HR: 08:45h
AN: PP51G-04    [Abstracts]
TI: Nitrogen isotopic composition of ammonium released during diagenesis of organic matter under steady-state and non-steady state conditions in marine anoxic sediments
AU: * Prokopenko, M G
EM: prokopen@usc.edu
AF: Departement of Earth Sciences, USC, 3651 Trousedale, SCI-117, Los Angeles, CA 90089 United States
AU: Hammond, D E
EM: dhammond@usc.edu
AF: Departement of Earth Sciences, USC, 3651 Trousedale, SCI-117, Los Angeles, CA 90089 United States
AU: Stott, L
EM: stott@usc.edu
AF: Departement of Earth Sciences, USC, 3651 Trousedale, SCI-117, Los Angeles, CA 90089 United States
AU: Spivack, A J
EM: spivack@gso.uri.edu
AF: Graduate School of Oceanography, URI, South Ferry Road, Narragansett, RI 02882 United States
AB: The $\delta$15N of organic matter in marine sediments has been successfully used as a paleoceanographic proxy. However, diagenesis may alter the original isotopic ratio. One approach to evaluate the sign and magnitude of the diagenetic impact is to measure the isotopic composition of ammonium that is produced during the degradation of nitrogenous organic matter (N-org). If diagenesis of organic matter is the only process affecting ammonium concentration and its isotopic composition, the $\delta$15N of ammonium should reflect the isotopic ratio of the N-org plus fractionation, if any. We evaluated the impact of diagenesis on $\delta$15N of N-org on a time scale of few thousand years in two California Borderland Basins, Santa Barbara (SBB) and San Nicolas (SNB). We found ammonium to be about 2.5 \permil heavier than the bulk N-org. One interpretation is that N-org consists of multiple, isotopically distinct fractions of organic matter with variable lability. The most labile fraction must be 2.5 \permil heavier than the bulk N-org. The relatively large loss of N-org in SNB (50 %) should have resulted in a 2 \permil shift in $\delta$15N of N-org downcore, while in SBB, higher accumulation rates led to smaller loss of N-org (15%) and little change in its isotopic composition. Sediment from the Peru-Chile margin, collected during ODP Legs 201 and 202, was used to evaluate the diagenetic impact on a time scale of millennia to millions of years. In rapidly accumulating anoxic coastal sediments with constant $\delta$15N in the N-org , we found diagenetic fractionation is less than 1 \permil. Several other ODP sites are also strongly influenced by non-steady state conditions, and the isotopic composition of pore water ammonium primarily reflects changes in $\delta$15N of the source organic matter through time. Preliminary model results show that isotopically heavy ammonium released from organic matter deposited within the last 12-15 Kyr has diffused about 30 m into the sediments. This result underscores the importance of recognition and proper evaluation of non-steady state conditions.
DE: 4805 Biogeochemical cycles (1615)
DE: 4825 Geochemistry
DE: 4870 Stable isotopes
DE: 4203 Analytical modeling
DE: 4802 Anoxic environments
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting