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

Abstract. The availability of the micronutrient iron (Fe) in surface waters determines primary production, N 2 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 (O 2 ) 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 O 2 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 O 2 . An observed dFe decrease from 10 to 5 nmol L −1 coincided with an O 2 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 O 2 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 O 2 -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 O 2 concentrations in OMZs may result in increases in Fe, Mn and Co concentrations.