Hydrocarbon gas in the Costa Rica subduction zone: primary composition and post-genetic alteration

At the active continental margin off Costa Rica substantial amounts of hydrocarbon gases are encountered in sediments. The molecular composition (C1–C3) of free hydrocarbon gas as well as the isotopic composition (δ13C of methane and ethane and D of methane) was analysed on core samples (ranging between 50 and 380 m depth) collected at sites 1040–1043 which was drilled during ODP Leg 170. In addition, the molecular composition of the C1–C3 hydrocarbons and the δ13C composition of C1 and C2 hydrocarbons was determined on adsorbed gas from selected depth intervals at Site 1041 (50–380 mbsf). The molecular composition, and stable carbon and hydrogen isotope signature of low molecular weight hydrocarbons from core sediments and gas pockets indicate that most of the gas was generated by microbial CO2-reduction. Beside δ13C values of about –80‰ for methane (which is typical for microbially- generated methane) extremely light δ13C values of –55‰ were measured for ethane. The carbon isotope composition of methane and ethane, as well as the C1/(C2+C3) ratio display distinct trends with increasing depth. Gas mixing calculations indicate that the percentage of thermally-generated ethane increases from 10% at about 75 mbsf to almost 80% at 380 mbsf. The fraction of thermogenic methane in this depth interval is calculated to range from 0.03 to 1.8% of the total methane. The small contribution of thermogenic methane would increase the δ13C value by <1‰. Therefore, the increase of δ13C of methane (by about 12‰) with depth cannot be explained by gas mixing alone. Instead, the observed δ13C trend is caused by successive isotope depletion of the methane precursor within the sedimentary organic matter due to progressing microbial gas generation.