New constraints on the formation of hydrocarbon-derived low magnesium calcite at brine seeps in the Gulf of Mexico

Hydrocarbon-rich brine seeps - representing extreme environments typified by halophilic and chemotrophic organisms - may provide insight into the ecology of microorganisms on an early Earth and possibly other planetary bodies. Authigenic low magnesium calcite (LMC) is common at brine seeps and represents an archive of fluid compositions, fluid origin, and the conditions under which this carbonate mineral formed. Here we present the analysis of authigenic LMC from three hydrocarbon-rich brine seeps (GB260, similar to 460 m; GB697, similar to 1280 m; AC601, similar to 2340 m) from the Gulf of Mexico. The formation of LMC was favored by brine fluids with extremely low Mg/Ca mole ratios (e.g. <2) relative to seawater (Mg/Ca mole ratio of similar to 5.2). Petrographic observations reveal that the studied LMC is of primary origin. Its delta C-13 values vary from -53.5 parts per thousand to 14.0 parts per thousand, indicating variable carbon sources that include C-13-depleted methane, crude oil, seawater dissolved inorganic carbon, and C-13-enriched residual carbonate species reflecting methanogenesis. In contrast, LMC shows predominantly relatively high delta C-13(org) values (from -39.0 parts per thousand to -25.7 parts per thousand), suggesting an unrecognized local source of nutrients fueling the local production of biomass. The delta O-18 values of LMC (as high as 6.0 parts per thousand) are too high for precipitation in equilibrium with seawater. probably reflecting the involvement of deep formation fluids. The wide range of Sr-87/Sr-88 ratios and the enrichment of Sr and Ba in LMC is in accordance with pore fluids deriving from the dissolution of the Louann salt or alteration of silicates in deep sediments. Given the stability of LMC, its petrographic and geochemical characterization achieved in this study can be used to better constrain brine seep activities throughout the geological record. (C) 2019 Elsevier B.V. All rights reserved.