Thermochemical or microbial sulfate reduction: determining the driver of native sulfur formation in the subsurface
Abstract
Large native sulfur deposits can be found as part of carbonate caprock assemblages at the top of or lateral to salt diapirs. Carbonate caprocks are formed when hydrocarbons come in contact with gypsum/anhydrite associated with salt diapirs. Presumably, sulfate-reducing microbes drive carbonate caprock formation by replacing gypsum with carbonate derived from hydrocarbon oxidation through microbial sulfate reduction (MSR), which is known to produce sulfide. For the production of native sulfur associated with carbonate caprock, it has been assumed molecular oxygen (O2) is required. Using geochemical data and new insights on sulfur microbiology, Labrado et al. demonstrated O2 availability is not a prerequisite to form native sulfur deposits. To avoid high sulfide levels in an O2-free environment, sulfate-reducing bacteria are postulated to cooperate with methane-oxidizing and possibly methanogenic microbes to create native sulfur as a nontoxic product. To achieve this, sulfate-reducing bacteria either have to alter the mode of their metabolic pathway or employ a different sulfate reduction mechanism altogether.
There are three issues at hand. First, there is the possibility native sulfur formation does not require any microbial involvement because, at temperatures higher than 120 ºC, thermochemical sulfate reduction (TSR) can produce native sulfur spontaneously. Second, tracking down an unknown microbial process/pathway is difficult. Third, little is known about the environmental parameters under which the inferred microbial process occurred. To tackle these issues, we combined petrographic observations, high-resolution sulfur isotope analyses, and clumped isotope signatures of carbonate caprock from Damon Mound (TX) to better characterize the temperature, formation fluids, hydrocarbon sources, and series of geologic events. We conclude that genesis of native sulfur appears to be driven by meso/thermophilic microbes and not TSR, which is inhibited in the observed temperature range. Meteoric waters only contributed to the genesis of the latest generation of carbonate veins, corroborating our hypothesis that native sulfur is formed by microbes in the absence of molecular oxygen. References Labrado, A.L., Brunner, B., Bernasconi, S.M., and Peckmann, J. (2019). Frontiers in Microbiology,10.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.U11C..03L
- Keywords:
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- 0810 Post-secondary education;
- EDUCATION;
- 0815 Informal education;
- EDUCATION