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.
Highlights
Epigenetic Native Sulfur DepositsNative sulfur is formed by a number of abiotic and biological processes in a multitude of settings such as deeply buried sediments by thermochemical sulfate reduction (Warren, 2006), seafloor hydrothermal systems (Butterfield et al, 2011; Seewald et al, 2015), at volcanoes, at arctic glaciers (Grasby et al, 2003), in lake sediments (Philip et al, 1994; Lindtke et al, 2011), in shallow marine sediments, in sulfidic cave systems, or at the seafloor as filamentous sulfur
Several lines of evidence indicate that the genesis of native sulfur in epigenetic native sulfur deposits (ENSDs) has occurred in absence of O2
This does not preclude the genesis of native sulfur by oxidation with O2, as there are examples where that option appears realistic, such as in intermittent oxidation of sulfide (Jassim et al, 1999) or the oxidation of sulfide in a soil-influenced environment (Peckmann et al, 1999)
Summary
Native sulfur is formed by a number of abiotic and biological processes in a multitude of settings such as deeply buried sediments by thermochemical sulfate reduction (Warren, 2006), seafloor hydrothermal systems (Butterfield et al, 2011; Seewald et al, 2015), at volcanoes (e.g., in Chile; Ferraris and Vila, 1990), at arctic glaciers (Grasby et al, 2003), in lake sediments (Philip et al, 1994; Lindtke et al, 2011), in shallow marine sediments (e.g., through the giant sulfur bacterium Thiomargarita namibiensis; Schulz and Schulz, 2005), in sulfidic cave systems (e.g., incomplete sulfide oxidation by Sulfurovumlike Epsilonproteobacteria; Hamilton et al, 2015), or at the seafloor as filamentous sulfur (i.e., by Beggiatoa; Jørgensen et al, 2010 or by Arcobacter; Wirsen et al, 2002; Sievert et al, 2007). To generate the largest native sulfur caprock deposit with 89 million tons of native sulfur (Boling dome; Long, 1992a), approximately 44 million tons or 1.4 · 1012 moles of O2 would be required, not including O2 lost to hydrocarbon oxidation If such a supply is provided by oxygenated water to the subsurface and assuming a solubility of 250 μmol O2 per liter freshwater, this corresponds to approximately 5,100 km of oxygenated water, or for ease of comparison, to 9 months of draining of the Amazon River. This notion has been eloquently summarized by Machel (1992): “Economically viable deposits of native sulfur usually are formed by only one process: inorganic oxidation of H2S by molecular oxygen.” The disparity between the paradigm that O2 is required and the absence of direct evidence that O2 is available in the needed quantities to drive the process resulted in a conundrum that has persisted for over 100 years
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