Experimental Inhibition of Carbonate Precipitation by Sulfite Minerals

Introduction: Volcanic and hydrothermal supply of reducing gases to the early atmosphere of Mars likely surpassed the slow supply of oxidizing power from hydrogen escape to space, resulting in a neutral or mildly reducing atmosphere and in geochemical cycles that operated very differently than they do on the modern Earth. We have recently suggested that one such difference between the modern and ancient geochemical cycle of sulfur is the possible existence of a SO2 climate feedback on early Mars [1], analogous to the negative climate feedback between atmospheric CO2 concentrations and the rate of silicate weathering on the modern Earth [2]. Being both a more effective greenhouse gas (when added to a CO2-rich atmosphere) and a stronger acid than CO2, small atmospheric concentrations of SO2 suffice for it to become the dominant control on climate, on the pH of surface waters and through these on the rate of silicate weathering and supply of alkalinity to surface water bodies. Precipitation of hannebachite (CaSO3·/2H2O) is an alkalinity, and therefore climate-dependent mineral sink for SO2, which completes the requirements for operation of a negative climate feedback. A prediction from the aqueous chemistry of SO2 is that under these conditions sulfite minerals should precipitate at the expense of carbonates [1]. In a series of mineral precipitation experiments we show that this is indeed the case and that oxidation of the precipitates gives rise to mineral assemblages similar to those detected on the surface of Mars.