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Published April 15, 2014 | Supplemental Material + Published
Journal Article Open

SQUID–SIMS is a useful approach to uncover primary signals in the Archean sulfur cycle


Many aspects of Earth's early sulfur cycle, from the origin of mass-anomalous fractionations to the degree of biological participation, remain poorly understood—in part due to complications from postdepositional diagenetic and metamorphic processes. Using a combination of scanning high-resolution magnetic superconducting quantum interference device (SQUID) microscopy and secondary ion mass spectrometry (SIMS) of sulfur isotopes (^(32)S, ^(33)S, and ^(34)S), we examined drill core samples from slope and basinal environments adjacent to a major Late Archean (∼2.6–2.5 Ga) marine carbonate platform from South Africa. Coupled with petrography, these techniques can untangle the complex history of mineralization in samples containing diverse sulfur-bearing phases. We focused on pyrite nodules, precipitated in shallow sediments. These textures record systematic spatial differences in both mass-dependent and mass-anomalous sulfur-isotopic composition over length scales of even a few hundred microns. Petrography and magnetic imaging demonstrate that mass-anomalous fractionations were acquired before burial and compaction, but also show evidence of postdepositional alteration 500 million y after deposition. Using magnetic imaging to screen for primary phases, we observed large spatial gradients in Δ^(33)S (>4‰) in nodules, pointing to substantial environmental heterogeneity and dynamic mixing of sulfur pools on geologically rapid timescales. In other nodules, large systematic radial δ^(34)S gradients (>20‰) were observed, from low values near their centers increasing to high values near their rims. These fractionations support hypotheses that microbial sulfate reduction was an important metabolism in organic-rich Archean environments—even in an Archean ocean basin dominated by iron chemistry.

Additional Information

© 2014 National Academy of Sciences. Freely available online through the PNAS open access option. Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved March 10, 2014 (received for review December 4, 2013). This paper benefited greatly from the thoughtful comments of two anonymous reviewers. The Agouron Institute and National Aeronautic and Space Administration Exobiology Award NNX09AM91G supported this work. Author contributions: W.W.F., D.A.F., T.D.R., J.L.K., and J.M.E. designed research; W.W.F., D.A.F., J.E.J., and T.D.R. performed research; W.W.F., D.A.F., J.E.J., T.D.R., Y.G., J.L.K., and J.M.E. contributed new reagents/analytic tools; W.W.F., D.A.F., J.E.J., T.D.R., Y.G., and J.M.E. analyzed data; and W.W.F., D.A.F., and J.E.J. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1322577111/-/DCSupplemental.

Attached Files

Published - PNAS-2014-Fischer-5468-73.pdf

Supplemental Material - pnas.201322577SI.pdf

Supplemental Material - sd01.txt


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