GLACIAL-INTERGLACIAL DIFFERENCES IN CIRCULATION AND CARBON CYCLING WITHIN THE UPPER WESTERN NORTH-ATLANTIC

We investigated glacial-interglacial changes in the circulation and ca rbon cycling in the western North Atlantic subtropical gyre using hydr ographic data and downcore records of the stable isotopic compositions of individual shells of Bahamian benthic foraminifera. Potential temp erature-salinity-depth relations show that modern thermocline (similar to 200-1000 m) and deep (similar to 1000-2000 m) waters in the Provid ence Channels, Bahamas, originate in the Sargasso Sea and are typical of the subtropical gyre. Gradients in the stable isotopic compositions of late Holocene Planulina and Cibicidoides species from the bank mar gins (similar to 400 to 1500 m depth) reflect temperature, nutrient, a nd isotopic gradients of modern subtropical gyre waters. The differenc e between the delta(18)O of glacial maximum and late Holocene foramini fera is similar to 2.1 parts per thousand for the upper 900 m of the w ater column and similar to 1.6 parts per thousand for deeper waters, i ndicating that these waters were similar to 4 degrees C and similar to 2 degrees C cooler, respectively, during glacial time. The glacial te mperature gradient (dT/dz) was similar to today, while the base of the thermocline was similar to 100 m shallower. These results differ sign ificantly from our earlier results from multiple shell delta(18)O anal yses, which implied upper thermocline waters were only similar to 1 de grees C cooler and dT/dz was greater during the glacial maximum. The d ifference occurs because bioturbation adversely affects multiple shell analyses of glacial-aged samples from shallow water depths. At all de pths above 1500 m, foraminiferal delta(13)C are greater during the gla cial maximum than the late Holocene by at least 0.1 to 0.2 parts per t housand (as much as 0.6 parts per thousand in the lower thermocline), indicating that nutrient concentrations throughout the thermocline wer e reduced and there was no oxygen minimum zone during the glacial maxi mum. This suggests greater, more uniform ventilation of the thermoclin e. Results of single and multiple shell delta(13)C analyses of glacial age foraminifera compare favorably because samples most affected by m ixing correspond to water depths where the glacial-interglacial change of delta(13)C was small. Cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopyc nal surfaces outcrop, including large areas of the subtropical ocean. A shallower thermocline base is consistent with southward migration of the northern edge of the subtropical gyre or increased mode water pro duction. Enhanced thermocline ventilation is consistent with more vigo rous winds and all isopycnal surfaces outcropping in the area of Ekman downwelling.