Sulfur dioxide inhibits calcium carbonate precipitation: Implications for early Mars and Earth

[1] Recent studies have suggested a role for sulfur dioxide (SO2) in maintaining relatively warm surface temperatures on early Mars. Here we show experimentally, that SO2 concentrations orders of magnitude lower than those required for it to have been of climatic importance strongly affect the aqueous chemistry and the precipitated mineral assemblage. At near-neutral pH, part-per-billion concentrations of SO2 prevent the formation of calcium carbonate in favor of hannebachite, a hydrated calcium sulfite. In the presence of iron, possible precursors to phyllosilicate minerals and iron carbonate co-precipitate with hannebachite. This provides an explanation for the existence of early Noachian phyllosilicates in the apparent dearth of outcrop-scale calcium carbonates. Oxidation of this precipitated assemblage produces sulfates, iron oxides and acidity, consistent with evidence for late Noachian – early Hesperian acid-sulfate dominated environments. For early Earth, the results allow placing an upper limit on atmospheric SO2 concentrations for any period in which carbonates exist in the geologic record. Citation: Halevy, I., and D. P. Schrag (2009), Sulfur dioxide inhibits calcium carbonate precipitation: Implications for early Mars and Earth, Geophys. Res. Lett., 36, L23201, doi:10.1029/ 2009GL040792.