Computer Science
Scientific paper
Nov 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.5384l&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 71, Issue 22, p. 5384-5404.
Computer Science
10
Scientific paper
We report 15N/14N ratios of porewater nitrate in sediments from the Bering Sea basin, where microbial nitrate reduction has been identified as a significant sink for fixed nitrogen (N). Strong 15N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration [NO3-] decreases (δ15N generally reaches 25 35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction (ɛcell) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of ɛcell, while uncertain, show a negative correlation with bottom water [O2]; we propose that this relates to the [NO3-] at the depth of denitrification. The N isotope effect at the scale of nitrate sediment water exchange (ɛapp) is ˜0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. ɛapp is much lower than ɛcell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of 15N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to greater diffusive distances for nitrate to the depth of denitrification. The data suggest that low bottom water [O2] tends to yield more complete expression of ɛcell at the sediment water scale, due to higher [NO3-] at the depth of denitrification. While porewater ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O2] bottom waters, the efflux and subsequent oxidation of ammonium enriched in 15N by incomplete nitrification can significantly enhance the total net isotope effect of sedimentary N loss (ɛsed, equivalent to ɛapp but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean ɛsed of ˜4‰ (˜2‰ if restricted to seafloor below 1 km depth).
Brunelle Brigitte G.
Cane Greg
Granger Julie
Hoffmann Sharon
Lehmann Moritz F.
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