IMPLICATIONS OF ANOMALOUS D33S FOR EVOLUTION OF ATMOSPHERIC OXYGEN, CONTINENTAL WEATHERING, AND PALEOPROTEROZOIC GLACIATIONS

Many sulfur-bearing minerals older than ~2 Ga show a mass-independent fractionation (MIF) in their sulfur isotopic composition.  The origin of this MIF has been attributed to the gas phase photolysis of atmospheric S species in an oxygen-depleted atmosphere.  Therefore, systematic MIF variations may be used as proxy for interpreting long-term and short-term fluctuations in atmospheric oxygen concentrations and may provide insight into the factors controlling extreme climatic events such as global glaciations. 

Glacial diamictites from the Huronian Supergroup of southern Ontario, Canada and stratigraphically equivalent units on continents other than North America suggest that such a global glaciation may have existed between ~2.4 and ~2.2 Ga.  We report the multiple sulfur isotopic composition (³²S, ³³S, ³⁴S) of Cr-reducible sulfur (CRS) from detailed sampling of a drill core through the earliest Huronian glacial interval made up of the Ramsay Lake diamictite, the underlying McKim Formation, and overlying Pecors Formation.  Through the drill core, D³³S values of CRS range between 0.22 to ~ 0 ‰ and vary systematically with depth.  Within 6 meters of the contact of the McKim Formation and the overlying diamictite, D³³S decreases from 0.22 to 0.16 ‰, immediately above the diamictite to ~5 meters into the overlying Pecors Formation, D³³S is within analytical error of 0 ‰, and from ~ 6 - 15 meters above the diamictite D³³S increases uniformly from 0 to 0.1 ‰.  The small measured range of the D³³S values from the McKim and Pecors Formations is consistent with the observed contraction in the mass-independent record of Earth’s early sulfur cycle between ~ 2.5 to 2.0 Ga.  The systematic variation of D³³S with depth, however, implies a significant relationship between atmospheric oxygen levels and the weathering rates of continental sulfides, sulfates, and organic sulfur, and the global Paleoproterozoic glaciation.