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

Henrik Drake, Martin J. Whitehouse, Christine Heim, Peter W. Reiners, Mikael Tillberg, K. Johan Hogmalm, Mark Dopson, Curt Broman, Mats E. Åström

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

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 34S relative to 32S. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the δ34Spyrite values are up to +132‰V-CDT and with a total range of 186‰. The lightest δ34Spyrite values (−54‰) suggest very large fractionation during MSR from an initial sulfate with δ34S values (δ34Ssulfate,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 δ34Spyrite 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 δ34Spyrite values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific δ34Spyrite variability and overall average δ34Spyrite values (+11 to +16‰) lower than the anticipated δ34Ssulfate,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 δ13C values in cogenetic calcite suggest that the superheavy δ34Spyrite 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 34S-rich pyrite.

Original languageEnglish (US)
Pages (from-to)556-574
Number of pages19
JournalGeobiology
Volume16
Issue number5
DOIs
StatePublished - Sep 2018

Keywords

  • continental crust
  • deep biosphere
  • microbial sulfate reduction
  • pyrite
  • sulfur isotopes

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Environmental Science(all)
  • Earth and Planetary Sciences(all)

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