Ocean "Conveyor Belt" Sustains Sea Life, Study Says

John Roach
for National Geographic News
June 15, 2004

An estimated three-quarters of all marine life is maintained by a single ocean-circulation pattern in the Southern Hemisphere that pulls nutrient-rich waters from the deep ocean, brings them to the surface, and distributes them around the world.

"This is really something," said Jorge Sarmiento, a professor of atmospheric and ocean sciences at Princeton University in New Jersey. Sarmiento made the discovery using sophisticated computer models.

The nutrient-rich waters help feed phytoplankton, single-celled plants at the bottom of the marine food chain that live at the ocean surface. As phytoplankton die, some slowly sink, decomposing along the way and carrying nutrients to the deep ocean.

For years, scientists have wondered how these sinking nutrients, which seem lost to the deep sea, get back to the surface. Sarmiento's model, which links the resupply of the sunken nutrients to a single circulation pattern, provides an answer.

Arnold Gordon, a physical oceanographer with Columbia University in New York, said that, while the circulation patterns in the southern oceans have been known for some time, "that they play such an essential role in the large-scale nutrient budget of the upper ocean layers as proposed by Sarmiento is new and important."

Barrier Layer

Key to Sarmiento's model is the small difference in density among the three distinct layers of water in the Southern Ocean, the name some people use for the waters around Antarctica. These differences in density allow for mixing across the layers.

"Think of the ocean as consisting of three layers," he said. "At the surface to a depth of about 100 meters (330 feet), we have the layer of the ocean where there is enough light for photosynthesis to occur. Below this we have what I call the barrier layer."

The barrier layer is what oceanographers refer to as the thermocline (because it is where temperature changes rapidly with depth) or the pycnocline (because it is where density changes rapidly with depth).

Since density variations in the ocean are due mostly to temperature, the terms "thermocline" and "pycnocline" are interchangeable, according to Sarmiento.

The barrier layer averages about 500 to 1,000 meters (1,600 to 3,300 feet) in depth. The deep ocean, which has an average thickness of about 3 kilometers (2 miles), lies below the barrier layer.

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