DEEP THERMOHALINE CIRCULATION IN THE LOW-LATITUDE ATLANTIC DURING THELAST GLACIAL

Present-day low-latitude eastern and western Atlantic basins are geoch emically distinct below the sill depth of the Mid-Atlantic Ridge. Whil e Antarctic Bottom Water (AABW) circulates freely in the western Atlan tic, now into the eastern Atlantic is restricted below 4 km which resu lts in filling the abyssal depths of this basin with water of geochemi cal similarity to nutrient depleted North Atlantic Deep Water. Using c arbon isotopes and Cd/Ca ratios in benthic foraminifera we reconstruct the geochemistry of these basins during the last glacial maximum. Res ults indicate that deep eastern and western Atlantic basins became geo chemically identical during the last glacial. This was achieved by sho aling of the upper surface of AABW above the sill depth of the Mid-Atl antic Ridge, which allowed bottom waters in both basins to be filled w ith the same water mass. Although AABW became the dominant water mass in the deep eastern Atlantic basin during the glacial, Holocene-glacia l delta(13)C-PO4 shifts in this basin are in Redfield proportions, unl ike the disproportionate Holocene-glacial delta(13)C-PO4 shifts observ ed in the Southern Ocean. By examining the composition of deep and int ermediate waters throughout the Atlantic, we show that this effect was induced by a change in gradient of the delta(13)C-PO4 deepwater mixin g line during glacial times. Evidence from high-latitude planktonic da ta suggests that the change in gradient of the deepwater mixing line w as brought about through a significant reduction in the thermodynamic effect on Southern Ocean surface waters. By using coupled delta(13)C-P O4 data to constrain the composition of end member water masses in the glacial Atlantic, we conclude that deep waters in the low-latitude gl acial Atlantic were composed of a mixture of northern and southern sou rce waters in a ratio of 1:3.