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

Abstract

<p><strong>Abstract.</strong> The availability of the micronutrient iron (Fe) in surface waters determines primary production, N<sub>2</sub> 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<sub>2</sub>) 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<sub>2</sub> concentrations (< 50 μmol kg<sup>−1</sup>), 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<sub>2</sub>. An observed dFe decrease from 10 to 5 nmol L<sup>−1</sup> coincided with an O<sub>2</sub> increase from 30 to 50 μmol kg<sup>−1</sup> 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<sub>2</sub> 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<sub>2</sub>-depleted subsurface to surface waters (0.08–13.5 μmol m<sup>−2</sup> d<sup>−1</sup>) 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<sup>−2</sup> d<sup>−1</sup>). 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<sub>2</sub> concentrations in OMZs may result in increases in Fe, Mn and Co concentrations.</p>

Trace Element Biogeochemistry in the High‐Latitude North Atlantic Ocean: Seasonal Variations and Volcanic Inputs

“…Dissolved Mn, Co and Fe are micronutrients, and in Figure 8 fall on the trajectory of the other nutrient‐type elements (e.g., Cd), but less of their variance is explained by PC1, indicating that processes other than biological uptake and remineralization influenced their distributions. While these redox sensitive elements have elevated concentrations in oxygen minimum zones as a result of benthic supply (Bruland et al, 2005; Rapp et al, 2019; Schroller‐Lomnitz et al, 2019), no association with low oxygen is notable in Figure 8 and their variance is not explained by the oxygen/salinity correlated PC2. As might be expected, the dissolved concentrations of Mn, Co, and Fe therefore appear to be determined by a combination of factors including supply strength, biological uptake and remineralization, characteristics and abundance of particles, and water mass age and mixing.…”

“…Cd), but less of their variance is explained by PC1, indicating that processes other than biological uptake and remineralization influenced their distributions. Whilst these redox sensitive elements have elevated concentrations in oxygen minimum zones as a result of benthic supply (Bruland et al, 2005;Rapp et al, 2019;Schroller-Lomnitz et al, 2019), no association with low oxygen is notable in Fig. 8 and their variance is not explained by the oxygen/salinity correlated PC2.…”

El Niño‐Driven Oxygenation Impacts Peruvian Shelf Iron Supply to the South Pacific Ocean

“…Cobalt and Mn are other redox‐sensitive micronutrient trace metals (TMs) that exhibit increased water column concentrations under low oxygen conditions, likely due to enhanced release from sediments and slow oxidation kinetics (Hawco et al, ; Noble et al, ; Rapp et al, ). In contrast, the nonredox sensitive TMs Cd and Ni show oceanic vertical profiles that are strongly regulated by biological uptake and remineralization (Bruland & Lohan, ).…”

A novel device for trace metal‐clean sampling of bottom water and suspended particles at the ocean's lower boundary: The Benthic Trace Profiler

“…During October, the concentrations of Ni, Cu, Zn, and Cd were around half or less than half compared to the March campaign. This observation could be related to enhanced TM sequestration by phytoplankton and organic material during and after the productive season (Sundby et al 1986;Kremling et al 1997;Brügmann et al 1998;Pohl and Hennings 2005;Bruland and Lohan 2006;Scholz et al 2011;Noble et al 2012;Rigaud et al 2013;Rapp et al 2019). In addition, more reducing and sulfidic conditions in the bottom water and/or sediment before/during the October campaign may have led to TM removal into sulfide minerals (Sundby et al 1986;Brügmann et al 1998;Pohl and Hennings 2005;Rigaud et al 2013).…”