Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic)

We present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen (DIN: NO3-, NO2-, NH4+) and oxygen (O-2) from 7 sites with variable sand content along 18 degrees N offshore Mauritania (NW Africa). Bottom water O-2 concentrations at the shallowest station were hypoxic (42 mu M) and increased to 125 mu M at the deepest site (1113 m). Total oxygen uptake rates were highest on the shelf (-10.3 mmol O-2 m(-2) d(-1)) and decreased quasi-exponentially with water depth to 3.2 mmol O-2 m(-2) d(-1). Average denitrification rates estimated from a flux balance decreased with water depth from 2.2 to 0.2 mmol N m(-2) d(-1). Overall, the sediments acted as net sink for DIN. Observed increases in delta N-15(NO3) and delta O-18(NO3) in the benthic chamber deployed on the shelf, characterized by muddy sand, were used to calculate apparent benthic nitrate fractionation factors of 8.0 parts per thousand ((15)epsilon(app)) and 14.1 parts per thousand ((18)epsilon(app)). Measurements of delta N-15(NO2) further demonstrated that the sediments acted as a source of N-15 depleted NO2-. These observations were analyzed using an isotope box model that considered denitrification and nitrification of NH4+ and NO2-. The principal findings were that (i) net benthic N-14/N-15 fractionation (epsilon(DEN)) was 12.9 +/- 1.7 parts per thousand, (ii) inverse fractionation during nitrite oxidation leads to an efflux of isotopically light NO2- (-22 +/- 1.9 parts per thousand), and (iii) direct coupling between nitrification and denitrification in the sediment is negligible. Previously reported epsilon(DEN) for fine-grained sediments are much lower (4-8 parts per thousand). We speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater-seawater exchange in permeable sediments and the hypoxic, high productivity environment. Although not without uncertainties, the results presented could have important implications for understanding the current state of the marine N cycle. (C) 2014 Elsevier Ltd. All rights reserved.