HR: 0800h
AN: PP21B-1561    [Abstracts]
TI: Millennial-Scale Variations of Nitrogen Isotopes and Export Proxies in the Subarctic Pacific During MIS 3: Evidence for an Oceanic Fertility Switch?
AU: * Galbraith, E D
EM: egalbraith@eos.ubc.ca
AF: University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AU: Schmittner, A
EM: aschmittner@coas.oregonstate.edu
AF: Oregon State University, 104 COAS Admin Bldg, Corvalis, OR 97331 United States
AU: Pedersen, T F
EM: tfp@uvic.ca
AF: University of Victoria, 3800 Finnerty Road, Victoria, BC V8W 3P6 Canada
AB: Millennial-scale variability has previously been observed in both the nitrogen isotopic ratio (δ15N) and paleoproductivity proxies in sediments of the Arabian Sea and the Californian margin. Here we show that hemipelagic sediments from a seamount in the Gulf of Alaska (ODP site 887) record similar millennial variability in δ15N and biogenic components during Marine Isotope Stage 3, with a tantalizing resemblance to North Atlantic temperature records. The subarctic Pacific record can be interpreted in several ways. One possibility is that periods of intense aridity promoted enhanced dust transport to the North Pacific, alleviating Fe limitation and allowing more complete utilization of the available nitrate. Although the age constraints are insufficient to distinguish stadials from interstadials, one would expect stadial periods to be more arid and, hence, to be correlated with high δ15N and productivity. However, in this case the Gulf of Alaska δ15N record would be antiphased with δ15N records of the Oregon and California margins, which seems unlikely. Alternatively, Fe could have been delivered by enhanced offshore transport of shelf material during periods of sea level rise, resulting in a positive phasing between the Gulf of Alaska and other δ15N records, but without providing a mechanistic link between them. A more parsimonious explanation is that nitrogen isotopes and export productivity varied in phase, on millennial timescales, at all of these locations. We propose that this resulted from millennial changes in the distribution of nutrients and oxygen in the ocean, caused by changes in the physical circulation. During interstadials, strong NADW formation maintained relatively low deep water nutrient concentrations, ensuring a rich supply of nutrients in the upper ocean. The cessation of NADW formation during stadials allowed the migration of nutrients to the deep sea, stripping the upper ocean of its potential fertility. The marine nitrogen cycle responded to these events through the modulation of thermocline suboxia by upper ocean fertility. During interstadials when the upper ocean was relatively nutrient-rich, expanded thermocline suboxia drove accelerated denitrification, producing an increase in δ15N near denitrification zones. When the upper ocean became nutrient-poor during stadials, thermocline suboxia contracted and δ15N fell to lower values. Enhanced ventilation in much of the upper ocean, particularly in the North Pacific, may have further increased the subsurface oxygen supply during stadials. If valid, this mechanism would have had some effect on atmospheric trace gases including carbon dioxide and nitrous oxide.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0469 Nitrogen cycling
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 4845 Nutrients and nutrient cycling (0470, 1050)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005