Upper Ocean Thermal Gradients in the Eastern Equatorial Atlantic: Implications for Interhemispheric Heat Transport Since 150 ka

Ocean Drilling Project (ODP) site 662A (water depth 3814 m) in the eastern equatorial Atlantic contains a continuous sediment record for the past 150,000 years, comprised of calcium carbonate ooze with high sedimentation rates (approximately 5 cm/kyr). Site 662A is located on the eastern flank of the mid-Atlantic ridge directly beneath the zone of equatorial divergence associated with the South Equatorial Current (SEC). At present, sea surface temperature (SST) is highest and salinity is lowest at this location during boreal winter, when the intertropical convergence zone (ITCZ) is displaced farthest to the south. The water column is most stratified and maximum advection of heat and salt from the southern hemisphere to the northern hemisphere also occurs during boreal winter. Foraminiferal Mg/Ca ratios and δ18Oc records were generated for two species of planktonic foraminifera ( Globigerinoides sacculifer, a surface dweller and Neogloboquadrina dutertrei, a thermocline dweller). These measurements can then be used to estimate changes in the local δ18O of seawater after correcting for global ice volume. At site 662A, changes in salinity can be attributed to changes in evaporation/precipitation, with increases in salinity reflected by a relative increase in local δ18Osw. Foraminiferal Mg/Ca paleotemperature data for G. sacculifer indicate that sea surface temperature for the last 150,000 yrs ranged from approximately 21°C to 25°C with a temperature change of ~4°C during glacial terminations 1 and 2. Local δ18Osw calculated from the G. sacculifer record suggest that sea surface salinities were generally lower relative to the Holocene during marine isotope stage 5, and higher during MIS 2 -- 4. A comparable Mg/Ca record for N. dutertrei is being generated to reconstruct changes in the thermal structure of the water column. Preliminary results suggest that N. dutertrei Mg/Ca paleotemperatures are more variable than those of G. sacculifer and may indicate that thermocline waters are decoupled from the surface. Our data suggest the upper water column was more stratified during MIS 5 than during MIS 2 -- 3, indicating stronger thermohaline circulation during this interglacial period. Alternatively, lower salinities may offset increased cross equatorial heat advection during MIS 5, such that changes in salinity in the eastern equatorial Atlantic may act as a global climate buffer.