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Dispatch from the Deep: Global Ocean Circulation and Deep Sea Temperatures

Map Ocean

oof! Greetings from the deck of the Atlantis over the Juan de Fuca Ridge, where we're all getting tossed about today! At this point, I don't really get seasick anymore, but the ocean swells passing under the ship create a pretty wild ride sometimes! Yikes! I'd better tie down my pen!

Out on the ocean, the wind is the main culprit in generating these surface movements. The wind creates a current in the surface water; that current is then modified by the shapes of the continents and by the Earth's rotation. If you don't spend much time sailing the great oceans, you might not be familiar with the global patterns of ocean circulation, but sailors have known about them for centuries. You probably have heard of one of these large currents, though—the Gulf Stream—when you listen to weather reports. The Gulf Stream is a key player in global ocean circulation. It carries warm water from the Caribbean along the eastern coast of America and then across to Europe, warming climate in those areas.

But global ocean circulation doesn't just happen on the surface. Deeper down in the ocean, too far below for me to feel it up here on deck, water is moving too. This is because ocean water has different densities. The colder the water, the heavier it is; the hotter the water, the lighter it is. Density is also based on the salinity, or salt content, of ocean water; water with lots of salt is more dense, and thus heavier, than water with less salt. Thus, the denser, heavier water sinks and the less dense, lighter water rises.

To picture how the ocean moves, imagine a kind of conveyor belt from the bottom of the ocean up to the surface. Water, like air, moves because of differences in temperature; hot water in hot water, for example, won't move. But hot water in slightly less hot water will rise; hot water in cold water will rise even more rapidly. It can get pretty warm at the surface of the ocean; I usually swim in water that reaches a balmy 80ºF. But beaches are on the very tippy-top of the ocean; the deepest part of the ocean measured so far goes down about 11 miles! No matter how warm the surface of the ocean gets, the ocean's huge volume and deep basins keep temperatures at the bottom of the ocean at only slightly above freezing. At the abyssal zone, the part of the sea closest to the vents, it's way too cold to even dip a pinky into the water. (Not that you could dip a pinky in—the pressure is too great to allow any break in ALVIN's seal!) In fact, no matter how warm it is up top, by the time the sub has sunk to a depth of just 1,000 meters (3,280 feet), the water temperature is about 40ºF. From there it continues to drop until, 7,200 feet down, where the smokers are, the temperature is only about 2º to 3ºC (35ºF)—just above freezing!

Map Ocean Ridge

So where does this cold water come from? Melting glaciers form the cold water that then sinks near the poles, pushing warmer water up and across the globe. The result is a complex, three-dimensional system of global ocean circulation that is based on the simple idea that colder, denser water sinks beneath warmer, less dense water. These movements mean that the world's oceans are being "stirred," spreading nutrients and oxygen. (They also regulate global climate; this is why global climate change, and the resulting melting of glaciers, has the potential to have such a devastating effect on every part of the globe!)

You might think that deep sea volcanic activity would heat up the deep ocean waters and it does—but only slightly. The mid-ocean ridge system, which is really a long string of submarine volcanoes, supports hydrothermal vents: deep sea hot springs. These vents spew super-heated mineral-rich water (sometimes called hydrothermal fluid). That water escapes from the vents at temperatures of up to 400ºC (about 750ºF)! The really cool thing is that the release of this warm water allows living things to thrive within the vent fluid, but all that toasty warmth dissipates pretty fast. Organisms have to stay within a meter (about three feet) of a vent in order to stay warm; beyond a meter of the vent, the seawater is already at 35ºF, essentially the temperature of water in the abyssal zone. So the cold waters of the deep oceans cool the hot fluids of the vents very effectively. It's kind of like being in a cold room that has a fireplace; to stay warm, you need to stay right in front of that fireplace!

If you get too close to the fire, of course, you'll singe your clothes and eyebrows! It's the same for the ALVIN. As the ALVIN moves around the vents, the pilots have to be very careful not to get too close to the vent fluid. Not all of the ALVIN is made of metal, and the heat discharged at the vent could damage plastic tubing, hoses, and other non-metal equipment. Yesterday one of the ALVIN pilots told me about the time the crew got too close and melted the hose cable of the ALVIN's hydraulically controlled arm! They had to scrap a whole day's work because the robotic arm got cooked!

To prevent these costly (and time-wasting!) mistakes, the pilots get a lot of extra training. Every fifth dive, two pilots go down on the ALVIN. The second pilot is there to receive extra sub operation training, all under the watchful eye of the master pilot. PIT ("pilot in training") dives are very important; each new pilot has to develop an instinct to know exactly how close to different spots on the chimney he or she can bring the sub. At the end of each dive, when the ALVIN comes back on board ship, the tech team examines every part of the submersible for signs of damage from the heat and pressure. And the Atlantis carries enough spare parts to rebuild the entire sub! When you're out here you can't just drive to the hardware store to pick up a new "widget"!

Dark, freezing, under pressure and far from the hardware store! It sure is tough down at the deep sea vents! But so exciting—an unknown world, down at the bottom of the sea. I hope I get picked to go down!

Wishing you a warm and breezy day,
Pat

American Museum of Natural History

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Phone: 212-769-5100

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