δ 44/42 Ca in gas hydrates, porewaters and authigenic carbonates from Niger Delta sediments

Abstract

We report Ca isotope ratios for gas hydrates, pore waters and authigenic carbonate concretions recovered by coring at cold seep locations from the Niger Delta deep province. Our aim in this study has been to understand the mechanisms controlling Ca-isotope fractionation during biogeochemical processes at cold seeps. Carbonate precipitation in cold seep environments is closely related to the anaerobic oxidation of methane (AOM), which is mediated by a consortium of micro-organisms and releases bicarbonate (HCO3) and sulfide (HS) into surrounding pore waters [1]. We found that carbonate precipitation causes Ca enrichment in the surrounding porewaters, as expected from previous work demonstrating that calcite prefers isotopically light Ca [2]. Our measured δCa values for pore-waters are consistent with values estimated previously by Teichert et al. [3]. In our studied sediment cores, the highest δCa ratios in pore-waters were measured at the AOM sediment depth (dashed line in Fig.1), indicating on-going carbonate formation at these horizons. Porewater and carbonate δCa measurements may therefore offere potential to identify the location and intensity of AOM processes now and in the past. We will also discuss the mineralogical control of Ca isotope fractionation at cold seeps.