GLACIAL INTERGLACIAL VARIATIONS IN PRODUCTION AND NITROGEN-FIXATION IN THE CARIACO BASIN DURING THE LAST 580 KYR/

The effect of sea level change on nutrient supply to the anoxic Cariac o demonstrates the fundamental importance of nitrogen (N-2) fixation a nd phosphate to oceanic production. As N-2 fixation produces biomass o f low delta(15)N and has been reported to be an important component of the nitrogen cycle in the modern Cariaco Basin, we propose that it co ntributes to the light interglacial delta(15)N (similar to 2% - 3%) va lues observed in the Ocean Drilling Program (ODP) site 1002 sediment r ecord. During the glacials the sediments are bioturbated (oxic conditi ons) with low total organic caebon (TOC) contents and sedimentary delt a(15)N values of similar to 5%, suggesting that nitrogen (N-2) fixatio n contributed little to the N nutrition of Cariaco surface waters. The most plausible explanation for the inferred glacial/interglacial chan ges in N-2 fixation in the Cariaco is that they have occurred in respo nse to variations in the N/P ratio of the nutrient supply, driven by c hanges in denitrification.