Plass, Anna, Schlosser, Christian , Sommer, Stefan, Dale, Andrew W. , Achterberg, Eric P. and Scholz, Florian (2020) The control of hydrogen sulfide on benthic iron and cadmium fluxes in the oxygen minimum zone off Peru. Biogeosciences (BG), 17 . pp. 3685-3704. DOI 10.5194/bg-17-3685-2020.

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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 Fe and Cd in the oxygen minimum zone off Peru. We combine bottom water profiles, pore water profiles, as well as benthic fluxes determined from pore water profiles and in-situ from 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 Fe release from sediments into bottom waters. Diffusive Fe fluxes and Fe fluxes from benthic chamber incubations are roughly consistent (0.3–17.1 mmol m−2 y−1), 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. Removal of dissolved Fe after its release to anoxic bottom waters is rapid in the first 4 m away from the seafloor (half-life < 3 min) which hints to oxidative removal by nitrite or interaction with particles in the benthic boundary layer. Benthic flux estimates of Cd are indicative of a flux into the sediment within the oxygen minimum zone. Fluxes from benthic chamber incubations (up to 22.6 µmol m−2 y−1) exceed the diffusive fluxes (< 1 µmol m−2 y−1) by a factor > 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 co-varies with H2S concentrations in the pore water of surface sediments, we argue that Cd removal is mediated by precipitation of CdS within the chamber. A mass balance approach, taking into account the contributions of diffusive fluxes and fluxes measured in benthic chambers as well as Cd delivery with organic material 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 as well as 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 an 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.

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