Mechanisms of trace metal enrichment in submarine, methane-derived carbonate chimneys from the Gulf of Cadiz

Abstract

Methane oxidation and sulphate reduction are the key processes driving the precipitation of carbonates and iron sulphides in methane-seeps under anaerobic conditions. These carbonates incorporate trace metals from several sources, such as dissolved species in seawater and the surrounding sediments. Thus, it is possible to infer significant information about the development of methane-derived carbonates from their bulk composition. However, post-depositional processes change relative trace metal concentrations and mask this information. In order to study mechanisms of trace metal accumulation despite interference from these post-depositional processes, we examined submarine carbonate chimneys from different sites on the seafloor of the Gulf of Cadiz using analysis of bulk composition and electron microprobe composition of carbonates, detrital minerals, authigenic pyrite, and iron oxyhydroxides. Positive correlations between Si, Al, Ca and Fe and trace element contents and electron microprobe results suggest that the main processes of trace metal incorporation into chimneys are: (1) inclusion and cementation of sediment material into the chimneys (Ti, Zr, Rb, Cr and some rare earth elements); (2) substitution of Ca in microcrystalline carbonates (Ba, Sr); and (3) assimilation into authigenic pyrite (Mo, Co, Ni, Pb, As, Zn, V and U), a process enhanced by bacterial sulphate reduction and development of framboidal textures. We also calculated several redox indices to evaluate the effects of exposure to oxygenated waters on the composition of chimneys. Ni/Co and V/Cr ratios, EFMo/EFU and the mean and standard deviation of iron mineral sizes pointed to oxygen-rich settings. Geochemical compositions of selected minerals and positive correlations between trace metals and representative indices can be used to determine the source and enrichment mechanism of trace metals in authigenic carbonates developed in methane-seeps as a consequence of anaerobic oxidation of methane.