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Dale, A.W.* ; Sommer, S.* ; Ryabenko, E.* ; Noffke, A.L.* ; Bohlen, L.* ; Wallmann, K.* ; Stolpovsky, K.* ; Greinert, J.* ; Pfannkuche, O.*

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

Geochim. Cosmochim. Acta 134, 234-256 (2014)

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We present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen (DIN: NO₃⁻, NO₂⁻, NH₄⁺) and oxygen (O₂) from 7 sites with variable sand content along 18°N offshore Mauritania (NW Africa). Bottom water O₂ concentrations at the shallowest station were hypoxic (42 μM) and increased to 125 μM at the deepest site (1113 m). Total oxygen uptake rates were highest on the shelf (−10.3 mmol O₂ m⁻² d⁻¹) and decreased quasi-exponentially with water depth to −3.2 mmol O₂ 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 δ¹⁵N_NO3 and δ¹⁸O_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‰ (¹⁵ε_app) and 14.1‰ (¹⁸ε_app). Measurements of δ¹⁵N_NO2 further demonstrated that the sediments acted as a source of ¹⁵N 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 ¹⁴N/¹⁵N fractionation (ε_DEN) was 12.9 ± 1.7‰, (ii) inverse fractionation during nitrite oxidation leads to an efflux of isotopically light NO₂⁻ (−22 ± 1.9‰), and (iii) direct coupling between nitrification and denitrification in the sediment is negligible. Previously reported ε_DEN for fine-grained sediments are much lower (4–8‰). 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.

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