Biogeochemical Fractionation of MIF Sulfate
Abstract
Sulfur isotopes carry some of the most varied signatures of any stable isotope system; the four stable isotopes allow mass-dependent and mass-independent (MIF) isotopic effects to be identified and placed in the context of natural processes. Both atmospheric and biological processes produce characteristic arrays which allow them to be recognized in the rock record. We present the results of quadruple sulfur isotope analysis of pyrite collected from the 2990-2780 Ma Witwatersrand Supergroup. Isotopic analysis was completed using the Sensitive High Resolution Ion Microprobe - Stable Isotope (SHRIMP-SI) at the Research School of Earth Sciences, Australian National University. Uncertainties in Δ33S and Δ36S were ca. 0.1 ‰ and 0.4 ‰ respectively. The analysed pyrite grains returned Δ33S values of -0.6 to -0.9 ‰ and Δ36S values of -1.5 to -2.5 ‰, with these data plotting along a line of slope -7 which was offset from the origin by approximately half a per mil (Figure 1). When compared with available data, this line passes through data collected from Archean sulfate minerals, which showed similar Δ33S values but fall on the MIF array of Δ36S/Δ33S = -0.9. We suggest that these data represent microbial sulfate reduction of MIF sulfate, where this reaction has forced the isotopic composition of the reduction products off the MIF array downwards along a slope of -7, which is the proposed slope for microbial sulfate reduction. Shen et al. (2009) have previously suggested that microbial reduction of MIF sulfur species would cause this shift to more negative Δ36S without a significant change in Δ33S. Therefore, these data suggest that mass-dependent processes, such as microbial sulfate reduction, can significantly alter the isotopic composition of MIF species during the Archean, causing deviation from the array which is presently used to identify mass-independent fractionation.
Shen, Y., Farquhar, J., Masterson, A., Kaufman, A. J. & Buick, R. (2009). Evaluating the role of microbial sulfate reduction in the early Archean using quadruple isotope systematics. Earth and Planetary Science Letters, 279, 383-391. https//doi.org/10/1016/j.epsl.2009.01.018- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V31B..03T
- Keywords:
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- 9820 Techniques applicable in three or more fields;
- GENERAL OR MISCELLANEOUSDE: 1030 Geochemical cycles;
- GEOCHEMISTRYDE: 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1094 Instruments and techniques;
- GEOCHEMISTRY