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 (O2) from 7 sites with variable sand content along 18°N offshore Mauritania (NW Africa). Bottom water O2 concentrations at the shallowest station were hypo...
Published in: | Geochimica et Cosmochimica Acta |
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Elsevier
2014
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Online Access: | https://oceanrep.geomar.de/id/eprint/24425/ https://oceanrep.geomar.de/id/eprint/24425/1/Dale%20et.al.pdf https://oceanrep.geomar.de/id/eprint/24425/7/leg_NetworkLink%281%29.kml https://doi.org/10.1016/j.gca.2014.02.026 |
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ftoceanrep:oai:oceanrep.geomar.de:24425 2023-05-15T17:35:43+02:00 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 Pfannkuche, Olaf 2014-06-01 text other https://oceanrep.geomar.de/id/eprint/24425/ https://oceanrep.geomar.de/id/eprint/24425/1/Dale%20et.al.pdf https://oceanrep.geomar.de/id/eprint/24425/7/leg_NetworkLink%281%29.kml https://doi.org/10.1016/j.gca.2014.02.026 en eng Elsevier https://oceanrep.geomar.de/id/eprint/24425/1/Dale%20et.al.pdf https://oceanrep.geomar.de/id/eprint/24425/7/leg_NetworkLink%281%29.kml Dale, A. W. , Sommer, S., Ryabenko, E., Noffke, A., Bohlen, L., Wallmann, K. , Stolpovsky, K., Greinert, J. and Pfannkuche, O. (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 <https://doi.org/10.1016/j.gca.2014.02.026>. doi:10.1016/j.gca.2014.02.026 cc_by info:eu-repo/semantics/restrictedAccess Article PeerReviewed 2014 ftoceanrep https://doi.org/10.1016/j.gca.2014.02.026 2023-04-07T15:13:02Z 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. Article in Journal/Newspaper North Atlantic OceanRep (GEOMAR Helmholtz Centre für Ocean Research Kiel) Geochimica et Cosmochimica Acta 134 234 256 |
institution |
Open Polar |
collection |
OceanRep (GEOMAR Helmholtz Centre für Ocean Research Kiel) |
op_collection_id |
ftoceanrep |
language |
English |
description |
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. |
format |
Article in Journal/Newspaper |
author |
Dale, Andrew W. Sommer, Stefan Ryabenko, Evgenia Noffke, Anna Bohlen, Lisa Wallmann, Klaus Stolpovsky, Konstantin Greinert, Jens Pfannkuche, Olaf |
spellingShingle |
Dale, Andrew W. Sommer, Stefan Ryabenko, Evgenia Noffke, Anna Bohlen, Lisa Wallmann, Klaus Stolpovsky, Konstantin Greinert, Jens Pfannkuche, Olaf Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic) |
author_facet |
Dale, Andrew W. Sommer, Stefan Ryabenko, Evgenia Noffke, Anna Bohlen, Lisa Wallmann, Klaus Stolpovsky, Konstantin Greinert, Jens Pfannkuche, Olaf |
author_sort |
Dale, Andrew W. |
title |
Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic) |
title_short |
Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic) |
title_full |
Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic) |
title_fullStr |
Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic) |
title_full_unstemmed |
Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic) |
title_sort |
benthic nitrogen fluxes and fractionation of nitrate in the mauritanian oxygen minimum zone (eastern tropical north atlantic) |
publisher |
Elsevier |
publishDate |
2014 |
url |
https://oceanrep.geomar.de/id/eprint/24425/ https://oceanrep.geomar.de/id/eprint/24425/1/Dale%20et.al.pdf https://oceanrep.geomar.de/id/eprint/24425/7/leg_NetworkLink%281%29.kml https://doi.org/10.1016/j.gca.2014.02.026 |
genre |
North Atlantic |
genre_facet |
North Atlantic |
op_relation |
https://oceanrep.geomar.de/id/eprint/24425/1/Dale%20et.al.pdf https://oceanrep.geomar.de/id/eprint/24425/7/leg_NetworkLink%281%29.kml Dale, A. W. , Sommer, S., Ryabenko, E., Noffke, A., Bohlen, L., Wallmann, K. , Stolpovsky, K., Greinert, J. and Pfannkuche, O. (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 <https://doi.org/10.1016/j.gca.2014.02.026>. doi:10.1016/j.gca.2014.02.026 |
op_rights |
cc_by info:eu-repo/semantics/restrictedAccess |
op_doi |
https://doi.org/10.1016/j.gca.2014.02.026 |
container_title |
Geochimica et Cosmochimica Acta |
container_volume |
134 |
container_start_page |
234 |
op_container_end_page |
256 |
_version_ |
1766134970837893120 |