11Sediments in oxygen-depleted marine environments can be an important sink or 12 source of bio-essential trace metals in the ocean. However, the key mechanisms 13 controlling the release from or burial of trace metals in sediments are not exactly 14 understood. Here, we investigate the benthic biogeochemical cycling of Fe and Cd in 15 the oxygen minimum zone off Peru. We combine bottom water profiles, pore water 16 profiles, as well as benthic fluxes determined from pore water profiles and in-situ from 17 benthic chamber incubations along a depth transect at 12° S. In agreement with 18 previous studies, both concentration-depth profiles and in-situ benthic fluxes indicate 19 a Fe release from sediments into bottom waters. Diffusive Fe fluxes and Fe fluxes from 20 benthic chamber incubations are roughly consistent (0.3 -17.1 mmol m -2 y -1 ), 21 indicating that diffusion is the main transport mechanism of dissolved Fe across the 22 sediment-water interface. The occurrence of mats of sulfur oxidizing bacteria on the 23 seafloor represents an important control on the spatial distribution of Fe fluxes by 24 regulating hydrogen sulfide (H2S) concentrations and, potentially, Fe sulfide 25 precipitation within the surface sediment. Removal of dissolved Fe after its release to 26 anoxic bottom waters is rapid in the first 4 m away from the seafloor (half-life < 3 min) 27 which hints to oxidative removal by nitrite or interaction with particles in the benthic 28 boundary layer. Benthic flux estimates of Cd are indicative of a flux into the sediment 29 within the oxygen minimum zone. Fluxes from benthic chamber incubations (up to 22.6 30 µmol m -2 y -1 ) exceed the diffusive fluxes (< 1 µmol m -2 y -1 ) by a factor > 25, indicating 31 that downward diffusion of Cd across the sediment-water interface is of subordinate 32 importance for Cd removal from benthic chambers. As Cd removal in benthic chambers 33 co-varies with H2S concentrations in the pore water of surface sediments, we argue 34 that Cd removal is mediated by precipitation of CdS within the chamber. A mass 35 balance approach, taking into account the contributions of diffusive fluxes and fluxes 36 measured in benthic chambers as well as Cd delivery with organic material suggests 37 that CdS precipitation in the near-bottom water could make an important contribution 38 to the overall Cd mass accumulation in the sediment solid phase. According to our 39 results, the solubility of trace metal sulfide minerals (Cd << Fe) is a key-factor 40 controlling trace metal removal and consequently the magnitude as well as the 41 temporal and spatial heterogeneity of sedimentary fluxes. We argue that depending on 42 their sulfide solubility, sedimentary source or sink fluxes of trace metals will change 43 differentially as a result of declining oxygen concentrations and an associated 44

X‐ray fluorescence (XRF) core scanning of marine and lake sediments has been extensively used to study changes in past environmental and climatic processes over a range of timescales. The interpretation of XRF‐derived element ratios in paleoclimatic and paleoceanographic studies primarily considers differences in the relative abundances of particular elements. Here we present new XRF core scanning data from two long sediment cores in the Andaman Sea in the northern Indian Ocean and show that sea level related processes influence terrigenous inputs based proxies such as Ti/Ca, Fe/Ca, and elemental concentrations of the transition metals (e.g., Mn). Zr/Rb ratios are mainly a function of changes in median grain size of lithogenic particles and often covary with changes in Ca concentrations that reflect changes in biogenic calcium carbonate production. This suggests that a common process (i.e., sea level) influences both records. The interpretation of lighter element data (e.g., Si and Al) based on low XRF counts is complicated as variations in mean grain size and water content result in systematic artifacts and signal intensities not related to the Al or Si content of the sediments. This highlights the need for calibration of XRF core scanning data based on discrete sample analyses and careful examination of sediment properties such as porosity/water content for reliably disentangling environmental signals from other physical properties. In the case of the Andaman Sea, reliable extraction of a monsoon signal requires accounting for the sea level influence on the XRF data.

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.

An extensive data set of biogenic silica (BSi) fluxes is presented for the Peruvian oxygen minimum zone (OMZ) at 11°S and 12°S. Each transect extends from the shelf to the upper slope (∼1,000 m) and dissects the permanently anoxic waters between ∼200 and 500 m water depth. BSi burial (2,100 mmol m−2 yr−1) and recycling fluxes (3,300 mmol m−2 yr−1) were highest on the shelf with mean preservation efficiencies (34% ± 15%) that exceed the global mean of 10%–20%. BSi preservation was highest on the inner shelf (up to 56%), decreasing to 7% and 12% under anoxic waters and below the OMZ, respectively. The data suggest that the main control on BSi preservation is the rate at which reactive BSi is transported away from undersaturated surface sediments by burial and bioturbation to the underlying saturated sediment layers where BSi dissolution is thermodynamically and/or kinetically inhibited. BSi burial across the entire Peruvian margin between 3°S to 15°S and down to 1,000 m water depth is estimated to be 0.1–0.2 Tmol yr−1; equivalent to 2%–7% of total burial on continental margins. Existing global data permit a simple relationship between BSi rain rate to the seafloor and the accumulation of unaltered BSi, giving the possibility to reconstruct rain rates and primary production from the sediment archive in addition to benthic Si turnover in global models.

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