Coupled nitrification-denitrification in sediment of the eastern Bering Sea shelf leads to 15N enrichment of fixed N in shelf waters

J. Granger, M. G. Prokopenko, Daniel Mikhail Sigman, C. W. Mordy, Z. M. Morse, L. V. Morales, R. N. Sambrotto, B. Plessen

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Abstract

We studied the nitrogen biogeochemistry of the ice-covered eastern Bering Sea shelf using the isotope ratios (15N/14N and 18O/16O) of NO3- and other N species. The 15N/14N of late winter NO3 - on the shelf decreases inshore and is inversely correlated with bottom water [NH4+], consistent with an input of low- 15N/14N NO3- from partial nitrification of NH4+ remineralized from the sediments. An inshore 15N/14N increase in total dissolved N (TDN) suggests that (1) the sediment-derived NH4+ is elevated in 15N due to the same partial nitrification that yields the low- 15N/14N NO3-, and (2) 15N-deplete NO3- from partial nitrification within the sediments is denitrified to N2. The proportion of newly nitrified NO3- on the shelf, evidenced by an inshore decrease in NO3-18O/16O, is correlated with the N deficit, further implicating nitrification coupled to denitrification; however, a simple N isotope budget indicates a comparable rate of denitrification supported by diffusion of NO3- into the sediments. The isotopic impact of benthic N loss is further demonstrated by a correlation between the 15N/14N of shelf surface sediment and the N deficit of the overlying water column, both of which increase inshore and northward, as well as by Arctic NO3- isotope data indicating that the fixed N transported through Bering Strait has a 15N/14N higher than is found in the open Bering Sea. The significant net isotope effect of benthic N loss on the Bering shelf, 6-8 ‰, is at odds with previous assumptions regarding the global ocean's N isotope budget.

Original languageEnglish (US)
Article numberC11006
JournalJournal of Geophysical Research: Oceans
Volume116
Issue number11
DOIs
StatePublished - 2011

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Forestry
  • Oceanography
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Palaeontology

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