Methane and sulfur cycling in terrestrial hydrocarbon seeps

Hydrocarbon seeps are ubiquitous in marine and terrestrial environments where gaseous fluids with unconsolidated, fine-grained sediments ascend along fractures prior to being discharged on seafloor or land surface. Complex geological and microbial processes are involved in the sequestration of photosynthetically produced organic carbon into deep subsurface environments and cycling of methane and carbon dioxide back to atmosphere. Extensive studies conducted on marine settings indicate that geochemical stratification in sediment porewater is dynamically regulated by various microbial processes. Whether the experience accumulated over the decadal observation on marine settings could be applied to shallow and deep biosphere beneath terrestrial hydrocarbon seeps remains poorly constrained. To address the issue about how carbon and sulfur compounds were cycled in terrestrial hydrocarbon seeps, this presentation summarized the results obtained from samples collected in two sites (one at 60C and the other at 27C) of southwestern Taiwan. These sites characterized by continuously voluminous discharge of hydrocarbons were considered as the model analogs that would provide better constraints on microbial processes at ambient and high temperatures in seep-related subsurface environments. Our findings indicated that sulfate reduction and methanogenesis were active at temperatures up to 80C. Sulfate reducing and fermentative populations shifted substantially upon incubations at different temperatures, suggesting that degradation of organic carbon could only proceed with collaborative interactions among metabolisms. The proliferation of mesophilic sulfate reduction in sulfate-deprived terrestrial environments appears to be best facilitated by atmospheric oxidation of pyrite inherited in sediments. Sulfate produced in surface environments migrated downward to fuel sulfate reduction coupled to anaerobic methane oxidation near the sulfate-to-methane transition. Of various methanogenic pathways, acetoclastic methanogenesis appears to fractionate carbon isotopes at a very small magnitude. This when combined with molecular results, field observations and mass balance calculation suggests that thermophilic methanogens produce thermogenesis-like methane at rates sufficiently high to account for the methane accumulation in deep, hot reservoirs. Thermogenic methane potentially overprinted by microbial methane at great depths was subject to further modification by methanogenesis and methane oxidation in sediments near surface. Overall, metabolic and geochemical stratification comparable with those in marine settings formed as the results of the interplay between the upward transport of gaseous, reduced, diluted fluids and the downward diffusion of oxidized, solute-enriched fluids. In contrary to marine settings, methane flux exceeds the capacity of microbial consumption, releasing methane directly to the atmosphere at quantities at least five orders of magnitude greater than those at the air-seawater interface.