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.
Impact of atmospheric deposition of anthropogenic and natural trace metals on Northwestern Mediterranean surface waters: A box model assessment