Dissolved benthic phosphate, iron and carbon fluxes in the Mauritanian upwelling system and implications for ongoing deoxygenation.

Schroller-Lomnitz, Ulrike, Hensen, Christian, Dale, Andrew W., Scholz, Florian, Clemens, David, Sommer, Stefan, Noffke, Anna and Wallmann, Klaus (2019) Dissolved benthic phosphate, iron and carbon fluxes in the Mauritanian upwelling system and implications for ongoing deoxygenation. Deep Sea Research Part I: Oceanographic Research Papers, 143 . pp. 70-84. DOI 10.1016/j.dsr.2018.11.008.

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Supplementary data:

Abstract

Highlights

• next to organic matter degradation, bioirrigation and bottom water percolation through permeable surface sediments enhances benthic TPO43- and Fe2+ release
• changes in bottom water oxygenation induce slight changes benthic TPO43- and Fe2+ release rates measured in 2011 and 2014
• deoxygenation experiments imply enhanced TPO43- and Fe2+ release at ongoing deoxygenation in the Mauritanian OMZ

Abstract

Benthic fluxes of total dissolved phosphate (TPO43-), dissolved iron (Fe2+), and dissolved inorganic carbon (DIC) were determined in situ using benthic chambers at nine stations along a depth transect between 47 and 1108 m water depth at 18 °N off Mauritania (NW Africa) during the upwelling season in 2014 (RV Meteor cruise M107). Bottom water oxygen (O2) concentrations were always ≥ 25 µM, and all fluxes (TPO43-, Fe2+, DIC) were consistently directed from the sediments into the bottom water. The highest benthic TPO43- release of 0.2 ± 0.07 mmol m2 d-1 was found at 47 m water depth (50 µM O2). The highest diffusive Fe2+ flux of 0.03 mmol m2 d-1, determined from porewater Fe2+ concentrations, occurred at 67 m water depth (27 µM O2). This was much lower than the detrital Fe supply as indicated by constant Fe/Al ratios along the depth transect. TPO43- release rates decreased concurrently with DIC flux and water depth. A difference of up to one order of magnitude between benthic chamber and diffusive TPO43- fluxes indicated that the total TPO43- release was strongly enhanced by bioirrigation.

The observed fluxes were similar to those measured during an earlier cruise in 2011, generally indicating comparable release rates during both upwelling seasons. Furthermore, ex situ oxygen manipulation experiments showed an increase of the nutrient release (e.g. TPO43-, Fe2+) after seven days of anoxic bottom water conditions. The fluxes were enhanced by a factor of 1.4 for P and 7.3 for Fe compared to the measured release under natural conditions and reached values as high as those measured in the anoxic oxygen minimum zone off Peru. Our observations support the hypothesis that increasing deoxygenation of the oceans will likely enhance sedimentary TPO43- and Fe2+ release and thus contribute to a positive feedback mechanism with increasing nutrient levels and increased ocean productivity.

Document Type: Article
Keywords: Benthic fluxes, Phosphate, Iron, Carbon, Mauritanian Upwelling, Oxygen Minimum Zone
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > SFB 754
OceanRep > SFB 574 > B5
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.dsr.2018.11.008
ISSN: 0967-0637
Projects: SFB754
Expeditions/Models/Experiments:
Date Deposited: 03 Dec 2018 10:14
Last Modified: 28 Jan 2019 11:49
URI: http://oceanrep.geomar.de/id/eprint/44787

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