ATMOSPHERIC CHEMISTRY OF SO₂ DURING THE EARLIEST HURONIAN GLACIATION

The Huronian Supergroup of southern Ontario, Canada preserves a record of Paleoproterozoic glaciations in a succession of three diamictite-bearing intervals deposited between ~2.4 and ~2.2 Ga. We report δ³⁴S, Δ³³S, and Δ³⁶S of sulfide sulfur from ~40 drill-core samples taken over ~150 m through the earliest glacial interval, represented by the diamictite of the Ramsay Lake Formation, the underlying McKim Formation, and overlying Pecors Formation. Measured sulfur isotope compositions exhibit Δ³³S values of -0.06 to 0.27‰ that are small but analytically resolvable from zero. δ³⁴S values from the same samples range from -0.3 to 7.8‰ VCDT. Simple mass-dependent processes that fractionate ³⁴S to this degree will only produce Δ³³S variability <<0.1‰. Multiple sulfur isotope compositions from the McKim and Pecors Formations, therefore, preserve a record of anomalous S isotope fractionation that is an order of magnitude smaller than the record present in rocks older than ~2.5 Ga. Experimental studies indicate that the magnitude and sign of Δ³⁶S and Δ³³S covariation is a unique characteristic of the gas-phase photochemical reactions responsible for anomalous S isotope fractionation. In the drill-core samples, Δ³⁶S and Δ³³S covary strongly and indicate that, unlike in the Archean atmosphere, the anomalous fractionation was not produced by photolysis of SO₂ at wavelengths <220 nm. Both the operation of SO₂ photolysis and the preservation of its photochemical products depend strongly on the oxygen content of the contemporary atmosphere and, in concert with recent suggestions, the Δ³⁶S and Δ³³S systematics suggest that pO₂ > 10^-2 to 10^-5 PAL before and after the first Huronian glacial interval. Down-core profiles of Δ³⁶S and Δ³³S show that the atmospheric chemistry of SO₂ evolved during the Ramsay Lake glacial interval, and preserve evidence for dynamic chemical behavior reminiscent of the modern atmosphere. Accordingly, models of the interplay between Paleoproterozoic atmospheric evolution and climate during the initiation and termination of the earliest Huronian glaciation may actually find firm observational footing in the sulfur multiple isotope record present in ice cores and atmospheric aerosols.