Mineralogical and stable isotopic characterization of authigenic carbonate from a hydrocarbon seep site, Gulf of Mexico slope: Possible relation to crude oil degradation

One of the characteristic features on the cold deep seafloor on Atwater Valley (AT) Block 425 (1930 m water depth) in the Gulf of Mexico is the occurrence of seepage of crude oil and hydrocarbon gases associated with thermogenic gas hydrate and authigenic carbonate deposits. Mineralogical, petrographic and isotopic analyses revealed that the precipitation of these carbonates was related to the microbial oxidation of hydrocarbons. Carbonate deposits are composed of heavily oil-stained, small nodules to large cobbles of dense limestones of fine-grained textures. Other carbonate fragments are composed of small intraclasts, pellets and some bivalve shell fragments cemented together by aragonite cement. XRD analysis and petrographic investigations revealed that the authigenic carbonate is predominantly composed of aragonite that occurs as microcrystalline matrix and cement. Microcrystalline, sparitic and botryoidal acicular aragonite crystals are observed as void-filling cement. The $δ$18O values of the authigenic carbonate (3.80 to 4.16‰ V-PDB) suggest that the precipitating fluid has equilibrated with the cold bottom seawater. The calculated $δ$18O values (0.18 to 0.54‰ SMOW) of the precipitating fluid show subtle deviation from the values reported for the present bottom seawater. This is attributed possibly to the precipitation from pore water with temperature ranged from 3.3 to 1.7 °C, which is less than the measured bottom seawater (4 °C). However, the mixing of the marine pore water with some deep hydrocarbon-rich brines or hydrate-water would provide a source of 18O-enrichment fluid for the precipitation of the authigenic carbonates. Based on the predominance of aragonite with botryoidal and clotted fabric, the occurrence of pyrite and from the oxygen isotopic data, it is plausible that the authigenic carbonates may have formed near the sediment/water interface in an oxygen-depleted environment with low temperature. The narrow range of the $δ$13C values of the authigenic carbonate (− 23.88 to − 28.62‰ V-PDB) suggests that a single source of carbon is predominant. Based on the moderately low $δ$13C values and the occurrence of spots of crude oil coatings of the carbonate samples, in addition to the application of a carbon isotope mass balance model, the carbon involving in the formation of the carbonates of the AT 425 seep site could be mainly derived from microbial degradation of crude oil. Microbial oxidation of non-methane thermogenic hydrocarbon gases such as ethane, propane and isobutane would play some role as a source of carbonate-carbon.