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

Abstract

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 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) and leachable particulate (Lp; >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 (<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 (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.

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.…”

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, 2016;Noble et al, 2012;Rapp et al, 2019). In contrast, the nonredox sensitive TMs Cd and Ni show oceanic vertical profiles that are strongly regulated by biological uptake and remineralization (Bruland & Lohan, 2006).…”

Climate-Biogeochemistry Interactions in the Tropical Ocean: Data Collection and Legacy

“…For cruises from 2008 to 2013, PTFE-coated 8 l GO-FLO bottles (General Oceanics) were mounted on a Kevlar wire with sample handling and preservation as per Chever et al (2015). From 2014 onward, 24 Ocean Test Equipment (OTE) samplers were deployed mounted on a powder coated sampling CTD (Sea-Bird SBE25) rosette using a Kevlar conducting cable with sample handling and preservation as per Rapp et al (2019).…”