Rare earth elements in cold seep carbonates from the Niger delta

Abstract

The circulation of methane-rich fluids at cold seeps often leads to precipitation of authigenic carbonates mediated by microbial activity, which can be used as archives of fluid seepage on continental margins. In this study, we have investigated the potential of using rare earth element (REE) concentrations in authigenic carbonates for tracing fluid sources and providing information on associated biogeochemical processes at cold seeps. We have examined various chemical procedures for the analysis of REE in authigenic carbonates, and present a robust method using dilute acetic acid solution that allows the extraction of REE from carbonate phases with reduced contamination from Fe–Mn oxides and detrital minerals. Using this method, we analysed a series of carbonate samples collected at various deep-sea sites of active fluid seepage on the Niger Delta for their REE and trace element concentrations, δ 13C and, δ 18O values and 87Sr/ 86Sr isotope compositions. We show that the REE composition of cold seep carbonates is controlled primarily by that of the fluid from which they have precipitated. Our results also suggest that significant REE fractionation can occur during carbonate precipitation, which can be related to various degrees of REE complexation by organic versus carbonate ligands in response to changing pore water chemical composition for parameters such as carbonate alkalinity.