Insa Rapp

Upwelling ocean currents associated with oxygen minimum zones (OMZs) supply nutrients fuelling intense marine productivity. Perturbations in the extent and intensity of OMZs are projected in the future, but it is currently uncertain how this will impact fluxes of redox‐sensitive trace metal micronutrients to the surface ocean. Here we report seawater concentrations of Fe, Mn, Co, Cd, and Ni alongside the redox indicator iodide/iodate in the Peruvian OMZ during the 2015 El Niño event. The El Niño drove atypical upwelling of oxygen‐enriched water over the Peruvian Shelf, resulting in oxidized iodine and strongly depleted Fe (II), total dissolved Fe, and reactive particulate Fe concentrations relative to non‐El Niño conditions. Observations of Fe were matched by the redox‐sensitive micronutrients Co and Mn, but not by non‐redox‐sensitive Cd and Ni. These observations demonstrate that oxygenation of OMZs significantly reduces water column inventories of redox‐sensitive micronutrients, with potential impacts on ocean productivity.

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 it 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 5 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) TM data collected at seven 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 (−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 (dissolved iron) 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; assessed using Al as indicator for lithogenic inputs) 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) driven by turbulent mixing and vertical advection were an order of magnitude larger than atmospheric deposition fluxes (0.63–1.43 µmol m⁻² d⁻¹; estimated using dAl inventories in the surface mixed layer) in the continental slope and shelf region. 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.

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.