Isotopic fingerprints of benthic nitrogen cycling in the Peruvian oxygen minimum zone.

Dale, Andrew W. , Bourbonnais, A., Altabet, M., Wallmann, Klaus and Sommer, Stefan (2019) Isotopic fingerprints of benthic nitrogen cycling in the Peruvian oxygen minimum zone. Geochimica et Cosmochimica Acta, 245 . pp. 406-425. DOI 10.1016/j.gca.2018.10.025.

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Abstract

Stable isotopes (15,14N, 18,16O) of dissolved inorganic nitrogen (N) were measured in sediment porewaters and benthic flux chambers across the Peruvian oxygen minimum zone (OMZ) from 74 to 1000 m water depth. Sediments at all locations were net consumers of bottom water NO3−. In waters shallower than 400 m, this sink was largely attributed to dissimilatory nitrate reduction to ammonium (DNRA) by filamentous nitrate-storing bacteria (Marithioploca and Beggiatoa) and to denitrification by foraminifera. The apparent N isotope effect of benthic NO3− loss (15εapp) was 7.4 ± 0.7‰ at microbial mat sites and 2.5 ± 0.9‰ at the lower fringe of the OMZ (400 m) where foraminifera were abundant. The OMZ sediments were a source of 15N-enriched NO2− (28.9 to 65.5‰) and NH4+ (19.4–20.5‰) to the bottom water. Model simulations generally support a previous hypothesis attributing the 15NH4+ enrichment to a coupling between DNRA and anammox (termed DAX) using biologically-stored NO3− from Marithioploca and NH4+ from the porewater. The model predicts that 40% of NO3− that is actively transported into the sediment by Marithioploca is reduced to N2 by this pathway. DAX enhances N2 fluxes by a factor of 2–3 and accounts for 70% of fixed N loss to N2. Moreover, because most of the ambient porewater NH4+ is generated by DNRA, up to two-thirds of biologically-transported NO3− could end up being lost to N2. This challenges the premise that Marithioploca-dominated sediments tend to conserve fixed N. By limiting the flux of 15NH4+ back to the ocean, DAX also tends to decrease benthic N fractionation. Tracking the fate of NH4+ once it leaves the sediment is critical for understanding how the benthos contributes to N isotope signals in the water column.

Document Type: Article
Keywords: Anaerobic ammonium oxidation, Marine-sediments, Nitrate reduction, DNRA, Benthic nitrogen cycle, Sulfur bacteria, Peru, isotope, Marithioploca, model, sediment
Research affiliation: OceanRep > SFB 754 > B6
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > SFB 754 > B5
OceanRep > SFB 754 > B1
OceanRep > SFB 754
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.gca.2018.10.025
ISSN: 0016-7037
Projects: SFB754
Expeditions/Models/Experiments:
Date Deposited: 04 Dec 2018 08:33
Last Modified: 06 Dec 2018 12:23
URI: http://oceanrep.geomar.de/id/eprint/44808

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