WEBVTT 00:01.167 --> 00:04.497 RONALD SMITH: We are starting a brand new subject today. 00:04.500 --> 00:07.470 It's one though that is integral with the course, and 00:07.467 --> 00:09.867 that's the study of the oceans. 00:09.867 --> 00:13.727 As we'll see, not only the atmosphere and the ocean 00:13.733 --> 00:18.803 interact, each influences the other which makes it necessary 00:18.800 --> 00:20.900 to understand both. 00:20.900 --> 00:27.130 But also, to some extent, they obey similar laws of physics. 00:27.133 --> 00:32.603 For example, questions of static stability, when will a 00:32.600 --> 00:35.900 column of air turn over, when will a column of water turn 00:35.900 --> 00:41.630 over, what makes the winds blow, what makes the ocean 00:41.633 --> 00:45.373 currents move, Coriolis plays-- 00:45.367 --> 00:47.067 Coriolis force plays a similar role in both. 00:47.067 --> 00:50.897 So this would be a time not only for you to learn a few 00:50.900 --> 00:54.830 things about the ocean, but to establish more linkages 00:54.833 --> 00:57.173 between the things you've learned before and the things 00:57.167 --> 00:58.167 you're learning now. 00:58.167 --> 01:00.527 Now that we're halfway, or almost halfway through the 01:00.533 --> 01:04.933 course, linkages are a big part of the course. 01:04.933 --> 01:08.233 Where you find connections between things we've already 01:08.233 --> 01:10.203 done, and things we're learning now. 01:10.200 --> 01:14.470 So try to flag those whenever you come across them, I think 01:14.467 --> 01:17.767 it'll help you learn the material more and it'll 01:17.767 --> 01:21.167 establish a sense, I hope, of unity in the course. 01:21.167 --> 01:24.867 Where things begin to gel, and become easier, because a very 01:24.867 --> 01:28.467 limited set of physical principles can apply to a wide 01:28.467 --> 01:31.827 range of geophysical phenomena. 01:36.133 --> 01:39.533 OK, so what we're going to do today. 01:39.533 --> 01:43.233 I'll probably just do the first two, we'll talk about 01:43.233 --> 01:45.803 the bathymetry of the oceans. 01:45.800 --> 01:51.330 The word bathymetry here means the study of the ocean depth. 01:51.333 --> 01:53.333 So it's pretty straightforward, just how deep 01:53.333 --> 01:53.973 is the ocean. 01:53.967 --> 01:56.267 That's the subject of bathymetry. 01:56.267 --> 01:59.697 We're going to see, however, that ties very closely to 01:59.700 --> 02:00.730 plate tectonics. 02:00.733 --> 02:03.673 Now if you were taking a course in geology, which my 02:03.667 --> 02:07.727 department offers a number of, you would study plate 02:07.733 --> 02:10.103 tectonics in quite some detail. 02:10.100 --> 02:14.270 It's the modern paradigm to understand how the continents 02:14.267 --> 02:16.867 were formed, and the oceans are formed, and a number of 02:16.867 --> 02:18.697 the things you see in rocks. 02:18.700 --> 02:21.470 We're just going to touch on it briefly, because this isn't 02:21.467 --> 02:24.597 a course in geology, but I need to do it because you 02:24.600 --> 02:27.530 can't understand the bathymetry of the oceans 02:27.533 --> 02:30.503 without understanding a bit about plate tectonics. 02:30.500 --> 02:32.730 Has anybody had a course in geology where they talked 02:32.733 --> 02:35.803 about plate tectonics? 02:35.800 --> 02:38.170 Sarah has--So you're going to see that stuff coming back, 02:38.167 --> 02:39.167 but fairly quickly. 02:39.167 --> 02:41.527 I'm not going to spend too much time on it. 02:41.533 --> 02:44.233 We'll spend most of today talking about temperature and 02:44.233 --> 02:47.833 salinity, and we might get into ocean currents today, but 02:47.833 --> 02:50.773 if not that'll be the subject of next time. 02:50.767 --> 02:54.097 And then biological productivity will eventually, 02:54.100 --> 02:58.930 maybe Friday or next week, get into El Nino. 02:58.933 --> 03:02.173 Sarah reminded me that there's no chapter in your book on 03:02.167 --> 03:05.997 oceanography, so we're kind of on our own here. 03:06.000 --> 03:09.530 These notes will be posted, but there is a section on El 03:09.533 --> 03:12.203 Nino, and you're going to want to read that, and that'll give 03:12.200 --> 03:14.830 a little background information about the oceans. 03:14.833 --> 03:16.973 And there may be a few little other bits and pieces 03:16.967 --> 03:19.897 scattered throughout the textbook on oceanography. 03:19.900 --> 03:23.470 So take advantage of what you have there, but realize there 03:23.467 --> 03:26.497 isn't a great deal, and so you'll have to rely fairly 03:26.500 --> 03:27.930 heavily on the notes here. 03:30.533 --> 03:35.133 OK, so some of you have seen this, probably 03:35.133 --> 03:36.533 most of you seen this. 03:36.533 --> 03:41.433 The basic idea behind plate tectonics is that the 03:41.433 --> 03:48.503 spherical skin of the earth, this shallow layer called the 03:48.500 --> 03:53.370 crust that's rather rigid floating above a deeper 03:53.367 --> 03:58.727 semi-liquid mantle, that crust is broken into several 03:58.733 --> 04:02.203 discrete plates. 04:02.200 --> 04:04.800 The Indo-Australian plate, the African plate, the South 04:04.800 --> 04:08.670 American plate, and those plates, as they move around 04:08.667 --> 04:11.927 remain rigid. 04:11.933 --> 04:15.473 And so the interactions, or the interesting parts, occur 04:15.467 --> 04:19.527 at the boundaries where one rigid plate butts up against 04:19.533 --> 04:22.933 another and some kind of interaction occurs. 04:22.933 --> 04:26.403 Maybe it's a subduction zone, maybe it's a new--new plate 04:26.400 --> 04:30.270 material being formed, but that's the way we try to 04:30.267 --> 04:32.467 understand the structure of the earth these days is to 04:32.467 --> 04:37.267 understand where are these plates, and how do their edges 04:37.267 --> 04:40.597 deform as the plates move around. 04:40.600 --> 04:45.670 So for example, that boundary there is a mid-ocean ridge, or 04:45.667 --> 04:48.997 a so called mid-ocean spreading center, you can tell 04:49.000 --> 04:49.830 that from the arrow. 04:49.833 --> 04:55.673 So new ocean crust is being formed at that point, and so 04:55.667 --> 04:59.297 this plate is moving away from that plate at a certain rate. 04:59.300 --> 05:02.370 Up here in the North Atlantic as well, you've got the plates 05:02.367 --> 05:05.397 pulling away from each other, and new ocean 05:05.400 --> 05:07.600 crust is being formed. 05:07.600 --> 05:11.630 In a line like that look, this artist has used kind of a cold 05:11.633 --> 05:14.403 front symbol, I don't know where he got that idea from. 05:14.400 --> 05:17.530 But the basic idea here is that that symbol, for this 05:17.533 --> 05:20.533 artist, is referring to a subduction zone, where one 05:20.533 --> 05:25.033 plate is being drawn down under another. 05:25.033 --> 05:28.903 And so crustal material is being disappeared, it's being 05:28.900 --> 05:34.070 returned down into the mantle of the earth. 05:34.067 --> 05:41.627 Notice that some plates are consisting of ocean only, some 05:41.633 --> 05:44.833 plates are consisting of a combination of continental 05:44.833 --> 05:55.303 crust and ocean crust. OK, so that's the basic idea behind 05:55.300 --> 05:56.570 plate tectonics. 05:59.067 --> 06:04.297 The plates are shown also in this cartoon, and once again 06:04.300 --> 06:08.930 note that for example, this large Pacific plate, fairly 06:08.933 --> 06:14.033 rigid and giant in size, is ocean crust only. 06:14.033 --> 06:17.303 Whereas most of the others, not the Nazca plate, but the 06:17.300 --> 06:21.970 Australian plate, has ocean crust plus continental crust. 06:21.967 --> 06:26.597 The Eurasian plate has both, North American plate has a big 06:26.600 --> 06:28.170 chunk of ocean in it. 06:28.167 --> 06:30.227 So when you think about plates. 06:30.233 --> 06:34.303 Remember the earlier, the thing that preceded this, was 06:34.300 --> 06:37.300 the theory of continental drift. 06:37.300 --> 06:41.870 Continental drift was the idea that people noticed this nice 06:41.867 --> 06:46.297 jigsaw fit, for example, between South America and the 06:46.300 --> 06:48.100 bite of Africa. 06:48.100 --> 06:52.600 Or the way this coast kind of could fit in against here. 06:52.600 --> 06:55.270 Or the fact that the rocks here, were similar to the 06:55.267 --> 06:57.297 rocks there up in Scotland. 06:57.300 --> 07:00.830 So the idea was that, early on, that the continents may 07:00.833 --> 07:01.403 have moved. 07:01.400 --> 07:05.830 But the early idea was that they moved through the ocean, 07:05.833 --> 07:10.233 the continents plowed their way through ocean crust to 07:10.233 --> 07:14.603 move around on our planet. 07:14.600 --> 07:18.300 However that was soon found to be incorrect, because the 07:18.300 --> 07:23.730 physics of trying to push a continent through ocean crust 07:23.733 --> 07:25.333 was shown to be impossible. 07:25.333 --> 07:29.403 Instead this is the vision that now seems to be the right 07:29.400 --> 07:33.570 one, where you've got plates sometimes consisting of 07:33.567 --> 07:35.627 continents and oceans. 07:35.633 --> 07:38.973 They move relative to another, relative to one another, and 07:38.967 --> 07:41.497 all the action is right at their boundaries, where 07:41.500 --> 07:44.730 there's creation of new crust or the destruction of crust. 07:44.733 --> 07:49.533 So this is the conceptual model that seems to fit all 07:49.533 --> 07:54.673 the data that we have. 07:54.667 --> 07:58.997 And what would drive that kind of motion, well it's basically 07:59.000 --> 08:00.700 part of the mantle convection. 08:00.700 --> 08:04.400 So there's heat being generated in the interior of 08:04.400 --> 08:07.800 the earth, by the decay of radioactive elements, like 08:07.800 --> 08:12.070 uranium and so on, decays naturally, releases heat. 08:12.067 --> 08:14.927 The interior of the earth heats up, and that 08:14.933 --> 08:17.533 destabilizes the lapse rate if you like. 08:17.533 --> 08:20.733 It basically, if you're heating this fluid from below, 08:20.733 --> 08:24.473 you're destabilizing it and convection begins. 08:24.467 --> 08:28.727 And as part of that convection cell, then you get spreading 08:28.733 --> 08:32.533 centers for the crust and then subduction zones where some of 08:32.533 --> 08:36.133 that crustal material is drawn back into the mantle, and 08:36.133 --> 08:37.903 melted, and returned. 08:37.900 --> 08:44.970 So it's not a crust only phenomenon,, it's driven by 08:44.967 --> 08:48.497 mantle convection, but for our purposes we're interested 08:48.500 --> 08:51.970 primarily in what it does to the crust of the earth. 08:55.267 --> 08:58.527 So going back into geologic time, we 08:58.533 --> 08:59.703 can see this happening. 08:59.700 --> 09:03.800 So here's the present day, and then we go back in time to the 09:03.800 --> 09:07.500 Cretaceous, the Jurassic, the Triassic, and Permian. 09:07.500 --> 09:12.930 Here's a geologic time scale, the age of the earth, this is 09:12.933 --> 09:18.103 in thousands--in millions of years, the age of the earth is 09:18.100 --> 09:23.400 back here around five, roughly five billion years ago. 09:23.400 --> 09:26.670 And the first one of these diagrams that the artist is 09:26.667 --> 09:30.527 showing is the Permian, which is about 255 09:30.533 --> 09:31.573 million years ago. 09:31.567 --> 09:34.897 At that point all the continents were together, in a 09:34.900 --> 09:38.200 giant super continent called Pangaea. 09:38.200 --> 09:43.370 And then as time progressed it split up, first with a seaway 09:43.367 --> 09:45.997 that came through this way, and then eventually you begin 09:46.000 --> 09:49.700 to get the Atlantic ocean opening up, and today you have 09:49.700 --> 09:50.630 something like this. 09:50.633 --> 09:54.333 So we're going from 255 million years ago for the 09:54.333 --> 09:58.473 Permian, and then stepping forward to the Triassic, the 09:58.467 --> 10:00.467 Jurassic, the Cretaceous. 10:00.467 --> 10:04.067 So when the dinosaurs roamed the earth, 10:04.067 --> 10:06.727 it was in this stage. 10:06.733 --> 10:11.633 And when humans evolved, well it was already 10:11.633 --> 10:13.103 looking like this. 10:13.100 --> 10:17.800 So humans never saw this configuration, humans evolved 10:17.800 --> 10:19.330 just in the last couple of million years. 10:19.333 --> 10:25.333 So we've been looking at that structure for our 10:25.333 --> 10:26.573 evolutionary history. 10:29.833 --> 10:33.633 Now, so what does this mean for the structure of the 10:33.633 --> 10:34.833 oceans and the continents? 10:34.833 --> 10:40.933 So here's a section through let's take it through here I 10:40.933 --> 10:44.573 guess an east-west, section. 10:44.567 --> 10:46.897 I think it's in the southern hemisphere, let's check that. 10:49.867 --> 10:52.027 Yeah, so there's South America, so this is an 10:52.033 --> 10:59.673 east-west, section through the South Atlantic ocean. 10:59.667 --> 11:03.467 It shows the Pacific Ocean plate, which is continental 11:03.467 --> 11:05.897 crust being subducted below the 11:05.900 --> 11:09.000 mountains of South America. 11:09.000 --> 11:13.500 As that material is drawn down into the mantle, it melts. 11:13.500 --> 11:18.730 As it melts, some lava, some magma, has come off of that. 11:18.733 --> 11:21.803 That then make their way upwards to cause the volcanoes 11:21.800 --> 11:25.570 along the west coast of South America. 11:25.567 --> 11:28.497 Otherwise the rest of that material is just lost down 11:28.500 --> 11:30.800 into the mantle. 11:30.800 --> 11:33.530 When you get over into the Atlantic Ocean, there's a 11:33.533 --> 11:37.103 rigid boundary there that's part of the same plate. 11:37.100 --> 11:40.200 Continental crust, and ocean crust, part of the same plate, 11:40.200 --> 11:42.300 but there's a spreading center. 11:42.300 --> 11:45.600 That's where molten material is coming up again and 11:45.600 --> 11:50.330 solidifying as it cools to form new ocean crust. So this 11:50.333 --> 11:53.933 is moving away, while new ocean crust is being created 11:53.933 --> 11:55.433 right at that point. 11:55.433 --> 11:59.303 And then over here, that's a rigid-rigid connection, so 11:59.300 --> 12:03.800 that once again is part of the same plate with no crust being 12:03.800 --> 12:06.830 lost or gained at that particular boundary. 12:06.833 --> 12:10.103 But the point I want to make here is that there are two 12:10.100 --> 12:15.330 types of crust. Continental crust is generally of a 12:15.333 --> 12:21.073 lighter material, and floats a bit higher in the semi-molten 12:21.067 --> 12:24.067 parts of the mantle, whereas the ocean crust is a little 12:24.067 --> 12:29.367 bit denser and floats a bit lower. 12:29.367 --> 12:36.297 So you've got basically a lower floating ocean crust 12:36.300 --> 12:42.270 here, and a less dense higher floating continental crust. 12:42.267 --> 12:44.867 And then when you fill that with water the water has 12:44.867 --> 12:47.467 nothing to do with this of course, but the addition of 12:47.467 --> 12:50.967 water makes it seem like an ocean to us. 12:50.967 --> 12:54.727 There happens to be enough water in the ocean to 12:54.733 --> 12:59.433 generally cover the ocean crust, but not to cover the 12:59.433 --> 13:00.203 continents. 13:00.200 --> 13:03.700 Now if there were twice as much liquid water available on 13:03.700 --> 13:07.370 the planet, that distinction would be less important 13:07.367 --> 13:10.067 because the water level would be here, and it would cover 13:10.067 --> 13:15.427 both the ocean crust and the continental crust. But with 13:15.433 --> 13:18.733 the amount of water that we have, it means that the 13:18.733 --> 13:23.403 continental crust usually sticks above sea level and the 13:23.400 --> 13:27.370 ocean crust does not, it's submerged below sea level. 13:27.367 --> 13:30.127 Remember that amount of water is quite 13:30.133 --> 13:31.633 unconnected with any of this. 13:31.633 --> 13:36.273 The amount of water we have on the planet probably did come 13:36.267 --> 13:39.827 out of the interior of the planet over geologic time, but 13:39.833 --> 13:43.033 it's just an accident that it happens to be deep enough to 13:43.033 --> 13:46.903 cover the ocean crust but not the continental crust. And 13:46.900 --> 13:49.370 we'll find a few exceptions to that when we look 13:49.367 --> 13:52.127 more closely as well. 13:52.133 --> 13:55.003 Any questions on this? 13:55.000 --> 13:56.030 Yes. 13:56.033 --> 13:56.273 STUDENT: So what are the differences between the ocean 13:56.267 --> 13:57.527 crust and continental crust besides just their density? 14:01.833 --> 14:02.703 PROFESSOR: Well, they're made of slightly 14:02.700 --> 14:05.600 different chemical compositions. 14:05.600 --> 14:10.700 Normally there's more, I think quartz generally in these 14:10.700 --> 14:15.270 rocks, a light mineral allows it to be a little bit denser, 14:15.267 --> 14:16.797 a little bit less dense. 14:16.800 --> 14:22.370 And other denser minerals are found more prolifically in the 14:22.367 --> 14:26.127 ocean crust, which makes it a little bit denser. 14:29.133 --> 14:32.003 OK, we'll come back to that. 14:32.000 --> 14:35.300 But I wanted to make this point, so when you then take 14:35.300 --> 14:42.170 a, what's called, make a hypsometric curve, I'm going 14:42.167 --> 14:45.527 to focus on the bar graph over here on the left. 14:45.533 --> 14:50.133 This is elevation in meters above and below sea level, sea 14:50.133 --> 14:52.173 level is marked at zero here. 14:52.167 --> 14:56.167 Which I remind you is a somewhat arbitrary choice, it 14:56.167 --> 14:59.797 depends on how much water we have in the oceans. 14:59.800 --> 15:02.730 And over geologic time, that has probably 15:02.733 --> 15:03.433 changed a little bit. 15:03.433 --> 15:06.803 For example, when you have an ice age you store some of the 15:06.800 --> 15:10.470 water up on the continents in the form of glaciers, and sea 15:10.467 --> 15:11.797 level drops a little bit. 15:11.800 --> 15:14.900 So that is kind of an arbitrary reference point, but 15:14.900 --> 15:17.530 it's commonly used and so we'll use it here. 15:17.533 --> 15:20.273 Whats shown in the bar graph then, is the percent of the 15:20.267 --> 15:24.367 earth's surface that lies, for example, between sea level and 15:24.367 --> 15:26.027 one kilometer above. 15:26.033 --> 15:29.403 And it's about 20% of the earth's surface. 15:29.400 --> 15:32.470 Between one kilometer and two kilometers above sea level, 15:32.467 --> 15:35.327 it's about 5% of the earth's surface. 15:35.333 --> 15:38.073 And you find some parts on the continents that are even 15:38.067 --> 15:42.127 higher, even up to four, but you know in fact Mount Everest 15:42.133 --> 15:44.973 is up here somewhere, there's even a little bit of land that 15:44.967 --> 15:49.667 lies 9000 meters above sea level. 15:49.667 --> 15:54.067 Going down below sea level, you find there's not much land 15:54.067 --> 15:56.067 at one kilometer, two kilometers, and three 15:56.067 --> 15:57.727 kilometers below sea level. 15:57.733 --> 16:01.533 But a lot at four, five, and six 16:01.533 --> 16:03.433 kilometers below sea level. 16:03.433 --> 16:06.033 Now this is a bit of a surprise because if the earth 16:06.033 --> 16:11.203 was just a rough surface, had been roughened by some 16:11.200 --> 16:17.500 process, it would have kind of a normal distribution for this 16:17.500 --> 16:18.670 hypsometric curve. 16:18.667 --> 16:21.527 It would have some average height, and then less above, 16:21.533 --> 16:22.233 and less below. 16:22.233 --> 16:26.933 But actually no, this has a double peak, a very 16:26.933 --> 16:28.973 interesting double peak. 16:28.967 --> 16:31.527 And of course that has to do with the point I already made. 16:31.533 --> 16:35.803 There are two types of crust here, this is continental 16:35.800 --> 16:41.000 crust and this is ocean crust. So this plate tectonics that 16:41.000 --> 16:44.100 gives us the two types of crust, ocean crust and 16:44.100 --> 16:48.730 continental crust, is the cause for this double peak in 16:48.733 --> 16:53.433 the hypsometric curve for land elevation. 16:53.433 --> 16:56.273 And again it just so happens that we have an amount of 16:56.267 --> 17:01.267 water that puts most of this down below sea level, and some 17:01.267 --> 17:05.297 or most of the continents just at sea 17:05.300 --> 17:07.970 level or slightly above. 17:12.400 --> 17:18.270 So now we can turn to the particular features, 17:18.267 --> 17:20.597 bathymetric features in the ocean. 17:20.600 --> 17:24.430 I'm going to talk about the abyssal plane, which are these 17:24.433 --> 17:29.273 flat lying parts of the ocean bottom. 17:29.267 --> 17:32.567 And then some of these other things that have to do with 17:32.567 --> 17:34.497 plate tectonics like the mid-ocean ridges, the 17:34.500 --> 17:37.330 trenches, and then some other features as well. 17:41.133 --> 17:43.433 The way we know all of this by the way is from 17:43.433 --> 17:44.803 acoustic depth profiling. 17:44.800 --> 17:49.500 So you take a ship, and it sends out an acoustic signal, 17:49.500 --> 17:53.130 a sound wave, and you bounce it off the bottom and you time 17:53.133 --> 17:55.903 how long it takes that signal to go down to the bottom of 17:55.900 --> 17:58.100 the ocean and back up again. 17:58.100 --> 18:01.370 And in the old days you just had a single pinger going 18:01.367 --> 18:04.997 right directly down, so you'd have to take the ship back and 18:05.000 --> 18:07.970 forth on a very complicated long route 18:07.967 --> 18:08.897 to map out the ocean. 18:08.900 --> 18:12.030 But now, they can send it out in a fan with different 18:12.033 --> 18:14.133 acoustic beams going in different directions, so you 18:14.133 --> 18:18.333 can do a single swath as you go along and get depth over 18:18.333 --> 18:21.873 some range to your left and to your right as 18:21.867 --> 18:24.327 the ship sails along. 18:24.333 --> 18:28.403 Also in some cases, but I won't be emphasizing it here, 18:28.400 --> 18:30.400 you can look for other reflections 18:30.400 --> 18:32.830 off subsurface layers. 18:32.833 --> 18:36.903 For example, that sound wave may go down into the ocean 18:36.900 --> 18:39.130 crust or the sediments a little bit, and bounce back 18:39.133 --> 18:42.703 up, giving you ideas of what's going on below the 18:42.700 --> 18:44.200 bottom of the ocean. 18:44.200 --> 18:48.600 And that'd be useful if you're doing geological surveys of 18:48.600 --> 18:51.530 the ocean crust. But for our purposes we're just going to 18:51.533 --> 18:58.073 be using that to map out the actual ocean depth itself. 18:58.067 --> 18:59.567 Stop me if you have questions here. 19:02.267 --> 19:06.567 Sound moves rapidly in seawater, the speed of sound 19:06.567 --> 19:09.797 in air is about 300 meters per second, speed of sound in 19:09.800 --> 19:12.670 water I think is three or four times that, it's really--it 19:12.667 --> 19:13.927 goes quite rapidly. 19:13.933 --> 19:16.933 Nonetheless it's still a finite speed, and you can 19:16.933 --> 19:21.003 easily time how long it takes for that acoustic signal to 19:21.000 --> 19:27.000 get back to your receiver and get the depth from that. 19:27.000 --> 19:32.700 So this is a cartoon just showing some of the features. 19:32.700 --> 19:35.970 For example, there's a section of abyssal plane, that kind of 19:35.967 --> 19:39.097 flat lying section. 19:39.100 --> 19:41.670 Flat in part, because it's composed of sediments that 19:41.667 --> 19:46.227 have fallen from above, and as they fall they fill in the 19:46.233 --> 19:49.733 cracks first, and then as you get quite a pile of sediments 19:49.733 --> 19:52.873 it tends to give you a flat surface. 19:52.867 --> 19:56.627 A mid-ocean ridge, one of these spreading centers where 19:56.633 --> 20:00.573 magmas are coming up and forming new ocean crust, tends 20:00.567 --> 20:04.427 to be elevated because those rocks are still hot, and less 20:04.433 --> 20:09.873 dense, and they float a bit higher than cold ocean crust 20:09.867 --> 20:11.667 until they cool down. 20:11.667 --> 20:14.167 And then they sink a little bit away from 20:14.167 --> 20:16.127 the spreading center. 20:16.133 --> 20:18.973 But that could come up a bit, if this abyssal plane is at 20:18.967 --> 20:22.697 five kilometers, then this mid-ocean ridge might only be 20:22.700 --> 20:27.630 two or three kilometers below the ocean surface. 20:27.633 --> 20:34.433 You can have undersea volcanoes, that'll start 20:34.433 --> 20:37.933 building from the ocean floor, just like volcanoes on land 20:37.933 --> 20:40.373 start to build from the continental 20:40.367 --> 20:43.027 surface and build upwards. 20:43.033 --> 20:45.903 Undersea volcanoes build up from the ocean floor, and in 20:45.900 --> 20:52.870 some cases they will not reach the ocean surface, in which 20:52.867 --> 20:55.327 case they're called seamounts. 20:55.333 --> 20:57.933 I don't know why they've drawn this the way they have, 20:57.933 --> 21:01.103 indicating that these have penetrated the earth's 21:01.100 --> 21:02.330 surface, the ocean surface. 21:02.333 --> 21:06.703 Usually the word seamount is confined to an undersea 21:06.700 --> 21:09.830 volcano that has not reached the earth's surface, so I 21:09.833 --> 21:12.303 disagree with the artist a little bit here. 21:12.300 --> 21:17.700 On occasion you find them with flat tops, which means that at 21:17.700 --> 21:22.600 one point they reached the ocean surface and were leveled 21:22.600 --> 21:24.700 by wave action. 21:24.700 --> 21:27.100 And now they've settled back down a little bit, so you'll 21:27.100 --> 21:31.300 find undersea volcanoes some of them with a flat surface, 21:31.300 --> 21:34.430 those are called guyots. 21:34.433 --> 21:37.103 And then the ocean trenches, where you get the subducting 21:37.100 --> 21:43.970 plates, are the deepest parts of the ocean generally. 21:43.967 --> 21:45.897 Then I wanted to make this point about 21:45.900 --> 21:47.370 a continental shelf. 21:47.367 --> 21:51.027 So there's a continent, there's a continental shelf, 21:51.033 --> 21:54.233 there's the drop down to the abyssal plane. 21:54.233 --> 22:00.303 You see then geologically, a continental shelf is really 22:00.300 --> 22:03.000 part of the continent. 22:03.000 --> 22:07.570 It just so happens that the water level is high enough so 22:07.567 --> 22:11.627 it's covered up slightly some of this, making it appear on a 22:11.633 --> 22:15.633 map of the earth that its ocean, but geologically it is 22:15.633 --> 22:18.273 continental. 22:18.267 --> 22:20.667 And there should be very little confusion about the 22:20.667 --> 22:23.767 two, because this is going to be a very shallow ocean, 22:23.767 --> 22:27.827 probably only 100 or 200 meters deep. 22:27.833 --> 22:31.533 Whereas this is five kilometers deep, so it's going 22:31.533 --> 22:37.673 to be pretty clear to separate geologic continental 22:37.667 --> 22:40.927 structures from geologic ocean crustal structures. 22:40.933 --> 22:43.303 Because they really are at a very different level, even 22:43.300 --> 22:47.400 though there's enough water at the present time, especially 22:47.400 --> 22:51.730 with the glaciers mostly melted at this time to make a 22:51.733 --> 22:55.533 little thin layer of water covering 22:55.533 --> 22:56.803 those continental shelves. 23:01.167 --> 23:06.127 So now here's the whole, the whole world ocean, and the 23:06.133 --> 23:08.633 color scheme is not quantified on here, but I can tell you 23:08.633 --> 23:10.073 basically what's going on. 23:10.067 --> 23:14.667 All the deep blue is abyssal plane, about 23:14.667 --> 23:16.527 five kilometers deep. 23:16.533 --> 23:21.803 And you see it a lot of places. 23:21.800 --> 23:26.200 These little lines that run up through the middle of oceans, 23:26.200 --> 23:29.270 for example in the South Pacific and all the way up 23:29.267 --> 23:31.727 through the Atlantic, the south and the north Atlantic, 23:31.733 --> 23:36.303 those are spreading centers, mid-ocean ridges. 23:36.300 --> 23:40.870 And then the trenches don't show up well on this diagram, 23:40.867 --> 23:43.867 but they are the darker blue still. 23:43.867 --> 23:47.667 And you see a little thin line along there that's a trench, 23:47.667 --> 23:51.267 you see one here, you see an important one here, and up 23:51.267 --> 23:55.227 along here, and even along the tip of Aleutian Islands. 23:55.233 --> 23:58.933 So those are the subduction zones. 23:58.933 --> 24:03.033 You also see a scattering of sea mounts various places, and 24:03.033 --> 24:04.133 other features. 24:04.133 --> 24:07.873 But I'd say oh and you see the, for example, right along 24:07.867 --> 24:13.127 there, you see a nice example of continental material with 24:13.133 --> 24:16.133 just enough water over it so we would call that ocean, but 24:16.133 --> 24:19.203 geologically it is continent. 24:19.200 --> 24:22.170 That's the continental shelves, 24:22.167 --> 24:24.527 continental shelf area. 24:24.533 --> 24:28.033 So this is the geometry in which then we'll be studying 24:28.033 --> 24:29.333 ocean currents and so on. 24:29.333 --> 24:36.533 It's going to be constrained by this pattern of depth, and 24:36.533 --> 24:39.103 the thing that's going to be very important to us is the 24:39.100 --> 24:42.300 way that these continents tend to break up 24:42.300 --> 24:45.870 the oceans into segments. 24:45.867 --> 24:50.667 For example, Asia with Australia included, and North 24:50.667 --> 24:53.927 and South America, break up the Pacific ocean into kind of 24:53.933 --> 24:55.833 a north, south, oriented ocean. 24:58.733 --> 25:02.573 North America, South America, compared with Europe and 25:02.567 --> 25:04.797 Africa, break up the Atlantic again into a north, south, 25:04.800 --> 25:05.730 oriented ocean. 25:05.733 --> 25:09.373 The Indian Ocean is a little bit different because Asia 25:09.367 --> 25:12.797 fills the northern hemisphere, most of it, down to about say 25:12.800 --> 25:14.900 20 degrees north latitude. 25:14.900 --> 25:17.930 So the Indian Ocean is primarily an ocean just in the 25:17.933 --> 25:20.033 southern hemisphere. 25:20.033 --> 25:23.873 And then, the one gigantic exception to this is the 25:23.867 --> 25:26.867 so-called Southern Ocean. 25:26.867 --> 25:29.297 The term for that strip of ocean that goes all the way 25:29.300 --> 25:33.730 around the globe, it's usually called the Southern Ocean in 25:33.733 --> 25:36.003 oceanography. 25:36.000 --> 25:38.800 And if you go far enough south in the Pacific you join onto 25:38.800 --> 25:42.600 it, the Atlantic you join onto it, the Indian you join on. 25:42.600 --> 25:45.670 And as we'll see, you can have ocean currents here that go 25:45.667 --> 25:47.567 right around the globe. 25:47.567 --> 25:51.127 Whereas at all these higher latitudes, any ocean current 25:51.133 --> 25:54.333 that moves east-west is going to hit a continent and is 25:54.333 --> 25:56.773 going to have to wrap back around. 25:56.767 --> 26:01.327 So you get what are called gyres, in most of the oceans, 26:01.333 --> 26:03.973 because they're confined by these 26:03.967 --> 26:07.327 north-south oriented barriers. 26:07.333 --> 26:11.033 So gyres are the things we'll be studying for most of the 26:11.033 --> 26:13.573 oceans, but not in the southern ocean. 26:13.567 --> 26:15.527 There we'll be able to look at a current that goes all the 26:15.533 --> 26:19.433 way, all the way around the globe. 26:19.433 --> 26:20.703 Questions on this? 26:23.433 --> 26:28.433 So we'll zoom in a little bit to--so all the major oceans, 26:28.433 --> 26:29.703 there's the Atlantic ocean. 26:32.733 --> 26:37.403 A little bit of a trench here, but generally it's abyssal 26:37.400 --> 26:39.970 plane, and it is with a mid-ocean 26:39.967 --> 26:41.427 spreading center here. 26:41.433 --> 26:45.303 By the way, so that's spreading which now these 26:45.300 --> 26:47.930 continents were originally joined together, and you can 26:47.933 --> 26:52.103 see how well they fit from a jigsaw puzzle point of view. 26:52.100 --> 26:58.200 That spreading is at about the rate the way I remember this 26:58.200 --> 27:02.570 it's at about the rate that your fingernails grow, so it's 27:02.567 --> 27:06.197 a couple of centimeters per year basically. 27:06.200 --> 27:11.100 So this is still widening today, and at the end of the 27:11.100 --> 27:12.530 year it's going to be a few centimeters-- 27:12.533 --> 27:15.473 the Atlantic ocean is going to be a few centimeters wider 27:15.467 --> 27:18.267 than it is today. 27:18.267 --> 27:22.067 And that's a slow process, but when you take that speed and 27:22.067 --> 27:25.497 multiply it over millions of years, you can see how you can 27:25.500 --> 27:29.800 get hundreds or even thousands of kilometers of ocean width 27:29.800 --> 27:34.870 generated by that slow spreading. 27:34.867 --> 27:39.667 And then once again, you see some continental shelf area up 27:39.667 --> 27:41.867 in here, covered by water, but 27:41.867 --> 27:45.697 generally part of the continent. 27:45.700 --> 27:49.930 And you see a big piece of that down here as well, around 27:49.933 --> 27:52.073 Africa a little bit too. 27:52.067 --> 27:55.927 And all of the Mediterranean sea, most of it is you'd 27:55.933 --> 27:58.533 probably call it ocean--continental crust 27:58.533 --> 28:01.733 rather than ocean crust. 28:01.733 --> 28:06.333 Pacific ocean, vast areas of abyssal plane, but there are 28:06.333 --> 28:12.003 trench systems hard to see, but the black lines. 28:12.000 --> 28:14.500 Mid-ocean ridge that comes down here, was spreading, and 28:14.500 --> 28:18.470 then lots of ocean crust here. 28:18.467 --> 28:21.527 And then up along California, you're having some motions. 28:21.533 --> 28:25.033 It's more of a transform fault, where things are moving 28:25.033 --> 28:28.773 parallel to one another, not exactly a spreading center, 28:28.767 --> 28:32.967 not exactly subduction, but some complicated combination 28:32.967 --> 28:35.567 of the two including lateral slip. 28:38.433 --> 28:41.173 And so we'll see, once again, we'll get gyres in the 28:41.167 --> 28:46.227 Northern and Southern Pacific constrained by the barrier 28:46.233 --> 28:50.433 affect of the continents to the east and west. 28:50.433 --> 28:55.673 Indian Ocean bathymetry, big continental a big abyssal 28:55.667 --> 28:59.597 planes, and then some curious old, these appear to be old 28:59.600 --> 29:03.170 spreading centers that are no longer active. 29:03.167 --> 29:05.127 Maybe Erin can tell us more about that. 29:05.133 --> 29:07.433 But anyway there's some structures there, but they 29:07.433 --> 29:10.033 don't appear to be active spreading centers. 29:10.033 --> 29:12.533 And here's some of the subduction zones over in the 29:12.533 --> 29:13.803 Pacific there. 29:16.000 --> 29:18.230 The Arctic Ocean has some interesting structure, but 29:18.233 --> 29:19.433 it's pretty passive too. 29:19.433 --> 29:26.433 There's two deep basins--most of this is continental shelf, 29:26.433 --> 29:35.003 pretty shallow, depth the order of a few hundred meters. 29:35.000 --> 29:39.630 But there are a couple of deep parts as well, that go down to 29:39.633 --> 29:41.033 several kilometers. 29:41.033 --> 29:45.733 And then a ridge, I think a passive one, that's called the 29:45.733 --> 29:48.803 Lomonosov Ridge, but I don't think that's an active 29:48.800 --> 29:49.500 spreading center. 29:49.500 --> 29:51.500 So this has got some interesting structure, but 29:51.500 --> 29:55.200 it's more of a passive structure at the moment, it's 29:55.200 --> 29:57.200 not an active spreading center. 30:00.333 --> 30:05.103 OK, so that's an introduction to the shape 30:05.100 --> 30:06.330 of the ocean basins. 30:06.333 --> 30:07.503 Are there any questions on that? 30:11.233 --> 30:16.333 I want to turn now to ocean properties, ocean water 30:16.333 --> 30:17.373 properties. 30:17.367 --> 30:22.097 Sea surface temperature, that we measure from ships, from 30:22.100 --> 30:24.170 instruments below the ships, and from satellites. 30:26.733 --> 30:30.803 We can measure salinity from ships, to get into the deep 30:30.800 --> 30:33.330 ocean we need to put something down into the ocean, I'll show 30:33.333 --> 30:35.573 you how we do that. 30:35.567 --> 30:39.867 For ocean topography, which I'll define later, a little 30:39.867 --> 30:43.067 irregularity in the ocean surface, we use 30:43.067 --> 30:44.267 satellites for that. 30:44.267 --> 30:47.597 And then for ocean currents we use all those things ships, 30:47.600 --> 30:48.900 floats, and moorings. 30:48.900 --> 30:51.370 So before we're done, I'm going to talk about how we 30:51.367 --> 30:55.097 measure all these things, and understand a bit about the 30:55.100 --> 30:58.470 ocean water and how it's moving. 30:58.467 --> 31:06.727 So here is a map taken in late August, I think of the year 31:06.733 --> 31:11.203 2000 from satellites, showing the sea surface temperature, 31:11.200 --> 31:17.030 SST. The color is in Celsius, and you see that in the 31:17.033 --> 31:20.303 tropical regions you're getting up to temperatures 28 31:20.300 --> 31:20.570 and higher. 31:20.567 --> 31:25.797 Remember 27, 28, was the threshold for hurricanes. 31:25.800 --> 31:29.370 So you are getting a lot of warm ocean temperature that 31:29.367 --> 31:31.967 support hurricanes, some of that however is right at the 31:31.967 --> 31:35.627 equator and so you couldn't have hurricanes forming there 31:35.633 --> 31:37.633 because you don't have the Coriolis force. 31:37.633 --> 31:40.903 But in other parts you get the warm water extending far 31:40.900 --> 31:44.030 enough away from the equator, so you could have hurricanes. 31:44.033 --> 31:46.003 I pointed out when we were talking about hurricanes, two 31:46.000 --> 31:49.100 interesting places. 31:49.100 --> 31:55.470 The Western tropical South Pacific, where you don't have 31:55.467 --> 31:58.167 hurricanes because it's too cold, and you see it there. 31:58.167 --> 32:00.727 It's a cold ocean current, the Humboldt Current coming up 32:00.733 --> 32:06.703 here, taking cold water from the Southern Ocean, peeling 32:06.700 --> 32:09.870 some of it off, bring it up here, and keeping that part of 32:09.867 --> 32:12.527 the world ocean cool. 32:12.533 --> 32:16.773 And you see something very similar here, where cold 32:16.767 --> 32:18.827 waters being peeled off and come up here to keep the 32:18.833 --> 32:22.873 southern tropical Atlantic a bit cool as well. 32:22.867 --> 32:27.697 Otherwise it's pretty warm in the tropics, except where 32:27.700 --> 32:29.030 you're getting these cool currents. 32:29.033 --> 32:32.233 The California current does a little bit of cooling in this 32:32.233 --> 32:35.003 region, and the return from the Gulfstream does some 32:35.000 --> 32:39.270 cooling on the eastern side of the north Atlantic there, so 32:39.267 --> 32:42.097 that's that now. 32:42.100 --> 32:45.600 But if you get a mental picture of this, be very 32:45.600 --> 32:48.630 careful what you do with it, because you could 32:48.633 --> 32:49.833 be very much misled. 32:49.833 --> 32:54.933 This is sea surface temperature, when I go down 32:54.933 --> 32:58.503 even just one kilometer in the ocean, or especially if I go 32:58.500 --> 33:00.470 down to the ocean bottom, it looks 33:00.467 --> 33:02.927 nothing at all like this. 33:02.933 --> 33:05.103 So temperature is not vertically homogeneous. 33:08.433 --> 33:11.473 As I'll show you later on, this warm water that can form 33:11.467 --> 33:16.597 near the tropics, forms a rather thin layer floating on 33:16.600 --> 33:21.100 the cold water that fills most of the world ocean. 33:21.100 --> 33:27.930 So this is not a picture that can then be transferred down 33:27.933 --> 33:31.003 into the ocean very deep at all. 33:31.000 --> 33:33.970 In fact in some cases, you may only be able to go down a few 33:33.967 --> 33:38.127 hundred meters before this picture changes rather 33:38.133 --> 33:39.373 dramatically. 33:41.800 --> 33:45.330 Salinity is the other important property we track in 33:45.333 --> 33:50.003 ocean water, and here's a map of the sea surface salinity. 33:50.000 --> 33:53.600 So just the surface, just the salinity you would measure 33:53.600 --> 33:56.270 from a ship if you took a surface sample of water and 33:56.267 --> 33:58.397 analyzed it. 33:58.400 --> 34:06.270 And well, the most remarkable thing is the narrow range. 34:06.267 --> 34:11.327 And you get some salinities as fresh as maybe 31 or 32 parts 34:11.333 --> 34:13.603 per thousand. 34:13.600 --> 34:18.800 And some, perhaps in the Mediterranean Sea, and the Red 34:18.800 --> 34:23.370 Sea, Gulf of Aden perhaps, getting up to 38, 39. 34:23.367 --> 34:27.927 But generally that's the full range of ocean salinity. 34:27.933 --> 34:31.333 So you've heard me say, occasionally, that sea surface 34:31.333 --> 34:34.973 salinity is 35 parts per thousand, of course that's not 34:34.967 --> 34:38.327 a very precise statement. 34:38.333 --> 34:43.173 But it only goes about plus or minus four parts per thousand 34:43.167 --> 34:47.167 around that mean value of 35 parts per thousand. 34:47.167 --> 34:53.227 The reason for this must be that the ocean mixes itself 34:53.233 --> 34:57.603 occasionally, to maintain this kind of 34:57.600 --> 35:02.370 rather homogeneous salinity. 35:02.367 --> 35:04.727 At least compared to the rate at which you're adding 35:04.733 --> 35:07.533 freshwater, or the rate at which you're adding or 35:07.533 --> 35:08.773 subtracting salt. 35:08.767 --> 35:13.467 So the basic story behind this narrow range of salinity, is 35:13.467 --> 35:16.167 that the ocean is, at least for salt, 35:16.167 --> 35:19.897 relatively well mixed. 35:19.900 --> 35:22.630 Now if it were completely well mixed, it would have the same 35:22.633 --> 35:24.033 salinity everywhere. 35:24.033 --> 35:27.903 If it were vigorously being stirred, like a vigorous spoon 35:27.900 --> 35:31.000 stirring in a pot, any differences would be 35:31.000 --> 35:31.830 immediately removed. 35:31.833 --> 35:36.073 So it's not perfectly well stirred, but is reasonably 35:36.067 --> 35:40.227 well stirred, giving you this rather homogeneous salinity 35:40.233 --> 35:43.133 over the world ocean. 35:43.133 --> 35:47.133 So that's lesson number one, but at the next level though 35:47.133 --> 35:51.733 of detail, we can notice that there are some variations. 35:51.733 --> 35:54.033 And they probably make sense to us because it's in 35:54.033 --> 36:03.203 these--it's in the so called Intertropical Convergence 36:03.200 --> 36:07.830 Zone, or the belt of tropical rainforests, that you get a 36:07.833 --> 36:09.633 little bit lower salinity. 36:09.633 --> 36:12.773 That's right through here, and you see it here, and a little 36:12.767 --> 36:14.127 bit down through here. 36:14.133 --> 36:17.603 So there's a lot of rain falling on the ocean there, 36:17.600 --> 36:21.330 fresh water coming down and diluting the salt a little 36:21.333 --> 36:26.433 bit, giving you a smaller salinity along the equator. 36:26.433 --> 36:30.033 And then as you move north and south from that into the belt 36:30.033 --> 36:34.173 of deserts, the descending branch of the Hadley cell, 36:34.167 --> 36:37.427 with very little precipitation and some evaporation. 36:37.433 --> 36:40.203 Remember when you evaporate seawater, you leave the salt 36:40.200 --> 36:43.500 behind, and so the salinity is going to be increased. 36:43.500 --> 36:47.430 And you see the increased salinity there in both the 36:47.433 --> 36:49.973 Northern and Southern hemisphere, connected with the 36:49.967 --> 36:50.897 belt of deserts. 36:50.900 --> 36:53.530 And then you get up in the mid-latitudes and once again 36:53.533 --> 36:56.773 you've got the frontal storms, cold fronts, warm fronts, 36:56.767 --> 36:58.967 bringing rain and that once again 36:58.967 --> 37:01.797 dilutes the surface salinity. 37:01.800 --> 37:05.300 So a lot of things we've spoken about before, in terms 37:05.300 --> 37:07.830 of the general circulation of the atmosphere and where it 37:07.833 --> 37:13.873 rains and where it doesn't, are reflected in this plot of 37:13.867 --> 37:17.367 sea surface salinity. 37:17.367 --> 37:21.427 Here again it'd be dangerous though, to try to imagine that 37:21.433 --> 37:24.403 pattern would extend down to the oceans very far. 37:24.400 --> 37:27.230 Other things will take place that will prevent this from 37:27.233 --> 37:32.603 being the pattern deeper down in the ocean. 37:32.600 --> 37:33.830 Questions on that? 37:37.033 --> 37:41.333 OK, now I want to tell you about how we get how we 37:41.333 --> 37:46.903 measure ocean properties down into the ocean. 37:46.900 --> 37:50.700 The old way to do this, and I'll talk about the new ways 37:50.700 --> 37:54.670 too, the old way for about 50 years, the predominant way for 37:54.667 --> 37:57.567 sounding the ocean, for getting temperature and 37:57.567 --> 38:01.827 salinity profiles with depth, was the Nansen bottle. 38:01.833 --> 38:05.273 And here's a diagram, here's a picture of one, mounted on a 38:05.267 --> 38:09.197 cable that's about to be put down into the ocean. 38:09.200 --> 38:13.870 And here is a cartoon of what happens after it's put down 38:13.867 --> 38:16.397 into the ocean and is triggered, so that it tips 38:16.400 --> 38:18.670 over and the valves close on it. 38:18.667 --> 38:22.927 Now I have one of those with me, and I want to show how 38:22.933 --> 38:27.473 this thing works because it's kind of a clever gadget. 38:27.467 --> 38:30.627 And although it's not being used much anymore, much of 38:30.633 --> 38:33.703 what we know about the world ocean came 38:33.700 --> 38:35.630 from this simple device. 38:35.633 --> 38:37.803 Again it's called a Nansen bottle, it was designed by the 38:37.800 --> 38:42.370 famous Norwegian explorer Fridtjof Nansen. 38:42.367 --> 38:48.397 And it is composed of a hollow metal tube, fairly thin and 38:48.400 --> 38:53.830 its steel, with a valve at the top and the bottom, that is 38:53.833 --> 38:57.433 currently If I look through this it's open, I've got both 38:57.433 --> 39:00.303 valves open. 39:00.300 --> 39:05.030 And there is a tie rod that connects the two valves, and 39:05.033 --> 39:09.733 so when this tie rod shifts relative to the bottle itself, 39:09.733 --> 39:13.473 it'll close both valves simultaneously. 39:13.467 --> 39:17.667 So there's also a couple of housings for thermometers on 39:17.667 --> 39:20.627 the outside, so you can get the temperature of the water 39:20.633 --> 39:22.233 at that depth as well. 39:22.233 --> 39:27.203 So imagine that you've come you've taken your ship to a 39:27.200 --> 39:37.930 given location, and you've got a long cable coming up to a 39:37.933 --> 39:43.973 winch and a pulley, that then takes that cable overboard and 39:43.967 --> 39:47.997 down to whatever depths you want to go to. 39:48.000 --> 39:49.570 So how do we get started with that? 39:49.567 --> 39:52.767 Well the winch operator gets some of the cable in the 39:52.767 --> 39:57.967 water, and then the scientist would lean over and attach the 39:57.967 --> 40:00.927 first one of these Nansen bottles to this cable. 40:00.933 --> 40:03.733 So imagine you've got a cable coming down like this, and 40:03.733 --> 40:07.173 you're leaning over the side of the ship, and you reach out 40:07.167 --> 40:10.027 and you fasten this thing in by putting the cable right 40:10.033 --> 40:13.273 down in there and pushing that little pin over. 40:13.267 --> 40:15.827 And then down here the cable comes through there, and you 40:15.833 --> 40:18.103 lock it with a turn buckle. 40:18.100 --> 40:21.670 So it's locked securely at the bottom, not so 40:21.667 --> 40:24.727 securely at the top. 40:24.733 --> 40:28.133 Once you get it on there, you wave to the winch operator, 40:28.133 --> 40:30.233 and he puts that cable down. 40:30.233 --> 40:34.603 And you'd repeat that about 20 times over the next hour or 40:34.600 --> 40:39.000 two, and when you're all done you've got a cable overboard, 40:39.000 --> 40:40.970 perhaps going down to five kilometers. 40:40.967 --> 40:43.597 You can do this all the way down to the abyssal plane, and 40:43.600 --> 40:47.170 along this cable you have Nansen bottles, perhaps as 40:47.167 --> 40:52.627 many as 20 of them. 40:52.633 --> 40:56.073 When that's all in place, then you reach over one more time 40:56.067 --> 40:58.967 and you fasten a little thing called a messenger, a little 40:58.967 --> 41:05.197 brass cylinder that slides down the cable, hits this 41:05.200 --> 41:12.030 little plate, knocking free this pin, and now the weight 41:12.033 --> 41:15.273 of the bottle begins to act. 41:15.267 --> 41:24.327 Let's see if I can do this now, OK I'm having trouble 41:24.333 --> 41:25.403 doing it with my bum arm here. 41:25.400 --> 41:32.200 But I'm pushing this thing down, and as I do so it is has 41:32.200 --> 41:38.130 fallen away to an orientation like this, and now both valves 41:38.133 --> 41:43.473 are closed, trapping water from that level. 41:43.467 --> 41:45.967 So when it's brought up, you've got samples of the 41:45.967 --> 41:47.127 water at each of these depths. 41:47.133 --> 41:50.133 Also, when you flip over these thermometers, they're so 41:50.133 --> 41:53.003 called reversing thermometers, and when they're flipped over 41:53.000 --> 41:57.530 the mercury column breaks, and so you lock in the temperature 41:57.533 --> 41:59.303 at that depth. 41:59.300 --> 42:01.800 And when you bring it up, you've got a record of what 42:01.800 --> 42:03.100 the temperature was at that depth. 42:03.100 --> 42:07.000 So by flipping it over, you lock in the temperature, and 42:07.000 --> 42:09.600 by flipping it over and closing the valves, you lock 42:09.600 --> 42:12.370 in the sample of water from that depth. 42:12.367 --> 42:15.627 Then over the next couple of hours you bring that cable 42:15.633 --> 42:19.273 back to the surface, and each time when a Nansen bottle 42:19.267 --> 42:21.727 emerges, you reach over, disconnect it, 42:21.733 --> 42:22.973 and put it in a rack. 42:22.967 --> 42:25.767 And after several hours of work, you've got data from the 42:25.767 --> 42:26.767 whole sounding. 42:26.767 --> 42:29.967 And over a period of 50 years or so throughout the 40's, 42:29.967 --> 42:34.597 50's, 60's, 70's, and 80's, this was used to map out most 42:34.600 --> 42:37.000 of the world ocean in terms of its temperature 42:37.000 --> 42:39.470 and salinity structure. 42:39.467 --> 42:42.527 Questions on that? 42:42.533 --> 42:45.733 You can pass that around if you like, so people can get a 42:45.733 --> 42:47.073 sense for it. 42:47.067 --> 42:50.667 Now my first oceanographic cruise I managed to embarrass 42:50.667 --> 42:59.527 myself, by failing to get that Nansen bottle properly secured 42:59.533 --> 43:01.533 before I waved to the winch operator to 43:01.533 --> 43:02.333 tell him to go down. 43:02.333 --> 43:06.103 So the very first one of these I did, one of the Nansen 43:06.100 --> 43:12.070 bottles came up totally destroyed, crushed. 43:12.067 --> 43:15.767 And can you figure out how that would have happened? 43:15.767 --> 43:20.697 So what happened is, it wasn't secured properly at the top. 43:20.700 --> 43:24.930 It had flipped, I didn't notice I turned my back away, 43:24.933 --> 43:28.333 but it flipped before it entered the water. 43:28.333 --> 43:33.533 So both valves had closed with air inside, and then it went 43:33.533 --> 43:34.233 down to depth. 43:34.233 --> 43:37.073 And of course, the pressure is enormous at the bottom of the 43:37.067 --> 43:39.697 ocean, you can do a quick hydrostatic calculation to 43:39.700 --> 43:41.930 understand you can get hundreds of atmospheres of 43:41.933 --> 43:43.133 pressure down there. 43:43.133 --> 43:45.533 And it just took that bottle with air inside, and just 43:45.533 --> 43:49.703 crushed it like a Coke can in your fist. And it came up all 43:49.700 --> 43:52.570 wrinkled, and my chief scientist was not very happy 43:52.567 --> 43:57.567 with me but I learned to do better the next time. 43:57.567 --> 44:01.727 Anyway it's important historical, but also to 44:01.733 --> 44:04.473 understand how that was done. 44:07.200 --> 44:13.300 Today it's done a bit differently, it's done with a 44:13.300 --> 44:14.970 CTD and a rosette. 44:14.967 --> 44:18.627 Now CTD is an abbreviation meaning conductivity 44:18.633 --> 44:20.233 temperature depth. 44:20.233 --> 44:25.433 It's an electronic device, the conductivity part is similar 44:25.433 --> 44:29.333 to what we did on the river lab. 44:29.333 --> 44:32.703 As you lower this thing down into the ocean, it measures 44:32.700 --> 44:35.930 the electrical conductivity and from that you can 44:35.933 --> 44:38.703 determine the salinity. 44:38.700 --> 44:41.030 It also has a thermistor on there, so you can measure 44:41.033 --> 44:44.173 temperature, and it's got a pressure sensor on there so 44:44.167 --> 44:45.267 you can measure depth. 44:45.267 --> 44:48.527 So as you lower this down, in real time because there's a 44:48.533 --> 44:51.633 cable coming back to the surface, you're getting a 44:51.633 --> 44:54.333 detailed profile of temperature, 44:54.333 --> 44:56.873 salinity, and depth. 44:56.867 --> 44:59.267 That's a great system. 44:59.267 --> 45:03.997 However that of course would not give you a water sample, 45:04.000 --> 45:08.800 so if you want to do any kind of water chemistry a CTD would 45:08.800 --> 45:10.170 not be sufficient. 45:10.167 --> 45:14.127 The beauty of the old Nansen bottle was, you got a water 45:14.133 --> 45:15.033 sample as well. 45:15.033 --> 45:17.133 So if you later on decide you want to do any kind of 45:17.133 --> 45:19.603 chemistry, you had that water sample. 45:19.600 --> 45:23.070 The CTD is a great advance on that, but it doesn't give you 45:23.067 --> 45:23.967 the water sample. 45:23.967 --> 45:26.997 So what's usually done today, is that they have a set of 45:27.000 --> 45:31.630 water collecting bottles arranged around the perimeter, 45:31.633 --> 45:34.473 and that's called the rosette. 45:34.467 --> 45:38.067 And they've got an electronic control, where the valves on 45:38.067 --> 45:42.067 these can be closed on command. 45:42.067 --> 45:44.227 So in addition so you send this down to the bottom the 45:44.233 --> 45:47.333 ocean, profiling conductivity, temperature, depth. 45:47.333 --> 45:52.633 And then every, I don't know, 500 meters or so, you might 45:52.633 --> 45:56.533 close one of the valves on these bottles and you get a 45:56.533 --> 45:58.533 water sample from that depth as well. 45:58.533 --> 46:01.433 So if you were to go out on an oceanographic cruise today, 46:01.433 --> 46:05.733 you'd be largely using the CTD and the rosette to do vertical 46:05.733 --> 46:10.973 profiling of temperature and salinity. 46:10.967 --> 46:12.197 Questions on that? 46:15.833 --> 46:19.133 So there's one, they brought it back on board and they're, 46:19.133 --> 46:22.103 I guess, drawing samples off of the, from the different 46:22.100 --> 46:25.330 water bottles there. 46:25.333 --> 46:29.503 Now technology is advancing even beyond that, however. 46:29.500 --> 46:32.600 Remember even with that one, you have to get a ship there, 46:32.600 --> 46:34.530 you've got to get a cable over that can go all the way to the 46:34.533 --> 46:38.433 bottom, and that can take a couple hours to do, and ship 46:38.433 --> 46:39.633 time is very expensive. 46:39.633 --> 46:43.203 So now the field is moving in the direction of these 46:43.200 --> 46:51.470 autonomous explorers, where it's an unmanned vehicle that 46:51.467 --> 46:55.227 you launch from a ship but then it pretty much goes its 46:55.233 --> 46:56.633 own way, and you can control it. 46:56.633 --> 47:00.973 You talk to it with sonar, sending acoustic signals to it 47:00.967 --> 47:05.467 which it can receive, and it will then cruise around in the 47:05.467 --> 47:08.527 ocean up, or down, or laterally, measuring. 47:08.533 --> 47:11.133 Of course you don't get water samples from this but you get 47:11.133 --> 47:14.073 temperature, conductivity, and depth, because this 47:14.067 --> 47:15.997 has a CTD on board. 47:16.000 --> 47:18.400 And you can control it and move it wherever you want, 47:18.400 --> 47:21.130 back and forth. 47:21.133 --> 47:23.833 In one area, if you think conditions might be changing, 47:23.833 --> 47:27.033 or traversing larger parts of the ocean, if you want to map 47:27.033 --> 47:31.803 out a big piece of the ocean volume. 47:31.800 --> 47:35.200 So this is a new thing coming on, it's quite exciting to 47:35.200 --> 47:37.970 have this, and you can be back in your office actually in a 47:37.967 --> 47:40.727 way, and this data is coming out on your screen. 47:40.733 --> 47:43.403 It's pretty remarkable what this will mean for 47:43.400 --> 47:44.130 oceanography. 47:44.133 --> 47:46.773 This is just getting started, so this is really opening up 47:46.767 --> 47:50.827 brand new doors for understanding the oceans with 47:50.833 --> 47:53.933 these autonomous explorers. 47:53.933 --> 47:55.203 Any questions on that? 47:57.533 --> 47:59.833 OK. 47:59.833 --> 48:02.633 So what do you get from this then? 48:02.633 --> 48:04.273 We're almost out of time, but here's a 48:04.267 --> 48:08.027 typical ocean sounding. 48:08.033 --> 48:12.873 Temperature, salinity, and density, versus depth. 48:12.867 --> 48:16.897 So on this plot, zero is taken to be sea level, and this goes 48:16.900 --> 48:19.530 down to about 4000 meters which is almost the depth to 48:19.533 --> 48:21.403 the abyssal plane. 48:21.400 --> 48:25.900 Very often you find rapid changes at first, and when the 48:25.900 --> 48:30.400 temperature drops from warm surface to colder values 48:30.400 --> 48:33.800 beneath, that region of strong temperature gradient is 48:33.800 --> 48:36.500 referred to as the thermocline. 48:36.500 --> 48:38.500 And you'll hear that term over and over again, it's very 48:38.500 --> 48:41.130 important in the ocean, this thermocline. 48:41.133 --> 48:44.233 In this particular sounding, it started about 200 meters 48:44.233 --> 48:47.003 below the surface, and by the time you got down to 500 or 48:47.000 --> 48:49.730 600 meters, you're at a temperature of about four 48:49.733 --> 48:53.303 degrees Celsius, and then eventually down to just one 48:53.300 --> 48:54.600 degree Celsius. 48:54.600 --> 49:00.770 Whereas at the surface you had 25, 26, 27 degrees Celsius. 49:00.767 --> 49:07.697 The salinity in this case was large at the surface, 34.9, 49:07.700 --> 49:10.830 became a bit fresher as you dropped down through what's 49:10.833 --> 49:15.033 called the halocline, and then became slowly a little bit 49:15.033 --> 49:16.533 saltier beneath. 49:19.200 --> 49:25.330 Now the density of seawater is controlled primarily by the 49:25.333 --> 49:28.803 temperature and the salinity. 49:28.800 --> 49:32.230 The density is a very important quantity, but if you 49:32.233 --> 49:34.833 know temperature and you know salinity, you can compute or 49:34.833 --> 49:37.073 you can measure the density. 49:37.067 --> 49:41.397 So what's plotted in this final panel, is the density 49:41.400 --> 49:46.600 derived from the measured temperature and salinity. 49:46.600 --> 49:51.300 And it shows a lower density water near the surface by the 49:51.300 --> 49:56.970 way, the units are in grams per cubic centimeter here, 49:56.967 --> 49:59.597 remember fresh water has a density of about 49:59.600 --> 50:02.170 one in those units. 50:02.167 --> 50:05.097 So this is a little bit denser than fresh water, and gets 50:05.100 --> 50:07.530 even denser by the time you get down to the bottom 50:07.533 --> 50:11.203 primarily because of the temperature in this case. 50:11.200 --> 50:13.770 The colder temperature is giving rise to the lower--to 50:13.767 --> 50:19.327 the higher density in the deep ocean. 50:23.100 --> 50:25.570 I think I'm out of time, so we're going to continue this 50:25.567 --> 50:28.827 next time and talk about the concept of static stability. 50:28.833 --> 50:31.273 When will water remain in layers? 50:31.267 --> 50:33.797 When will it overturn to form convection? 50:33.800 --> 50:35.630 And this will be the starting point for that discussion.