:01:29.210 --> 00:01:30.530 align:middle line:90% and in the ocean. 00:01:30.530 --> 00:01:34.490 align:middle line:84% Second thing is that the pH, the hydrogen ion content, 00:01:34.490 --> 00:01:39.170 align:middle line:84% of the ocean, is a major master variable for many chemical 00:01:39.170 --> 00:01:41.730 align:middle line:84% and biological processes in the oceans. 00:01:41.730 --> 00:01:44.050 align:middle line:84% And inorganic carbonate equilibria 00:01:44.050 --> 00:01:47.490 align:middle line:84% play an important role in determining oceanic pH. 00:01:47.490 --> 00:01:51.330 align:middle line:84% So it's important to understand equilibrium chemistry 00:01:51.330 --> 00:01:54.410 align:middle line:84% of the inorganic carbon system in seawater. 00:01:54.410 --> 00:01:58.220 align:middle line:84% Third, and related to the pH factor, 00:01:58.220 --> 00:02:01.140 align:middle line:84% is that bicarbonate is third most concentrated anion 00:02:01.140 --> 00:02:01.880 align:middle line:90% in the ocean. 00:02:01.880 --> 00:02:04.580 align:middle line:84% So it's one of the major ions in seawater. 00:02:04.580 --> 00:02:06.620 align:middle line:84% And finally, the marine carbon cycle 00:02:06.620 --> 00:02:11.660 align:middle line:84% is a key factor in determining the fate of anthropogenic CO2 00:02:11.660 --> 00:02:15.740 align:middle line:84% and the sorts of societal relevance these days as well. 00:02:15.740 --> 00:02:17.600 align:middle line:84% And finally, a little note on the bottom, 00:02:17.600 --> 00:02:19.500 align:middle line:90% which I won't say out loud. 00:02:19.500 --> 00:02:20.660 align:middle line:90% Sorry. 00:02:20.660 --> 00:02:26.220 align:middle line:84% For those of you who aren't [? pleased ?] by any of this. 00:02:26.220 --> 00:02:31.020 align:middle line:84% So first, I'm going to give, without any numbers, a really 00:02:31.020 --> 00:02:35.620 align:middle line:84% basic and simple overview of carbon cycling in the ocean. 00:02:35.620 --> 00:02:39.980 align:middle line:84% The oceans are connected to three boundaries, at least 00:02:39.980 --> 00:02:42.060 align:middle line:84% three that I can think of at the moment-- 00:02:42.060 --> 00:02:45.005 align:middle line:84% land, atmosphere, and the sea floor. 00:02:45.005 --> 00:02:46.380 align:middle line:84% And there are exchanges of carbon 00:02:46.380 --> 00:02:49.780 align:middle line:90% across all of those boundaries. 00:02:49.780 --> 00:02:53.560 align:middle line:84% Weathering processes on land deliver both bicarbonate, 00:02:53.560 --> 00:02:55.740 align:middle line:84% and what I forgot to write down, your organic carbon 00:02:55.740 --> 00:02:59.920 align:middle line:84% to rivers, which then carry those materials into the oceans. 00:02:59.920 --> 00:03:04.520 align:middle line:84% The oceans and the atmospherics change CO2. 00:03:04.520 --> 00:03:09.160 align:middle line:84% Once the CO2 is into the ocean, it's 00:03:09.160 --> 00:03:14.360 align:middle line:84% distributed between the various components of the carbonic acid 00:03:14.360 --> 00:03:18.760 align:middle line:84% system, I guess you'd call it-- carbonic acid and CO2. 00:03:18.760 --> 00:03:20.720 align:middle line:84% And then carbonic acid dissociates 00:03:20.720 --> 00:03:23.200 align:middle line:84% to form bicarbonate, which in turn dissociates 00:03:23.200 --> 00:03:24.460 align:middle line:90% to form carbonate. 00:03:24.460 --> 00:03:27.340 align:middle line:84% So we often talk about total CO2 in the oceans. 00:03:27.340 --> 00:03:30.240 align:middle line:84% And what we mean is the sum of the carbonate ion 00:03:30.240 --> 00:03:36.720 align:middle line:84% plus bicarbonate ion plus carbonic acid H2CO3 plus CO2. 00:03:36.720 --> 00:03:39.680 align:middle line:90% Another thing you'll often see. 00:03:39.680 --> 00:03:43.880 align:middle line:84% In this lecture, I'll use CO2 and carbonic acid 00:03:43.880 --> 00:03:45.280 align:middle line:90% interchangeably. 00:03:45.280 --> 00:03:47.120 align:middle line:84% When we talk about the thermodynamics 00:03:47.120 --> 00:03:49.960 align:middle line:84% of the system in the ocean, what we generally do 00:03:49.960 --> 00:03:57.890 align:middle line:84% is add together CO2 plus H2CO3, and we call it H2CO2 star. 00:03:57.890 --> 00:04:03.250 align:middle line:84% So just remember, that's the sum of carbon dioxide plus H2CO3. 00:04:03.250 --> 00:04:10.690 align:middle line:84% And just so in case anybody asks, it's mostly CO2. 00:04:10.690 --> 00:04:14.450 align:middle line:84% There's intense inorganic carbon cycle-- 00:04:14.450 --> 00:04:17.029 align:middle line:84% organic and inorganic carbon cycle-- in the ocean. 00:04:17.029 --> 00:04:20.450 align:middle line:84% In the surface ocean, CO2 is taken up during photosynthesis 00:04:20.450 --> 00:04:22.050 align:middle line:90% to form organic matter. 00:04:22.050 --> 00:04:25.290 align:middle line:84% And to a lesser extent, but still an important extent, 00:04:25.290 --> 00:04:27.970 align:middle line:84% carbonate ion is taken up by organisms which 00:04:27.970 --> 00:04:30.530 align:middle line:90% make calcium carbonate shells. 00:04:30.530 --> 00:04:33.850 align:middle line:84% So organic carbon and calcium carbonate 00:04:33.850 --> 00:04:35.930 align:middle line:84% are both formed in the surface ocean. 00:04:35.930 --> 00:04:38.530 align:middle line:84% Those degrade to some extent in the surface ocean, 00:04:38.530 --> 00:04:40.690 align:middle line:84% but they fall out-- some of it falls out 00:04:40.690 --> 00:04:43.770 align:middle line:84% as particles from the surface ocean into the deep ocean. 00:04:43.770 --> 00:04:47.890 align:middle line:84% Once there, respiration causes the decomposition 00:04:47.890 --> 00:04:51.050 align:middle line:84% of organic matter, again, releasing CO2 into the water. 00:04:51.050 --> 00:04:53.850 align:middle line:84% And chemical processes lead to the dissolution 00:04:53.850 --> 00:04:55.660 align:middle line:84% of calcium carbonate, which releases 00:04:55.660 --> 00:04:58.940 align:middle line:90% carbonate ion into the water. 00:04:58.940 --> 00:05:03.883 align:middle line:84% Almost all the organic matter is decomposed before particles 00:05:03.883 --> 00:05:05.300 align:middle line:84% reach the sea floor, and something 00:05:05.300 --> 00:05:08.100 align:middle line:84% like half of the calcium carbonate is decomposed. 00:05:08.100 --> 00:05:10.780 align:middle line:84% But those processes continue in the sea floor, 00:05:10.780 --> 00:05:12.940 align:middle line:84% where, again, respiration releases 00:05:12.940 --> 00:05:16.260 align:middle line:84% CO2 from organic matter, and carbonate dissolution 00:05:16.260 --> 00:05:18.740 align:middle line:84% releases carbonate ion from the dissolution 00:05:18.740 --> 00:05:20.420 align:middle line:90% of calcium carbonate. 00:05:20.420 --> 00:05:24.620 align:middle line:84% And a tiny fraction of the original organic matter 00:05:24.620 --> 00:05:27.640 align:middle line:84% is left buried in the sediments, and a more significant, 00:05:27.640 --> 00:05:29.140 align:middle line:84% but still reasonably small fraction 00:05:29.140 --> 00:05:33.180 align:middle line:84% of the calcium carbonate is also buried in sediments. 00:05:33.180 --> 00:05:35.740 align:middle line:84% There's a very quick nonquantitative overview 00:05:35.740 --> 00:05:41.150 align:middle line:84% of the carbon cycling in the oceans. 00:05:41.150 --> 00:05:45.300 align:middle line:84% And we're going to have one more carbon cycle overhead. 00:05:45.300 --> 00:05:50.810 align:middle line:84% And I've chosen one done by Eric Sundquist almost 15 years ago. 00:05:50.810 --> 00:05:53.060 align:middle line:84% So the numbers in the atmosphere and the surface ocean 00:05:53.060 --> 00:05:54.390 align:middle line:90% are you're a bit out of date. 00:05:54.390 --> 00:05:56.110 align:middle line:84% But I like this picture because it 00:05:56.110 --> 00:05:59.350 align:middle line:84% has a time scale on the left-hand side, which 00:05:59.350 --> 00:06:02.830 align:middle line:84% is very useful in thinking about the different reservoirs. 00:06:02.830 --> 00:06:04.910 align:middle line:84% First thing to note is one you know about already 00:06:04.910 --> 00:06:08.990 align:middle line:84% is that CO2 in the atmosphere is increasing 00:06:08.990 --> 00:06:12.230 align:middle line:84% and continues to increase at a rapid rate. 00:06:12.230 --> 00:06:15.310 align:middle line:84% A small amount of that CO2 is taken up 00:06:15.310 --> 00:06:17.110 align:middle line:84% in the surface ocean, which is shown 00:06:17.110 --> 00:06:21.230 align:middle line:84% by this little downward arrow with a 2 and 3 next to it here. 00:06:21.230 --> 00:06:24.630 align:middle line:84% The surface ocean reservoir is approximately the same size 00:06:24.630 --> 00:06:25.410 align:middle line:90% as the atmosphere. 00:06:25.410 --> 00:06:30.270 align:middle line:84% [INAUDIBLE] Oh, didn't I write units? 00:06:30.270 --> 00:06:32.290 align:middle line:84% In my original slide, the units were on there. 00:06:32.290 --> 00:06:33.960 align:middle line:90% But it's-- 00:06:33.960 --> 00:06:35.430 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:06:35.430 --> 00:06:38.830 align:middle line:84% WILLIAM MARTIN: Yeah, petagrams of carbon and petagrams 00:06:38.830 --> 00:06:41.430 align:middle line:84% of carbon per year are the units. 00:06:41.430 --> 00:06:48.250 align:middle line:84% And a petagram is 10 to the 15th grams. 00:06:48.250 --> 00:06:52.120 align:middle line:90% 00:06:52.120 --> 00:06:54.880 align:middle line:84% So the surface ocean reservoir is about the same size 00:06:54.880 --> 00:06:56.640 align:middle line:90% as the atmospheric reservoir. 00:06:56.640 --> 00:07:00.960 align:middle line:84% And if you move over to the left to the time scale, 00:07:00.960 --> 00:07:05.400 align:middle line:84% the interactions between oceanic surface and the atmosphere 00:07:05.400 --> 00:07:07.280 align:middle line:90% have a time scale of years. 00:07:07.280 --> 00:07:11.440 align:middle line:84% If you look below the surface ocean into the deep ocean, 00:07:11.440 --> 00:07:15.720 align:middle line:84% that is the largest of the reservoirs that are cycled 00:07:15.720 --> 00:07:20.060 align:middle line:84% on this annual to 10,000-year timescale. 00:07:20.060 --> 00:07:23.600 align:middle line:84% So the deep ocean is a very large reservoir of carbon. 00:07:23.600 --> 00:07:27.460 align:middle line:84% So it's the largest reservoir among these, 00:07:27.460 --> 00:07:32.560 align:middle line:84% and it's cycling time is on the order of hundreds to 1,000 00:07:32.560 --> 00:07:34.440 align:middle line:90% years. 00:07:34.440 --> 00:07:37.560 align:middle line:84% The next important reservoir is what 00:07:37.560 --> 00:07:39.460 align:middle line:84% is called reactive marine sediments. 00:07:39.460 --> 00:07:41.120 align:middle line:84% How big this reservoir is-- actually, 00:07:41.120 --> 00:07:43.720 align:middle line:84% it's a little arbitrary, and it depends how you define it. 00:07:43.720 --> 00:07:48.400 align:middle line:84% But however you define it, it's a large reservoir, 00:07:48.400 --> 00:07:53.380 align:middle line:84% and it's cycled on a timescale of thousands of years. 00:07:53.380 --> 00:07:57.820 align:middle line:84% Below this, by far the largest reservoir on the Earth's surface 00:07:57.820 --> 00:08:00.520 align:middle line:84% is the Earth's crust, which is an enormous reservoir, 00:08:00.520 --> 00:08:03.260 align:middle line:84% but its cycling time is measured in hundreds of thousands 00:08:03.260 --> 00:08:03.760 align:middle line:90% of years. 00:08:03.760 --> 00:08:06.400 align:middle line:84% So it's important for a very long time-scale processes. 00:08:06.400 --> 00:08:10.580 align:middle line:90% 00:08:10.580 --> 00:08:13.580 align:middle line:84% Where am I going back [INAUDIBLE]? 00:08:13.580 --> 00:08:16.900 align:middle line:84% From this carbon cycle picture, we 00:08:16.900 --> 00:08:20.920 align:middle line:84% said already that once CO2 is dissolved in the ocean, 00:08:20.920 --> 00:08:24.020 align:middle line:84% it becomes distributed between the different inorganic carbon 00:08:24.020 --> 00:08:29.140 align:middle line:84% species, which are CO2 plus carbonic acid, H2CO3, which 00:08:29.140 --> 00:08:32.980 align:middle line:90% we lump together as H2CO3 star. 00:08:32.980 --> 00:08:35.340 align:middle line:84% Carbonic acid dissociates to some extent 00:08:35.340 --> 00:08:39.380 align:middle line:84% to form bicarbonate, which in turn dissociates to some extent 00:08:39.380 --> 00:08:40.580 align:middle line:90% to form carbonate. 00:08:40.580 --> 00:08:44.380 align:middle line:84% So we talk about total CO2 as the sum of all those species 00:08:44.380 --> 00:08:46.700 align:middle line:90% in seawater. 00:08:46.700 --> 00:08:50.150 align:middle line:84% We know very well the equilibrium constants 00:08:50.150 --> 00:08:53.830 align:middle line:84% for the dissociation of carbonic acid to bicarbonate 00:08:53.830 --> 00:08:57.750 align:middle line:84% and for the dissociation of bicarbonate to carbonate. 00:08:57.750 --> 00:09:01.870 align:middle line:84% And so what we have here is a system of five unknowns. 00:09:01.870 --> 00:09:06.170 align:middle line:84% So the unknowns are total CO2, the carbonate system components, 00:09:06.170 --> 00:09:07.830 align:middle line:90% and H+. 00:09:07.830 --> 00:09:10.870 align:middle line:84% We know the two dissociation constants. 00:09:10.870 --> 00:09:13.910 align:middle line:90% And if we measure two things-- 00:09:13.910 --> 00:09:16.330 align:middle line:84% if, for example, total CO2 and pH-- 00:09:16.330 --> 00:09:19.390 align:middle line:84% we can calculate the speciation of the inorganic carbon 00:09:19.390 --> 00:09:20.750 align:middle line:90% system in seawater. 00:09:20.750 --> 00:09:23.350 align:middle line:84% What I'm going to show next is a plot 00:09:23.350 --> 00:09:25.770 align:middle line:84% where I've assumed a value for the total CO2, 00:09:25.770 --> 00:09:27.750 align:middle line:84% a reasonable value for the oceans. 00:09:27.750 --> 00:09:30.910 align:middle line:84% And then I've calculated the speciation-- so the distribution 00:09:30.910 --> 00:09:34.550 align:middle line:84% of inorganic carbon between carbonic acid, bicarbonate, 00:09:34.550 --> 00:09:38.910 align:middle line:84% and carbonate ion in the oceans as a function of pH. 00:09:38.910 --> 00:09:42.550 align:middle line:84% And that's shown here with concentration 00:09:42.550 --> 00:09:46.430 align:middle line:84% on the vertical axis and pH, which, of course, is 00:09:46.430 --> 00:09:49.280 align:middle line:84% the negative log of the hydrogen ion activity 00:09:49.280 --> 00:09:51.200 align:middle line:90% on the horizontal axis. 00:09:51.200 --> 00:09:54.160 align:middle line:84% And the only point I want to make about this particular plot 00:09:54.160 --> 00:09:57.240 align:middle line:84% is that at seawater pH, which is about pH 8, 00:09:57.240 --> 00:10:01.340 align:middle line:84% by far the dominant component of the inorganic carbon 00:10:01.340 --> 00:10:02.980 align:middle line:84% system in seawater is bicarbonate, 00:10:02.980 --> 00:10:07.520 align:middle line:84% which makes up about 90% of the inorganic carbon at seawater pH. 00:10:07.520 --> 00:10:10.120 align:middle line:84% The next most abundant is carbonate ion, which 00:10:10.120 --> 00:10:12.080 align:middle line:90% is about 10% of the total. 00:10:12.080 --> 00:10:18.800 align:middle line:84% And CO2 or H2CO3 is another almost factor of 10 less 00:10:18.800 --> 00:10:19.920 align:middle line:90% than that. 00:10:19.920 --> 00:10:23.140 align:middle line:84% But normally, when you see a plot like this, 00:10:23.140 --> 00:10:25.520 align:middle line:84% you won't see it plotted the way I have here, 00:10:25.520 --> 00:10:28.200 align:middle line:84% with pH on the horizontal axis and concentration 00:10:28.200 --> 00:10:29.340 align:middle line:90% on the vertical axis. 00:10:29.340 --> 00:10:33.880 align:middle line:84% Instead, you'll see it plotted like this, where pH, again, 00:10:33.880 --> 00:10:36.280 align:middle line:84% is on the horizontal axis, but the vertical axis 00:10:36.280 --> 00:10:37.340 align:middle line:90% is a log scale. 00:10:37.340 --> 00:10:39.640 align:middle line:84% So it's the log of the concentration. 00:10:39.640 --> 00:10:41.640 align:middle line:84% And that has-- plots like this are 00:10:41.640 --> 00:10:46.970 align:middle line:84% called [INAUDIBLE] an acid which dissociates to a conjugate base. 00:10:46.970 --> 00:10:51.330 align:middle line:84% The distribution of acid and basic species 00:10:51.330 --> 00:10:56.010 align:middle line:84% on a log-log plot as a function pH is called the Bjerrum plot. 00:10:56.010 --> 00:10:59.803 align:middle line:84% And it has one feature which I think is really handy 00:10:59.803 --> 00:11:02.470 align:middle line:84% and which will come up later in the lecture, so I want to point, 00:11:02.470 --> 00:11:03.530 align:middle line:90% it out here. 00:11:03.530 --> 00:11:07.450 align:middle line:84% If you write the expression for the first dissociation 00:11:07.450 --> 00:11:10.210 align:middle line:84% constant of carbonic acid-- so for dissociation 00:11:10.210 --> 00:11:15.970 align:middle line:84% of H2CO3 to HCO3 minus, which I've shown on the bottom-- 00:11:15.970 --> 00:11:20.750 align:middle line:84% and I've lost my arrow here, so I can't point at it, 00:11:20.750 --> 00:11:25.250 align:middle line:84% but it's shown on the bottom at the end of that blue arrow. 00:11:25.250 --> 00:11:28.070 align:middle line:84% You can take the log of both sides of that equation and get 00:11:28.070 --> 00:11:31.970 align:middle line:84% the log of the dissociation constant equals the log of H+, 00:11:31.970 --> 00:11:34.850 align:middle line:84% plus the log of the ratio of bicarbonate to carbonate 00:11:34.850 --> 00:11:37.210 align:middle line:90% to H2CO3 concentrations. 00:11:37.210 --> 00:11:39.930 align:middle line:84% At the point where those two concentrations are equal, 00:11:39.930 --> 00:11:42.130 align:middle line:84% that is my [? big ?] [INAUDIBLE], 00:11:42.130 --> 00:11:45.630 align:middle line:84% which is that point right there, then 00:11:45.630 --> 00:11:48.310 align:middle line:84% it's the log of 1, which is zero. 00:11:48.310 --> 00:11:53.510 align:middle line:84% So at the pH where the acid and basic forms are equal, then 00:11:53.510 --> 00:11:57.790 align:middle line:84% the pH is equal to the negative log of the dissociation 00:11:57.790 --> 00:11:59.210 align:middle line:90% constant of the acid. 00:11:59.210 --> 00:12:03.870 align:middle line:84% So the pH equals the pKa at the point where the concentrations 00:12:03.870 --> 00:12:05.767 align:middle line:90% intersect on this Bjerrum plot. 00:12:05.767 --> 00:12:07.350 align:middle line:84% That's an important thing to remember, 00:12:07.350 --> 00:12:09.390 align:middle line:90% which will come up later. 00:12:09.390 --> 00:12:10.770 align:middle line:90% So remember that. 00:12:10.770 --> 00:12:14.690 align:middle line:90% 00:12:14.690 --> 00:12:18.515 align:middle line:90% [? Good. ?] You hold that. 00:12:18.515 --> 00:12:20.390 align:middle line:84% So we're going to do one more thing before we 00:12:20.390 --> 00:12:23.190 align:middle line:90% get into the nitty-gritty here. 00:12:23.190 --> 00:12:26.470 align:middle line:84% We might ask the question if [INAUDIBLE] CO2 and pH 00:12:26.470 --> 00:12:29.670 align:middle line:84% are the ideal things to measure in seawater in order 00:12:29.670 --> 00:12:32.230 align:middle line:84% to determine the carbonate speciation 00:12:32.230 --> 00:12:34.150 align:middle line:90% between its inorganic forms. 00:12:34.150 --> 00:12:37.470 align:middle line:84% And the way we'll look at that here 00:12:37.470 --> 00:12:39.830 align:middle line:84% is to do a hypothetical experiment in which we 00:12:39.830 --> 00:12:44.240 align:middle line:84% mix one water mass, which has this distinct pH and total CO2, 00:12:44.240 --> 00:12:45.720 align:middle line:84% with another water mass, which also 00:12:45.720 --> 00:12:47.880 align:middle line:90% has a known pH and total CO2. 00:12:47.880 --> 00:12:50.280 align:middle line:84% We make a 50/50 mixture of those. 00:12:50.280 --> 00:12:53.800 align:middle line:84% Then the total CO2 of the mixture 00:12:53.800 --> 00:12:56.760 align:middle line:84% will be the weighted average of the total CO2s 00:12:56.760 --> 00:12:59.560 align:middle line:84% of the individual water masses of pH. 00:12:59.560 --> 00:13:01.880 align:middle line:90% PH does not mix conservatively. 00:13:01.880 --> 00:13:04.760 align:middle line:84% So if we want to know the pH of the mixture, 00:13:04.760 --> 00:13:07.800 align:middle line:84% we're going to need to know something else about those water 00:13:07.800 --> 00:13:08.800 align:middle line:90% masses. 00:13:08.800 --> 00:13:15.080 align:middle line:84% So the question is, what other thing might we want to know? 00:13:15.080 --> 00:13:19.120 align:middle line:84% And something that I'm sure you talked about 00:13:19.120 --> 00:13:22.250 align:middle line:84% and you know already is that salinity mixes conservatively. 00:13:22.250 --> 00:13:24.000 align:middle line:84% So what if the other thing we want to know 00:13:24.000 --> 00:13:27.960 align:middle line:84% is actually related to the major ion composition of the oceans? 00:13:27.960 --> 00:13:30.320 align:middle line:84% That will mean that it will mix conservatively too. 00:13:30.320 --> 00:13:35.040 align:middle line:84% So if we look at a quantity that's 00:13:35.040 --> 00:13:38.000 align:middle line:84% related to the major ion composition of seawater. 00:13:38.000 --> 00:13:42.970 align:middle line:84% And what I've shown here is a table from the Zeebe book 00:13:42.970 --> 00:13:44.530 align:middle line:84% that I put in the reading list that I 00:13:44.530 --> 00:13:47.630 align:middle line:84% recommend for everybody because it's very clearly written. 00:13:47.630 --> 00:13:50.310 align:middle line:84% And it's a great book, and for the chemistry students, 00:13:50.310 --> 00:13:52.290 align:middle line:84% it's something you might consider getting. 00:13:52.290 --> 00:13:53.990 align:middle line:84% It's not overwhelmingly expensive, 00:13:53.990 --> 00:13:56.543 align:middle line:90% and it's a very valuable book. 00:13:56.543 --> 00:13:58.210 align:middle line:84% I wouldn't understand this stuff as well 00:13:58.210 --> 00:14:01.690 align:middle line:84% if I didn't read that book [INAUDIBLE] selection. 00:14:01.690 --> 00:14:04.730 align:middle line:84% So let's look at the charge balance in seawater. 00:14:04.730 --> 00:14:08.470 align:middle line:84% And what this table shows is on the left-hand side, 00:14:08.470 --> 00:14:11.810 align:middle line:84% the major cations in seawater, the concentration, 00:14:11.810 --> 00:14:17.590 align:middle line:84% and the total charge, which is the charge on the ion 00:14:17.590 --> 00:14:18.710 align:middle line:90% times its concentration. 00:14:18.710 --> 00:14:21.650 align:middle line:84% So the total charge attributed to sodium ions 00:14:21.650 --> 00:14:23.550 align:middle line:84% is equal to the total concentration. 00:14:23.550 --> 00:14:26.670 align:middle line:84% Since it's ionic charge is 1 for magnesium, 00:14:26.670 --> 00:14:30.370 align:middle line:84% the total charge is twice its concentration, and so on. 00:14:30.370 --> 00:14:34.630 align:middle line:84% So if we add all those up, we get the total positive-- 00:14:34.630 --> 00:14:38.210 align:middle line:84% the sum of positive charges attributable to the major ions 00:14:38.210 --> 00:14:39.250 align:middle line:90% in seawater. 00:14:39.250 --> 00:14:43.660 align:middle line:84% On the left-hand side are shown at cations, 00:14:43.660 --> 00:14:46.020 align:middle line:84% and the same calculation for them. 00:14:46.020 --> 00:14:47.860 align:middle line:90% And for this group of anions. 00:14:47.860 --> 00:14:49.900 align:middle line:84% We add up the total charge, and we 00:14:49.900 --> 00:14:53.360 align:middle line:84% get an amount that's about 2.2 millimoles per kilogram 00:14:53.360 --> 00:14:57.860 align:middle line:84% less than the total charge of cations in seawater. 00:14:57.860 --> 00:15:02.260 align:middle line:84% So is seawater positively charged? 00:15:02.260 --> 00:15:03.300 align:middle line:90% No. 00:15:03.300 --> 00:15:05.100 align:middle line:84% So obviously, I've left something out, 00:15:05.100 --> 00:15:07.440 align:middle line:84% or he has left something out of this table. 00:15:07.440 --> 00:15:11.860 align:middle line:84% Could you tell me what's left out of the table? 00:15:11.860 --> 00:15:13.560 align:middle line:84% Bicarbonate, he's left out of the table. 00:15:13.560 --> 00:15:14.480 align:middle line:90% That's the major one. 00:15:14.480 --> 00:15:16.897 align:middle line:84% There are a couple other things that are left out as well. 00:15:16.897 --> 00:15:19.100 align:middle line:90% What else is left out? 00:15:19.100 --> 00:15:21.000 align:middle line:90% OK, I'll tell you. 00:15:21.000 --> 00:15:23.180 align:middle line:84% Carbonate ion is left out as well. 00:15:23.180 --> 00:15:26.540 align:middle line:84% And borate-- the borate ion is also left out. 00:15:26.540 --> 00:15:30.260 align:middle line:84% What do these things have in common? 00:15:30.260 --> 00:15:33.260 align:middle line:84% Those missing anions-- bicarbonate, 00:15:33.260 --> 00:15:34.980 align:middle line:90% carbonate and borate-- 00:15:34.980 --> 00:15:39.300 align:middle line:84% are all the conjugate bases of weak acids. 00:15:39.300 --> 00:15:44.630 align:middle line:84% So bicarbonate and carbonate ion are conjugate bases of H2CO3 00:15:44.630 --> 00:15:46.550 align:middle line:90% and HCO3-. 00:15:46.550 --> 00:15:49.030 align:middle line:84% And the borate ion is the conjugate base 00:15:49.030 --> 00:15:51.470 align:middle line:90% of the weak acid, boric acid. 00:15:51.470 --> 00:15:53.530 align:middle line:84% And why do these get special treatment? 00:15:53.530 --> 00:15:55.710 align:middle line:84% Well, they get special treatment because they serve 00:15:55.710 --> 00:15:58.190 align:middle line:90% to buffer the pH of seawater. 00:15:58.190 --> 00:16:02.910 align:middle line:84% So if you add, say, hydrogen ion to seawater, then part of it 00:16:02.910 --> 00:16:06.950 align:middle line:84% will be used to convert carbonate and bicarbonate 00:16:06.950 --> 00:16:10.930 align:middle line:84% to carbonic acid, and to convert borate to boric acid. 00:16:10.930 --> 00:16:13.190 align:middle line:84% So the pH change in seawater resulting 00:16:13.190 --> 00:16:14.790 align:middle line:84% from your addition of hydrogen ion 00:16:14.790 --> 00:16:16.470 align:middle line:84% will be less than it would have been 00:16:16.470 --> 00:16:21.990 align:middle line:84% if these conjugate bases of weak acids were present in seawater. 00:16:21.990 --> 00:16:24.367 align:middle line:84% So this group of anions in seawater 00:16:24.367 --> 00:16:25.950 align:middle line:84% gets special treatment because they're 00:16:25.950 --> 00:16:29.510 align:middle line:84% very important to determining the pH of the [INAUDIBLE]. 00:16:29.510 --> 00:16:32.510 align:middle line:90% 00:16:32.510 --> 00:16:35.890 align:middle line:84% This collection of [INAUDIBLE], and a few others, 00:16:35.890 --> 00:16:39.050 align:middle line:84% which we'll talk about later, but these are the main ones, 00:16:39.050 --> 00:16:42.310 align:middle line:84% contribute to what we call the alkalinity of seawater. 00:16:42.310 --> 00:16:45.170 align:middle line:84% And the alkalinity is defined in terms 00:16:45.170 --> 00:16:48.570 align:middle line:84% of the acid-neutralizing capacity of seawater. 00:16:48.570 --> 00:16:51.970 align:middle line:84% And to read the definition, it's alkalinity 00:16:51.970 --> 00:16:55.730 align:middle line:84% and moles per kilogram is equal to the amount of strong acid 00:16:55.730 --> 00:16:58.730 align:middle line:84% that must be added to 1 kilogram of seawater 00:16:58.730 --> 00:17:02.530 align:middle line:84% to make its pH equal to that of the second equivalence point 00:17:02.530 --> 00:17:04.490 align:middle line:90% of the CO2 system. 00:17:04.490 --> 00:17:07.170 align:middle line:90% And that pH is about 4.3. 00:17:07.170 --> 00:17:10.369 align:middle line:84% So we're going to talk about just what the heck that means 00:17:10.369 --> 00:17:13.770 align:middle line:84% and why this is the definition now. 00:17:13.770 --> 00:17:18.210 align:middle line:84% But we're going to start first by talking a little bit more 00:17:18.210 --> 00:17:21.450 align:middle line:84% about the components that make up the alkalinity of seawater 00:17:21.450 --> 00:17:22.810 align:middle line:90% at pH 8. 00:17:22.810 --> 00:17:26.410 align:middle line:84% And if you look in the Zeebe book, it's a term that-- 00:17:26.410 --> 00:17:27.522 align:middle line:90% I think is unique to him. 00:17:27.522 --> 00:17:29.730 align:middle line:84% But it's a useful thing to remember even if you don't 00:17:29.730 --> 00:17:30.830 align:middle line:90% remember what he calls it. 00:17:30.830 --> 00:17:32.890 align:middle line:84% He calls it the practical alkalinity. 00:17:32.890 --> 00:17:36.700 align:middle line:84% And that's the alkalinity that you can use for most 00:17:36.700 --> 00:17:39.020 align:middle line:84% calculations in the oceans, again, 00:17:39.020 --> 00:17:43.980 align:middle line:84% equal to the concentration of bicarbonate plus twice carbonate 00:17:43.980 --> 00:17:51.020 align:middle line:84% ion plus borate plus hydroxide minus H+. 00:17:51.020 --> 00:17:54.260 align:middle line:84% One question you might ask is, why is it carbonate ion 00:17:54.260 --> 00:17:56.600 align:middle line:90% multiplied times 2? 00:17:56.600 --> 00:17:59.260 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:17:59.260 --> 00:18:01.300 align:middle line:84% WILLIAM MARTIN: It's not that the charge is 2. 00:18:01.300 --> 00:18:03.600 align:middle line:84% We'll talk about that a little bit later. 00:18:03.600 --> 00:18:04.840 align:middle line:90% I'll show that explicitly. 00:18:04.840 --> 00:18:06.180 align:middle line:90% But it's because-- 00:18:06.180 --> 00:18:07.380 align:middle line:90% AUDIENCE: There are two. 00:18:07.380 --> 00:18:08.880 align:middle line:84% WILLIAM MARTIN: Because it takes you 00:18:08.880 --> 00:18:12.780 align:middle line:84% have to add two protons to it to convert it to H2CO3. 00:18:12.780 --> 00:18:16.940 align:middle line:90% 00:18:16.940 --> 00:18:19.820 align:middle line:84% And you can see from this what should have been obvious 00:18:19.820 --> 00:18:21.982 align:middle line:84% from those Bjerrum plots that I showed before, 00:18:21.982 --> 00:18:23.940 align:middle line:84% and that I'm going to keep showing until you're 00:18:23.940 --> 00:18:26.740 align:middle line:84% sick of looking at them, is that bicarbonate 00:18:26.740 --> 00:18:32.480 align:middle line:84% is the predominant component of alkalinity at pH 8 in seawater. 00:18:32.480 --> 00:18:34.660 align:middle line:90% And the next is carbonate. 00:18:34.660 --> 00:18:36.210 align:middle line:90% And following that is borate. 00:18:36.210 --> 00:18:40.510 align:middle line:84% And then the water alkalinity contributes a very small amount. 00:18:40.510 --> 00:18:43.550 align:middle line:84% But we'll go back to just what the definition of alkalinity 00:18:43.550 --> 00:18:44.050 align:middle line:90% means. 00:18:44.050 --> 00:18:46.258 align:middle line:84% Remember, it's defined in terms of the amount of acid 00:18:46.258 --> 00:18:53.070 align:middle line:84% that you have to add to reduce the pH of a seawater sample 00:18:53.070 --> 00:18:56.910 align:middle line:90% from its in situ pH to 4.3. 00:18:56.910 --> 00:18:59.510 align:middle line:90% So why those numbers? 00:18:59.510 --> 00:19:01.950 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:19:01.950 --> 00:19:04.790 align:middle line:90% WILLIAM MARTIN: All right. 00:19:04.790 --> 00:19:06.918 align:middle line:84% I'm not actually sure why I stuck this in here, 00:19:06.918 --> 00:19:08.710 align:middle line:84% but it's the same Bjerrum plot I showed you 00:19:08.710 --> 00:19:11.590 align:middle line:84% before with the exception that I've added borate to it to, 00:19:11.590 --> 00:19:14.830 align:middle line:84% and I've shown pH 8, which is where 00:19:14.830 --> 00:19:18.390 align:middle line:84% your seawater sample starts before you start adding acid 00:19:18.390 --> 00:19:19.230 align:middle line:90% to it. 00:19:19.230 --> 00:19:25.030 align:middle line:84% And you can see from looking at this plot qualitatively what's 00:19:25.030 --> 00:19:26.530 align:middle line:90% going to happen as you add acid. 00:19:26.530 --> 00:19:28.830 align:middle line:84% You move to the left on this diagram. 00:19:28.830 --> 00:19:33.070 align:middle line:84% And so carbonate and bicarbonate are converted to H2CO3, 00:19:33.070 --> 00:19:37.080 align:middle line:84% and borate is converted to boric acid. 00:19:37.080 --> 00:19:40.840 align:middle line:84% We'll look in a little more detail now. 00:19:40.840 --> 00:19:46.420 align:middle line:84% This is what we call a titration curve for seawater. 00:19:46.420 --> 00:19:50.020 align:middle line:84% What that means is it's a plot of-- 00:19:50.020 --> 00:19:51.520 align:middle line:84% well, when you do a titration, you'd 00:19:51.520 --> 00:19:55.000 align:middle line:84% have a beaker of water with a pH electrode to an acid base 00:19:55.000 --> 00:19:57.300 align:middle line:84% titration with a pH electrode stuck in it. 00:19:57.300 --> 00:19:59.520 align:middle line:84% You add acid, and you measure pH. 00:19:59.520 --> 00:20:02.600 align:middle line:84% Then you add more acid, and you measure pH again. 00:20:02.600 --> 00:20:06.480 align:middle line:84% And as you do that from its starting point 00:20:06.480 --> 00:20:09.620 align:middle line:84% until the pH gets very low, you get a titration curve, 00:20:09.620 --> 00:20:13.000 align:middle line:84% which is just a plot of pH as a function of the amount of acid 00:20:13.000 --> 00:20:14.320 align:middle line:90% you've added. 00:20:14.320 --> 00:20:19.520 align:middle line:84% And Scott may have noticed that this plot is a little strange. 00:20:19.520 --> 00:20:22.620 align:middle line:84% And it's a little strange because when I made it, 00:20:22.620 --> 00:20:25.620 align:middle line:84% I wanted to be confident that I was doing it right. 00:20:25.620 --> 00:20:28.860 align:middle line:84% So I did it under conditions that I normally use to titrate. 00:20:28.860 --> 00:20:30.460 align:middle line:90% I work in sediment-poor waters. 00:20:30.460 --> 00:20:31.700 align:middle line:90% So we have tiny samples. 00:20:31.700 --> 00:20:34.210 align:middle line:84% So this is actually the titration of a 1 and 1/2 mL 00:20:34.210 --> 00:20:38.210 align:middle line:84% sample of seawater with 1 millimolar HCl. 00:20:38.210 --> 00:20:41.070 align:middle line:84% If you talk to a real ocean chemist, 00:20:41.070 --> 00:20:44.090 align:middle line:84% they don't use 1 and 1/2 mLs of seawater [? to do ?] this. 00:20:44.090 --> 00:20:45.810 align:middle line:90% They use more like 250 mLs. 00:20:45.810 --> 00:20:47.310 align:middle line:84% So the numbers would look different, 00:20:47.310 --> 00:20:50.910 align:middle line:84% but the curve would look the same. 00:20:50.910 --> 00:20:54.210 align:middle line:84% So what we want to talk about is the shape of this curve. 00:20:54.210 --> 00:20:56.890 align:middle line:84% And you can see that when you start at pH 8, 00:20:56.890 --> 00:21:00.930 align:middle line:84% there's a steep part of the curve where the pH decreases 00:21:00.930 --> 00:21:02.390 align:middle line:90% pretty fast as you add acid. 00:21:02.390 --> 00:21:04.490 align:middle line:84% Then there's a pretty long, quite flat part 00:21:04.490 --> 00:21:07.410 align:middle line:84% of the curve where the pH decreases only slowly as you 00:21:07.410 --> 00:21:08.650 align:middle line:90% add the acid. 00:21:08.650 --> 00:21:13.970 align:middle line:84% And then right around pH 4 and 1/2 or so, there's another steep 00:21:13.970 --> 00:21:17.170 align:middle line:84% part of the curve where the pH changes rapidly as you add more 00:21:17.170 --> 00:21:18.370 align:middle line:90% acid. 00:21:18.370 --> 00:21:21.610 align:middle line:84% Now remember, I said the definition of alkalinity 00:21:21.610 --> 00:21:24.250 align:middle line:84% has you titrating seawater from its starting point 00:21:24.250 --> 00:21:26.810 align:middle line:90% down to a pH of 4.3. 00:21:26.810 --> 00:21:30.170 align:middle line:84% What's special about this pH of 4.3? 00:21:30.170 --> 00:21:33.510 align:middle line:84% So to look at that, here in blue it's 00:21:33.510 --> 00:21:36.470 align:middle line:84% the same titration curve I've shown you already. 00:21:36.470 --> 00:21:40.750 align:middle line:84% And in red, I plotted the slope of the titration curve. 00:21:40.750 --> 00:21:43.990 align:middle line:84% And I should tell you, I left out-- 00:21:43.990 --> 00:21:45.930 align:middle line:84% it plotted against an invisible axis, 00:21:45.930 --> 00:21:48.070 align:middle line:84% so I didn't want the plot to be too cluttered. 00:21:48.070 --> 00:21:49.550 align:middle line:90% So that's not important. 00:21:49.550 --> 00:21:52.910 align:middle line:84% But just remember, it's not plotted against this pH axis 00:21:52.910 --> 00:21:53.630 align:middle line:90% here. 00:21:53.630 --> 00:21:56.350 align:middle line:84% The important thing is that now, if you 00:21:56.350 --> 00:22:02.110 align:middle line:84% look at this steep part of the curve between pH 5 and pH 4, 00:22:02.110 --> 00:22:05.530 align:middle line:90% you see there's a very sharp-- 00:22:05.530 --> 00:22:11.070 align:middle line:84% the slope of the curve of pH versus acid 00:22:11.070 --> 00:22:16.990 align:middle line:84% added gets steeper and steeper until it reaches a point where 00:22:16.990 --> 00:22:19.850 align:middle line:84% the slope of the curve is at a minimum, 00:22:19.850 --> 00:22:21.690 align:middle line:90% and then it starts to increase. 00:22:21.690 --> 00:22:24.510 align:middle line:84% That point is called an inflection point. 00:22:24.510 --> 00:22:27.610 align:middle line:84% And it's readily determined in a titration. 00:22:27.610 --> 00:22:30.110 align:middle line:84% So this is a point that you can determine readily 00:22:30.110 --> 00:22:32.360 align:middle line:90% and reproducibly in a titration. 00:22:32.360 --> 00:22:34.600 align:middle line:84% It's the point in the titration curve 00:22:34.600 --> 00:22:36.480 align:middle line:84% where there's an inflection point, which 00:22:36.480 --> 00:22:40.180 align:middle line:84% is where the slope goes from decreasing to increasing. 00:22:40.180 --> 00:22:45.800 align:middle line:84% So now we've shown that it's a readily identifiable spot 00:22:45.800 --> 00:22:47.020 align:middle line:90% in the titration curve. 00:22:47.020 --> 00:22:48.360 align:middle line:90% So that makes it useful. 00:22:48.360 --> 00:22:49.980 align:middle line:84% And we've equated it to alkalinity. 00:22:49.980 --> 00:22:54.360 align:middle line:84% But really, we should say why that point in the titration 00:22:54.360 --> 00:22:58.060 align:middle line:84% curve is equal to the alkalinity as we've defined it in seawater. 00:22:58.060 --> 00:23:01.200 align:middle line:90% And so we'll do one more thing. 00:23:01.200 --> 00:23:03.880 align:middle line:84% Again, here's that same brand Bjerrum pot again, 00:23:03.880 --> 00:23:08.880 align:middle line:84% which shows the speciation of inorganic carbon and borate 00:23:08.880 --> 00:23:11.400 align:middle line:84% and boric acid systems as a function of pH. 00:23:11.400 --> 00:23:14.880 align:middle line:84% And to its left, I've defined the alkalinity 00:23:14.880 --> 00:23:17.580 align:middle line:84% in the seawater sample, which is equal to-- 00:23:17.580 --> 00:23:20.840 align:middle line:84% I'm going to say it, at the risk of boring you just 00:23:20.840 --> 00:23:22.120 align:middle line:90% to make the point clear. 00:23:22.120 --> 00:23:25.400 align:middle line:84% It's equal to the sum of carbonate plus twice bicarbonate 00:23:25.400 --> 00:23:30.090 align:middle line:84% plus borate plus hydroxide minus H+. 00:23:30.090 --> 00:23:32.970 align:middle line:84% Now, on the bottom plot, what I've done 00:23:32.970 --> 00:23:38.730 align:middle line:84% is I've added up bicarbonate plus twice carbonate plus borate 00:23:38.730 --> 00:23:41.270 align:middle line:84% plus hydroxide as a function of pH 00:23:41.270 --> 00:23:43.910 align:middle line:90% just from this speciation curve. 00:23:43.910 --> 00:23:46.970 align:middle line:84% And that would be the sum of those components 00:23:46.970 --> 00:23:50.670 align:middle line:84% as you add acid to your sample and the pH decreases. 00:23:50.670 --> 00:23:55.970 align:middle line:84% And I've also plotted the hydrogen ion concentration, 00:23:55.970 --> 00:23:57.470 align:middle line:90% again as a function of pH. 00:23:57.470 --> 00:23:58.970 align:middle line:90% So you start at pH 8. 00:23:58.970 --> 00:24:01.970 align:middle line:84% The hydrogen concentration is 10 to the minus 8. 00:24:01.970 --> 00:24:08.370 align:middle line:84% And as you add acid, obviously the H+ decreases. 00:24:08.370 --> 00:24:12.730 align:middle line:84% And well, on a log-log plot, H+ versus pH is just a-- has 00:24:12.730 --> 00:24:15.750 align:middle line:90% a slope of minus 1. 00:24:15.750 --> 00:24:19.410 align:middle line:84% But the important point here is that eventually, 00:24:19.410 --> 00:24:23.850 align:middle line:84% as you titrate your seawater sample, you get to one spot 00:24:23.850 --> 00:24:29.220 align:middle line:84% where the sum of twice carbonate plus bicarbonate plus borate 00:24:29.220 --> 00:24:34.060 align:middle line:84% plus OH- equals the concentration of hydrogen ion. 00:24:34.060 --> 00:24:37.020 align:middle line:84% And it turns out that point is exactly at the inflection 00:24:37.020 --> 00:24:38.780 align:middle line:90% point in the titration curve. 00:24:38.780 --> 00:24:40.780 align:middle line:90% And so what is that? 00:24:40.780 --> 00:24:43.660 align:middle line:84% If you look again at the expression for total alkalinity, 00:24:43.660 --> 00:24:47.180 align:middle line:84% when the sum of these things equals 00:24:47.180 --> 00:24:52.260 align:middle line:84% the hydrogen-ion concentration, then the alkalinity is zero. 00:24:52.260 --> 00:24:54.300 align:middle line:84% So at that point in your titration, 00:24:54.300 --> 00:24:58.180 align:middle line:84% you've added enough acid to make the alkalinity zero. 00:24:58.180 --> 00:24:59.700 align:middle line:84% So the amount of acid you've added 00:24:59.700 --> 00:25:02.700 align:middle line:84% is just equal to the alkalinity of your water sample. 00:25:02.700 --> 00:25:07.980 align:middle line:84% So that shows you why this particular point 00:25:07.980 --> 00:25:12.780 align:middle line:84% in the titration curve is chosen to define the alkalinity. 00:25:12.780 --> 00:25:15.700 align:middle line:84% And it also shows that the quantity that you determine 00:25:15.700 --> 00:25:20.380 align:middle line:84% this way is equal to that net sum 00:25:20.380 --> 00:25:24.020 align:middle line:84% of conjugate bases of weak acids, which contribute 00:25:24.020 --> 00:25:27.510 align:middle line:90% to buffering seawater. 00:25:27.510 --> 00:25:31.170 align:middle line:84% So that's what the alkalinity is and why it's useful to us. 00:25:31.170 --> 00:25:34.310 align:middle line:90% 00:25:34.310 --> 00:25:37.910 align:middle line:84% But never to leave a simple thing simple enough 00:25:37.910 --> 00:25:39.710 align:middle line:90% to understand-- 00:25:39.710 --> 00:25:42.050 align:middle line:84% and there's actually a reason for doing this. 00:25:42.050 --> 00:25:44.410 align:middle line:90% I regret this choice of words. 00:25:44.410 --> 00:25:46.590 align:middle line:84% It's not a more precise definition 00:25:46.590 --> 00:25:48.230 align:middle line:84% than the one I've just shown you. 00:25:48.230 --> 00:25:50.510 align:middle line:84% It's a different way of saying the same thing. 00:25:50.510 --> 00:25:54.890 align:middle line:84% And the reason for doing it this way is that seawater, of course, 00:25:54.890 --> 00:25:58.750 align:middle line:84% isn't composed-- the only weak acids in seawater 00:25:58.750 --> 00:26:01.590 align:middle line:84% are carbonic acid and boric acid. 00:26:01.590 --> 00:26:04.270 align:middle line:84% There are a number of minor components, which also 00:26:04.270 --> 00:26:06.030 align:middle line:90% contribute to the alkalinity. 00:26:06.030 --> 00:26:07.580 align:middle line:84% And although for most calculations, 00:26:07.580 --> 00:26:09.330 align:middle line:84% you don't have to worry about it too much, 00:26:09.330 --> 00:26:11.710 align:middle line:84% there are some circumstances in which you need to. 00:26:11.710 --> 00:26:14.330 align:middle line:84% And examples of these are phosphoric acid. 00:26:14.330 --> 00:26:18.510 align:middle line:84% You know that phosphate is present in seawater. 00:26:18.510 --> 00:26:21.110 align:middle line:90% Ammonia is present in seawater. 00:26:21.110 --> 00:26:23.250 align:middle line:84% Silicic acid is present in seawater. 00:26:23.250 --> 00:26:24.870 align:middle line:84% And under some circumstances, these 00:26:24.870 --> 00:26:27.730 align:middle line:84% could contribute in a significant way 00:26:27.730 --> 00:26:29.210 align:middle line:90% to the alkalinity. 00:26:29.210 --> 00:26:30.790 align:middle line:84% Remember, when you do your titration, 00:26:30.790 --> 00:26:33.490 align:middle line:84% you're titrating all the conjugate bases. 00:26:33.490 --> 00:26:37.512 align:middle line:84% And so if you want to interpret it accurately, 00:26:37.512 --> 00:26:39.970 align:middle line:84% you should take into account these minor components as well 00:26:39.970 --> 00:26:42.050 align:middle line:84% as the major ones I've discussed already. 00:26:42.050 --> 00:26:43.890 align:middle line:84% And what this definition does is give you 00:26:43.890 --> 00:26:47.570 align:middle line:84% a really simple way of figuring out 00:26:47.570 --> 00:26:50.210 align:middle line:84% which components of your water sample 00:26:50.210 --> 00:26:55.550 align:middle line:84% you should include in the alkalinity definition. 00:26:55.550 --> 00:26:58.067 align:middle line:84% It's dangerous to read this, [? because it's ?] confusing. 00:26:58.067 --> 00:26:59.650 align:middle line:84% But I'll read it anyway, and then I'll 00:26:59.650 --> 00:27:01.970 align:middle line:84% say how to use it, which is simple. 00:27:01.970 --> 00:27:05.290 align:middle line:84% So total alkalinity of a natural water is thus-- 00:27:05.290 --> 00:27:09.090 align:middle line:84% and I guess Dixon alone knows what this "thus" means-- 00:27:09.090 --> 00:27:12.810 align:middle line:84% is defined as the number of moles of hydrogen ion equivalent 00:27:12.810 --> 00:27:14.550 align:middle line:84% to the excess of proton acceptors-- 00:27:14.550 --> 00:27:16.330 align:middle line:90% and here's the important part-- 00:27:16.330 --> 00:27:19.530 align:middle line:84% bases formed from weak acids with the dissociation constant 00:27:19.530 --> 00:27:21.710 align:middle line:90% less than 10 to the minus 4.5. 00:27:21.710 --> 00:27:26.720 align:middle line:84% So a pKa larger than 4.5 over proton [? donors, ?] 00:27:26.720 --> 00:27:30.780 align:middle line:84% acids with K greater than 10 to the minus 4.5, 00:27:30.780 --> 00:27:36.420 align:middle line:84% or a pKa less than 4.5 [INAUDIBLE] 00:27:36.420 --> 00:27:37.940 align:middle line:90% in 1 kilogram of sample. 00:27:37.940 --> 00:27:41.540 align:middle line:84% Remember that I pointed out that on these Bjerrum plots, which 00:27:41.540 --> 00:27:46.860 align:middle line:84% are log of concentration versus log versus pH, that the point-- 00:27:46.860 --> 00:27:49.120 align:middle line:84% look at CO2 and bicarbonate here. 00:27:49.120 --> 00:27:51.500 align:middle line:84% Remember I pointed out before, that at the point where 00:27:51.500 --> 00:27:55.300 align:middle line:84% these two intersect, the pH is equal to the PK. 00:27:55.300 --> 00:28:01.120 align:middle line:84% So given this definition up here, and a Bjerrum plot, 00:28:01.120 --> 00:28:03.940 align:middle line:84% you can say for sure that bicarbonate contributes 00:28:03.940 --> 00:28:05.820 align:middle line:84% to the alkalinity of the seawater 00:28:05.820 --> 00:28:09.860 align:middle line:84% because its PK is clearly greater than 4.5, 00:28:09.860 --> 00:28:11.800 align:middle line:90% which is that dotted line there. 00:28:11.800 --> 00:28:18.180 align:middle line:84% And you can say the same thing for borate and boric acid. 00:28:18.180 --> 00:28:20.700 align:middle line:90% The PK is whatever it is-- 00:28:20.700 --> 00:28:24.110 align:middle line:90% 9.5. 00:28:24.110 --> 00:28:26.550 align:middle line:90% Let's see. 00:28:26.550 --> 00:28:27.970 align:middle line:90% Those are the major components. 00:28:27.970 --> 00:28:29.930 align:middle line:84% Well, sulfate is also a major component. 00:28:29.930 --> 00:28:34.410 align:middle line:84% But since its intersection is off the plot here, 00:28:34.410 --> 00:28:35.870 align:middle line:84% we won't talk about that, but we'll 00:28:35.870 --> 00:28:37.012 align:middle line:90% talk about a similar thing. 00:28:37.012 --> 00:28:38.470 align:middle line:84% If you look at the minor components 00:28:38.470 --> 00:28:40.550 align:middle line:84% in the part of the plot that's below, 00:28:40.550 --> 00:28:44.390 align:middle line:84% look at the top one-- fluoride and HF. 00:28:44.390 --> 00:28:46.530 align:middle line:84% Hydrofluoric acid is a strong acid. 00:28:46.530 --> 00:28:48.090 align:middle line:90% Its pKa is what? 00:28:48.090 --> 00:28:51.090 align:middle line:90% 2.5, which is greater than 4.5. 00:28:51.090 --> 00:28:53.070 align:middle line:84% So that means that in this definition, 00:28:53.070 --> 00:28:56.270 align:middle line:84% HF is a proton donor, which means that it appears 00:28:56.270 --> 00:29:00.550 align:middle line:84% as a negative term, just like H+ in the alkalinity definition, 00:29:00.550 --> 00:29:04.110 align:middle line:84% if you want to include HF and fluoride. 00:29:04.110 --> 00:29:07.530 align:middle line:84% [INAUDIBLE] Phosphoric acid is a triprotic acid. 00:29:07.530 --> 00:29:15.590 align:middle line:84% So it's H3PO4, which dissociates to H2PO4 to H2PO4- to HPO42- 00:29:15.590 --> 00:29:17.470 align:middle line:90% to PO43-. 00:29:17.470 --> 00:29:22.320 align:middle line:84% If you look at phosphate in this plot, and you see that up there? 00:29:22.320 --> 00:29:23.555 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:29:23.555 --> 00:29:26.160 align:middle line:90% 00:29:26.160 --> 00:29:30.440 align:middle line:84% WILLIAM MARTIN: So here's phosphate and the phosphate HPO4 00:29:30.440 --> 00:29:31.280 align:middle line:90% pair. 00:29:31.280 --> 00:29:34.960 align:middle line:90% The PK is whatever it is. 00:29:34.960 --> 00:29:39.040 align:middle line:90% [INAUDIBLE] It's about 9. 00:29:39.040 --> 00:29:41.620 align:middle line:84% And so phosphate does contribute to alkalinity. 00:29:41.620 --> 00:29:44.580 align:middle line:84% And it takes 2 protons to get to 4.5. 00:29:44.580 --> 00:29:47.680 align:middle line:84% So it contributes 2 units to alkalinity. 00:29:47.680 --> 00:29:53.600 align:middle line:84% HPO42-, again, you can see with its acid base pair, 00:29:53.600 --> 00:29:58.120 align:middle line:90% that pKa is about 6. 00:29:58.120 --> 00:30:01.240 align:middle line:84% So HPO42- also contributes to alkalinity. 00:30:01.240 --> 00:30:04.120 align:middle line:84% But you see the dominant component of phosphoric acid 00:30:04.120 --> 00:30:07.720 align:middle line:90% system is HPO4-. 00:30:07.720 --> 00:30:11.180 align:middle line:84% So that's the dominant component at pH 4.5. 00:30:11.180 --> 00:30:14.040 align:middle line:84% So it does not contribute to the alkalinity. 00:30:14.040 --> 00:30:15.640 align:middle line:84% And here, this is an example of why 00:30:15.640 --> 00:30:19.440 align:middle line:84% I said carbonate doesn't contribute 2 units because it 00:30:19.440 --> 00:30:24.440 align:middle line:84% has a charge of minus 2 HPO4- has a charge minus 1, 00:30:24.440 --> 00:30:26.540 align:middle line:84% but it does not contribute to alkalinity 00:30:26.540 --> 00:30:30.260 align:middle line:84% because when you take a seawater sample and add acid to it, 00:30:30.260 --> 00:30:35.060 align:middle line:84% you don't convert H2PO4- to anything. 00:30:35.060 --> 00:30:37.820 align:middle line:84% That's the end point of the titration. 00:30:37.820 --> 00:30:42.440 align:middle line:84% And in fact, if you follow phosphoric acid 00:30:42.440 --> 00:30:49.060 align:middle line:84% one more to H3PO4, you can see that its PK is less than 2. 00:30:49.060 --> 00:30:52.700 align:middle line:84% So it's a strong acid, and it contributes a negative. 00:30:52.700 --> 00:30:57.280 align:middle line:84% H3PO4 appears as minus H3PO4 in the alkalinity equation. 00:30:57.280 --> 00:31:00.220 align:middle line:90% 00:31:00.220 --> 00:31:01.740 align:middle line:90% I'm sure that's all very clear. 00:31:01.740 --> 00:31:04.520 align:middle line:84% But after I've made it complicated, 00:31:04.520 --> 00:31:08.060 align:middle line:84% I'll do one more thing to make it simple again. 00:31:08.060 --> 00:31:09.660 align:middle line:84% There are several different terms 00:31:09.660 --> 00:31:11.560 align:middle line:84% that are used in the alkalinity regimes. 00:31:11.560 --> 00:31:14.980 align:middle line:84% One is carbonate alkalinity-- bicarbonate plus twice 00:31:14.980 --> 00:31:17.580 align:middle line:90% concentration of carbonate ion. 00:31:17.580 --> 00:31:21.910 align:middle line:84% Water alkalinity is OH minus H. And I 00:31:21.910 --> 00:31:25.950 align:middle line:84% should say that sometimes those two are lumped together, 00:31:25.950 --> 00:31:29.070 align:middle line:84% and the sum of them is called carbonate alkalinity just 00:31:29.070 --> 00:31:30.010 align:middle line:90% to be confusing. 00:31:30.010 --> 00:31:31.390 align:middle line:90% I don't know what [INAUDIBLE]. 00:31:31.390 --> 00:31:34.990 align:middle line:84% Borate alkalinity is equal to the concentration of borate ion. 00:31:34.990 --> 00:31:39.030 align:middle line:84% And again, we get back to this practical alkalinity equation, 00:31:39.030 --> 00:31:43.350 align:middle line:84% which is the alkalinity that's due to the carbonate water 00:31:43.350 --> 00:31:44.530 align:middle line:90% and borate systems. 00:31:44.530 --> 00:31:47.350 align:middle line:84% And that's what you can use for most of your calculations 00:31:47.350 --> 00:31:48.950 align:middle line:90% in seawater. 00:31:48.950 --> 00:31:52.070 align:middle line:84% But in some applications, such as the things 00:31:52.070 --> 00:31:54.913 align:middle line:84% that I work on, [INAUDIBLE] sediment-poor waters where 00:31:54.913 --> 00:31:56.330 align:middle line:84% things are a little bit different, 00:31:56.330 --> 00:31:59.110 align:middle line:84% you need to think about the contributions of ammonia, 00:31:59.110 --> 00:32:03.110 align:middle line:84% the phosphoric acid system, sulfide, and silicic acid 00:32:03.110 --> 00:32:03.935 align:middle line:90% to alkalinity. 00:32:03.935 --> 00:32:05.310 align:middle line:84% In that case, you have to go back 00:32:05.310 --> 00:32:07.430 align:middle line:84% to the more complicated plot I showed before 00:32:07.430 --> 00:32:09.330 align:middle line:84% and figure out what you have to include. 00:32:09.330 --> 00:32:15.390 align:middle line:90% 00:32:15.390 --> 00:32:17.230 align:middle line:84% So we've talked about three quantities 00:32:17.230 --> 00:32:20.640 align:middle line:84% that you can measure in a seawater sample-- the total CO2, 00:32:20.640 --> 00:32:23.440 align:middle line:90% alkalinity, and pH. 00:32:23.440 --> 00:32:25.980 align:middle line:84% I'm not going to go into any detail about measurements. 00:32:25.980 --> 00:32:28.460 align:middle line:84% I've tried that before, and it was only confusing. 00:32:28.460 --> 00:32:30.520 align:middle line:90% So we're not going to do that. 00:32:30.520 --> 00:32:34.120 align:middle line:84% But I'll just note that you can measure total CO2-- 00:32:34.120 --> 00:32:35.820 align:middle line:90% well, there's a few ways. 00:32:35.820 --> 00:32:38.370 align:middle line:84% One, you can do a titration and get a titration 00:32:38.370 --> 00:32:39.620 align:middle line:90% curve like the one I've shown. 00:32:39.620 --> 00:32:40.995 align:middle line:84% And you can do some curve fitting 00:32:40.995 --> 00:32:43.000 align:middle line:90% to calculate the total CO2. 00:32:43.000 --> 00:32:46.940 align:middle line:84% The second and most common way is to acidify a sample, 00:32:46.940 --> 00:32:50.280 align:middle line:84% strip the CO2 out, and then quantify the amount of CO2 00:32:50.280 --> 00:32:52.240 align:middle line:84% that you've generated by stripping it 00:32:52.240 --> 00:32:56.120 align:middle line:90% all out of the seawater. 00:32:56.120 --> 00:33:00.680 align:middle line:84% Those of you who go to Tuesday chemistry talks, 00:33:00.680 --> 00:33:03.560 align:middle line:84% you might have seen one a couple weeks ago where 00:33:03.560 --> 00:33:05.480 align:middle line:84% another method was shown in which 00:33:05.480 --> 00:33:07.680 align:middle line:84% you strip the CO2 out of seawater 00:33:07.680 --> 00:33:09.940 align:middle line:84% by equilibrating it with a sodium-- 00:33:09.940 --> 00:33:13.320 align:middle line:84% [INAUDIBLE] gradient with a sodium hydroxide solution. 00:33:13.320 --> 00:33:16.490 align:middle line:84% Then you measure the effect of the CO2 00:33:16.490 --> 00:33:19.090 align:middle line:84% in decreasing the conductivity of the sodium hydroxide 00:33:19.090 --> 00:33:20.130 align:middle line:90% solution. 00:33:20.130 --> 00:33:24.050 align:middle line:84% But the bottom line is that all of these-- 00:33:24.050 --> 00:33:27.370 align:middle line:84% that a good routine measurement of total CO2 00:33:27.370 --> 00:33:30.330 align:middle line:84% gives you a precision of about 2 micromoles per kilogram, which 00:33:30.330 --> 00:33:32.370 align:middle line:90% is about a tenth of a percent. 00:33:32.370 --> 00:33:36.450 align:middle line:84% And the people who are really proud of their seawater work 00:33:36.450 --> 00:33:38.770 align:middle line:90% get to even better than that. 00:33:38.770 --> 00:33:40.830 align:middle line:84% But not just precision is important, 00:33:40.830 --> 00:33:42.970 align:middle line:90% but accuracy is important, too. 00:33:42.970 --> 00:33:50.210 align:middle line:84% And in order to make sure that everyone's measurements can 00:33:50.210 --> 00:33:52.330 align:middle line:84% be looked at on the same scale, so there's 00:33:52.330 --> 00:33:55.530 align:middle line:84% no inter-laboratory differences, Andrew Dixon 00:33:55.530 --> 00:34:00.170 align:middle line:84% has made available large quantities of reference seawater 00:34:00.170 --> 00:34:01.910 align:middle line:90% that's very well-characterized. 00:34:01.910 --> 00:34:04.430 align:middle line:84% And so to calibrate your total CO2 measurements, 00:34:04.430 --> 00:34:08.449 align:middle line:84% you calibrate against reference seawater. 00:34:08.449 --> 00:34:10.850 align:middle line:84% The second thing we've talked about is alkalinity. 00:34:10.850 --> 00:34:13.690 align:middle line:84% And as I've shown, it's measured by titration. 00:34:13.690 --> 00:34:16.500 align:middle line:84% You can measure it by generating a titration curve, 00:34:16.500 --> 00:34:19.520 align:middle line:84% and then doing curve-fitting to calculate the alkalinity. 00:34:19.520 --> 00:34:22.060 align:middle line:84% Or you can do what's called a gram titration, which 00:34:22.060 --> 00:34:24.040 align:middle line:84% if you ever need to do it, you'll learn about, 00:34:24.040 --> 00:34:26.620 align:middle line:84% but I'm not going to talk about it. 00:34:26.620 --> 00:34:29.820 align:middle line:84% And again, the precision is on the order 00:34:29.820 --> 00:34:31.560 align:middle line:90% of 2 micromoles per kilogram. 00:34:31.560 --> 00:34:34.139 align:middle line:84% So again, about a tenth of a percent. 00:34:34.139 --> 00:34:37.780 align:middle line:84% And the calibration is the same referenced seawater 00:34:37.780 --> 00:34:41.420 align:middle line:84% that's used for total CO2 calibrations. 00:34:41.420 --> 00:34:45.420 align:middle line:84% And finally, you could measure the pH. 00:34:45.420 --> 00:34:48.000 align:middle line:84% And that's done either with an electrochemical measurement-- 00:34:48.000 --> 00:34:51.219 align:middle line:84% so that's pH-sensitive glass electrode, 00:34:51.219 --> 00:34:55.540 align:middle line:90% or with pH-sensitive dyes. 00:34:55.540 --> 00:35:00.980 align:middle line:84% And the important-- again, very good pH measurements 00:35:00.980 --> 00:35:05.180 align:middle line:84% have precision of about 0.002 pH units. 00:35:05.180 --> 00:35:09.380 align:middle line:84% But the important thing here is that the calibration 00:35:09.380 --> 00:35:14.740 align:middle line:84% has to be against buffers that are made for use in seawater. 00:35:14.740 --> 00:35:16.360 align:middle line:84% So there are buffers that are made up 00:35:16.360 --> 00:35:18.880 align:middle line:84% in an artificial seawater medium. 00:35:18.880 --> 00:35:21.840 align:middle line:84% And again, it's Dixon who makes these buffers. 00:35:21.840 --> 00:35:27.880 align:middle line:84% [? Just ?] a CO2 system dictator, I guess. 00:35:27.880 --> 00:35:29.520 align:middle line:90% But that's an important point. 00:35:29.520 --> 00:35:33.860 align:middle line:90% 00:35:33.860 --> 00:35:36.360 align:middle line:84% The equal-- well, especially if you're doing electrochemical 00:35:36.360 --> 00:35:41.120 align:middle line:84% measurements, you cannot calibrate versus buffers that 00:35:41.120 --> 00:35:44.688 align:middle line:84% don't have an ionic strength equal to that of seawater. 00:35:44.688 --> 00:35:45.480 align:middle line:90% Just remember that. 00:35:45.480 --> 00:35:46.500 align:middle line:90% It's very important. 00:35:46.500 --> 00:35:48.137 align:middle line:90% AUDIENCE: [INAUDIBLE]. 00:35:48.137 --> 00:35:49.220 align:middle line:90% WILLIAM MARTIN: All right. 00:35:49.220 --> 00:35:52.740 align:middle line:84% Where can we [? find ?] [? it? ?] It's like being 00:35:52.740 --> 00:35:53.240 align:middle line:90% a lawyer. 00:35:53.240 --> 00:35:57.400 align:middle line:84% You should never proceed to a slide 00:35:57.400 --> 00:35:58.820 align:middle line:90% when you're not sure what it is. 00:35:58.820 --> 00:36:02.820 align:middle line:90% 00:36:02.820 --> 00:36:05.240 align:middle line:84% We have what I call the "more than I ever wanted to know" 00:36:05.240 --> 00:36:06.080 align:middle line:90% section. 00:36:06.080 --> 00:36:10.880 align:middle line:90% And in a way, it's-- 00:36:10.880 --> 00:36:14.650 align:middle line:84% well, it's a detail, but its actually an important detail 00:36:14.650 --> 00:36:18.290 align:middle line:84% if you want to use all the measurements correctly. 00:36:18.290 --> 00:36:21.670 align:middle line:84% And the first part of this section is about pH scales. 00:36:21.670 --> 00:36:23.370 align:middle line:90% We talked about this already. 00:36:23.370 --> 00:36:25.130 align:middle line:84% For laboratory chemistry and for work 00:36:25.130 --> 00:36:27.030 align:middle line:84% that you might have done as an undergraduate, 00:36:27.030 --> 00:36:29.570 align:middle line:84% you undoubtedly, whether you knew it or not, 00:36:29.570 --> 00:36:33.090 align:middle line:84% calibrated your pH measurements against low ionic strength 00:36:33.090 --> 00:36:35.850 align:middle line:90% buffers that were calibrated. 00:36:35.850 --> 00:36:38.490 align:middle line:84% Is it still called the NBS pH scale? 00:36:38.490 --> 00:36:39.250 align:middle line:90% Or is it called-- 00:36:39.250 --> 00:36:40.050 align:middle line:90% AUDIENCE: NIST. 00:36:40.050 --> 00:36:41.472 align:middle line:90% WILLIAM MARTIN: NIST pH scale. 00:36:41.472 --> 00:36:42.930 align:middle line:84% And the important thing about these 00:36:42.930 --> 00:36:46.050 align:middle line:84% is they're buffers that are made up in low ionic strength 00:36:46.050 --> 00:36:47.110 align:middle line:90% solutions. 00:36:47.110 --> 00:36:49.530 align:middle line:84% Seawater has a high ionic strength. 00:36:49.530 --> 00:36:54.370 align:middle line:84% And you cannot use these low ionic strength buffers 00:36:54.370 --> 00:36:57.210 align:middle line:84% to calibrate measurements that are made in seawater. 00:36:57.210 --> 00:37:00.250 align:middle line:90% [INAUDIBLE] 00:37:00.250 --> 00:37:03.770 align:middle line:84% So the NBS, or NIST, pH scale has been abandoned 00:37:03.770 --> 00:37:06.090 align:middle line:90% for measurements in seawater. 00:37:06.090 --> 00:37:09.650 align:middle line:84% And if you ever see anybody calibrating their pH electrode, 00:37:09.650 --> 00:37:11.640 align:middle line:90% then this buffers. 00:37:11.640 --> 00:37:13.640 align:middle line:84% When they want to make measurements in seawater, 00:37:13.640 --> 00:37:15.980 align:middle line:90% you can hit them over the head. 00:37:15.980 --> 00:37:17.400 align:middle line:90% Say, no, you can't do that. 00:37:17.400 --> 00:37:19.780 align:middle line:84% And I'll show you the consequence 00:37:19.780 --> 00:37:21.280 align:middle line:90% of that on the next slide here. 00:37:21.280 --> 00:37:23.940 align:middle line:90% 00:37:23.940 --> 00:37:27.860 align:middle line:84% Analyzing hydrogen ion concentration in seawater 00:37:27.860 --> 00:37:29.420 align:middle line:84% actually yields a measurement which 00:37:29.420 --> 00:37:31.460 align:middle line:84% gives you the sum of the free hydrogen 00:37:31.460 --> 00:37:35.300 align:middle line:90% ion plus bisulfate plus HF. 00:37:35.300 --> 00:37:37.740 align:middle line:84% And so if you want to actually know, 00:37:37.740 --> 00:37:41.540 align:middle line:84% for your purposes, the free hydrogen ion concentration, 00:37:41.540 --> 00:37:44.940 align:middle line:84% you have to know the contributions of bisulfate 00:37:44.940 --> 00:37:45.660 align:middle line:90% and HF. 00:37:45.660 --> 00:37:49.260 align:middle line:84% Of course, we know the total sulfate concentration 00:37:49.260 --> 00:37:51.700 align:middle line:84% because it's conservative, and it's a major ion. 00:37:51.700 --> 00:37:54.780 align:middle line:84% In the same way, we know the total fluoride concentration. 00:37:54.780 --> 00:37:57.120 align:middle line:84% But if we want to calculate a species, 00:37:57.120 --> 00:38:02.820 align:middle line:84% we need to know the dissociation constants for HSO4, 00:38:02.820 --> 00:38:06.680 align:middle line:84% or the acidity constant for sulfuric acid 00:38:06.680 --> 00:38:11.150 align:middle line:84% and for hydrofluoric acid very well. 00:38:11.150 --> 00:38:14.830 align:middle line:84% They, who shall remain undefined, 00:38:14.830 --> 00:38:17.150 align:middle line:84% have decided it's better to-- but we can send you 00:38:17.150 --> 00:38:18.830 align:middle line:90% the picture-- 00:38:18.830 --> 00:38:21.110 align:middle line:90% to define a new pH scale. 00:38:21.110 --> 00:38:26.890 align:middle line:84% And in fact, I got tired of this argument, so I haven't kept up. 00:38:26.890 --> 00:38:29.350 align:middle line:84% And I don't know which of these two options 00:38:29.350 --> 00:38:31.670 align:middle line:84% they've settled on, if either one. 00:38:31.670 --> 00:38:33.750 align:middle line:84% The important thing is just to make sure 00:38:33.750 --> 00:38:36.390 align:middle line:84% that you understand if you do work in [INAUDIBLE], 00:38:36.390 --> 00:38:38.750 align:middle line:84% that there are different pH scales. 00:38:38.750 --> 00:38:41.330 align:middle line:84% And that you need to know which one you're using, 00:38:41.330 --> 00:38:45.470 align:middle line:84% and that if you're using H+ measurements on one of these 00:38:45.470 --> 00:38:48.310 align:middle line:84% scales, you need to use equilibrium constants 00:38:48.310 --> 00:38:49.770 align:middle line:90% on the same scale. 00:38:49.770 --> 00:38:51.510 align:middle line:90% That's the important thing. 00:38:51.510 --> 00:38:55.270 align:middle line:84% So the two scales-- one is called the total pH scale. 00:38:55.270 --> 00:38:59.030 align:middle line:84% It includes free hydrogen ion plus bisulfate 00:38:59.030 --> 00:39:02.150 align:middle line:84% and neglects the contribution of HF. 00:39:02.150 --> 00:39:07.190 align:middle line:84% Or the seawater pH scale, which includes free H+ plus bisulfate 00:39:07.190 --> 00:39:08.150 align:middle line:90% plus HF. 00:39:08.150 --> 00:39:10.200 align:middle line:84% There are two different pH scales. 00:39:10.200 --> 00:39:16.680 align:middle line:84% And I've said here that the pH measurements are reported 00:39:16.680 --> 00:39:20.130 align:middle line:84% on the total hydrogen ion scale, which was certainly true 00:39:20.130 --> 00:39:21.380 align:middle line:90% at the time I made this slide. 00:39:21.380 --> 00:39:22.640 align:middle line:90% I'm not so sure anymore. 00:39:22.640 --> 00:39:24.560 align:middle line:84% But there are also equilibrium constants 00:39:24.560 --> 00:39:27.480 align:middle line:84% that are reported on the total hydrogen ion scale. 00:39:27.480 --> 00:39:30.480 align:middle line:84% And the same is true for the seawater scale. 00:39:30.480 --> 00:39:32.780 align:middle line:84% There are pH, seawater scale measurements, 00:39:32.780 --> 00:39:35.720 align:middle line:84% and equilibrium constants that are on the seawater 00:39:35.720 --> 00:39:40.800 align:middle line:90% scale, hydrogen ion scale. 00:39:40.800 --> 00:39:42.400 align:middle line:84% You can convert between the scales 00:39:42.400 --> 00:39:51.720 align:middle line:84% if you know the HSO4 complexation constant and the HF 00:39:51.720 --> 00:39:53.560 align:middle line:90% complexation constant. 00:39:53.560 --> 00:39:55.800 align:middle line:84% And here's the difference between the pH 00:39:55.800 --> 00:39:57.880 align:middle line:84% scales between the seawater scale 00:39:57.880 --> 00:40:02.860 align:middle line:84% and the total hydrogen 9 scale at pH 8 is about 0.01 pH units, 00:40:02.860 --> 00:40:05.240 align:middle line:90% or about 2%. 00:40:05.240 --> 00:40:08.500 align:middle line:84% The difference between the NBS, or NIST, scale, 00:40:08.500 --> 00:40:12.100 align:middle line:84% and the total hydrogen ion scale is about 0.1 pH unit, 00:40:12.100 --> 00:40:16.500 align:middle line:84% or 20% That's why you should never, ever calibrate 00:40:16.500 --> 00:40:18.620 align:middle line:84% your electrode for use in seawater 00:40:18.620 --> 00:40:22.280 align:middle line:84% on the NIST buffer with NIST buffers, 00:40:22.280 --> 00:40:24.340 align:middle line:90% because there's a large error. 00:40:24.340 --> 00:40:30.380 align:middle line:84% And a good pH measurement has a precision of 0.002 pH units. 00:40:30.380 --> 00:40:33.100 align:middle line:84% So these scales are important if you 00:40:33.100 --> 00:40:35.320 align:middle line:84% want to interpret the measurements accurately. 00:40:35.320 --> 00:40:38.420 align:middle line:90% 00:40:38.420 --> 00:40:41.140 align:middle line:84% There's a second important piece of this "more than you ever 00:40:41.140 --> 00:40:44.420 align:middle line:84% wanted to know" section, and it's about equilibrium constants 00:40:44.420 --> 00:40:46.300 align:middle line:90% for calculations in seawater. 00:40:46.300 --> 00:40:51.540 align:middle line:84% And just as an example, I've taken the dissociation 00:40:51.540 --> 00:40:56.340 align:middle line:84% of bicarbonate into H+ plus CO32 minus and written 00:40:56.340 --> 00:41:03.100 align:middle line:84% the equilibrium constant, which is the product of H+ and CO32- 00:41:03.100 --> 00:41:05.220 align:middle line:90% divided by HCO3-. 00:41:05.220 --> 00:41:08.080 align:middle line:90% You all know that. 00:41:08.080 --> 00:41:11.590 align:middle line:84% In an ideal solution, again, this equilibrium constant 00:41:11.590 --> 00:41:15.190 align:middle line:84% is just the product of the concentrations in that solution. 00:41:15.190 --> 00:41:19.410 align:middle line:84% To clarify what that means, in an ideal solution, 00:41:19.410 --> 00:41:23.470 align:middle line:84% the concentrations of all the solutes approach zero. 00:41:23.470 --> 00:41:25.190 align:middle line:84% What that means is that there are 00:41:25.190 --> 00:41:27.950 align:middle line:84% no interactions between the ions that 00:41:27.950 --> 00:41:30.850 align:middle line:84% are in the solution and the water molecules 00:41:30.850 --> 00:41:32.350 align:middle line:84% that they're dissolved in, and there 00:41:32.350 --> 00:41:36.150 align:middle line:84% are no interactions between the different ions in solution. 00:41:36.150 --> 00:41:40.710 align:middle line:84% So that's a hypothetical ideal solution. 00:41:40.710 --> 00:41:45.830 align:middle line:84% But in a real solution, these ion solvent and ion-ion 00:41:45.830 --> 00:41:48.070 align:middle line:90% interactions are important. 00:41:48.070 --> 00:41:51.110 align:middle line:84% And so in a real solution, you would write the [INAUDIBLE] 00:41:51.110 --> 00:41:54.150 align:middle line:84% of every one of these concentration terms 00:41:54.150 --> 00:41:57.310 align:middle line:84% would be multiplied by a corresponding activity 00:41:57.310 --> 00:41:58.550 align:middle line:90% coefficient. 00:41:58.550 --> 00:42:01.830 align:middle line:84% This activity coefficient are generally less than 1 00:42:01.830 --> 00:42:04.720 align:middle line:84% for ions in solution, and they account 00:42:04.720 --> 00:42:08.720 align:middle line:84% for electrostatic interactions between the ions 00:42:08.720 --> 00:42:11.680 align:middle line:84% and between the ions and the solvent. 00:42:11.680 --> 00:42:13.480 align:middle line:84% Have we talked about this at all? 00:42:13.480 --> 00:42:14.080 align:middle line:90% AUDIENCE: No. 00:42:14.080 --> 00:42:14.872 align:middle line:90% WILLIAM MARTIN: OK. 00:42:14.872 --> 00:42:16.120 align:middle line:90% Well, just explain-- 00:42:16.120 --> 00:42:18.720 align:middle line:90% OK, well, just explain-- 00:42:18.720 --> 00:42:21.240 align:middle line:84% I think it's pretty easy to explain why [? this ?] happens. 00:42:21.240 --> 00:42:25.040 align:middle line:84% You know that the water molecules, H2O, and the hydrogen 00:42:25.040 --> 00:42:27.480 align:middle line:84% and the organization of the atoms in the molecule 00:42:27.480 --> 00:42:31.120 align:middle line:84% is such that there's a small positive charge on one 00:42:31.120 --> 00:42:33.560 align:middle line:84% side of the water molecule and a small negative charge 00:42:33.560 --> 00:42:34.380 align:middle line:90% on the other side. 00:42:34.380 --> 00:42:36.260 align:middle line:84% AUDIENCE: We did do h-bond dipole. 00:42:36.260 --> 00:42:37.152 align:middle line:90% WILLIAM MARTIN: OK. 00:42:37.152 --> 00:42:37.860 align:middle line:90% So you know that. 00:42:37.860 --> 00:42:41.420 align:middle line:84% So if you put an ion in solution it, of course, 00:42:41.420 --> 00:42:42.720 align:middle line:90% is the center charge. 00:42:42.720 --> 00:42:46.200 align:middle line:84% And so what happens is the water molecules, since they're polar, 00:42:46.200 --> 00:42:50.040 align:middle line:84% they're dipoles, they are rearranged 00:42:50.040 --> 00:42:52.560 align:middle line:90% around the iodine solution. 00:42:52.560 --> 00:42:55.300 align:middle line:84% So those are ion-solvent interactions. 00:42:55.300 --> 00:42:56.800 align:middle line:84% And they're important in determining 00:42:56.800 --> 00:42:59.840 align:middle line:84% the activity, the chemical potential 00:42:59.840 --> 00:43:02.360 align:middle line:90% of the ions in the solution. 00:43:02.360 --> 00:43:06.310 align:middle line:84% And that obviously if you have two ions in solution, 00:43:06.310 --> 00:43:08.350 align:middle line:84% then they feel each other's electric fields. 00:43:08.350 --> 00:43:10.730 align:middle line:84% They affect each other's behaviors as well. 00:43:10.730 --> 00:43:12.890 align:middle line:84% And so these activity coefficients 00:43:12.890 --> 00:43:16.930 align:middle line:84% are necessary to account for these ion solvent and ion-ion 00:43:16.930 --> 00:43:18.050 align:middle line:90% interactions. 00:43:18.050 --> 00:43:22.370 align:middle line:84% And so the equilibrium constant is 00:43:22.370 --> 00:43:26.730 align:middle line:84% a product of the activities of the ion in solution 00:43:26.730 --> 00:43:28.710 align:middle line:84% rather than their concentrations. 00:43:28.710 --> 00:43:31.930 align:middle line:84% And the activity of an [? iodine ?] solution 00:43:31.930 --> 00:43:37.802 align:middle line:84% is its activity coefficient multiplied by its concentration. 00:43:37.802 --> 00:43:38.490 align:middle line:90% Now, there's-- 00:43:38.490 --> 00:43:42.770 align:middle line:84% AUDIENCE: [INAUDIBLE] determine those activities. 00:43:42.770 --> 00:43:44.330 align:middle line:90% Are they calculated? 00:43:44.330 --> 00:43:46.370 align:middle line:84% WILLIAM MARTIN: They're calculated. 00:43:46.370 --> 00:43:48.930 align:middle line:90% They're calculated on-- 00:43:48.930 --> 00:43:51.630 align:middle line:84% Scott and [? Meg ?] are explaining [INAUDIBLE]. 00:43:51.630 --> 00:43:55.090 align:middle line:84% And Meg is our activity [? trawl ?] fishing expert. 00:43:55.090 --> 00:43:56.150 align:middle line:90% But they're calculated. 00:43:56.150 --> 00:43:59.690 align:middle line:84% And in fact, a single ion activity coefficient 00:43:59.690 --> 00:44:03.390 align:middle line:90% is a theoretical construct. 00:44:03.390 --> 00:44:05.635 align:middle line:84% AUDIENCE: You can learn how to do it, too. 00:44:05.635 --> 00:44:06.510 align:middle line:90% WILLIAM MARTIN: Yeah. 00:44:06.510 --> 00:44:11.350 align:middle line:90% 00:44:11.350 --> 00:44:15.030 align:middle line:84% So in seawater, it has an ionic strength 00:44:15.030 --> 00:44:19.650 align:middle line:84% of about 0.7, which is a high ionic strength. 00:44:19.650 --> 00:44:22.710 align:middle line:84% I'll have you take that as [INAUDIBLE]. 00:44:22.710 --> 00:44:24.410 align:middle line:84% And because it's a high ionic strength, 00:44:24.410 --> 00:44:28.230 align:middle line:84% it's very difficult to calculate activity coefficients that 00:44:28.230 --> 00:44:30.550 align:middle line:90% are applicable to seawater. 00:44:30.550 --> 00:44:35.430 align:middle line:84% And for that reason, what's done is that what are called 00:44:35.430 --> 00:44:39.010 align:middle line:84% constant ionic medium equilibrium constants are used. 00:44:39.010 --> 00:44:41.230 align:middle line:84% These constants are measured in solutions 00:44:41.230 --> 00:44:44.750 align:middle line:84% with the same proportions of major ions as seawater. 00:44:44.750 --> 00:44:48.630 align:middle line:84% They're measured as a function of temperature and salinity. 00:44:48.630 --> 00:44:50.830 align:middle line:84% And the pressure dependence is generally 00:44:50.830 --> 00:44:53.390 align:middle line:84% calculated from thermodynamic data. 00:44:53.390 --> 00:44:57.410 align:middle line:84% So these are equilibrium constants, 00:44:57.410 --> 00:44:59.270 align:middle line:84% which are applicable to solutions 00:44:59.270 --> 00:45:03.440 align:middle line:84% that have the same ionic composition as seawater. 00:45:03.440 --> 00:45:09.680 align:middle line:84% So now the dissociation constant for bicarbonate to carbonate 00:45:09.680 --> 00:45:11.600 align:middle line:84% would be written like this, where 00:45:11.600 --> 00:45:15.160 align:middle line:84% it's the total hydrogen-ion concentration multiplied 00:45:15.160 --> 00:45:18.520 align:middle line:84% by the total carbonate HCO3 minus concentration. 00:45:18.520 --> 00:45:21.440 align:middle line:84% And now again, we're back to concentrations 00:45:21.440 --> 00:45:25.400 align:middle line:84% because the activity coefficient is all subsumed 00:45:25.400 --> 00:45:29.332 align:middle line:90% into the equilibrium constant. 00:45:29.332 --> 00:45:31.100 align:middle line:90% But what's this T? 00:45:31.100 --> 00:45:34.220 align:middle line:84% We've talked about what the T on H+ means. 00:45:34.220 --> 00:45:36.320 align:middle line:84% That means it's the hydrogen-ion concentration 00:45:36.320 --> 00:45:40.200 align:middle line:84% on the total hydrogen-ion concentration scales. 00:45:40.200 --> 00:45:45.400 align:middle line:84% The CO3 to minus T actually is an important thing 00:45:45.400 --> 00:45:48.100 align:middle line:84% because there's very little free carbonate ion in seawater. 00:45:48.100 --> 00:45:52.840 align:middle line:84% Most of it is complex to the major ions, major cations. 00:45:52.840 --> 00:45:55.040 align:middle line:84% But in these equilibrium constants, 00:45:55.040 --> 00:46:01.410 align:middle line:84% the total-- the sum of carbonate plus MgCO3 plus CaCO3 dissolved 00:46:01.410 --> 00:46:03.710 align:middle line:90% in solution that we include. 00:46:03.710 --> 00:46:06.450 align:middle line:90% 00:46:06.450 --> 00:46:09.530 align:middle line:84% These seawater constants, and as I've written it 00:46:09.530 --> 00:46:13.730 align:middle line:84% on the total hydrogen-ion scale, are available in the literature 00:46:13.730 --> 00:46:15.990 align:middle line:84% as functions of temperature, salinity, and pressure. 00:46:15.990 --> 00:46:18.190 align:middle line:84% And those are the ones you want to use for seawater. 00:46:18.190 --> 00:46:22.550 align:middle line:90% 00:46:22.550 --> 00:46:26.970 align:middle line:84% So now we know all about total CO2 alkalinity and pH. 00:46:26.970 --> 00:46:28.890 align:middle line:84% And we know about the equilibrium constants 00:46:28.890 --> 00:46:31.130 align:middle line:84% that are used to calculate the speciation 00:46:31.130 --> 00:46:34.010 align:middle line:84% of the inorganic carbon system in seawater. 00:46:34.010 --> 00:46:37.130 align:middle line:84% So we're ready to work with the carbonate system in seawater. 00:46:37.130 --> 00:46:40.450 align:middle line:84% And what we have is the total CO2, 00:46:40.450 --> 00:46:45.210 align:middle line:84% which is a sum of CO2, bicarbonate, and carbonate, 00:46:45.210 --> 00:46:46.650 align:middle line:90% alkalinity-- 00:46:46.650 --> 00:46:49.290 align:middle line:84% which I've read so many times you 00:46:49.290 --> 00:46:52.210 align:middle line:84% don't want to hear it again-- plus or minus minor components 00:46:52.210 --> 00:46:55.090 align:middle line:84% that you may or may not want to include. 00:46:55.090 --> 00:46:57.710 align:middle line:84% And we have another expression here, 00:46:57.710 --> 00:47:01.100 align:middle line:84% which is a mass balance for borate. 00:47:01.100 --> 00:47:04.140 align:middle line:84% So the total borate is equal to boric acid plus borate. 00:47:04.140 --> 00:47:07.180 align:middle line:84% And remember that total borate is conserved in seawater. 00:47:07.180 --> 00:47:08.820 align:middle line:90% It's conservative in seawater. 00:47:08.820 --> 00:47:12.100 align:middle line:84% So this value is just a constant times the salinity. 00:47:12.100 --> 00:47:17.540 align:middle line:84% We have 1, 2, 3, 4 equilibrium constants-- two 00:47:17.540 --> 00:47:20.700 align:middle line:84% for the carbonate system, one for boric acid, 00:47:20.700 --> 00:47:22.420 align:middle line:90% and one for water. 00:47:22.420 --> 00:47:28.400 align:middle line:84% And if you add up the unknowns and the number of equations, 00:47:28.400 --> 00:47:31.260 align:middle line:84% and if you measure total CO2, alkalinity, 00:47:31.260 --> 00:47:32.740 align:middle line:84% and salinity, what you'll find is 00:47:32.740 --> 00:47:35.140 align:middle line:84% that you have seven equations and seven unknowns. 00:47:35.140 --> 00:47:38.220 align:middle line:84% And you can solve this system for the speciation 00:47:38.220 --> 00:47:43.420 align:middle line:84% of the inorganic carbon system, boric acid, and water. 00:47:43.420 --> 00:47:47.100 align:middle line:90% 00:47:47.100 --> 00:47:49.300 align:middle line:84% And just to show this one more time. 00:47:49.300 --> 00:47:52.820 align:middle line:84% That's actually what I did to calculate this Bjerrum plot. 00:47:52.820 --> 00:47:57.550 align:middle line:84% And I'm going to use it just one more time. 00:47:57.550 --> 00:48:00.270 align:middle line:84% I think it's not going to appear again anymore-- use it 00:48:00.270 --> 00:48:04.150 align:middle line:84% just one more time to show you some useful approximations 00:48:04.150 --> 00:48:07.350 align:middle line:84% for back of the envelope calculations. 00:48:07.350 --> 00:48:11.150 align:middle line:84% And the first one is that you see that at pH 8, 00:48:11.150 --> 00:48:15.990 align:middle line:84% total CO2 is made up of bicarbonate plus carbonate 00:48:15.990 --> 00:48:18.070 align:middle line:90% plus a small amount of CO2. 00:48:18.070 --> 00:48:21.550 align:middle line:84% So if you want to do quick, fast calculations, the way it's done 00:48:21.550 --> 00:48:26.190 align:middle line:84% is just to ignore the CO2 contribution. 00:48:26.190 --> 00:48:29.430 align:middle line:84% For alkalinity, you can do the same thing. 00:48:29.430 --> 00:48:32.510 align:middle line:84% Twice the carbonate plus bicarbonate, 00:48:32.510 --> 00:48:42.310 align:middle line:84% and in this simple treatment it ignores borate and OH- and H+. 00:48:42.310 --> 00:48:44.190 align:middle line:84% And if you use these two approximations, 00:48:44.190 --> 00:48:46.670 align:middle line:84% then you can calculate the bicarbonate ion concentration 00:48:46.670 --> 00:48:48.310 align:middle line:84% roughly as the difference between 00:48:48.310 --> 00:48:50.050 align:middle line:90% alkalinity and total CO2. 00:48:50.050 --> 00:48:52.590 align:middle line:90% 00:48:52.590 --> 00:48:56.430 align:middle line:84% One point to note is that, as we'll see, 00:48:56.430 --> 00:48:57.930 align:middle line:84% you may have seen already, and I'll 00:48:57.930 --> 00:49:00.130 align:middle line:84% show you if you haven't seen it already, 00:49:00.130 --> 00:49:03.990 align:middle line:84% as when seawater leaves the surface ocean, 00:49:03.990 --> 00:49:05.250 align:middle line:90% it has a certain pH. 00:49:05.250 --> 00:49:07.810 align:middle line:84% As it circulates through the deep ocean, 00:49:07.810 --> 00:49:10.730 align:middle line:84% you might say as it ages, its pH decreases. 00:49:10.730 --> 00:49:15.490 align:middle line:84% And so as seawater ages, its pH drops below pH 8. 00:49:15.490 --> 00:49:17.970 align:middle line:84% It goes in the left direction on this plot. 00:49:17.970 --> 00:49:20.250 align:middle line:84% And the thing that's worth pointing out 00:49:20.250 --> 00:49:23.690 align:middle line:84% is that the further away from pH 8 you get, 00:49:23.690 --> 00:49:27.690 align:middle line:84% the lower the pH gets, the worse these approximations are. 00:49:27.690 --> 00:49:31.410 align:middle line:84% So that's something to keep in mind. 00:49:31.410 --> 00:49:38.170 align:middle line:84% So now we're going to talk about some biogeochemical processes. 00:49:38.170 --> 00:49:41.810 align:middle line:84% And we'll talk about how these biogeochemical processes affect 00:49:41.810 --> 00:49:43.890 align:middle line:90% total CO2 and alkalinity. 00:49:43.890 --> 00:49:46.050 align:middle line:84% And the first two processes on this page 00:49:46.050 --> 00:49:48.890 align:middle line:84% occur in the surface ocean, and they 00:49:48.890 --> 00:49:52.810 align:middle line:84% have to do with first, the production of organic matter 00:49:52.810 --> 00:49:54.360 align:middle line:90% by photosynthesis 00:49:54.360 --> 00:49:59.500 align:middle line:84% During this process, CO2 is taken up to form organic carbon 00:49:59.500 --> 00:50:01.140 align:middle line:90% and oxygen is released. 00:50:01.140 --> 00:50:06.140 align:middle line:84% Remember that CO2 does not contribute to the alkalinity. 00:50:06.140 --> 00:50:09.500 align:middle line:84% So if you take up CO2 into organic carbons, 00:50:09.500 --> 00:50:13.340 align:middle line:84% the changes are the total CO2 decreases 00:50:13.340 --> 00:50:16.500 align:middle line:84% by 1 mole per mole of organic matter form, 00:50:16.500 --> 00:50:19.300 align:middle line:84% and the alkalinity doesn't change. 00:50:19.300 --> 00:50:21.260 align:middle line:84% And there is a smaller effect, which 00:50:21.260 --> 00:50:24.300 align:middle line:84% is that organic matter that's formed in the surface ocean 00:50:24.300 --> 00:50:27.420 align:middle line:84% contains not just carbon, but it contains nitrogen as well. 00:50:27.420 --> 00:50:32.940 align:middle line:84% If this nitrogen is taken up from nitrate in the water, 00:50:32.940 --> 00:50:36.160 align:middle line:84% then for every nitrate that's taken up into organic matter, 00:50:36.160 --> 00:50:40.123 align:middle line:84% there's a small increase in alkalinity. 00:50:40.123 --> 00:50:41.540 align:middle line:84% So there's an increase of one unit 00:50:41.540 --> 00:50:43.220 align:middle line:84% in alkalinity for every nitrate that's 00:50:43.220 --> 00:50:45.180 align:middle line:90% taken up into organic matter. 00:50:45.180 --> 00:50:48.560 align:middle line:84% But since the ratio of nitrogen to carbon is 16 over 106, 00:50:48.560 --> 00:50:50.340 align:middle line:90% that's a small effect. 00:50:50.340 --> 00:50:53.070 align:middle line:84% So the formation of organic matter by photosynthesis 00:50:53.070 --> 00:50:55.750 align:middle line:84% causes a drop in total CO2 of one unit 00:50:55.750 --> 00:50:57.390 align:middle line:90% and no change in alkalinity. 00:50:57.390 --> 00:51:00.270 align:middle line:84% Now, if we go to the precipitation of calcium 00:51:00.270 --> 00:51:04.310 align:middle line:84% carbonate, which is formed by some of the organisms in surface 00:51:04.310 --> 00:51:06.630 align:middle line:90% ocean. 00:51:06.630 --> 00:51:10.190 align:middle line:84% That reaction is calcium plus carbonate ion forms 00:51:10.190 --> 00:51:11.430 align:middle line:90% calcium carbonate. 00:51:11.430 --> 00:51:13.390 align:middle line:84% You can see that, again, total CO2 00:51:13.390 --> 00:51:16.310 align:middle line:84% decreases by 1 moles for every mole of calcium carbonate 00:51:16.310 --> 00:51:17.030 align:middle line:90% formed. 00:51:17.030 --> 00:51:20.670 align:middle line:84% But now remember, every mole of carbonate ion 00:51:20.670 --> 00:51:23.150 align:middle line:84% contributes 2 moles to the alkalinity. 00:51:23.150 --> 00:51:25.630 align:middle line:84% So for every mole of calcium carbonate formed, 00:51:25.630 --> 00:51:29.510 align:middle line:84% there's a decrease in the alkalinity of 2 moles. 00:51:29.510 --> 00:51:32.550 align:middle line:90% 00:51:32.550 --> 00:51:34.910 align:middle line:84% One question you might ask is how these processes 00:51:34.910 --> 00:51:36.670 align:middle line:84% affect the ability of the ocean to take up 00:51:36.670 --> 00:51:38.790 align:middle line:90% CO2 from the atmosphere. 00:51:38.790 --> 00:51:45.290 align:middle line:84% If there's a-- well, the ocean always tends to equilibrium. 00:51:45.290 --> 00:51:46.910 align:middle line:84% The surface ocean tends to equilibrium 00:51:46.910 --> 00:51:49.830 align:middle line:84% with the atmosphere, which is determined by-- 00:51:49.830 --> 00:51:51.520 align:middle line:84% and you'll talk about this later-- 00:51:51.520 --> 00:51:53.260 align:middle line:90% via Henry's law constant. 00:51:53.260 --> 00:51:56.560 align:middle line:84% So that the CO2 content of the surface ocean in equilibrium 00:51:56.560 --> 00:51:59.000 align:middle line:84% with the atmosphere is equal to a constant times 00:51:59.000 --> 00:52:02.640 align:middle line:84% the partial pressure of CO2 in the atmosphere. 00:52:02.640 --> 00:52:05.260 align:middle line:84% And if there's a perturbation in the equilibrium, 00:52:05.260 --> 00:52:08.100 align:middle line:84% then the system will tend back towards equilibrium. 00:52:08.100 --> 00:52:10.960 align:middle line:84% So say you have processes in the surface ocean 00:52:10.960 --> 00:52:15.160 align:middle line:84% which remove CO2 from the surface ocean. 00:52:15.160 --> 00:52:18.560 align:middle line:84% That will tend to drive surface ocean towards taking up 00:52:18.560 --> 00:52:20.600 align:middle line:84% more CO2 from the atmosphere in order 00:52:20.600 --> 00:52:23.295 align:middle line:90% to come back into equilibrium. 00:52:23.295 --> 00:52:24.920 align:middle line:84% Scott's not [? scowling. ?] [INAUDIBLE] 00:52:24.920 --> 00:52:26.980 align:middle line:84% I haven't said anything [INAUDIBLE]. 00:52:26.980 --> 00:52:30.680 align:middle line:90% 00:52:30.680 --> 00:52:33.640 align:middle line:84% So what we can do, if we want to consider 00:52:33.640 --> 00:52:38.720 align:middle line:84% the effects of these processes on the ability of the surface 00:52:38.720 --> 00:52:41.160 align:middle line:84% ocean to take up CO2, what we can do, 00:52:41.160 --> 00:52:43.840 align:middle line:84% remember that I showed that if we measure 00:52:43.840 --> 00:52:47.740 align:middle line:84% total CO2 and alkalinity and the salinity of the water sample, 00:52:47.740 --> 00:52:51.490 align:middle line:84% we could calculate the inorganic carbon speciation. 00:52:51.490 --> 00:52:54.090 align:middle line:84% That means for a given total CO2 and alkalinity, we 00:52:54.090 --> 00:52:57.310 align:middle line:84% can calculate the CO2 concentration in the water. 00:52:57.310 --> 00:53:01.650 align:middle line:84% And so that's what I've done here. 00:53:01.650 --> 00:53:05.490 align:middle line:84% I've got alkalinity plotted on the vertical axis. 00:53:05.490 --> 00:53:08.010 align:middle line:84% Total CO2 on the horizontal axis. 00:53:08.010 --> 00:53:10.530 align:middle line:84% And what I've done is taken a whole bunch of pairs 00:53:10.530 --> 00:53:12.310 align:middle line:90% of alkalinity and total CO2s. 00:53:12.310 --> 00:53:15.250 align:middle line:84% And for each pair I've calculated the CO2 concentration 00:53:15.250 --> 00:53:16.690 align:middle line:90% at equilibrium. 00:53:16.690 --> 00:53:18.410 align:middle line:84% And I've used that set of calculations 00:53:18.410 --> 00:53:24.290 align:middle line:84% to draw contours of constant CO2 concentration. 00:53:24.290 --> 00:53:28.070 align:middle line:84% And once I've done that, I've taken a starting point, 00:53:28.070 --> 00:53:30.130 align:middle line:90% which is this black dot. 00:53:30.130 --> 00:53:34.650 align:middle line:84% I'll add a total alkalinity of 2,350 micromoles per kilogram 00:53:34.650 --> 00:53:38.270 align:middle line:84% and a total CO2 of 2,250 micromoles per kilogram. 00:53:38.270 --> 00:53:40.770 align:middle line:84% And I've said, first, suppose that we 00:53:40.770 --> 00:53:44.890 align:middle line:84% form 50 micromoles per kilogram of organic matter, what's 00:53:44.890 --> 00:53:48.210 align:middle line:84% the effect on the CO2 concentration 00:53:48.210 --> 00:53:52.070 align:middle line:84% in the sea at equilibrium in the seawater that we've left behind? 00:53:52.070 --> 00:53:55.270 align:middle line:84% So what we've said is that decreases the total CO2 00:53:55.270 --> 00:53:57.670 align:middle line:84% by 50 micromoles per kilogram, but doesn't 00:53:57.670 --> 00:53:59.310 align:middle line:90% change the alkalinity. 00:53:59.310 --> 00:54:04.935 align:middle line:84% So that's this black arrow moving to the left of this plot. 00:54:04.935 --> 00:54:06.310 align:middle line:84% And you can see that what happens 00:54:06.310 --> 00:54:12.630 align:middle line:84% is we've gone from a CO2 concentration of about 37 or 38 00:54:12.630 --> 00:54:15.330 align:middle line:84% down to a CO2 concentration of about 28. 00:54:15.330 --> 00:54:18.710 align:middle line:84% So we've decreased the CO2 concentration 00:54:18.710 --> 00:54:23.270 align:middle line:84% in the water sample as a result of removing organic matter. 00:54:23.270 --> 00:54:25.830 align:middle line:84% What that means is it increases-- 00:54:25.830 --> 00:54:28.230 align:middle line:84% it enhances-- that's the right way-- 00:54:28.230 --> 00:54:32.310 align:middle line:84% the surface ocean's ability to take up CO2 from the atmosphere. 00:54:32.310 --> 00:54:33.950 align:middle line:84% What if we do the same experiment 00:54:33.950 --> 00:54:36.230 align:middle line:84% with the formation of calcium carbonate? 00:54:36.230 --> 00:54:38.890 align:middle line:84% Remember, do a question from the same starting point. 00:54:38.890 --> 00:54:42.670 align:middle line:84% We're forming 50 micromoles per kilogram of calcium carbonate. 00:54:42.670 --> 00:54:46.110 align:middle line:84% But now the total CO2 decreases by 50. 00:54:46.110 --> 00:54:48.640 align:middle line:90% The alkalinity decreases by 100. 00:54:48.640 --> 00:54:55.480 align:middle line:84% So if we draw that vector, [INAUDIBLE], 00:54:55.480 --> 00:55:01.340 align:middle line:84% we decrease total CO2 by 50, decrease alkalinity by 100. 00:55:01.340 --> 00:55:06.560 align:middle line:84% And we find that now we've gone from a CO2 concentration 00:55:06.560 --> 00:55:12.400 align:middle line:84% of about 37 or 38 to a CO2 concentration of about 50. 00:55:12.400 --> 00:55:16.480 align:middle line:84% So removing calcium carbonate from the surface ocean 00:55:16.480 --> 00:55:19.900 align:middle line:84% actually increases its CO2 concentration, 00:55:19.900 --> 00:55:25.080 align:middle line:84% so decreases its ability to take up CO2 from the atmosphere. 00:55:25.080 --> 00:55:27.280 align:middle line:84% So here, we have two processes which remove carbon 00:55:27.280 --> 00:55:28.380 align:middle line:90% from the surface ocean. 00:55:28.380 --> 00:55:30.440 align:middle line:84% One, the formation of organic matter 00:55:30.440 --> 00:55:32.400 align:middle line:84% enhances the surface ocean's ability 00:55:32.400 --> 00:55:34.440 align:middle line:84% to take up CO2 from the atmosphere. 00:55:34.440 --> 00:55:36.420 align:middle line:84% The other, the formation of calcium carbonate, 00:55:36.420 --> 00:55:39.700 align:middle line:84% reduces its ability to take up CO2 from the atmosphere. 00:55:39.700 --> 00:55:42.280 align:middle line:90% 00:55:42.280 --> 00:55:44.600 align:middle line:84% So there's a biogeochemical application 00:55:44.600 --> 00:55:47.770 align:middle line:84% of our [INAUDIBLE] knowledge of a inorganic carbonate 00:55:47.770 --> 00:55:50.490 align:middle line:90% equilibrium in the ocean. 00:55:50.490 --> 00:55:52.850 align:middle line:84% Now, what if we turn it around, and instead 00:55:52.850 --> 00:55:55.730 align:middle line:84% of looking at the formation of organic matter and calcium 00:55:55.730 --> 00:55:59.080 align:middle line:84% carbonate, if we look at the destruction of organic matter 00:55:59.080 --> 00:56:00.730 align:middle line:90% in calcium carbonate. 00:56:00.730 --> 00:56:05.490 align:middle line:84% So here, first we reverse photosynthesis, 00:56:05.490 --> 00:56:12.610 align:middle line:84% and oxic metabolism is used to oxidize organic matter releasing 00:56:12.610 --> 00:56:13.870 align:middle line:90% CO2 in the water. 00:56:13.870 --> 00:56:17.210 align:middle line:84% And it's the exact reverse of photosynthesis. 00:56:17.210 --> 00:56:21.850 align:middle line:84% Total CO2 increases by 1 mole per mole of organic matter 00:56:21.850 --> 00:56:24.610 align:middle line:84% that's oxidized by oxygen. Alkalinity 00:56:24.610 --> 00:56:28.610 align:middle line:84% doesn't change because it's CO2 that's released into the water. 00:56:28.610 --> 00:56:33.130 align:middle line:84% Now, if we consider calcium carbonate dissolution, then 00:56:33.130 --> 00:56:35.570 align:middle line:84% again, the reverse of what we talked about before. 00:56:35.570 --> 00:56:39.110 align:middle line:84% For every mole of calcium carbonate that's dissolved, 00:56:39.110 --> 00:56:43.890 align:middle line:84% it releases 1 mole of total CO2 into the water. 00:56:43.890 --> 00:56:47.180 align:middle line:84% And since it releases it into the water as carbonate ion, 00:56:47.180 --> 00:56:49.140 align:middle line:84% it increases the alkalinity of the water 00:56:49.140 --> 00:56:53.380 align:middle line:84% by 2 moles per mole of calcium carbonate that's dissolved. 00:56:53.380 --> 00:56:56.820 align:middle line:84% And if you're like me and you're interested in carbonate ion 00:56:56.820 --> 00:56:58.420 align:middle line:84% concentrations in seawater, you might 00:56:58.420 --> 00:57:02.380 align:middle line:84% ask how these processes affect the carbonate ion concentration 00:57:02.380 --> 00:57:05.060 align:middle line:84% with seawater as it ages during its transit 00:57:05.060 --> 00:57:06.320 align:middle line:90% through the deep ocean. 00:57:06.320 --> 00:57:08.980 align:middle line:84% So remember, we probably haven't talked about this. 00:57:08.980 --> 00:57:12.500 align:middle line:90% 00:57:12.500 --> 00:57:18.100 align:middle line:84% Say a parcel of deep water forms in the North Atlantic. 00:57:18.100 --> 00:57:19.820 align:middle line:84% The large scale circulation takes us 00:57:19.820 --> 00:57:22.420 align:middle line:84% from the North Atlantic to the Southern Ocean, 00:57:22.420 --> 00:57:25.740 align:middle line:84% and eventually finds its way up to the Pacific Ocean. 00:57:25.740 --> 00:57:28.540 align:middle line:84% As it makes that transit, both organic matter and calcium 00:57:28.540 --> 00:57:30.120 align:middle line:90% carbonate fall into the water. 00:57:30.120 --> 00:57:32.980 align:middle line:84% Organic matter can decompose, releasing CO2. 00:57:32.980 --> 00:57:36.500 align:middle line:84% Calcium carbonate may dissolve, releasing both total CO2 00:57:36.500 --> 00:57:38.480 align:middle line:90% and alkalinity. 00:57:38.480 --> 00:57:40.580 align:middle line:84% So we might ask how some of those processes 00:57:40.580 --> 00:57:44.870 align:middle line:84% affects the carbonate ion concentration and also 00:57:44.870 --> 00:57:49.470 align:middle line:84% the pH of deep water as it ages in the ocean. 00:57:49.470 --> 00:57:52.110 align:middle line:84% I've done exactly the same thing as I did before. 00:57:52.110 --> 00:57:54.590 align:middle line:84% I've taken a whole bunch of pairs of alkalinity 00:57:54.590 --> 00:57:58.430 align:middle line:84% and total CO2 concentrations, and from those concentrations, 00:57:58.430 --> 00:58:02.150 align:middle line:84% I've calculated carbonate ion concentration for each pair. 00:58:02.150 --> 00:58:05.190 align:middle line:84% And I've used a set of carbonate ion values 00:58:05.190 --> 00:58:09.010 align:middle line:84% to draw contours of constant carbonate ion concentration. 00:58:09.010 --> 00:58:12.410 align:middle line:84% And again, I've taken a starting point, 00:58:12.410 --> 00:58:15.990 align:middle line:84% which is 2,300 micromoles per kilogram with alkalinities, 00:58:15.990 --> 00:58:19.430 align:middle line:84% 2,050 micromoles per kilogram of total CO2, 00:58:19.430 --> 00:58:23.630 align:middle line:84% and I've drawn vectors which represent the oxidation 00:58:23.630 --> 00:58:25.670 align:middle line:84% of organic matter and the dissolution of calcium 00:58:25.670 --> 00:58:26.910 align:middle line:90% carbonate. 00:58:26.910 --> 00:58:31.590 align:middle line:84% The oxidation of organic matter of 50 micromoles per kilogram 00:58:31.590 --> 00:58:33.590 align:middle line:84% of organic matter adds 50 micromoles 00:58:33.590 --> 00:58:37.210 align:middle line:84% per kilogram of total CO2 but doesn't add any alkalinity. 00:58:37.210 --> 00:58:41.030 align:middle line:84% So if we do that, we follow this vector here, 00:58:41.030 --> 00:58:42.650 align:middle line:84% we go from our starting point, which 00:58:42.650 --> 00:58:46.570 align:middle line:84% was at 180 micromoles per kilogram of carbonate ion 00:58:46.570 --> 00:58:52.450 align:middle line:84% concentration, and we end up at about 150 micromoles 00:58:52.450 --> 00:58:55.090 align:middle line:90% per kilogram of carbonate ion. 00:58:55.090 --> 00:58:58.410 align:middle line:84% So the oxidation of organic matter 00:58:58.410 --> 00:59:01.690 align:middle line:84% by oxic respiration in the ocean decreases the carbonate ion 00:59:01.690 --> 00:59:05.050 align:middle line:90% concentration for the seawater. 00:59:05.050 --> 00:59:07.010 align:middle line:84% Second thing I've done is say suppose 00:59:07.010 --> 00:59:10.490 align:middle line:84% we're dissolving calcium carbonate in that seawater 00:59:10.490 --> 00:59:11.230 align:middle line:90% sample. 00:59:11.230 --> 00:59:12.730 align:middle line:90% Same starting point. 00:59:12.730 --> 00:59:14.570 align:middle line:84% And we're dissolving 50 micromoles 00:59:14.570 --> 00:59:17.250 align:middle line:84% per kilogram of calcium carbonate, which 00:59:17.250 --> 00:59:20.450 align:middle line:84% adds 50 micromoles per kilogram of total CO2. 00:59:20.450 --> 00:59:24.810 align:middle line:84% And now also adds 100 micromoles per kilogram of alkalinity. 00:59:24.810 --> 00:59:28.810 align:middle line:84% So if we follow that vector from our starting 00:59:28.810 --> 00:59:31.450 align:middle line:84% point up to its ending point, we've 00:59:31.450 --> 00:59:35.650 align:middle line:84% added 50 micromoles per kilogram of total CO2, 100 of alkalinity. 00:59:35.650 --> 00:59:39.610 align:middle line:84% We find that it started at 180 micromoles per kilogram 00:59:39.610 --> 00:59:43.180 align:middle line:84% carbonate ion in the water and ended at 220. 00:59:43.180 --> 00:59:46.180 align:middle line:90% So we've added carbonate ion. 00:59:46.180 --> 00:59:48.260 align:middle line:84% We've increased the carbonate ion concentration 00:59:48.260 --> 00:59:50.940 align:middle line:84% of the seawater by dissolving calcium carbonate. 00:59:50.940 --> 00:59:54.620 align:middle line:84% So oxidation of organic matter in a seawater sample 00:59:54.620 --> 00:59:57.660 align:middle line:84% decreases its carbonate ion concentration. 00:59:57.660 --> 01:00:00.660 align:middle line:84% Dissolution of calcium carbonate increases its bicarbonate ion 01:00:00.660 --> 01:00:01.560 align:middle line:90% concentration. 01:00:01.560 --> 01:00:16.000 align:middle line:90% 01:00:16.000 --> 01:00:20.620 align:middle line:84% So what if we look at some observation data. 01:00:20.620 --> 01:00:23.700 align:middle line:90% We can [INAUDIBLE]. 01:00:23.700 --> 01:00:28.340 align:middle line:84% I plotted-- I've shown G06 data for the North Atlantic 01:00:28.340 --> 01:00:30.540 align:middle line:84% on the left-hand side, for the North 01:00:30.540 --> 01:00:33.500 align:middle line:84% Pacific on the right-hand side, alkalinity 01:00:33.500 --> 01:00:36.740 align:middle line:84% in the top, total CO2 in the middle, carbonate ion 01:00:36.740 --> 01:00:37.700 align:middle line:90% in the bottom. 01:00:37.700 --> 01:00:41.270 align:middle line:84% And I've mentioned already that deep water in the North Atlantic 01:00:41.270 --> 01:00:42.590 align:middle line:90% is relatively young. 01:00:42.590 --> 01:00:46.190 align:middle line:84% Deep water in the North Pacific is relatively old. 01:00:46.190 --> 01:00:48.790 align:middle line:84% So it's had a lot of organic matter, oxidation, 01:00:48.790 --> 01:00:51.178 align:middle line:84% and calcium carbonate dissolution within it 01:00:51.178 --> 01:00:53.470 align:middle line:84% during its transit from the North Atlantic to the North 01:00:53.470 --> 01:00:54.830 align:middle line:90% Pacific. 01:00:54.830 --> 01:00:59.390 align:middle line:84% And if you have incredibly good eyes, 01:00:59.390 --> 01:01:03.950 align:middle line:84% and you look at the deep water and see that the alkalinity has 01:01:03.950 --> 01:01:06.690 align:middle line:84% increased going from the North Atlantic, 01:01:06.690 --> 01:01:09.110 align:middle line:84% deep North Atlantic to the deep North Pacific. 01:01:09.110 --> 01:01:12.070 align:middle line:84% So what does that tell you has happened? 01:01:12.070 --> 01:01:14.070 align:middle line:84% If it's a single parcel of water that's 01:01:14.070 --> 01:01:15.910 align:middle line:84% made a transit from the deep North Atlantic 01:01:15.910 --> 01:01:17.990 align:middle line:84% to the deep North Pacific, what must 01:01:17.990 --> 01:01:20.210 align:middle line:84% have happened if the alkalinity does increase? 01:01:20.210 --> 01:01:25.350 align:middle line:90% 01:01:25.350 --> 01:01:27.650 align:middle line:84% Calcium carbonate has dissolved in the water. 01:01:27.650 --> 01:01:28.770 align:middle line:90% Go down one panel. 01:01:28.770 --> 01:01:30.110 align:middle line:90% We look at total CO2. 01:01:30.110 --> 01:01:32.670 align:middle line:84% And again, if you have much better eyes than I do, 01:01:32.670 --> 01:01:37.190 align:middle line:84% you can see that the total CO2 has increased as well. 01:01:37.190 --> 01:01:41.160 align:middle line:84% Now, total CO2 increase can be due to both organic matter 01:01:41.160 --> 01:01:44.420 align:middle line:84% oxidation and dissolution of calcium carbonate. 01:01:44.420 --> 01:01:47.100 align:middle line:90% But there's one more thing. 01:01:47.100 --> 01:01:50.360 align:middle line:84% Remember, what happens if all the organic matter oxidation 01:01:50.360 --> 01:01:52.460 align:middle line:90% occurs in a water sample? 01:01:52.460 --> 01:01:54.940 align:middle line:84% What happens to the carbonate ion concentration? 01:01:54.940 --> 01:01:56.440 align:middle line:90% AUDIENCE: [INAUDIBLE]. 01:01:56.440 --> 01:01:58.760 align:middle line:84% WILLIAM MARTIN: What happens if only calcium carbonate 01:01:58.760 --> 01:02:00.920 align:middle line:90% dissolution occured? 01:02:00.920 --> 01:02:02.360 align:middle line:90% It increases. 01:02:02.360 --> 01:02:05.060 align:middle line:84% Now if you look at the deep water again, 01:02:05.060 --> 01:02:09.560 align:middle line:84% and if you have even better eyes, 01:02:09.560 --> 01:02:11.560 align:middle line:84% the carbonate ion concentration that's 01:02:11.560 --> 01:02:13.960 align:middle line:84% calculated from alkalinity and total CO2 values 01:02:13.960 --> 01:02:16.060 align:middle line:84% in the deep North Atlantic is about 100, 01:02:16.060 --> 01:02:19.720 align:middle line:84% or a little more than 100, micromoles per kilogram. 01:02:19.720 --> 01:02:21.560 align:middle line:84% The same quantity in the deep North Pacific 01:02:21.560 --> 01:02:25.280 align:middle line:84% is about 90 micromoles per kilogram. 01:02:25.280 --> 01:02:29.660 align:middle line:84% So what two things must have been happening? 01:02:29.660 --> 01:02:30.580 align:middle line:90% AUDIENCE: [INAUDIBLE]. 01:02:30.580 --> 01:02:32.885 align:middle line:90% 01:02:32.885 --> 01:02:34.760 align:middle line:84% WILLIAM MARTIN: We know carbonate dissolution 01:02:34.760 --> 01:02:37.500 align:middle line:84% has occurred because the alkalinity increased. 01:02:37.500 --> 01:02:40.220 align:middle line:84% And now, since we know that carbonate ion concentration has 01:02:40.220 --> 01:02:42.700 align:middle line:84% decreased, we know that in addition to that, 01:02:42.700 --> 01:02:47.620 align:middle line:84% organic matter must have been oxidized along the way. 01:02:47.620 --> 01:02:49.780 align:middle line:90% I have one more picture. 01:02:49.780 --> 01:02:52.660 align:middle line:84% This is a picture that comes from Broecker and Peng, 01:02:52.660 --> 01:02:54.180 align:middle line:90% "Tracers in the Sea." 01:02:54.180 --> 01:02:56.120 align:middle line:84% And I'm not going to discuss it in detail, 01:02:56.120 --> 01:02:58.020 align:middle line:84% but I do recommend that you read this 01:02:58.020 --> 01:03:00.460 align:middle line:84% because it's a really good exercise in using 01:03:00.460 --> 01:03:02.040 align:middle line:90% these simple calculations. 01:03:02.040 --> 01:03:04.740 align:middle line:84% But I want to point out what's been done 01:03:04.740 --> 01:03:07.580 align:middle line:84% and just the very broad first brush 01:03:07.580 --> 01:03:10.940 align:middle line:90% conclusion from this picture. 01:03:10.940 --> 01:03:14.980 align:middle line:84% Was plotted in alkalinity versus total CO2 space. 01:03:14.980 --> 01:03:18.340 align:middle line:84% There's a bunch of alkalinity and total CO2 concentrations 01:03:18.340 --> 01:03:22.300 align:middle line:84% in temperate surface water around the oceans-- 01:03:22.300 --> 01:03:24.860 align:middle line:84% in Antarctic surface water, which 01:03:24.860 --> 01:03:26.700 align:middle line:84% we won't think about that, North Atlantic 01:03:26.700 --> 01:03:31.543 align:middle line:84% deep water, Antarctic deep water, and what is it? 01:03:31.543 --> 01:03:32.460 align:middle line:90% AUDIENCE: [INAUDIBLE]. 01:03:32.460 --> 01:03:35.540 align:middle line:84% WILLIAM MARTIN: North Indian and Pacific [INAUDIBLE] of water. 01:03:35.540 --> 01:03:39.110 align:middle line:84% So first, let's look going from surface water 01:03:39.110 --> 01:03:40.990 align:middle line:90% to Antarctic deep water. 01:03:40.990 --> 01:03:46.350 align:middle line:84% You can see that connecting the surface water to the deep water 01:03:46.350 --> 01:03:49.070 align:middle line:84% gives you a certain slope in alkalinity versus total CO2 01:03:49.070 --> 01:03:50.030 align:middle line:90% space. 01:03:50.030 --> 01:03:53.630 align:middle line:84% You can use the stoichiometric relationships 01:03:53.630 --> 01:03:58.110 align:middle line:84% where organic matter adds total CO2 but not alkalinity. 01:03:58.110 --> 01:04:01.710 align:middle line:84% Carbonate dissolution adds both alkalinity and total 01:04:01.710 --> 01:04:04.710 align:middle line:90% CO2 to interpret this slope. 01:04:04.710 --> 01:04:07.350 align:middle line:84% And if you do that, you find that the ratio 01:04:07.350 --> 01:04:10.830 align:middle line:84% of calcium carbonate dissolution to organic matter 01:04:10.830 --> 01:04:16.730 align:middle line:84% oxidation that's implied by this set of analyses is about 0.29, 01:04:16.730 --> 01:04:18.050 align:middle line:90% a little less than 0.3. 01:04:18.050 --> 01:04:22.310 align:middle line:84% So the amount of calcium carbonate dissolved is about 30% 01:04:22.310 --> 01:04:24.630 align:middle line:84% of the amount of organic carbon dioxide. 01:04:24.630 --> 01:04:27.310 align:middle line:84% Now what if you go from the Antarctic deep water 01:04:27.310 --> 01:04:29.970 align:middle line:90% to North Pacific deep water? 01:04:29.970 --> 01:04:32.790 align:middle line:90% It has a much steeper slope. 01:04:32.790 --> 01:04:37.040 align:middle line:84% And again, which means that more alkalinity is introduced 01:04:37.040 --> 01:04:40.657 align:middle line:84% in the water per unit of total CO2 01:04:40.657 --> 01:04:42.740 align:middle line:84% that's added to the water, which means, of course, 01:04:42.740 --> 01:04:45.360 align:middle line:84% that more calcium carbonate is dissolving. 01:04:45.360 --> 01:04:47.520 align:middle line:84% And if you interpret this slope, you 01:04:47.520 --> 01:04:50.600 align:middle line:84% see that the ratio of calcium carbonate dissolving 01:04:50.600 --> 01:04:54.480 align:middle line:84% to organic matter oxidizing is about 1. 01:04:54.480 --> 01:04:59.720 align:middle line:84% So the processes going from temperate surface water 01:04:59.720 --> 01:05:01.800 align:middle line:84% to Antarctic deep water are dominated 01:05:01.800 --> 01:05:04.100 align:middle line:84% by organic carbonate-- organic-- pardon 01:05:04.100 --> 01:05:07.320 align:middle line:90% me-- organic carbon oxidation. 01:05:07.320 --> 01:05:09.280 align:middle line:84% But if you go from Antarctic deep water 01:05:09.280 --> 01:05:11.280 align:middle line:84% to North Pacific deep water, then there's 01:05:11.280 --> 01:05:13.920 align:middle line:84% approximately equal amounts of carbonate dissolution 01:05:13.920 --> 01:05:16.720 align:middle line:90% and organic matter oxidation. 01:05:16.720 --> 01:05:21.840 align:middle line:90% And I'm going to stop there. 01:05:21.840 --> 01:05:24.960 align:middle line:84% So we've shown how biogeochemical processes affect 01:05:24.960 --> 01:05:27.140 align:middle line:84% total CO2 and alkalinity in the ocean. 01:05:27.140 --> 01:05:29.680 align:middle line:84% We've started to show how you can use distributions 01:05:29.680 --> 01:05:31.760 align:middle line:84% of total CO2 and alkalinity to learn 01:05:31.760 --> 01:05:34.010 align:middle line:84% about biogeochemical chemical processes. 01:05:34.010 --> 01:05:37.890 align:middle line:84% But we've not discussed the biological and chemical 01:05:37.890 --> 01:05:40.650 align:middle line:84% processes that control the formation and decomposition 01:05:40.650 --> 01:05:43.010 align:middle line:84% of organic matter, or the formation and dissolution 01:05:43.010 --> 01:05:44.810 align:middle line:90% of calcium carbonate. 01:05:44.810 --> 01:05:47.210 align:middle line:84% And another thing, which I didn't put up here, 01:05:47.210 --> 01:05:50.810 align:middle line:84% is that we haven't discussed ocean atmosphere interactions. 01:05:50.810 --> 01:05:52.490 align:middle line:84% Those are all things that are going 01:05:52.490 --> 01:05:55.650 align:middle line:84% to be applications of what I hope I've shown in the lecture 01:05:55.650 --> 01:05:59.130 align:middle line:84% today that are going to be discussed 01:05:59.130 --> 01:06:01.630 align:middle line:84% in a lot of detail later on in [INAUDIBLE]. 01:06:01.630 --> 01:06:04.770 align:middle line:90% [INAUDIBLE] And that's that. 01:06:04.770 --> 01:06:09.043 align:middle line:84% I finished early for once in my life. 01:06:09.043 --> 01:06:10.210 align:middle line:90% AUDIENCE: I have a question. 01:06:10.210 --> 01:06:11.252 align:middle line:90% WILLIAM MARTIN: Yeah, OK. 01:06:11.252 --> 01:06:15.650 align:middle line:84% AUDIENCE: On the North Atlantic alkalinity profiles, 01:06:15.650 --> 01:06:19.842 align:middle line:84% why is the alkalinity higher at the surface-- 01:06:19.842 --> 01:06:21.010 align:middle line:90% WILLIAM MARTIN: [INAUDIBLE]. 01:06:21.010 --> 01:06:22.810 align:middle line:90% AUDIENCE: The immediate surface. 01:06:22.810 --> 01:06:23.950 align:middle line:90% WILLIAM MARTIN: OK. 01:06:23.950 --> 01:06:26.410 align:middle line:84% Why is it higher at the surface than it is deep? 01:06:26.410 --> 01:06:28.610 align:middle line:84% AUDIENCE: [? Because ?] its alkalinity is not-- 01:06:28.610 --> 01:06:30.930 align:middle line:84% if photosynthesis hasn't contributed 01:06:30.930 --> 01:06:35.820 align:middle line:84% to alkalinity that much, what's the process that's decreasing 01:06:35.820 --> 01:06:37.360 align:middle line:90% the alkalinity at the surface? 01:06:37.360 --> 01:06:40.820 align:middle line:90% 01:06:40.820 --> 01:06:42.597 align:middle line:84% WILLIAM MARTIN: Well, is it right to look 01:06:42.597 --> 01:06:44.680 align:middle line:84% at what's increasing the alkalinity at the surface 01:06:44.680 --> 01:06:48.340 align:middle line:84% or what's decreasing the alkalinity as it goes deeper? 01:06:48.340 --> 01:06:51.300 align:middle line:84% I would interpret it more as what happens to the alkalinity 01:06:51.300 --> 01:06:53.378 align:middle line:84% as you go deeper in the water column. 01:06:53.378 --> 01:06:54.920 align:middle line:84% As you go deeper in the water column, 01:06:54.920 --> 01:06:57.460 align:middle line:90% organic matter is oxidized. 01:06:57.460 --> 01:06:59.980 align:middle line:84% And so that would decrease the alkalinity. 01:06:59.980 --> 01:07:02.460 align:middle line:90% No, it wouldn't. 01:07:02.460 --> 01:07:04.060 align:middle line:90% Scott, help. 01:07:04.060 --> 01:07:04.820 align:middle line:90% AUDIENCE: Sorry. 01:07:04.820 --> 01:07:07.320 align:middle line:84% WILLIAM MARTIN: I'm thinking of carbonate ion concentration. 01:07:07.320 --> 01:07:08.380 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:07:08.380 --> 01:07:10.740 align:middle line:84% WILLIAM MARTIN: Why the alkalinity in the North Atlantic 01:07:10.740 --> 01:07:13.080 align:middle line:84% decreases as a function of depth. 01:07:13.080 --> 01:07:14.480 align:middle line:90% Is that a water mass thing? 01:07:14.480 --> 01:07:18.540 align:middle line:90% 01:07:18.540 --> 01:07:22.260 align:middle line:84% AUDIENCE: It's just so [INAUDIBLE]. 01:07:22.260 --> 01:07:24.500 align:middle line:84% AUDIENCE: It depends on where you did the-- 01:07:24.500 --> 01:07:26.620 align:middle line:84% depends on where the [INAUDIBLE]. 01:07:26.620 --> 01:07:29.605 align:middle line:90% 01:07:29.605 --> 01:07:30.480 align:middle line:90% WILLIAM MARTIN: Yeah. 01:07:30.480 --> 01:07:32.260 align:middle line:84% Actually, Scott's absolutely right. 01:07:32.260 --> 01:07:33.635 align:middle line:84% Depends on where you're doing it. 01:07:33.635 --> 01:07:37.120 align:middle line:84% If you look at the envelope of surface concentrations, 01:07:37.120 --> 01:07:40.880 align:middle line:84% some of them are higher than the deeper values, and some of them 01:07:40.880 --> 01:07:41.760 align:middle line:90% are not. 01:07:41.760 --> 01:07:43.900 align:middle line:84% And is that dependant on salinity? 01:07:43.900 --> 01:07:47.140 align:middle line:84% AUDIENCE: A lot of it's salinity in the Atlantic. 01:07:47.140 --> 01:07:53.517 align:middle line:84% [INAUDIBLE] So go out in the yard. 01:07:53.517 --> 01:07:55.600 align:middle line:84% WILLIAM MARTIN: So alkalinity in the surface ocean 01:07:55.600 --> 01:07:59.160 align:middle line:84% is conservative, [? essentially ?] [INAUDIBLE] 01:07:59.160 --> 01:08:02.000 align:middle line:84% And so the variations in surface ocean 01:08:02.000 --> 01:08:05.640 align:middle line:84% alkalinity have to do with varying mixtures of seawater 01:08:05.640 --> 01:08:08.120 align:middle line:90% and fresh water. 01:08:08.120 --> 01:08:08.620 align:middle line:90% OK. 01:08:08.620 --> 01:08:11.360 align:middle line:90% Does that answer the question? 01:08:11.360 --> 01:08:15.977 align:middle line:84% AUDIENCE: So if you could take a few profiles from a small area, 01:08:15.977 --> 01:08:18.560 align:middle line:84% then you would say-- you would think that the alkalinity would 01:08:18.560 --> 01:08:21.800 align:middle line:90% just increase with depth? 01:08:21.800 --> 01:08:23.720 align:middle line:84% And so this plot has averaged out 01:08:23.720 --> 01:08:25.425 align:middle line:84% by the geographical areas [INAUDIBLE]. 01:08:25.425 --> 01:08:26.300 align:middle line:90% WILLIAM MARTIN: Yeah. 01:08:26.300 --> 01:08:29.569 align:middle line:84% This is actually all the alkalinity profiles 01:08:29.569 --> 01:08:31.689 align:middle line:84% that they measured during the GEOSECS expedition 01:08:31.689 --> 01:08:32.890 align:middle line:90% in the North Atlantic. 01:08:32.890 --> 01:08:35.130 align:middle line:84% That's why it's plotted as an envelope. 01:08:35.130 --> 01:08:38.649 align:middle line:84% It's actually a whole bunch of individual profiles plotted 01:08:38.649 --> 01:08:39.310 align:middle line:90% together. 01:08:39.310 --> 01:08:43.210 align:middle line:90% 01:08:43.210 --> 01:08:46.285 align:middle line:84% And the variations have a lot to do with water basins. 01:08:46.285 --> 01:08:48.410 align:middle line:84% AUDIENCE: And the other point [INAUDIBLE] not a lot 01:08:48.410 --> 01:08:51.850 align:middle line:90% of [INAUDIBLE]. 01:08:51.850 --> 01:08:58.010 align:middle line:84% [? the ?] [? North ?] [INAUDIBLE] I wouldn't expect 01:08:58.010 --> 01:09:01.330 align:middle line:84% a huge difference between surface and deep waters 01:09:01.330 --> 01:09:03.850 align:middle line:84% because of calcium carbonate to the solution. 01:09:03.850 --> 01:09:06.370 align:middle line:84% Not a lot of calcium [? carbonate ?] dissolves 01:09:06.370 --> 01:09:08.875 align:middle line:84% [? versus ?] Pacific [INAUDIBLE]. 01:09:08.875 --> 01:09:13.725 align:middle line:90% 01:09:13.725 --> 01:09:16.490 align:middle line:84% AUDIENCE: [? Also ?] surface deep waters forming in the North 01:09:16.490 --> 01:09:19.090 align:middle line:84% Atlantic, it's more similar to surface water [? that could ?] 01:09:19.090 --> 01:09:23.210 align:middle line:90% offset [INAUDIBLE]. 01:09:23.210 --> 01:09:26.130 align:middle line:84% WILLIAM MARTIN: Have you talked about water masses [INAUDIBLE] 01:09:26.130 --> 01:09:27.310 align:middle line:90% in immediate waters? 01:09:27.310 --> 01:09:29.579 align:middle line:90% AUDIENCE: [INAUDIBLE].