Abstract
In the deep biosphere, microbial sulfate reduction (MSR) is exploited for energy. Here, we show that, in fractured continental crystalline bedrock in three areas in Sweden, this process produced sulfide that reacted with iron to form pyrite extremely enriched in 34 S relative to 32 S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ34 Spyrite values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ34 Spyrite values (-54‰) suggest very large fractionation during MSR from an initial sulfate with δ34 S values (δ34 Ssulfate,0 ) of +14 to +28‰. Fractionation of this magnitude requires a slow MSR rate, a feature we attribute to nutrient and electron donor shortage as well as initial sulfate abundance. The superheavy δ34 Spyrite values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120±15‰) within single fracture intercepts in the boreholes, associated heavy average values up to +75‰ and heavy minimum δ34 Spyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ34 Spyrite variability and overall average δ34 Spyrite values (+11 to +16‰) lower than the anticipated δ34 Ssulfate,0 support this hypothesis. The superheavy pyrite found locally in the borehole intercepts thus represents a late stage in a much larger fracture system undergoing Rayleigh fractionation. Microscale Rb-Sr dating and U/Th-He dating of cogenetic minerals reveal that most pyrite formed in the early Paleozoic era, but crystal overgrowths may be significantly younger. The δ13 C values in cogenetic calcite suggest that the superheavy δ34 Spyrite values are related to organotrophic MSR, in contrast to findings from marine sediments where superheavy pyrite has been proposed to be linked to anaerobic oxidation of methane. The findings provide new insights into MSR-related S-isotope systematics, particularly regarding formation of large fractions of 34 S-rich pyrite.
Highlights
The generally large fractionation of stable sulfur isotopes (δ34S), due to faster turnover of 32S than 34S by microbial sulfate reduction (MSR) (Canfield, 2001), has made this isotope system one of the most extensively used for understanding both modern and ancient biogeochemical cycles (Canfield & Farquhar, 2009; Fike, Grotzinger, Pratt, & Summons, 2006; Johnston et al, 2005)
At the sulfate–methane transition zone (SMTZ) in modern marine sediments, in environments characterized by fast MSR rates associated with anaerobic oxidation of methane (AOM), δ34S of pyrite is typically in the range of +30 to +50‰ due to Rayleigh-type processes (Borowski, Rodriguez, Paull, & Ussler Iii, 2013; Jørgensen, Böttcher, Lüschen, Neretin, & Volkov, 2004), and in extreme cases, it has been reported as heavy as +114.3‰ (Lin et al, 2016)
These results demonstrate that the MSR occurring mainly in the early–mid Paleozoic in the upper kilometer of fractured crystalline bedrock was able to produce δ34Spyrite with the largest range and highest values yet observed (−54 to +132‰)
Summary
The generally large fractionation of stable sulfur isotopes (δ34S), due to faster turnover of 32S than 34S by microbial sulfate reduction (MSR) (Canfield, 2001), has made this isotope system one of the most extensively used for understanding both modern and ancient biogeochemical cycles (Canfield & Farquhar, 2009; Fike, Grotzinger, Pratt, & Summons, 2006; Johnston et al, 2005). The focus is on a mineral assemblage dominated by euhedral calcite, pyrite, and fluorite that yielded ages of 450–270 Ma in a few previous bulk Ar–Ar adularia and Sm–Nd fluorite-based determinations (Alm et al, 2005; Drake et al, 2009; Sandström et al, 2009) This assemblage has been linked to the Caledonian orogeny and the development of a several kilometer thick foreland basin (Cederbom, 2001; Guenthner, Reiners, Drake, & Tillberg, 2017) that was subsequently uplifted, eroded, and removed during Neogene erosional episodes (Guenthner et al, 2017; Japsen, Bidstrup, & Lidmar-B ergström, 2002). This mineral assemblage has fluid inclusion homogenization temperatures predominantly below 100°C (Alm et al, 2005; Drake & Tullborg, 2009; Sandström & Tullborg, 2009) that allows for a potential microbial influence in contrast to the omnipresent older mineral assemblages of clearly higher temperature hydrothermal character
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