Petrographic and geochemical characterization of the hydrocarbon-derived authigenic low-magnesium calcite at brine seeps in the Gulf of Mexico

Hydrocarbon-rich brine seep is one of the most hostile environment for organism in modern marine settings, which could provide insights into the microorganisms on the early earth or extraterrestrial bodies. Authigenic carbonates are common at brine seeps and record a robust fingerprint of fluid compositions, the conditions under which carbonates formed, and information regarding fluid-sediment and rock interactions. Here we present the analyses of authigenic carbonates from three hydrocarbon-rich brine seeps (GB260, ~460 m; GB697, ~1280 m; AC601, ~2340 m) in the Gulf of Mexico. The carbonates at all three sites are dominated by low-magnesium calcite (LMC). The formation of LMCs is induced from the brine fluids that with extremely low Mg/Ca mole ratio (e.g., < 2) relative to seawater (Mg/Ca mole ratio is ~5). Petrographic observations and elemental compositions reveal that the LMC is of primary origin. The δ¹⁸O values of the LMCs (as high as 6.0‰) are too high for precipitation in equilibrium with seawater, probably reflecting the involvement of deep formation fluids. The δ¹³C values of the LMCs range from -53.5‰ to 14.0‰, indicating complex carbon sources that include ¹³C-depleted methane, biodegraded crude oil, seawater CO₂, and ¹³C-enriched residual CO₂ from methanogenesis. In contrast, the LMCs show relatively moderate and constant δ¹³C_org values (from -39.0‰ to -25.7‰), suggesting an unrecognized local source of nutrients to fuel the production of biomass. High contens of Sr and Ba in LMCs regardless of variable ⁸⁷Sr/⁸⁶Sr ratios, between 0.707888 and 0.708800 for sites GB260 and AC601 and from 0.709937 to 0.710069 for site GB697. The wide range of ⁸⁷Sr/⁸⁶Sr ratios and the enrichment of Sr and Ba in LMC is in accordance with pore fluids deriving from the dissolution of Louann salt or alteration of silicates in the deep sediments. Given the stability of LMCs, the petrographic and geochemical characterization of LMCs obtained in this study can be used to identify and better constrain the brine seep activities throughout the geological record.