The control of hydrogen sulfide on benthic iron and cadmium fluxes in the oxygen minimum zone off Peru
Authors: Plass Anna , Schlosser Christian , Sommer Stefan , Dale Andrew W. , Achterberg Eric P. , Scholz Florian 
Published: July 2020

Abstract. Sediments in oxygen-depleted marine environments can be an important sink or
source of bio-essential trace metals in the ocean. However, the key
mechanisms controlling the release from or burial of trace metals in
sediments are not exactly understood. Here, we investigate the benthic
biogeochemical cycling of iron (Fe) and cadmium (Cd) in the oxygen minimum zone off Peru. We
combine bottom water and pore water concentrations, as well as benthic
fluxes determined from pore water profiles and from in situ benthic chamber
incubations, along a depth transect at 12∘ S. In agreement with
previous studies, both concentration–depth profiles and in situ benthic
fluxes indicate a release of Fe from sediments to the bottom water.
Diffusive Fe fluxes and Fe fluxes from benthic chamber incubations (−0.3 to −17.5 mmol m−2 yr−1) are broadly consistent at stations within the
oxygen minimum zone, where the flux magnitude is highest, indicating that
diffusion is the main transport mechanism of dissolved Fe across the
sediment–water interface. The occurrence of mats of sulfur-oxidizing
bacteria on the seafloor represents an important control on the spatial
distribution of Fe fluxes by regulating hydrogen sulfide (H2S)
concentrations and, potentially, Fe sulfide precipitation within the surface
sediment. Rapid removal of dissolved Fe after its release to anoxic bottom
waters hints at oxidative removal by nitrite and interactions with particles
in the near-bottom water column. Benthic flux estimates of Cd suggest a flux
into the sediment within the oxygen minimum zone. Fluxes from benthic
chamber incubations (up to 22.6 µmol m−2 yr−1) exceed
diffusive fluxes (<1 µmol m−2 yr−1) by a factor
of more than 25, indicating that downward diffusion of Cd across the
sediment–water interface is of subordinate importance for Cd removal from
benthic chambers. As Cd removal in benthic chambers covaries with H2S
concentrations in the pore water of surface sediments, we argue that Cd
removal is mediated by precipitation of cadmium sulfide (CdS) within the chamber water or
directly at the sediment–water interface. A mass balance approach, taking the contributions of diffusive and chamber fluxes as well as Cd
delivery with organic material into account, suggests that CdS precipitation in the
near-bottom water could make an important contribution to the overall Cd
mass accumulation in the sediment solid phase. According to our results, the
solubility of trace metal sulfide minerals (Cd ≪ Fe) is a
key factor controlling trace metal removal and, consequently, the magnitude and the temporal and spatial heterogeneity of sedimentary fluxes. We
argue that, depending on their sulfide solubility, sedimentary source or sink
fluxes of trace metals will change differentially as a result of declining
oxygen concentrations and the associated expansion of sulfidic surface
sediments. Such a trend could cause a change in the trace metal
stoichiometry of upwelling water masses with potential consequences for
marine ecosystems in the surface ocean.