The distribution of nitrate N-15/N-14 in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate

We report N-15/N-14 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 N-15 enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration [NO3-] decreases (delta N-15 generally reaches 25-35%). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction (epsilon(cell)) of 11% to 30 parts per thousand, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of epsilon(cell), while uncertain, show a negative correlation with bottom water [O-2]; 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 (epsilon(app)) is similar to 0 parts per thousand in two unreactive deep sites and is typically > 3 parts per thousand at more reactive sites at various depths. 8,PP is much lower than epsilon(cell) because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of N-15-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 [O-2] tends to yield more complete expression of epsilon(cell) 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 (εsed, equivalent to εapp but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean εsed of ∼4‰ (∼2‰ if restricted to seafloor below 1 km depth).