HR: 1340h
AN: OS33A-0580    [Abstracts]
TI: Nitrogen Isotopic Evidence for Climate-Induced Variations in Nitrogen Cycling in the Cariaco Basin Upwelling System from the Younger Dryas to Present
AU: * Hollander, D
EM: davidH@marine.usf.edu
AF: College of Marine Science, University of South Florida, St. Petersburg, FL 33701 United States
AU: Werne, J
EM: jwerne@umn.edu
AF: Large Lakes Observatory, University of Minesota/Duluth, Duluth, MN 55812 United States
AB: A high-resolution nitrogen isotopic study in the sediments of the Cariaco Basin, Venezuela, indicates that nitrogen cycling in the Cariaco Basin has varied dramatically over the past 12 14C kyr as a result of climate-induced variations in wind-driven upwelling intensity. The d15N of sedimentary organic matter (SOM) varies from as low as 1% during the Younger Dryas and later Holocene to values between 4 and 5.5% during the early Holocene (10 and 7 14C kybp). In the modern Cariaco Basin, strong correspondence between the d15N of thermocline nitrate, particulate organic nitrogen filtered from 275 and 1250 m water depth, and core-top SOM implies that the basin sediments accurately record the isotopic composition of thermocline nitrate (Thunnell et al., 2004). Thermocline nitrate in the Cariaco basin is 15N-depleted relative to deep-ocean nitrate, indicating the importance of newly fixed N to the planktonic ecosystem. d15N of SOM around 1% during the Younger Dryas and the mid-late Holocene indicate substantial changes in the nitrogen cycling and the overwhelming importance newly fixed N as a source for sedimentary N. At times of high productivity, low N:P ratios in surface waters and the proximity to a land source to supply needed Fe are factors capable of promoting enhanced N-fixation. Alternatively , either direct ammonium assimilation or the recently identified process of anaerobic ammonium oxidation could be responsible for the 15N-depleted nitrogen isotopic signature in the sediments during periods characterized by high productivity such as the Younger Dryas. Temporal variations in the biogeochemical cycling of nitrogen from the Younger Dryas to present will be discussed in the context of contemporary nitrogen cycling as identified by the CARIACO time-series, to previous sedimentary 15N-studies spanning glacial-interglacial periods in the Cariaco Basin (Haug et al., 1998), and to innovative studies of nitrogen cycling in other modern marine anoxic basins.
DE: 4215 Climate and interannual variability (3309)
DE: 4267 Paleoceanography
DE: 3022 Marine sediments--processes and transport
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1055 Organic geochemistry
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting