Geochemistry of Peruvian near-surface sediments

Sixteen short sediment cores were recovered from the upper edge (UEO), within (WO) and below (BO) the oxygen minimum zone (OMZ) off Peru during cruise 147 of R/V Sonne. Solids were analyzed for major/trace elements, total organic carbon, total inorganic carbon, total sulfur, the stable sulfur isotope composition (δ³⁴S) of pyrite, and sulfate reduction rates (SRR). Pore waters were analyzed for dissolved sulfate/sulfide and δ³⁴S of sulfate. In all cores highest SRR were observed in the top 5 cm where pore water sulfate concentrations varied little due to resupply of sulfate by sulfide oxidation and/or diffusion of sulfate from bottom water. δ³⁴S of dissolved sulfate showed only minor downcore increases. Strong ³²S enrichments in sedimentary pyrite (to −48‰ vs. V-CDT) are due to processes in the oxidative part of the sulfur cycle in addition to sulfate reduction. Manganese and Co are significantly depleted in Peruvian upwelling sediments most likely due to mobilization from particles settling through the OMZ, whereas release of both elements from reducing sediments only seems to occur in near-coastal sites. Cadmium, Mo and Re are exceptionally enriched in WO sediments (<600 m water depth). High Re and moderate Cd and Mo enrichments are seen in BO sediments (>600 m water depth). Re/Mo ratios indicate anoxic and suboxic conditions for WO and BO sediments, respectively. Cadmium and Mo downcore profiles suggest considerable contribution to UEO/WO sediments by a biodetrital phase, whereas Re presumably accumulates via diffusion across the sediment-water interface to precipitation depth. Uranium is distinctly enriched in WO sediments (due to sulfidic conditions) and in some BO sediments (due to phosphorites). Silver transfer to suboxic BO sediments is likely governed by diatomaceous matter input, whereas in anoxic WO sediments Ag is presumably trapped due to sulfide precipitation. Cadmium, Cu, Zn, Ni, Cr, Ag, and T1 predominantly accumulate via biogenic pre-concentration in plankton remains. Rhenium, Sb, As, V, U and Mo are enriched in accordance with seawater TE availability. Lead and Bi enrichment in UEO surface sediments is likely contributed by anthropogenic activity (mining). Accumulation rates of TOC, Cd, Mo, U, and V from Peruvian and Namibian sediments exceed those from the Oman Margin and Gulf of California due to enhanced preservation off Peru and Namibia.