Unprecedented 34 S-enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks

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 S_pyrite values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ³⁴ S_pyrite values (-54‰) suggest very large fractionation during MSR from an initial sulfate with δ³⁴ S values (δ³⁴ S_sulfate,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 S_pyrite 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 S_pyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ³⁴ S_pyrite variability and overall average δ³⁴ S_pyrite values (+11 to +16‰) lower than the anticipated δ³⁴ S_sulfate,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 S_pyrite 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.

Keywords: continental crust; deep biosphere; microbial sulfate reduction; pyrite; sulfur isotopes.