Formation of Large Native Sulfur Deposits Does Not Require Molecular Oxygen

Abstract

Large native (i.e., elemental) sulfur deposits can be part of caprock assemblages found on top of or in lateral position to salt diapirs and as stratabound mineralization in gypsum and anhydrite lithologies. Native sulfur is formed when hydrocarbons come in contact with sulfate minerals in presence of liquid water. The prevailing model for native sulfur formation in such settings is that sulfide produced by sulfate-reducing bacteria is oxidized to zero-valent sulfur in presence of molecular oxygen (O₂). Although possible, such a scenario is problematic because: (1) exposure to oxygen would drastically decrease growth of microbial sulfate-reducing organisms, thereby slowing down sulfide production; (2) on geologic timescales, excess supply with oxygen would convert sulfide into sulfate rather than native sulfur; and (3) to produce large native sulfur deposits, enormous amounts of oxygenated water would need to be brought in close proximity to environments in which ample hydrocarbon supply sustains sulfate reduction. However, sulfur stable isotope data from native sulfur deposits emplaced at a stage after the formation of the host rocks indicate that the sulfur was formed in a setting with little solute exchange with the ambient environment and little supply of dissolved oxygen. We deduce that there must be a process for the formation of native sulfur in absence of an external oxidant for sulfide. We hypothesize that in systems with little solute exchange, sulfate-reducing organisms, possibly in cooperation with other anaerobic microbial partners, drive the formation of native sulfur deposits. In order to cope with sulfide stress, microbes may shift from harmful sulfide production to non-hazardous native sulfur production. We propose four possible mechanisms as a means to form native sulfur: (1) a modified sulfate reduction process that produces sulfur compounds with an intermediate oxidation state, (2) coupling of sulfide oxidation to methanogenesis that utilizes methylated compounds, acetate or carbon dioxide, (3) ammonium oxidation coupled to sulfate reduction, and (4) sulfur comproportionation of sulfate and sulfide. We show these reactions are thermodynamically favorable and especially useful in environments with multiple stressors, such as salt and dissolved sulfide, and provide evidence that microbial species functioning in such environments produce native sulfur. Integrating these insights, we argue that microbes may form large native sulfur deposits in absence of light and external oxidants such as O₂, nitrate, and metal oxides. The existence of such a process would not only explain enigmatic occurrences of native sulfur in the geologic record, but also provide an explanation for cryptic sulfur and carbon cycling beneath the seabed.

Keywords: cryptic carbon cycling; cryptic sulfur cycling; isotope; methanogenesis; microbe; native sulfur; sulfur formation; sulfur reduction.

FIGURE 1

Sketches of classical epigenetic salt diapir caprock and stratabound sulfur deposit types. (A) Oxygen is delivered to the site where native sulfur is formed by infiltration of meteoric water. On its journey to the location where native sulfur is formed, O₂ has to pass hydrocarbon bearing strata. Solubility of O₂ may decrease with depth due to increased salinity. To maintain inflow of meteoric waters, the brine must be removed (modified from Ruckmick et al., 1979). (B) Oxygen is delivered to the site where native sulfur is formed by infiltration of meteoric water. On its journey to the location where native sulfur is formed, O₂ takes the same route as the hydrocarbons. Solubility of O₂ may decrease with depth due to increased salinity. To maintain inflow of meteoric waters, the brine must be removed (modified from Ruckmick et al., 1979).

FIGURE 2

Schematic of genesis of sulfide, native sulfur and carbonate minerals from hydrocarbons and calcium sulfate minerals. The red arrows with question marks indicate that native sulfur is either formed by an unknown microbial pathway or that an unknown oxidant is needed for the conversion of sulfide into native (zero-valent) sulfur. In cases where dolomite is formed, there must be a (unknown) source of magnesium. Dissolved sulfate in system (gray box) can become trapped in newly formed carbonate minerals as carbonate associated sulfate (CAS).

FIGURE 3

Genesis of native sulfur in presence (left) and absence (right) of O₂. (Left) In a system where native sulfur genesis is driven by supply with O_2, there is a competion between oxygen-consuming aerobic hydrocarbon oxidation and oxidation of sulfide to native sulfur. Replacement of gypsum with carbonate is confined to the interface between solid gypsum and hydrocarbons in order to maintain low-oxygen conditions required for sulfate-reducing bacteria using extracellular polymeric substances (EPS). Sulfur-oxidizing bacteria (SOB) could help produce native sulfur in close association with sulfate-reducing bacteria. (Right) In an oxygen-free environment, sulfate reduction can take place detached from gypsum surfaces at the hydrocarbon-brine interface because gypsum dissolution provides sulfate to the brine. Sulfur cycling may include simultaneous genesis and consumption of methane and sulfate, constituting complete cryptic carbon and sulfur cycles. Sulfide-oxidizing microbes (SOM) produce methane and native sulfur. Sulfur disproportionating bacteria (SDB) convert sulfur compounds with intermediate oxidation state or native sulfur into sulfate and sulfide. Finally, anearobic oxidation of methane (AOM) consumes sulfate and methane. Sulfide can react with native sulfur to form polysulfides, a reaction that is reverted during carbonate precipitation, due to local increase in acidity.

FIGURE 4

Carbon isotopes systematics in epigenetic sulfur deposits. Compilation of carbon isotope data from carbonates from epigenetic native sulfur deposits. Thermogenic or biogenic methane is isotopically very light and may have contributed to the formation of carbonates at various sites, such as the south-eastern Mediterranean Coastal Plain of Israel and Northern Sinai (Nissenbaum, 1984), the Hackberry salt dome, Louisiana, United States (McManus and Hanor, 1993); the Carpathian Foredeep, Poland (Parafiniuk et al., 1994; Böttcher and Parafiniuk, 1998), Sicily, Italy (Ziegenbalg et al., 2012), and within stratiform native sulfur deposits in the Castile anhydrite in the northwestern and west-central Delaware Basin, United States (Kirkland, 2014). The carbon isotope composition of carbonates from Damon Mound (this study) fall almost entirely into the range between oil-derived carbon and carbonate from seawater, but a contribution from methane cannot be excluded.

FIGURE 5

Sulfur isotopes systematics in epigenetic sulfur deposits. Compilation of data from United States Gulf Coast, Polish, and Sicilian native sulfur deposits, data normalized to presumed sulfur isotope composition of sulfate source (set to zero, data presented as an offset between isotope compositions; Δ³⁴S). The sulfur isotope compositon of native sulfur is lighter and the residual sulfates isotopically heavier than the original sulfate mineral. The isotope offset between native sulfur and original sulfate is much smaller than the offset between original sulfate and residual sulfate. However, the enrichment in ³⁴S does not exceed the theoretical maximum isotope fractionation for microbial sulfate reduction. Data sources: Huckley, Boling, Moss Bluff, and Spindletop domes (Feely and Kulp, 1957; Kyle and Agee, 1988); Damon Mound (new data); Poland (Parafiniuk et al., 1994); Sicily (Ziegenbalg et al., 2010).

FIGURE 6

Systems with ample and restricted fluid flow (i.e., removal of sulfate). Examples for sulfur isotope patterns in a system with ample, semi-restricted and restricted fluid flow. The input of sulfate from dissolution of gypsum/anhydrite (chosen δ³⁴S of +14‰), the sulfur isotope fractionation by sulfate reduction is assumed to be 75‰. (Top) If most of the sulfate that enters the system also leaves the system (ample fluid flow), the δ³⁴S of the residual sulfate (CAS) matches the δ³⁴S of original sulfate and the δ³⁴S of native sulfur is strongly offset to low values. (Middle) If half of the sulfate entering the system is converted to native sulfur, native sulfur and residual sulfate will have the same absolute isotopic offset from residual sulfate. (Bottom) If essentially all sulfate that enters the system is converted to native sulfur, the δ³⁴S of native sulfur matches the δ³⁴S of original sulfate, and the δ³⁴S of the residual sulfate is strongly offset to isotopically heavier values.