A BIOLOGICAL ORIGIN OF ELEMENTAL SULFUR IN SALT DOME CAP ROCK OF THE GULF COAST BASIN, USA

Salt dome cap rocks from the Boling and Main Pass Gulf Coast salt domes contain anomalously large elemental sulfur accumulations. Isotopic and petrographic data indicate complex histories of salt dome cap rock paragenesis for both domes. Whereas paragenetic complexity is in part due to the open nature of these hydrodynamic systems, a comprehensive understanding of elemental sulfur origin and concentration mechanisms is lacking. H2S oxidation to elemental sulfur is thought to occur via aerobic oxidation due to the volume of external oxidant required. Fluid flux calculations for seawater oxidants indicate the possibility, but improbability for aerobic oxidation as the sole means for elemental sulfur production. Petrographic analyses exhibit multiple generations of carbonate and sulfur growth, suggesting a system with episodic fluid influx. Sulfur crystals exhibit habits consistent with temperatures conducive to microbial processes. Large ranges in traditional sulfur isotope compositions (δ34S) has led some to infer microbial sulfate reduction and/or influx of sulfide-rich formation waters during salt dome cap rock formation. Ultimately, traditional sulfur isotope analyses alone cannot distinguish microbial, thermochemical, or exogenous sulfur sources. Multiple sulfur isotope (32S, 33S, 34S) datasets exhibit ranges that are classically interpreted as evidence of microbial sulfate reduction. However, further modeling indicates that sulfate reduction alone cannot produce the observed elemental sulfur isotopic compositions. Subsequent oxidation of sulfur and/or sulfide phases could explain observed isotopic compositions. Likewise, two end-member mixing of internal sulfide produced as a result of microbial sulfate reduction could also explain these observations. Ultimately, elemental sulfur from the Boling and Main Pass salt dome cap rock formed in a low temperature environment impacted by an active subsurface microbial biosphere