GG 140: The Atmosphere, the Ocean, and Environmental Change
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The Atmosphere, the Ocean, and Environmental Change
GG 140 - Lecture 19 - Ocean Bathymetry and Water Properties
Chapter 1: Plate Tectonics [00:00:00]
Professor Ron Smith:
We are starting a brand new subject today. It’s one though that is integral with the course, and that’s the study of the oceans. As we’ll see, not only the atmosphere and the ocean interact, each influences the other which makes it necessary to understand both. But also, to some extent, they obey similar laws of physics. For example, questions of static stability, when will a column of air turn over, when will a column of water turn over, what makes the winds blow, what makes the ocean currents move, Coriolis plays–Coriolis force plays a similar role in both.
So this would be a time not only for you to learn a few things about the ocean, but to establish more linkages between the things you’ve learned before and the things you’re learning now. Now that we’re halfway, or almost halfway through the course, linkages are a big part of the course. Where you find connections between things we’ve already done, and things we’re learning now. So try to flag those whenever you come across them, I think it’ll help you learn the material more and it’ll establish a sense, I hope, of unity in the course. Where things begin to gel, and become easier, because a very limited set of physical principles can apply to a wide range of geophysical phenomena.
OK, so what we’re going to do today. I’ll probably just do the first two, we’ll talk about the bathymetry of the oceans. The word bathymetry here means the study of the ocean depth. So it’s pretty straightforward, just how deep is the ocean. That’s the subject of bathymetry. We’re going to see, however, that ties very closely to plate tectonics. Now if you were taking a course in geology, which my department offers a number of, you would study plate tectonics in quite some detail. It’s the modern paradigm to understand how the continents were formed, and the oceans are formed, and a number of the things you see in rocks. We’re just going to touch on it briefly, because this isn’t a course in geology, but I need to do it because you can’t understand the bathymetry of the oceans without understanding a bit about plate tectonics.
Has anybody had a course in geology where they talked about plate tectonics? Sarah has–So you’re going to see that stuff coming back, but fairly quickly. I’m not going to spend too much time on it. We’ll spend most of today talking about temperature and salinity, and we might get into ocean currents today, but if not that’ll be the subject of next time. And then biological productivity will eventually, maybe Friday or next week, get into El Niño.
Sarah reminded me that there’s no chapter in your book on oceanography, so we’re kind of on our own here. These notes will be posted, but there is a section on El Niño, and you’re going to want to read that, and that’ll give a little background information about the oceans. And there may be a few little other bits and pieces scattered throughout the textbook on oceanography. So take advantage of what you have there, but realize there isn’t a great deal, and so you’ll have to rely fairly heavily on the notes here.
OK, so some of you have seen this, probably most of you seen this. The basic idea behind plate tectonics is that the spherical skin of the earth, this shallow layer called the crust that’s rather rigid floating above a deeper semi-liquid mantle, that crust is broken into several discrete plates. The Indo-Australian plate, the African plate, the South American plate, and those plates, as they move around remain rigid. And so the interactions, or the interesting parts, occur at the boundaries where one rigid plate butts up against another and some kind of interaction occurs. Maybe it’s a subduction zone, maybe it’s a new–new plate material being formed, but that’s the way we try to understand the structure of the earth these days is to understand where are these plates, and how do their edges deform as the plates move around.
So for example, that boundary there is a mid-ocean ridge, or a so called mid-ocean spreading center, you can tell that from the arrow. So new ocean crust is being formed at that point, and so this plate is moving away from that plate at a certain rate. Up here in the North Atlantic as well, you’ve got the plates pulling away from each other, and new ocean crust is being formed. In a line like that–look, this artist has used kind of a cold front symbol, I don’t know where he got that idea from. But the basic idea here is that that symbol, for this artist, is referring to a subduction zone, where one plate is being drawn down under another. And so crustal material is being disappeared, it’s being returned down into the mantle of the earth.
Notice that some plates are consisting of ocean only, some plates are consisting of a combination of continental crust and ocean crust. OK, so that’s the basic idea behind plate tectonics. The plates are shown also in this cartoon, and once again note that–for example, this large Pacific plate, fairly rigid and giant in size, is ocean crust only. Whereas most of the others, not the Nazca plate, but the Australian plate, has ocean crust plus continental crust. The Eurasian plate has both, North American plate has a big chunk of ocean in it.
So when you think about plates–Remember the earlier, the thing that preceded this, was the theory of continental drift. Continental drift was the idea that people noticed this nice jigsaw fit, for example, between South America and the bite of Africa. Or the way this coast kind of could fit in against here. Or the fact that the rocks here, were similar to the rocks there up in Scotland. So the idea was that, early on, that the continents may have moved. But the early idea was that they moved through the ocean, the continents plowed their way through ocean crust to move around on our planet.
However that was soon found to be incorrect, because the physics of trying to push a continent through ocean crust was shown to be impossible. Instead this is the vision that now seems to be the right one, where you’ve got plates sometimes consisting of continents and oceans. They move relative to another, relative to one another, and all the action is right at their boundaries, where there’s creation of new crust or the destruction of crust. So this is the conceptual model that seems to fit all the data that we have.
And what would drive that kind of motion, well it’s basically part of the mantle convection. So there’s heat being generated in the interior of the earth, by the decay of radioactive elements, like uranium and so on, decays naturally, releases heat. The interior of the earth heats up, and that destabilizes the lapse rate if you like. It basically, if you’re heating this fluid from below, you’re destabilizing it and convection begins. And as part of that convection cell, then you get spreading centers for the crust and then subduction zones where some of that crustal material is drawn back into the mantle, and melted, and returned. So it’s not a crust only phenomenon, it’s driven by mantle convection, but for our purposes we’re interested primarily in what it does to the crust of the earth.
So going back into geologic time, we can see this happening. So here’s the present day, and then we go back in time to the Cretaceous, the Jurassic, the Triassic, and Permian. Here’s a geologic time scale, the age of the earth, this is in thousands–in millions of years, the age of the earth is back here around five, roughly five billion years ago. And the first one of these diagrams that the artist is showing is the Permian, which is about 255 million years ago. At that point all the continents were together, in a giant super continent called Pangaea.
And then as time progressed it split up, first with a seaway that came through this way, and then eventually you begin to get the Atlantic ocean opening up, and today you have something like this. So we’re going from 255 million years ago for the Permian, and then stepping forward to the Triassic, the Jurassic, the Cretaceous. So when the dinosaurs roamed the earth, it was in this stage. And when humans evolved, well it was already looking like this. So humans never saw this configuration, humans evolved just in the last couple of million years. So we’ve been looking at that structure for our evolutionary history.
Now, so what does this mean for the structure of the oceans and the continents? So here’s a section through–let’s take it through here I guess–an east-west, section. I think it’s in the southern hemisphere, let’s check that. Yeah, so there’s South America, so this is an east-west, section through the South Atlantic ocean. It shows the Pacific Ocean plate, which is continental crust being subducted below the mountains of South America. As that material is drawn down into the mantle, it melts. As it melts, some lava, some magma, has come off of that. That then make their way upwards to cause the volcanoes along the west coast of South America. Otherwise the rest of that material is just lost down into the mantle.
When you get over into the Atlantic Ocean, there’s a rigid boundary there that’s part of the same plate. Continental crust, and ocean crust, part of the same plate, but there’s a spreading center. That’s where molten material is coming up again and solidifying as it cools to form new ocean crust. So this is moving away, while new ocean crust is being created right at that point. And then over here, that’s a rigid-rigid connection, so that once again is part of the same plate with no crust being lost or gained at that particular boundary.
But the point I want to make here is that there are two types of crust. Continental crust is generally of a lighter material, and floats a bit higher in the semi-molten parts of the mantle, whereas the ocean crust is a little bit denser and floats a bit lower. So you’ve got basically a lower floating ocean crust here, and a less dense higher floating continental crust. And then when you fill that with water–the water has nothing to do with this of course, but the addition of water makes it seem like an ocean to us.
There happens to be enough water in the ocean to generally cover the ocean crust, but not to cover the continents. Now if there were twice as much liquid water available on the planet, that distinction would be less important because the water level would be here, and it would cover both the ocean crust and the continental crust. But with the amount of water that we have, it means that the continental crust usually sticks above sea level and the ocean crust does not, it’s submerged below sea level.
Remember that amount of water is quite unconnected with any of this. The amount of water we have on the planet probably did come out of the interior of the planet over geologic time, but it’s just an accident that it happens to be deep enough to cover the ocean crust but not the continental crust. And we’ll find a few exceptions to that when we look more closely as well. Any questions on this? Yes.
Student: So what are the differences between the ocean crust and continental crust besides just their density?
Professor Ron Smith: Well, they’re made of slightly different chemical compositions. Normally there’s more, I think quartz generally in these rocks, a light mineral allows it to be a little bit denser, a little bit less dense. And other denser minerals are found more prolifically in the ocean crust, which makes it a little bit denser.
OK, we’ll come back to that. But I wanted to make this point, so when you then take a, what’s called, make a hypsometric curve, I’m going to focus on the bar graph over here on the left. This is elevation in meters above and below sea level, sea level is marked at zero here. Which I remind you is a somewhat arbitrary choice, it depends on how much water we have in the oceans. And over geologic time, that has probably changed a little bit. For example, when you have an ice age you store some of the water up on the continents in the form of glaciers, and sea level drops a little bit. So that is kind of an arbitrary reference point, but it’s commonly used and so we’ll use it here.
Whats shown in the bar graph then, is the percent of the earth’s surface that lies, for example, between sea level and one kilometer above. And it’s about 20% of the earth’s surface. Between one kilometer and two kilometers above sea level, it’s about 5% of the earth’s surface. And you find some parts on the continents that are even higher, even up to four, but you know in fact Mount Everest is up here somewhere, there’s even a little bit of land that lies 9000 meters above sea level.
Going down below sea level, you find there’s not much land at one kilometer, two kilometers, and three kilometers below sea level. But a lot at four, five, and six kilometers below sea level. Now this is a bit of a surprise because if the earth was just a rough surface, had been roughened by some process, it would have kind of a normal distribution for this hypsometric curve. It would have some average height, and then less above, and less below.
But actually no, this has a double peak, a very interesting double peak. And of course that has to do with the point I already made. There are two types of crust here, this is continental crust and this is ocean crust. So this plate tectonics that gives us the two types of crust, ocean crust and continental crust, is the cause for this double peak in the hypsometric curve for land elevation. And again it just so happens that we have an amount of water that puts most of this down below sea level, and some or most of the continents just at sea level or slightly above.
Chapter 2: Ocean Bathymetry [00:17:12]
So now we can turn to the particular features, bathymetric features in the ocean. I’m going to talk about the abyssal plane, which are these flat lying parts of the ocean bottom. And then some of these other things that have to do with plate tectonics like the mid-ocean ridges, the trenches, and then some other features as well. The way we know all of this by the way is from acoustic depth profiling.
So you take a ship, and it sends out an acoustic signal, a sound wave, and you bounce it off the bottom and you time how long it takes that signal to go down to the bottom of the ocean and back up again. And in the old days you just had a single pinger going right directly down, so you’d have to take the ship back and forth on a very complicated long route to map out the ocean. But now, they can send it out in a fan with different acoustic beams going in different directions, so you can do a single swath as you go along and get depth over some range to your left and to your right as the ship sails along.
Also in some cases, but I won’t be emphasizing it here, you can look for other reflections off subsurface layers. For example, that sound wave may go down into the ocean crust or the sediments a little bit, and bounce back up, giving you ideas of what’s going on below the bottom of the ocean. And that’d be useful if you’re doing geological surveys of the ocean crust. But for our purposes we’re just going to be using that to map out the actual ocean depth itself. Stop me if you have questions here.
Sound moves rapidly in seawater, the speed of sound in air is about 300 meters per second, speed of sound in water I think is three or four times that, it’s really–it goes quite rapidly. Nonetheless it’s still a finite speed, and you can easily time how long it takes for that acoustic signal to get back to your receiver and get the depth from that. So this is a cartoon just showing some of the features.
For example, there’s a section of abyssal plane, that kind of flat lying section. Flat in part, because it’s composed of sediments that have fallen from above, and as they fall they fill in the cracks first, and then as you get quite a pile of sediments it tends to give you a flat surface. A mid-ocean ridge, one of these spreading centers where magmas are coming up and forming new ocean crust, tends to be elevated because those rocks are still hot, and less dense, and they float a bit higher than cold ocean crust until they cool down. And then they sink a little bit away from the spreading center. But that could come up a bit, if this abyssal plane is at five kilometers, then this mid-ocean ridge might only be two or three kilometers below the ocean surface.
You can have undersea volcanoes, that’ll start building from the ocean floor, just like volcanoes on land start to build from the continental surface and build upwards. Undersea volcanoes build up from the ocean floor, and in some cases they will not reach the ocean surface, in which case they’re called seamounts. I don’t know why they’ve drawn this the way they have, indicating that these have penetrated the earth’s surface, the ocean surface. Usually the word seamount is confined to an undersea volcano that has not reached the earth’s surface, so I disagree with the artist a little bit here.
On occasion you find them with flat tops, which means that at one point they reached the ocean surface and were leveled by wave action. And now they’ve settled back down a little bit, so you’ll find undersea volcanoes some of them with a flat surface, those are called guyots. And then the ocean trenches, where you get the subducting plates, are the deepest parts of the ocean generally. Then I wanted to make this point about a continental shelf. So there’s a continent, there’s a continental shelf, there’s the drop down to the abyssal plane.
You see then geologically, a continental shelf is really part of the continent. It just so happens that the water level is high enough so it’s covered up slightly some of this, making it appear on a map of the earth that its ocean, but geologically it is continental. And there should be very little confusion about the two, because this is going to be a very shallow ocean, probably only 100 or 200 meters deep. Whereas this is five kilometers deep, so it’s going to be pretty clear to separate geologic continental structures from geologic ocean crustal structures. Because they really are at a very different level, even though there’s enough water at the present time, especially with the glaciers mostly melted at this time to make a little thin layer of water covering those continental shelves.
So now here’s the whole, the whole world ocean, and the color scheme is not quantified on here, but I can tell you basically what’s going on. All the deep blue is abyssal plane, about five kilometers deep. And you see it a lot of places. These little lines that run up through the middle of oceans, for example in the South Pacific and all the way up through the Atlantic, the south and the north Atlantic, those are spreading centers, mid-ocean ridges. And then the trenches don’t show up well on this diagram, but they are the darker blue still. And you see a little thin line along there that’s a trench, you see one here, you see an important one here, and up along here, and even along the tip of Aleutian Islands.
So those are the subduction zones. You also see a scattering of sea mounts various places, and other features. But I’d say–oh and you see the, for example, right along there, you see a nice example of continental material with just enough water over it so we would call that ocean, but geologically it is continent. That’s the continental shelves, continental shelf area.
So this is the geometry in which then we’ll be studying ocean currents and so on. It’s going to be constrained by this pattern of depth, and the thing that’s going to be very important to us is the way that these continents tend to break up the oceans into segments. For example, Asia with Australia included, and North and South America, break up the Pacific ocean into kind of a north, south, oriented ocean. North America, South America, compared with Europe and Africa, break up the Atlantic again into a north, south, oriented ocean. The Indian Ocean is a little bit different because Asia fills the northern hemisphere, most of it, down to about say 20 degrees north latitude. So the Indian Ocean is primarily an ocean just in the southern hemisphere.
And then, the one gigantic exception to this is the so-called Southern Ocean. The term for that strip of ocean that goes all the way around the globe, it’s usually called the Southern Ocean in oceanography. And if you go far enough south in the Pacific you join onto it, the Atlantic you join onto it, the Indian you join on. And as we’ll see, you can have ocean currents here that go right around the globe. Whereas at all these higher latitudes, any ocean current that moves east-west is going to hit a continent and is going to have to wrap back around. So you get what are called gyres, in most of the oceans, because they’re confined by these north-south oriented barriers. So gyres are the things we’ll be studying for most of the oceans, but not in the southern ocean. There we’ll be able to look at a current that goes all the way, all the way around the globe. Questions on this?
Chapter 3: Atlantic Ocean Bathymetry [00:26:23]
So we’ll zoom in a little bit to–so all the major oceans, there’s the Atlantic ocean. A little bit of a trench here, but generally it’s abyssal plane, and it is with a mid-ocean spreading center here. By the way, so that’s spreading which now these continents were originally joined together, and you can see how well they fit from a jigsaw puzzle point of view. That spreading is at about the rate–the way I remember this–it’s at about the rate that your fingernails grow, so it’s a couple of centimeters per year basically.
So this is still widening today, and at the end of the year it’s going to be a few centimeters–the Atlantic ocean is going to be a few centimeters wider than it is today. And that’s a slow process, but when you take that speed and multiply it over millions of years, you can see how you can get hundreds or even thousands of kilometers of ocean width generated by that slow spreading. And then once again, you see some continental shelf area up in here, covered by water, but generally part of the continent. And you see a big piece of that down here as well, around Africa a little bit too. And all of the Mediterranean sea, most of it is–you’d probably call it ocean–continental crust rather than ocean crust.
Chapter 4: Pacific Ocean Bathymetry [00:28:01]
Pacific ocean, vast areas of abyssal plane, but there are trench systems hard to see, but the black lines. Mid-ocean ridge that comes down here, was spreading, and then lots of ocean crust here. And then up along California, you’re having some motions. It’s more of a transform fault, where things are moving parallel to one another, not exactly a spreading center, not exactly subduction, but some complicated combination of the two including lateral slip.
And so we’ll see, once again, we’ll get gyres in the Northern and Southern Pacific constrained by the barrier affect of the continents to the east and west.
Chapter 5: Indian Ocean Bathymetry [00:28:50]
Indian Ocean bathymetry, big continental–a big abyssal planes, and then some curious old, these appear to be old spreading centers that are no longer active. Maybe Erin can tell us more about that. But anyway there’s some structures there, but they don’t appear to be active spreading centers. And here’s some of the subduction zones over in the Pacific there.
Chapter 6: Arctic Ocean Bathymetry [00:29:16]
The Arctic Ocean has some interesting structure, but it’s pretty passive too. There’s two deep basins–most of this is continental shelf, pretty shallow, depth the order of a few hundred meters. But there are a couple of deep parts as well, that go down to several kilometers. And then a ridge, I think a passive one, that’s called the Lomonosov Ridge, but I don’t think that’s an active spreading center. So this has got some interesting structure, but it’s more of a passive structure at the moment, it’s not an active spreading center.
Chapter 7: Measuring Ocean Surface Water Properties [00:30:00]
OK, so that’s an introduction to the shape of the ocean basins. Are there any questions on that? I want to turn now to ocean properties, ocean water properties. Sea surface temperature, that we measure from ships, from instruments below the ships, and from satellites. We can measure salinity from ships, to get into the deep ocean we need to put something down into the ocean, I’ll show you how we do that. For ocean topography, which I’ll define later, a little irregularity in the ocean surface, we use satellites for that. And then for ocean currents we use all those things ships, floats, and moorings. So before we’re done, I’m going to talk about how we measure all these things, and understand a bit about the ocean water and how it’s moving.
So here is a map taken in late August, I think of the year 2000 from satellites, showing the sea surface temperature, SST. The color is in Celsius, and you see that in the tropical regions you’re getting up to temperatures 28 and higher. Remember 27, 28, was the threshold for hurricanes. So you are getting a lot of warm ocean temperature that support hurricanes, some of that however is right at the equator and so you couldn’t have hurricanes forming there because you don’t have the Coriolis force.
But in other parts you get the warm water extending far enough away from the equator, so you could have hurricanes. I pointed out when we were talking about hurricanes, two interesting places. The Western tropical South Pacific, where you don’t have hurricanes because it’s too cold, and you see it there. It’s a cold ocean current, the Humboldt Current coming up here, taking cold water from the Southern Ocean, peeling some of it off, bring it up here, and keeping that part of the world ocean cool. And you see something very similar here, where cold waters being peeled off and come up here to keep the southern tropical Atlantic a bit cool as well. Otherwise it’s pretty warm in the tropics, except where you’re getting these cool currents. The California current does a little bit of cooling in this region, and the return from the Gulfstream does some cooling on the eastern side of the north Atlantic there, so that’s that now.
But if you get a mental picture of this, be very careful what you do with it, because you could be very much misled. This is sea surface temperature, when I go down even just one kilometer in the ocean, or especially if I go down to the ocean bottom, it looks nothing at all like this. So temperature is not vertically homogeneous. As I’ll show you later on, this warm water that can form near the tropics, forms a rather thin layer floating on the cold water that fills most of the world ocean. So this is not a picture that can then be transferred down into the ocean very deep at all. In fact in some cases, you may only be able to go down a few hundred meters before this picture changes rather dramatically.
Salinity is the other important property we track in ocean water, and here’s a map of the sea surface salinity. So just the surface, just the salinity you would measure from a ship if you took a surface sample of water and analyzed it. And well, the most remarkable thing is the narrow range. And you get some salinities as fresh as maybe 31 or 32 parts per thousand. And some, perhaps in the Mediterranean Sea, and the Red Sea, Gulf of Aden perhaps, getting up to 38, 39. But generally that’s the full range of ocean salinity. So you’ve heard me say, occasionally, that sea surface salinity is 35 parts per thousand, of course that’s not a very precise statement. But it only goes about plus or minus four parts per thousand around that mean value of 35 parts per thousand. The reason for this must be that the ocean mixes itself occasionally, to maintain this kind of rather homogeneous salinity. At least compared to the rate at which you’re adding freshwater, or the rate at which you’re adding or subtracting salt.
So the basic story behind this narrow range of salinity, is that the ocean is, at least for salt, relatively well mixed. Now if it were completely well mixed, it would have the same salinity everywhere. If it were vigorously being stirred, like a vigorous spoon stirring in a pot, any differences would be immediately removed. So it’s not perfectly well stirred, but is reasonably well stirred, giving you this rather homogeneous salinity over the world ocean.
So that’s lesson number one, but at the next level though of detail, we can notice that there are some variations. And they probably make sense to us because it’s in these–it’s in the so called Intertropical Convergence Zone, or the belt of tropical rainforests, that you get a little bit lower salinity. That’s right through here, and you see it here, and a little bit down through here. So there’s a lot of rain falling on the ocean there, fresh water coming down and diluting the salt a little bit, giving you a smaller salinity along the equator.
And then as you move north and south from that into the belt of deserts, the descending branch of the Hadley cell, with very little precipitation and some evaporation. Remember when you evaporate seawater, you leave the salt behind, and so the salinity is going to be increased. And you see the increased salinity there in both the Northern and Southern hemisphere, connected with the belt of deserts. And then you get up in the mid-latitudes and once again you’ve got the frontal storms, cold fronts, warm fronts, bringing rain and that once again dilutes the surface salinity.
So a lot of things we’ve spoken about before, in terms of the general circulation of the atmosphere and where it rains and where it doesn’t, are reflected in this plot of sea surface salinity. Here again it’d be dangerous though, to try to imagine that pattern would extend down to the oceans very far. Other things will take place that will prevent this from being the pattern deeper down in the ocean. Questions on that?
Chapter 8: Measuring Deep Water Properties [00:37:37]
OK, now I want to tell you about how we get–how we measure ocean properties down into the ocean. The old way to do this, and I’ll talk about the new ways too, the old way for about 50 years, the predominant way for sounding the ocean, for getting temperature and salinity profiles with depth, was the Nansen bottle. And here’s a diagram, here’s a picture of one, mounted on a cable that’s about to be put down into the ocean. And here is a cartoon of what happens after it’s put down into the ocean and is triggered, so that it tips over and the valves close on it.
Now I have one of those with me, and I want to show how this thing works because it’s kind of a clever gadget. And although it’s not being used much anymore, much of what we know about the world ocean came from this simple device. Again it’s called a Nansen bottle, it was designed by the famous Norwegian explorer Fridtjof Nansen. And it is composed of a hollow metal tube, fairly thin and its steel, with a valve at the top and the bottom, that is currently–If I look through this–it’s open, I’ve got both valves open. And there is a tie rod that connects the two valves, and so when this tie rod shifts relative to the bottle itself, it’ll close both valves simultaneously. So there’s also a couple of housings for thermometers on the outside, so you can get the temperature of the water at that depth as well.
So imagine that you’ve come–you’ve taken your ship to a given location, and you’ve got a long cable coming up to a winch and a pulley, that then takes that cable overboard and down to whatever depths you want to go to. So how do we get started with that? Well the winch operator gets some of the cable in the water, and then the scientist would lean over and attach the first one of these Nansen bottles to this cable. So imagine you’ve got a cable coming down like this, and you’re leaning over the side of the ship, and you reach out and you fasten this thing in by putting the cable right down in there and pushing that little pin over. And then down here the cable comes through there, and you lock it with a turn buckle. So it’s locked securely at the bottom, not so securely at the top.
Once you get it on there, you wave to the winch operator, and he puts that cable down. And you’d repeat that about 20 times over the next hour or two, and when you’re all done you’ve got a cable overboard, perhaps going down to five kilometers. You can do this all the way down to the abyssal plane, and along this cable you have Nansen bottles, perhaps as many as 20 of them. When that’s all in place, then you reach over one more time and you fasten a little thing called a messenger, a little brass cylinder that slides down the cable, hits this little plate, knocking free this pin, and now the weight of the bottle begins to act.
Let’s see if I can do this now, OK I’m having trouble doing it with my bum arm here. But I’m pushing this thing down, and as I do so it has fallen away to an orientation like this, and now both valves are closed, trapping water from that level. So when it’s brought up, you’ve got samples of the water at each of these depths. Also, when you flip over these thermometers, they’re so called reversing thermometers, and when they’re flipped over the mercury column breaks, and so you lock in the temperature at that depth. And when you bring it up, you’ve got a record of what the temperature was at that depth.
So by flipping it over, you lock in the temperature, and by flipping it over and closing the valves, you lock in the sample of water from that depth. Then over the next couple of hours you bring that cable back to the surface, and each time when a Nansen bottle emerges, you reach over, disconnect it, and put it in a rack. And after several hours of work, you’ve got data from the whole sounding. And over a period of 50 years or so throughout the 40’s, 50’s, 60’s, 70’s, and 80’s, this was used to map out most of the world ocean in terms of its temperature and salinity structure. Questions on that? You can pass that around if you like, so people can get a sense for it.
Now my first oceanographic cruise I managed to embarrass myself, by failing to get that Nansen bottle properly secured before I waved to the winch operator to tell him to go down. So the very first one of these I did, one of the Nansen bottles came up totally destroyed, crushed. And can you figure out how that would have happened? So what happened is, it wasn’t secured properly at the top. It flipped, I didn’t notice I turned my back away, but it had flipped before it entered the water.
So both valves had closed with air inside, and then it went down to depth. And of course, the pressure is enormous at the bottom of the ocean, you can do a quick hydrostatic calculation to understand you can get hundreds of atmospheres of pressure down there. And it just took that bottle with air inside, and just crushed it like a Coke can in your fist. And it came up all wrinkled, and my chief scientist was not very happy with me but I learned to do better the next time.
Anyway it’s important historical, but also to understand how that was done. Today it’s done a bit differently, it’s done with a CTD and a rosette. Now CTD is an abbreviation meaning conductivity temperature depth. It’s an electronic device, the conductivity part is similar to what we did on the river lab. As you lower this thing down into the ocean, it measures the electrical conductivity and from that you can determine the salinity. It also has a thermistor on there, so you can measure temperature, and it’s got a pressure sensor on there so you can measure depth. So as you lower this down, in real time because there’s a cable coming back to the surface, you’re getting a detailed profile of temperature, salinity, and depth. That’s a great system.
However that of course would not give you a water sample, so if you want to do any kind of water chemistry a CTD would not be sufficient. The beauty of the old Nansen bottle was, you got a water sample as well. So if you later on decide you want to do any kind of chemistry, you had that water sample. The CTD is a great advance on that, but it doesn’t give you the water sample. So what’s usually done today, is that they have a set of water collecting bottles arranged around the perimeter, and that’s called the rosette. And they’ve got an electronic control, where the valves on these can be closed on command.
So in addition–so you send this down to the bottom the ocean, profiling conductivity, temperature, depth. And then every, I don’t know, 500 meters or so, you might close one of the valves on these bottles and you get a water sample from that depth as well. So if you were to go out on an oceanographic cruise today, you’d be largely using the CTD and the rosette to do vertical profiling of temperature and salinity. Questions on that?
So there’s one, they brought it back on board and they’re, I guess, drawing samples off of the, from the different water bottles there. Now technology is advancing even beyond that, however. Remember even with that one, you have to get a ship there, you’ve got to get a cable over that can go all the way to the bottom, and that can take a couple hours to do, and ship time is very expensive. So now the field is moving in the direction of these autonomous explorers, where it’s an unmanned vehicle that you launch from a ship but then it pretty much goes its own way, and you can control it.
You talk to it with sonar, sending acoustic signals to it which it can receive, and it will then cruise around in the ocean up, or down, or laterally, measuring. Of course you don’t get water samples from this but you get temperature, conductivity, and depth, because this has a CTD on board. And you can control it and move it wherever you want, back and forth. In one area, if you think conditions might be changing, or traversing larger parts of the ocean, if you want to map out a big piece of the ocean volume. So this is a new thing coming on, it’s quite exciting to have this, and you can be back in your office actually in a way, and this data is coming out on your screen. It’s pretty remarkable what this will mean for oceanography. This is just getting started, so this is really opening up brand new doors for understanding the oceans with these autonomous explorers. Any questions on that?
OK. So what do you get from this then? We’re almost out of time, but here’s a typical ocean sounding. Temperature, salinity, and density, versus depth. So on this plot, zero is taken to be sea level, and this goes down to about 4000 meters which is almost the depth to the abyssal plane. Very often you find rapid changes at first, and when the temperature drops from warm surface to colder values beneath, that region of strong temperature gradient is referred to as the thermocline. And you’ll hear that term over and over again, it’s very important in the ocean, this thermocline. In this particular sounding, it started about 200 meters below the surface, and by the time you got down to 500 or 600 meters, you’re at a temperature of about four degrees Celsius, and then eventually down to just one degree Celsius. Whereas at the surface you had 25, 26, 27 degrees Celsius.
The salinity in this case was large at the surface, 34.9, became a bit fresher as you dropped down through what’s called the halocline, and then became slowly a little bit saltier beneath. Now the density of seawater is controlled primarily by the temperature and the salinity. The density is a very important quantity, but if you know temperature and you know salinity, you can compute or you can measure the density.
So what’s plotted in this final panel, is the density derived from the measured temperature and salinity. And it shows a lower density water near the surface–by the way, the units are in grams per cubic centimeter here, remember fresh water has a density of about one in those units. So this is a little bit denser than fresh water, and gets even denser by the time you get down to the bottom primarily because of the temperature in this case. The colder temperature is giving rise to the lower—to the higher density in the deep ocean.
I think I’m out of time, so we’re going to continue this next time and talk about the concept of static stability. When will water remain in layers? When will it overturn to form convection? And this will be the starting point for that discussion.
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