Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments

We report on a reconnaissance analysis of the geochemical composition of authigenic carbonates and sediment samples collected from various seepage sites on the Niger deep-sea fan. Our aim has been to investigate whether evidence for the presence of authigenic carbonates and gas hydrates within sediments is discernible from solid-phase sediment geochemistry. We show that sedimentary Sr/Ca and Mg/Ca ratios can be used to infer the presence of authigenic aragonite (Sr-rich) and Mg-rich carbonate phases (high-Mg calcite, dolomite) in cold seep settings. Using Sr/Ca and Mg/Ca ratios, the proportion (wt.%) of authigenic carbonates in Niger Fan sediments can be calculated from a mixing model between sediment fractions of terrigenous material, biogenic calcite, aragonite and high-Mg calcite. This approach was applied to high-resolution geochemical profiles along sediment cores recovered from various cold seep settings (mud volcano, diapirs, pockmarks, gas-hydrate bearing sediments). Our data reveal that authigenic carbonates occur as discrete phases in sediments from gas-hydrate-bearing areas, suggesting that such carbonate-rich sediment layers may represent paleo-indicators for ancient methane seepage in marine sediments, possibly associated to gas-hydrate reservoirs.