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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 16 Nov 2018

Submitted as: research article | 16 Nov 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Insa Rapp1, Christian Schlosser1, Jan-Lukas Menzel Barraqueta1,2, Bernhard Wenzel1, Jan Lüdke1, Jan Scholten3, Beat Gasser4, Patrick Reichert1, Martha Gledhill1, Marcus Dengler1, and Eric P. Achterberg1 Insa Rapp et al.
  • 1Helmholtz Centre for Ocean Research Kiel (GEOMAR), Wischhofstr. 1–3, 24148 Kiel, Germany
  • 2Department of Earth Sciences, Stellenbosch University, Stellenbosch, 7600, South Africa
  • 3Institute of Geosciences, Christian-Albrecht University Kiel (CAU), Otto-Hahn-Platz 1, 24118 Kiel, Germany
  • 4International Atomic Energy Agency (IAEA), Environment Laboratories, 4 Quai Antoine 1er, 98012 Monaco

Abstract. The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation and microbial community structure in large parts of the world's ocean, and thus plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g. Fe, manganese (Mn) and cobalt (Co)), with shelf sediments typically forming a key source. Over the last five decades, an expansion and intensification of OMZs has been observed and this trend is likely to proceed. However, it is unclear how trace metal (TM) distributions and transport are influenced by decreasing oxygen (O2) concentrations. Here we present dissolved (d; < 0.2 μm) and leachable particulate (Lp; > 0.2 μm) TM data collected at 7 stations along a 50 km transect in the Mauritanian shelf region. We observed enhanced concentrations of Fe, Co and Mn corresponding with low O2 concentrations (< 50 μmol kg−1), which were decoupled from major nutrients and nutrient-like and scavenged TMs (cadmium (Cd), lead (Pb), nickel (Ni) and copper (Cu)). Additionally, data from repeated station occupations indicated a direct link between dissolved and leachable particulate Fe, Co, Mn, and O2. An observed dFe decrease from 10 to 5 nmol L−1 coincided with an O2 increase from 30 to 50 μmol kg−1 and with a concomitant decrease in turbidity. The changes in Fe (Co and Mn) were likely driven by variations in their release from sediment pore water, facilitated by lower O2 concentrations and longer residence time of the water mass on the shelf. Variations in organic matter remineralization and lithogenic inputs (atmospheric deposition or sediment resuspension) only played a minor role in redox-sensitive TM variability. Vertical dFe fluxes from O2-depleted subsurface to surface waters (0.08–13.5 μmol m−2 d−1) were driven by turbulent mixing and vertical advection, and were an order of magnitude larger than atmospheric deposition fluxes (0.63–1.43 μmol m−2 d−1). Benthic fluxes are therefore the dominant dFe supply to surface waters on the continental margins of the Mauritanian upwelling region. Overall, our results indicated that the projected future decrease in O2 concentrations in OMZs may result in increases in Fe, Mn and Co concentrations.

Insa Rapp et al.
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Insa Rapp et al.
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