HR: 08:00h
AN: H51E-01 INVITED     [Abstracts]
TI: Coupled Nitrogen and Oxygen Isotope Measurements of Nitrate Along the Eastern North Pacific Margin
AU: * Sigman, D M
EM: sigman@princeton.edu
AF: Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544 United States
AU: Granger, J
EM: jgranger@eos.ubc.ca
AF: Department of Earth and Ocean Sciences, University of British Columbia, 6270 University Blvd., Vancouver, V6T 1Z4 Canada
AU: Lehmann, M F
EM: lehmann.moritz@uqam.ca
AF: GEOTOP-UQAM-McGill Research Centre, C.P. 8888, Succursale Centre-Ville, Montr‚al, H3C 3P8 Canada
AU: DiFiore, P
EM: pdifiore@Princeton.EDU
AF: Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544 United States
AU: van Geen, A
EM: avangeen@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, Route 9W, Palisades, NY 10964 United States
AB: Water column depth profiles along the eastern North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate N-15/N-14 and O-18/O-16 associated with denitrification in the oxygen-deficient waters of the eastern tropical North Pacific. The coupled variations in N-15/N-14 and O-18/O-16 suggest that the O and N isotope effects for denitrification are roughly equivalent at 20-25 permil. This is consistent with our culture study of denitrifiers in seawater medium but stands in contrast to the results of published freshwater studies, which suggest that the isotope effect for N is greater than that for O. The maximum in nitrate O-18/O-16 is somewhat shallower than the maxima in both nitrate N-15/N-14 and the nitrate deficit (as reconstructed from nitrate and phosphate concentrations). This reflects a depth-variant deviation toward higher nitrate O-18/O-16 (or lower N-15/N-14) than can be explained by denitrification if the N and O isotope effects of denitrification are indeed equal. We tentatively interpret the apparent deviation to result from the addition of low-N-15/N-14 nitrate to the shallow thermocline by the remineralization of newly fixed N. A simple model indicates that this remineralization has erased roughly half the nitrate deficit that would otherwise be present at 200 m depth in this region.
DE: 4805 Biogeochemical cycles (1615)
DE: 4802 Anoxic environments
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1615 Biogeochemical processes (4805)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting