Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
6442185 | Precambrian Research | 2016 | 10 Pages |
â¢In situ δ33S, δ34S and δ36S analysis of pyrite from a 3.52 Ga hydrothermal barite.â¢The Î33S-Î36S covariation in the pyrite is different from the co-existing barite.â¢Î36S data cannot be explained by microbial cycling or abiotic sulfate reduction.â¢Reworking of atmospheric sulfur with distinct Î36S/Î33S can explain covariations.â¢Measurement of 36S is crucial to distinguish biosignatures from abiotic reactions.
Anomalous fractionation of the minor isotopes of sulfur (Î33S, Î36S) in Archean pyrite is thought to reflect photochemical reactions in an anoxic atmosphere, with most samples falling along a reference array with Î36S/Î33S â â1. Small deviations from this array record microbial sulfate reduction or changes in atmospheric source reactions. Here, we argue that reworking of atmospheric sulfur with distinct minor sulfur isotope ratios (Î36S/Î33S â  â1) produced additional variability in sulfide Î33S and Î36S-values in a 3.52 Ga hydrothermal barite deposit at Londozi, Barberton Greenstone Belt, Swaziland. In situ measurement of the four stable sulfur isotopes in pyrite revealed Î36S-Î33S relationships and a Î36S/Î33S trend (â3.2 ± 0.4), which is significantly different from the co-variation between Î36S and Î33S in the co-existing barite that reflects ambient Paleoarchean seawater sulfate. This argues against biological or thermochemical sulfate reduction at the time of barite deposition, and requires incorporation of sulfide generated in a chemically distinct atmosphere before 3.52 Ga. We propose a model that combines reworking of this sulfur by hydrothermal leaching, deep mixing with juvenile sulfur and surface mixing with biogenic sulfide to explain the observed variation in δ34S, Î33S and Î36S. These interactions between abiotic and biological processes in the Londozi hydrothermal system complicate the interpretation of biosignatures based on deviations in Î33S and Î36S from the Archean reference array.