Geochemical characteristics of Chilean upwelling sediments at ∼36°S

Abstract

We examined the cycling of C, S, Fe and the trace elements (TE) Ag, Cd, Mo, Mn, Re and Se in Chilean upwelling sediments from water depths of 127–2744 m at 36°S. The sediments are clay-rich with moderate total organic carbon (TOC) and low biogenic CaCO 3 contents. For the shallow site (within oxygen minimum zone; OMZ), which shows some Thioploca biomass, highest observed TOC contents correspond to highest productivity-induced TOC-flux and/or enhanced preservation under O 2-deficient conditions. Pyrite formation may be restricted by the concurrent presence of elemental S in Thioploca and/or the availability of reactive iron. In the other sediments (> 370 m water depth) pyrite formation is strongly restricted due to the low availability of metabolizable TOC for sulfate reduction. Calculations of the degree of bacterially mediated remineralization suggest that more nitrogen than TOC was remineralized by sulfate reduction than expected from Redfield C/N relationship. Manganese is significantly depleted due to reductive dissolution of Mn-(hydr)oxides within all sediments. Mo is only significantly enriched vs. the lithogenic background in the shallow core, implying anoxic conditions. Contents of Re and Mo as well as Re/Mo ratios indicate suboxic conditions for the other cores. Cd and Re contents decrease with increasing water depth reflecting decreasing reducing conditions. In contrast, authigenic Ag contents increase with water depth suggesting enrichment controlled by seawater availability and/or opaline input (and not redox conditions). Elevated Se contents in all cores may be either due to bio-detrital input or diffusion from seawater. Accumulation rates of TOC, TS, and TE off Chile are in the same order as those from the Gulf of California and lower than those off Peru. Distinct seasonality in upwelling and dilution of Chilean sediments with terrigenous material lead to rather low metabolizable TOC contents and lower TE trapping capacity.