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Main points of today's lecture:

• Deep ocean currents
• Thermohaline circulation
• Primary deep water production
• General circulation patterns
• Conveyor belt

Insights:

• Deep water circulation, referred to as thermohaline circulation, arises from density differences between water masses produced by variations in water temperature (thermal effect) and salinity (haline effect).

• Most water masses in the bottom of the ocean are actually formed at the sea surface. For example, two of the cold, dense water masses circulating through the bottom of the Atlantic (AABW and NADW) are actually created offshore Antarctica and Greenland. Freezing winds cool down these surface waters which consequently sink to the bottom and flow eventually out of the Atlantic and into the Indian and Pacific Oceans.

• A recently developed deep ocean circulation model connects deep flow around the world with surface circulation. The conveyor belt model (Figure 5-16b of your text), largely driven by processes that occur in the North Atlantic, portrays the exchange of surface and deep water across the ocean basins compatible with general observations. Total round-trip time on the conveyor belt is about 1500 yr.

• Seas —like the Mediterranean and the Black Sea— are semi-enclosed oceans greatly influenced by continental processes, mainly evaporation which increases the salt content of the water, as well as precipitation and river runoff, which dilute the salinity.

Have you thought about... (answers need not be submitted!)

• How mobile is the water at the bottom of the ocean?

• Which of the oceans in the world is the saltiest, which is the coldest, which is the warmest?

The Deep Ocean contains abundant CO2 and O2, because these are easily dissolved in cold surface water that sinks to the ocean bottom from either the North or South Atlantic. O2 content controls the extent of oxidation of sediments arriving at the ocean floor. CO2 content controls the depth of the CCD (carbonate compensation depth), which determines how much carbonate is preserved on the ocean floor as sediments.

The temperature of bottom water formation determines how much CO2 can dissolve in water that sinks to the deep ocean. The rate of overturn of the oceans ("conveyor belt") determines the burial rate of C (carbon) from the atmosphere. Organic C (sinking from the surface waters as dead plants and animals) accumulates in the sediments at a rate dependent on the O2 content of the deep ocean.

Organic C in sediments is eventually reduced to methane (CH4) gas. Methane migrates upward and is trapped near the sea floor as frozen "gas hydrates" (methane combined with frozen water).


Large deposits of gas hydrates have been discovered recently along the margin of OR and WA, where sediments are being scraped off the subducting Juan de Fuca plate and added to the submarine part of North America.

Sedimentary rocks containing these frozen gas hydrates melt and bubble when they are brought to the surface because of the higher temperature and lower pressure compared with the sea floor.

It is a concern that massive amounts of gas hydrates (CH4) could be liberated from the bottom sediments during either lower sea level (lower pressure) or warmer deep ocean temperatures. This greenhouse gas could flood the oceans and the atmosphere in enormous quantities, leading to much warmer climate.

So, the Deep Ocean has enormous capacity to absorb and release greenhouse gases (CO2, CH4, and H2O). Knowledge of the rate, temperature and composition of seawater circulation through the deep ocean is vitally important is assessing long term climate change.

http://dusk.geo.orst.edu/oceans/deep_currents.html