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

Dale, Andrew W., Sommer, Stefan, Ryabenko, Evgenia, Noffke, Anna, Bohlen, Lisa, Wallmann, Klaus, Stolpovsky, Konstantin, Greinert, Jens and Pfannkuche, Olaf (2014) Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic). Geochimica et Cosmochimica Acta, 134 . pp. 234-256. DOI 10.1016/j.gca.2014.02.026.

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Abstract

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 δ15NNO3 and δ18ONO3 in the benthic chamber deployed on the shelf, characterized by muddy sand, were used to calculate apparent benthic nitrate fractionation factors of 8.0‰ (15εapp) and 14.1‰ (18εapp). Measurements of δ15NNO2 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 (εDEN) 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 εDEN 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.

Document Type: Article
Additional Information: WOS:000335136400014
Keywords: sediment, nitrogen, denitrification, nitrogen isotopes, benthic fractionation, oxygen minimum zone, Mauritania, Eastern Tropical North Atlantic (ETNA)
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > SFB 754 > B5
OceanRep > The Future Ocean - Cluster of Excellence
OceanRep > SFB 754
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems > DeepSea Monitoring
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > SFB 754 > B1
Kiel University
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.gca.2014.02.026
ISSN: 0016-7037
Projects: SFB754
Contribution Number:
ProjectNumber
DSM5
Expeditions/Models/Experiments:
Date Deposited: 06 May 2014 08:53
Last Modified: 15 Sep 2017 08:54
URI: http://oceanrep.geomar.de/id/eprint/24425

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