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Unprecedented 34S-enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks
Linnaeus University, Faculty of Health and Life Sciences, Department of Biology and Environmental Science.ORCID iD: 0000-0001-7230-6509
Swedish Museum of Natural History.
Georg August Univ, Germany.
Univ Arizona, USA.
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2018 (English)In: Geobiology, ISSN 1472-4677, E-ISSN 1472-4669, Vol. 16, no 5, p. 556-574Article in journal (Refereed) Published
Abstract [en]

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 S-34 relative to S-32. As documented by secondary ion mass spectrometry (SIMS) microanalyses, the S-34(pyrite) values are up to +132 parts per thousand V-CDT and with a total range of 186 parts per thousand. The lightest S-34(pyrite) values (-54 parts per thousand) suggest very large fractionation during MSR from an initial sulfate with S-34 values (S-34(sulfate,0)) of +14 to +28 parts per thousand. 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 S-34(pyrite) values were produced by Rayleigh fractionation effects in a diminishing sulfate pool. Large volumes of pyrite with superheavy values (+120 +/- 15 parts per thousand) within single fracture intercepts in the boreholes, associated heavy average values up to +75 parts per thousand and heavy minimum S-34(pyrite) values, suggest isolation of significant amounts of isotopically light sulfide in other parts of the fracture system. Large fracture-specific S-34(pyrite) variability and overall average S-34(pyrite) values (+11 to +16 parts per thousand) lower than the anticipated S-34(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 C-13 values in cogenetic calcite suggest that the superheavy S-34(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 S-34-rich pyrite.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2018. Vol. 16, no 5, p. 556-574
Keywords [en]
continental crust, deep biosphere, microbial sulfate reduction, pyrite, sulfur isotopes
National Category
Environmental Sciences
Research subject
Natural Science, Environmental Science
Identifiers
URN: urn:nbn:se:lnu:diva-77727DOI: 10.1111/gbi.12297ISI: 000441436700007PubMedID: 29947123OAI: oai:DiVA.org:lnu-77727DiVA, id: diva2:1248020
Projects
Characteristics and extent of microbial anaerobic methane oxidation and sulfate reduction in the deep terrestrial subsurface over geological time scalesProduction and consumption of the greenhouse gas methane in the crystalline bedroc
Funder
Swedish Research Council, 2017-05186Swedish Research Council Formas, 2017-00766Available from: 2018-09-13 Created: 2018-09-13 Last updated: 2018-09-13Bibliographically approved

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Drake, HenrikTillberg, MikaelDopson, MarkÅström, Mats E.

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