Sedimentary cycling and benthic fluxes of manganese, cobalt, nickel, copper, zinc and cadmium in the Peruvian oxygen minimum zone

Abstract

Marine sediments are an important source and sink of bio-essential trace metals to the ocean. However, the different mechanisms leading to trace metal release or burial are not fully understood and the associated fluxes are not well quantified. Here, we present sediment, pore water, sequential extraction and benthic flux data of Mn, Co, Ni, Cu, Zn and Cd along a latitudinal depth transect across the Peruvian oxygen minimum zone at 12°S. Sediments are depleted in Mn and Co compared to the lithogenic background. Diffusive Mn fluxes from the sediments into the bottom water (−26 to −550 μmol m−2 y−1) are largely consistent with the rate of Mn loss from the solid phase (−100 to −1160 μmol m−2 yr−1) suggesting that 50% or more of the sedimentary Mn depletion is attributed to benthic efflux. In contrast, benthic Co fluxes (~ −3 μmol m−2 yr−1) are lower than the rate of Co loss from the solid phase (up to −120 μmol m−2 yr−1), implying Co dissolution in the water column. The trace metals Ni, Cu, Zn and Cd are enriched within the sediments with respect to the lithogenic background. Uptake of Ni by phytoplankton in the photic zone and delivery with organic matter to the sediment surface can account for up to 100% of the excess Ni accumulation (87 to 180 μmol m−2 y−1) in shelf sediments near the coast, whereas at greater water depth additional scavenging by Mn- and Fe-oxides may contribute to Ni accumulation. Up to 20% of excess Cu (33 to 590 μmol m−2 y−1) and generally less than 20% of excess Zn (58 to 2170 μmol m−2 y−1) and Cd (6 to 260 μmol m−2 y−1) can be explained by delivery with fresh organic matter. Sequential extraction data suggest that the discrepancies between the known sources of Cd (and Cu) and their excess accumulation may be driven by the delivery of allochthonous sulphide minerals precipitated from the water column. Additionally, Cu may be scavenged by downward sinking organic material. In contrast, precipitation of Zn sulphide chiefly takes place in the sediment. Diffusive Zn fluxes into the sediment (21 to 1990 μmol m−2 y−1) match the excess Zn accumulation suggesting that Zn delivery is mediated by molecular diffusion from bottom waters. Considering the diverse behavioural pattern of trace metals observed in this study, we argue that declining oxygen and increasing hydrogen sulphide concentrations in a future ocean will modify trace metal fluxes at the seafloor and the trace metal stoichiometry of seawater.