The overturning circulation of the glacial Atlantic - A view from the top

Large scale ocean currents are reflected in the density structure of the ocean (geostrophic balance). Density structure for times in the past can be reconstructed using the oxygen isotopic composition of benthic foraminifera. In the upper ocean, currents are strong and density contrasts are large and can easily be resolved from the isotope data. Here we use sediment cores from the Bahamas and off the Atlantic coast of Florida to reconstruct the density structure of the waters in the Florida Straits during the Last Glacial Maximum. Today, the water flowing through the Florida Straits (30 Sv) contains both the western limb of the wind driven circulation of the subtropical gyre (17 Sv), and the northward flow of waters which compensate North Atlantic Deep Water export (13 Sv). We find evidence for substantially reduced flow relative to today (17 Sv), especially at intermediate depths. The simplest explanation for the reduced how is a severe reduction in the strength of the meridional overturning circulation during the Last Glacial Maximum.