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

Journal article
Authors H. Drake
M. J. Whitehouse
C. Heim
P. W. Reiners
M. Tillberg
Johan Hogmalm
M. Dopson
C. Broman
M. E. Astrom
Published in Geobiology
Volume 16
Issue 5
Pages 556-574
ISSN 1472-4677
Publication year 2018
Published at Department of Earth Sciences
Pages 556-574
Language en
Links dx.doi.org/10.1111/gbi.12297
Keywords continental crust, deep biosphere, microbial sulfate reduction, pyrite, sulfur isotopes, sulfur isotope fractionation, anaerobic methane oxidation, fatty-acid, biomarkers, reducing bacteria, fennoscandian shield, fluid inclusions, seawater sulfate, sulfide minerals, southern sweden, stable-isotopes, Life Sciences & Biomedicine - Other Topics, Environmental Sciences &, Ecology, Geology, ylor j, 1985, journal of general microbiology, v131, p631, ambers la, 1975, canadian journal of microbiology, v21, p1602
Subject categories Geology

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 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.

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