Micro-scale quadruple sulfur isotope analysis of pyrite from the ∼3480 Ma Dresser Formation: New insights into sulfur cycling on the early Earth

We report in situ quadruple sulfur isotope analysis (S-32, S-33, S-34 and S-36) of a pyritized microbial mat from the similar to 3480 Ma Dresser Formation, Pilbara Craton, Western Australia. These data yield positive delta S-34 and A.33S, indicative of sulfur sourced from a pool with similar character as the putative atmospheric elemental sulfur channel of Pavlov and Kasting (2002). Contrary to previous data from the Dresser Formation, however, this pyrite is heavily depleted in S-36 with a Delta S-36/Delta S-33 slope of c. -3.6, much steeper than slopes typically seen in other early Archean rock successions (Delta S-36/Delta S-33 approximate to -1) which suggests either a different atmospheric signature for deposited S or a different pool altogether. Significant micro-scale isotopic heterogeneity is observed within the microbial mat (delta S-34 = +1.6 parts per thousand to +6.7 parts per thousand; Delta S-33 = +0.4 parts per thousand to +2.6 parts per thousand; Delta S-36 = 3.1 parts per thousand to 8.1 parts per thousand), implying a role for microbial S metabolism. While metabolic S cycling has been shown to shift Delta S-36 to lower values, microbial metabolization of S does not appear sufficient to account for the full range of Delta S-36. We conclude that the isotopic composition of the pyrite was controlled by the relative proportions of mass independently fractionated (MIF) S-0 and sulfate-derived sulfur incorporated into polysulfide pyrite precursors during reactions in the microbial mat. The dominance of the MIF-S-0 isotopic signature (+delta S-34, +Delta S-33, -Delta S-36) indicates that contributions from the sulfate-derived sulfur pool were relatively small, consistent with low concentrations of sulfate in Archean seawater, and that contributions from a non-sulfate pool were significant. Micro-scale isotopic heterogeneity in the pyrite points to mixing between the two sulfur pools in selected micro-environments within the microbial mat. The particularly negative Delta S-36 observed here reveals a similar to 3480 Ma sulfur reservoir with novel Delta S-36/Delta S-33 chemistry whose significance now requires further investigation. (C) 2014 Elsevier B.V. All rights reserved.