Glacial‐interglacial differences in circulation and carbon cycling within the upper western North Atlantic

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Abstract
We investigated glacial‐interglacial changes in the circulation and carbon cycling in the western North Atlantic subtropical gyre using hydrographic data and downcore records of the stable isotopic compositions of individual shells of Bahamian benthic foraminifera. Potential temperature‐salinity‐depth relations show that modern thermocline (∼200–1000 m) and deep (∼1000–2000 m) waters in the Providence 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 margins (∼400 to 1500 m depth) reflect temperature, nutrient, and isotopic gradients of modern subtropical gyre waters. The difference between the δ18O of glacial maximum and late Holocene foraminifera is ∼2.1‰ for the upper 900 m of the water column and ∼1.6‰ for deeper waters, indicating that these waters were ∼4°C and ∼2°C cooler, respectively, during glacial time. The glacial temperature gradient (dT/dz) was similar to today, while the base of the thermocline was ∼100 m shallower. These results differ significantly from our earlier results from multiple shell δ18O analyses, which implied upper thermocline waters were only ∼1°C cooler and dT/dz was greater during the glacial maximum. The difference occurs because bioturbation adversely affects multiple shell analyses of glacial‐aged samples from shallow water depths. At all depths above 1500 m, foraminiferal δ13C are greater during the glacial maximum than the late Holocene by at least 0.1 to 0.2‰ (as much as 0.6‰ in the lower thermocline), indicating that nutrient concentrations throughout the thermocline were reduced and there was no oxygen minimum zone during the glacial maximum. This suggests greater, more uniform ventilation of the thermocline. Results of single and multiple shell δ13C analyses of glacial age foraminifera compare favorably because samples most affected by mixing correspond to water depths where the glacial‐interglacial change of δ13C was small. Cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopycnal 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 production. Enhanced thermocline ventilation is consistent with more vigorous winds and all isopycnal surfaces outcropping in the area of Ekman downwelling.