Deep thermohaline circulation in the low‐latitude Atlantic during the Last Glacial

Abstract

Present‐day low‐latitude eastern and western Atlantic basins are geochemically distinct below the sill depth of the Mid‐Atlantic Ridge. While Antarctic Bottom Water (AABW) circulates freely in the western Atlantic, flow into the eastern Atlantic is restricted below 4 km which results in filling the abyssal depths of this basin with water of geochemical similarity to nutrient depleted North Atlantic Deep Water. Using carbon isotopes and Cd/Ca ratios in benthic foraminifera we reconstruct the geochemistry of these basins during the last glacial maximum. Results indicate that deep eastern and western Atlantic basins became geochemically identical during the last glacial. This was achieved by shoaling of the upper surface of AABW above the sill depth of the Mid‐Atlantic Ridge, which allowed bottom waters in both basins to be filled with the same water mass. Although AABW became the dominant water mass in the deep eastern Atlantic basin during the glacial, Holocene‐glacial δ13C‐PO4 shifts in this basin are in Redfield proportions, unlike the disproportionate Holocene‐glacial δ13C‐PO4 shifts observed in the Southern Ocean. By examining the composition of deep and intermediate waters throughout the Atlantic, we show that this effect was induced by a change in gradient of the δ13C‐PO4 deepwater mixing line during glacial times. Evidence from high‐latitude planktonic data suggests that the change in gradient of the deepwater mixing line was brought about through a significant reduction in the thermodynamic effect on Southern Ocean surface waters. By using coupled δ13C‐PO4 data to constrain the composition of end member water masses in the glacial Atlantic, we conclude that deep waters in the low‐latitude glacial Atlantic were composed of a mixture of northern and southern source waters in a ratio of 1∶3.