The distribution of nitrate 15 N/ 14 N in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate Moritz F. Lehmann a, * , Daniel M. Sigman b , Daniel C. McCorkle c , Julie Granger d , Sharon Hoffmann c , Greg Cane e , Brigitte G. Brunelle b a Geochemistry and Geodynamics Research Center (GEOTOP-UQAM-McGill), University of Quebec at Montreal, Montreal, Que., Canada H3C 3P8 b Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, USA c Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA d Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 e Department of Earth System Science, University of California Irvine, Irvine, CA 92697-3100, USA Received 13 November 2006; accepted in revised form 22 July 2007; available onilne 7 September 2007 Abstract We report 15 N/ 14 N 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 15 N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration ½NO 3   decreases (d 15 N generally reaches 25–35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction (e cell ) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of e cell , while uncertain, show a negative correlation with bottom water [O 2 ]; we propose that this relates to the ½NO 3   at the depth of denitrification. The N isotope effect at the scale of nitrate sediment–water exchange (e app ) is 0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. e app is much lower than e cell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of 15 N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to great- er 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 e cell at the sediment–water scale, due to higher ½NO 3   at the depth of denitrification. While pore- water ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O 2 ] bottom waters, the efflux and subsequent oxidation of ammonium enriched in 15 N by incomplete nitri- fication can significantly enhance the total net isotope effect of sedimentary N loss (e sed , equivalent to e app but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean e sed of 4‰ (2‰ if restricted to seafloor below 1 km depth). Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Denitrification, the dissimilatory reduction of nitrate to N 2 O and N 2 , is the most important mechanism of N loss in the ocean (Codispoti and Christensen, 1985; Seitzinger, 1988; Middelburg et al., 1996). It occurs in the water col- umn where oxygen concentrations are below 5 lmol/L, 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.07.025 * Corresponding author. Present address: Institute for Environ- mental Geoscience, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland. Fax: +41 61 267 04 79. E-mail address: Moritz.Lehmann@unibas.ch (M.F. Lehmann). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 71 (2007) 5384–5404