Carbon-sulfur signals of methane versus crude oil diagenetic decomposition and U-Th age relationships for authigenic carbonates from asphalt seeps, southern Gulf of Mexico

Abstract

Offshore hydrocarbon accumulations in the Gulf of Mexico (GoM) are often accompanied by natural seepage of oil and gas from subsurface reservoirs into shallow sediments and the water column. This study investigated the temporal patterns and carbon-sulfur (C-S) coupling associated with authigenic carbonate samples recovered from surface sediments of a crude oil seepage site in southern GoM (Chapopote asphalt volcano, Bay of Campeche) using radioactive U-Th dates, and stable C, O, and S isotopes. The results were compared with data from multiple seep sites in the northern GoM where methane seepage is dominant along with non-methane hydrocarbons (ethane , propane, crude oil, etc.). U-Th age-dating of Chapopote seep carbonate samples yielded ages of 13.5 ka to 4.6 ka before present (BP), suggesting that Chapopote asphalt seepage has been ongoing for thousands of years. The results are also consistent with previous studies from the northern GoM that hypothesize that seeps along the GoM continental slope were active during the last deglaciation. δ13CCaCO3 and δ18OCaCO3 values from authigenic carbonates at Chapopote indicated a mixed contribution of methane and non-methane hydrocarbons to the dissolved inorganic carbon (DIC) pool, consistent with previous results. Comparison of δ13CCaCO3 vs. δ34SCRS (CRS = chromium reducible sulfur) from carbonate samples showed noticeable differences at the Chapopote seep site (average δ13CCaCO3 -25‰ VDPB, δ34SCRS -27‰ VCDT) relative to the methane seep-dominated samples from the northern GoM (average δ13CCaCO3 < -40‰ VDPB, δ34SCRS >0‰ VCDT). Our results point toward distinguishable differences in the paired δ13CDIC and δ34Ssulfide signatures produced via the diagenetic processes of sulfate-driven anaerobic oxidation of methane versus non-methane hydrocarbons. These results potentially provide an important proxy for identification of such diagenetic processes within the sedimentary records.