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Reversed flow of Atlantic deep water during the Last Glacial Maximum : Nature : Nature Publishing Group
The meridional overturning circulation (MOC) of the Atlantic Ocean is considered to be one of the most important components of the climate system. This is because its warm surface currents, such as the Gulf Stream, redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns, whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss, away from the atmosphere. Despite its significance for future climate, the operation of the MOC under contrasting climates of the past remains controversial. Nutrient-based proxies1, 2 and recent model simulations3 indicate that during the Last Glacial Maximum the convective activity in the North Atlantic Ocean was much weaker than at present. In contrast, rate-sensitive radiogenic 231Pa/230Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength4, 5, 6. Here we show that the basin-scale abyssal circulation of the Atlantic Ocean was probably reversed during the Last Glacial Maximum and was dominated by northward water flow from the Southern Ocean. These conclusions are based on new high-resolution data from the South Atlantic Ocean that establish the basin-scale north to south gradient in 231Pa/230Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of 231Pa/230Th outside the North Atlantic basin will enhance our understanding of past ocean circulation, provided that spatial gradients are carefully considered. This broader perspective suggests that the modern pattern of the Atlantic MOC--with a prominent southerly flow of deep waters originating in the North Atlantic--arose only during the Holocene epoch.

In plain English, this appears to state that the North Atlantic Drift or Gulf Stream were reversed during the last Ice age with the surface water flowing North South and the deep sea water flowing South North.  So rather than distributing warm tropical water towards Europe, it was distributing cold Arctic water towards Europe - similar to current southerly currents off Newfoundland. It is easy to sea why this may have caused/contributed to the ice age, but what caused the reversal?

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by Frank Schnittger (mail Frankschnittger at hot male dotty communists) on Wed Dec 8th, 2010 at 01:03:09 PM EST

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