Hello my name is David Schultz, and welcome to Our Earth, it's climate, History, and Processes. In this lecture we want to talk about how the ocean works on long time scales. And by long I mean hundreds, thousands, millions of years. To help us answer this question, I'm joined by Dr. Gregory Lane-Serff. He's a senior lecturer here at the University of Manchester in the School of Mechanical Aerospace and Civil Engineering. So Gregory, if we think about the global oceanic conveyor belt and the kind of circulation that goes on, we hear about the Thermohaline circulation or the Mariano overturning circulation. What is this and what does it mean? >> Okay, so, in the worlds oceans and particularly the arctic regions, we get the formation of dense cold waters by intense cooling that happens in those parts of the world. >> And the dense then sink down into the deep ocean and flow around the ocean at depth, initially. For example, southward through the Atlantic and also northward out to the Antarctic regions. Around Antarctica a few times perhaps and then up into the Indian and Pacific Oceans, before up-welling, by which I mean rising to the surface in those regions. And returning at surface level back through the Pacific and Indian Oceans. And back out through the Atlantic Ocean to return to where it started from. >> So, if we were to look at like any individual molecule of water, say, how long would it take to under go this circuit? >> So the longest time scales that we think happen with this are of the order of a thousand years. So, a way of thinking about that is, water that was passing between Iceland and Denmark at depth underneath the Viking explorers as they were trekking westwards to Greenland and North America is now, just about working its way back again, having gone all the way around that circuit and is now coming back through the North Atlantic, back up into the Arctic. So those are the scales we're talking about there. >> Okay, and is this picture an idealization? I mean, we see this picture in all kinds of textbooks, articles, how accurate is this depiction? >> Well, obviously it is rather smooth and simplified. When you get down to looking at the deep ocean basins, they're actually split up into lots of little sub-basins by ridges and channels and so on. So the dense waters it flows around rather than in a smooth conveyor belt, it more fills the basin and then flows through a channel or overflows or sails down into the next basin, mixing a bit as it goes. So it's more a sequence of basins like that, which it flows around. And then once it gets up into the surface ocean, as you've already commented there, are these complicated wind driven as well. So it is a very much averaged long term smoothed out picture of the flow. But it does give us a good picture of how things happen on these long time scales. >> So presumably we send instruments to sample the currents and so forth, and so but because the timescales are so slow we don't see the whole conveyor belt all depicted as one thing. We can only know about it from one basin to one basin, or within the golf stream moving from North America towards Europe. Is there any way that we can see the whole circuit being put together, illustrated? >> I mean, that is very hard. Because as you say, we've only been observing things for a relatively limited period of time. We can track traces for example, and that helps us to some extent. So for example, the traces that were effectively dumped on the surface ocean by the atmospheric atom bomb tests in the middle of the last century gives us a useful tracer that we can track. And there are other traces of that type, but in the end, we are only seeing segments of it by direct observation and most of our understanding of this comes by looking at models, where we can track particles within these ocean models to see how they move around. And it's from that that we've inferred that it's this thousand year time scale. >> And so the strength of this circulation changes over geologic time too. Sometimes it slows down, sometimes it speeds up. Do you want to comment on that? >> Yeah, so there's big changes particularly between the glacial and inter-glacial time scales there. At the moment there's a reasonably strong circulation. There's a suspicion that we can slow it down or cut it off if we have a lot of extra melting and extra melt water into the centers of the North Atlantic for example. And there is evidence of changes in the circulation that we look back over time, from deposits and so on. So we can see over the course, as you said, millions of years. >> And so a lot of the photosynthetic activity, the plankton and that obviously live in the surface layer, the mixed layer that we talked about in the last lecture. When these animals die, these plants and animals die, they sink down into the deeper ocean. How does the structure of the vertical ocean affect the that we call it, this raining of this organic debris down to deeper ocean? >> Well what we find is that for animals or plants in particular to grow, what they need is sunlight and nutrients. And what we find in the surface waters of the oceans is the nutrients that get depleted by these animals as they're growing there. As we saw in the last lecture, we can actually mix up in the winter and during that winter period nutrients are mixed up from depth. But during that winter period, if you look at an individual organism that's trying to photosynthesize, it's being moved over these big depths, spending a lot of time away from the sunlight. There's less sunlight anyway because it's winter and it's colder. So they've got nutrients but a lack of sunlight. In the summer or as the spring approaches, the sun comes out, the winds lessen, and you get a shallow layer formed at the surface, which now has plenty of nutrients in it. And at that point, the phytoplankton can take off, we get what's called a spring bloom, a sudden growth of biological activity, and on that phytoplankton, various sea plankton and other sea animals feed. Gradually, the tritors, bits of dirt organisms, fecal matter and so on descends through the ocean falling out of the mix layer into the deeper ocean and in that deep ocean, much of that material is in, what we call re-mineralized. That is it's turned back from complicated biological molecules back into nitrates, phosphates, the kind of nutrients we think of by consumption by small zooplankton but also by bacterial processes. And in fact most of the stuff that sinks down is remineralized back into nutrients provided there's enough oxygen in the deep ocean in order to do that. >> We like to think of the oceans as this big, homogeneous zone of activity. But really, there's quite a bit of structure in there. All this life doesn't just occur uniformly across the top of the ocean. There's actually specific regions, as you can see from this map that we're looking at now, where life is abundant. So if we look at this map, let's focus on the oceans, we've got reds, yellows, and greens that indicate regions of high chlorophyll concentration, as measured by satellites over many years. On the other hand, these blues and purples represent a relatively low chlorophyll concentrations. Chlorophyll is the active molecule that's critical to photosynthesis. So its present indicates this biological activity, or the photosynthesis going on. If you look at this productivity, in most of the tropics, where the water is relatively warm, it's actually void of photosynthesis, and it's actually as you go into the cooler waters, further northward and further southward, that you start to see appreciable amounts of photosynthetic activity. So, when these photosynthetic organisms are floating around near the surface, and then they die, they sink down into the deeper ocean, is that right? >> Yeah. >> And so they're providing food for scavengers in the deeper oceans. And in the case where these animals have hard shells, like the foraminifera or the diatoms, they get deposited on the ocean floor and form sedimentary rocks and so forth. As I mentioned before, this falling matter from the upper ocean to the bottom ocean is called marine snow. And in 1951 the environmentalist Rachel Carson first referred to the dust an organic material falling through the ocean, flake by flake, layer upon layer, the drift that's continued for hundreds of millions of years, the material of the most stupendous snow falls the Earth has ever seen. Now, later when people in submarines first observed and described this material, they recognized it as Rachel Carson's snowfall. Now, you mentioned this mixed layer and how important this mixed layer is for nutrient cycling and biological production. Now if we go from the annual cycle where the biological productivity changes on yearly times scales to much longer time scales, here is where it gets interesting. The character and outcome of this marine snow is determined by the structure of the thermocline which is in turn controlled by the climate that we have on Earth. Here's what I mean by this. Consider the earth's climate as it is now, with appreciable polar ice caps where we keep the polar surface ocean waters cold. Cold water has greater solubility for oxygen than warm water. Now specifically, 30% more oxygen can be dissolved in saline water at zero degrees Celsius than 25 degrees Celsius water. In the present climate, the oceanic conveyor belt carries down these cold oxygen rich waters to below the surface creating the strong thermocline and allowing aerobic organisms at depth to feed on the marine snow. This leads to a complete decay of the organic material, so very little organic material in the sediments occurs during the such climates. Because of the cold polar regions, we call this the icehouse climate. Now in contrast to other times in the past, the polar regions were quite warm and generally ice free. We call these climates hothouse climates, signifying a relatively weak pole to equator temperature difference. As a result, the down going water at the poles becomes relatively warm with much less oxygen in it, so there's fewer deep-water organisms that consume the marine snow. Also the circulation is rather sluggish because the thermocline is relatively weak during this period. And the result is that the marine snow, the organic material that's falling through the oceans, in this kind of climate experiences relatively little decay and you get more organic-rich sedimentary rocks in these hothouse climates. Now the burial of these organic materials leads to production of oil and gas in the oceanic sediments. In fact, the oil in the North Sea east of the United Kingdom comes from such periods of planetary warmth in the Carboniferous and in the late Mesozoic. The oil in the Gulf of Mexico comes from late Mesozoic deposits too. So to summarize today's lecture, we've seen that salinity and temperature both affect the density of ocean water, and this drives the global slow thermohaline circulation, otherwise known as the oceanic conveyor belt. We also discussed the difference in the ocean's circulations in the hothouse, in the icehouse climates. Now we discussed what this meant for the differences in terms of the thermacline and for the preservation of organic material in the deep oceans. Specifically, in the icehouse climate organic material almost completely decays and in the hothouse climate the organics don't decay and they are deposited on the ocean bottom. So, thanks for watching the video and thanks to my guest, Dr. Gregory Lane Serf.
