HR: 0800h
AN: PP21B-1383    [Abstracts]
TI: Evidence From Diatom-Bound d15N for Bering Sea Stratification During the Last Ice Age and a Causal Link to North Pacific Denitrification Changes
AU: * Brunelle, B G
EM: brunelle@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AU: Sigman, D M
AF: Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
AU: Cook, M S
AF: Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Keigwin, L D
AF: Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: Using a recently developed method for the isotopic analysis of diatom microfossil-bound N (Robinson et al. 2004), we have adapted the diatom cleaning protocol to be effective even in high-clay sediments of the Bering Sea. Newly generated profiles of diatom-bound d15N and opal content from the Bering Sea appear to reflect glacial-interglacial variations in local nutrient utilization. Interglacial conditions are characterized by low diatom-bound d15N (4-5 permil) and high opal content, indicative of relatively low levels of nutrient consumption and high diatom productivity, whereas glacial periods are predominantly characterized by high diatom-bound d15N ($\sim$8 permil) and low opal content. Taken together, these results suggest stratification of the Bering Sea upper water column during glacial times, with nutrient drawdown resulting from continued iron supply through atmospheric deposition. A steady increase in diatom-bound d15N and steady decrease in opal content throughout stages 5, 3, and 2 suggest a progressive increase in nutrient utilization. Two peaks in diatom-bound d15N are also observed upon deglaciation, before diatom-bound N relaxes to low Holocene d15N, and the presence of laminations during these periods suggests that local denitrification within the Bering Sea may have worked to raise the d15N of the nitrate feeding surface waters at these times. One interpretation of these data is that more complete consumption of nutrients in the Subarctic Pacific surface during the last glacial period lowered the nutrient content of the intermediate water formed in the North Pacific. This water then flowed equatorward to ventilate the low latitude thermocline. The decrease in the preformed nutrient concentration of thermocline water reduced productivity, subsurface O2 consumption, and denitrification in the low latitudes. These changes in denitrification, in turn, modulated the d15N of nitrate in the Bering Sea subsurface.
DE: 4805 Biogeochemical cycles (1615)
DE: 4870 Stable isotopes
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting