Fractionation of Sulfur and Oxygen isotopes during SO2 photolysis

The discovery of sulfur mass-independent fractionation (S-MIF) in ancient sedimentary rocks [1] has created the opportunity for quantitative constraints on the composition of the early atmosphere. Laboratory experiments have shown that S-MIF occurs during SO2 photodissociation at a variety of UV wavelengths [2, 3]. The source of the S-MIF in experiments at wavelengths ~ 200 nm is self-shielding in SO2 [4, 5]. Sulfur isotope substitution produces shifts in the locations of vibronic bands (see Fig. 2 in [4], and Fig. 5.2b in [5]). During photolysis saturation in the rovibronic lines of the most abundant SO2 isotopologue (SO2) results in a non-massdependent rate of photolysis in the rare isotopologues (i.e. self-shielding). Modeling SO2 photolysis in an atmosphere shows that after ~10% of SO2 has been dissociated, S-MIF signatures of ∆S ~ 10 ‰ are obtained, depending on the column abundance of SO2. The corresponding model ∆S/∆S ratios are ~ -3 to -0.5, depending on both the SO2 and CO2 column abundances in the model atmosphere. The oxygen isotopologues of SO2 will also undergo vibronic band shifts and self-shielding, and large O-MIF effects should be produced during SO2 photolysis. I am presently computing synthetic spectra for SO18O and SO17O isotopologues by the same method used for the sulfur isotopologues [4, 5], and will determine the magnitude of O-MIF expected. Oxygen exchange reactions must also be included. For example, SO2 exchange with H2O has been estimated to be rapid [6], and may be fast enough to preclude O-MIF from surviving in sulfates in the geologic record. In laboratory photolysis experiments oxygen exchange between SO2 and O may limit the magntiude of O-MIF in photolysis products such as O2. The sensitivity of O-MIF to these exchange reactions will be reported.