Observation of wavelength-sensitive mass-independent sulfur isotope effects during SO2 photolysis:: Implications for the early atmosphere

Mass-independent isotopic signatures for delta(33)S, delta(34)S, and delta(36)S produced in the photolysis of sulfur dioxide exhibit a strong wavelength dependence. Photolysis experiments with three light sources (ArF excimer laser (193 run), mercury resonance lamp (184.9 and 253.7 nm), and KrF excimer laser (248 nm)) are presented. Products of sulfur dioxide photolysis undertaken with 193-nm radiation exhibit characteristics that are similar to sulfur multiple-isotope data for terrestrial sedimentary rock samples older than 2450 Ma (reported by Farquhar et al. [2000a]), while photolysis experiments undertaken with radiation at other wavelengths (longer than 220 rim and at 184.9 rim) exhibit different characteristics. The spectral window between 190 and 220 nm falls between the Schumann-Runge bands of oxygen and the Hartley bands of ozone, and its absorption is therefore more sensitive to changes in altitude and atmospheric oxygen content than neighboring wavelengths. These two observations are used to suggest a link between sulfur dioxide photolysis at 193 nm and sulfur isotope anomalies in Archean rocks, This hypothesis includes the suggestion that UV wavelengths shorter than 200 nm penetrated deep in the Earth's atmosphere during the Archean. Potential implications of this hypothesis for the chemistry, composition, and UV absorption of the atmosphere are explored. We also explore the implications of these observations for documentation of bacterial sulfur metabolisms early in Earth's history.