Friday, November 5, 2010

Atlantic Ocean flow reversed 10,000 years ago, slowing down again

The global conveyer, or thermohaline system,
with surface currents in red, deep cold currents in blue (Image by Avs)

From ArsTechnica

The flow of top- and bottom-level currents in the Atlantic Ocean appear to be slowing down and may be due for a reversal like one that happened 10,000 years ago, according to new data.
By studying sediment samples, scientists have found that, some time after the last glacial maximum, the undercurrent of the Atlantic Ocean switched from flowing north to flowing south, thanks in large part to changing temperatures.

Recently, discussions about the flow of the ocean have centered around what role it plays in climate change.
The "conveyor belt" flow of the Atlantic Ocean, which currently goes south on the underside and north closer to the surface, helps regulate the temperature of the water and can distribute heat to normally cold areas.
If it were to stop, it might allow for a bit of localized cooling in areas that are otherwise melting.

To get a better history of the Atlantic's flow, researchers studied sediment samples, specifically looking for the elements protactinium and thorium.
They noted that a short time after the Last Glacial Maximum (LGM), the gradient of these elements in the North and South Atlantic reversed.

This indicates that the ocean used to flow north on the underside and south on the surface, the opposite of the way it does now.
The ocean was likely at an effective standstill at some point, probably around the beginning of the Holocene between 10,000 and 12,000 years ago.
The authors attribute the changes to surface cooling during the LGM, as well has an increase in seawater salinity in the Southern Ocean.

According to a graph in the
paper that shows water mass travel time in the ocean, the flow speed increased for a while, but has become lethargic in the past few thousand years, indicating it may be on its way to a stoppage.
Of course, it will likely be on the order of a thousand years or more before this happens naturally and, even if it does, the climate may be radically different by then anyway.

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