HR: 0800h
AN: B31A-0981    [Abstracts]
TI: Pore water profiles of dissolved N2 concentrations and δ15N-N2 in the Santa Barbara Basin: Evidence for non-diffusive nitrate flux into the sediment column
AU: * Prokopenko, M G
EM: mprokope@Princeton.EDU
AF: Department of Geosciences Princeton University, Guyot Hall Washington Rd, Princeton, NJ 08544 United States
AU: Sigman, D M
EM: sigman@Princeton.EDU
AF: Department of Geosciences Princeton University, Guyot Hall Washington Rd, Princeton, NJ 08544 United States
AU: Hammond, D E
EM: dhammond@usc.edu
AF: Department of Earth Sciences University of Southern California, 3651 Trousedale Prkw, Los Angeles, CA 90089 United States
AU: Barnett, B A
EM: bbarnett@Princeton.EDU
AF: Department of Geosciences Princeton University, Guyot Hall Washington Rd, Princeton, NJ 08544 United States
AU: Bender, M
EM: bender@Princeton.EDU
AF: Department of Geosciences Princeton University, Guyot Hall Washington Rd, Princeton, NJ 08544 United States
AU: Berelson, W
EM: berelson@usc.edu
AF: Department of Earth Sciences University of Southern California, 3651 Trousedale Prkw, Los Angeles, CA 90089 United States
AB: Recently constructed global N budgets indicate that sedimentary denitrification removes about 75 % of biologically available nitrogen from the ocean, which makes it the most important sink for the oceanic nitrate. However, the uncertainty of a factor of two or three exists in the estimates of benthic denitrification. The challenge of accurately quantifying the contribution of this process to the global nitrogen budget arises, in part, from the lack of detailed knowledge of the nitrogen metabolism of benthic microbial communities participating in the processes collectively termed as ''sedimentary denitrification''. Nitrogen isotopic gradients of pore water ammonium and nitrate have been previously used to place budgetary constraints on some of the microbial processes in anoxic and suboxic marine sediments. Here we present the profiles of concentrations and isotopic composition of dissolved N2 as well as N2/Ar ratios from the pore waters of anoxic sediments collected in the center of Santa Barbara Basin in May 2005. Concentration and N2/Ar ratio gradients indicate a 20 to 40 uM downcore increase in N2 over the upper 4 to 5 cm. Addition of N2 to the pore waters continues below 5 cm into the sediments, even though the concentrations of pore water nitrate approach the detection limit within 2 to 3 cm below the sediment surface. The newly added N2 is most likely of biological origin, as follows from the N2/Ar ratios. The downcore increase in N2 concentration is 2 to 3 times greater than predicted from denitrified bottom water nitrate (25 uM). The increase in N2 concentrations is accompanied by an increase in dinitrogen δ15N, from 0.4-0.7 per mil at the sediment-water interface to 0.9 - 1.3 per mil at depth (the range reflecting variations between stations). Isotopic mass balance requires that the added N2 has a δ15N of 7.5 - 12.5 per mil (± 3 per mil). The δ15N composition of organic matter deposited on the ocean floor is 7.5 per mil, while the δ15N of ammonium produced from the organic matter is ~ 10 per mil. The δ15N of the bottom water nitrate is between 8 and 12 per mil, depending on the magnitude of the nitrate deficit at the time of measurements. Therefore, our isotope data are consistent with N2 production from both organic nitrogen derived NH4+ and the bottom water NO3-, as in the anammox reaction. The deep N2 production and the amplitude of the deep N2 excess require a non-diffusive mechanism for NO3- supply, unless in situ oxidation of ammonium is producing excess NO3- or NO2- at depth.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0466 Modeling
DE: 0469 Nitrogen cycling
DE: 4804 Benthic processes, benthos (0408)
SC: Biogeosciences [B]
MN: Fall Meeting 2005