Distributions of sulfur isotopes in geological samples would provide
a record of atmospheric composition if the mechanism producing
the isotope effects could be described quantitatively. We
determined the UV absorption spectra of 32SO2, 33SO2, and 34SO2
and use them to interpret the geological record. The calculated
isotopic fractionation factors for SO2 photolysis give mass independent
distributions that are highly sensitive to the atmospheric
concentrations of O2, O3, CO2, H2O, CS2, NH3, N2O, H2S, OCS, and
SO2 itself. Various UV-shielding scenarios are considered and we
conclude that the negative 33S observed in the Archean sulfate
deposits can only be explained by OCS shielding. Of relevant
Archean gases, OCS has the unique ability to prevent SO2 photolysis
by sunlight at >202 nm. Scenarios run using a photochemical
box model show that ppm levels of OCS will accumulate in a
CO-rich, reducing Archean atmosphere. The radiative forcing, due
to this level of OCS, is able to resolve the faint young sun paradox.
Further, the decline of atmospheric OCS may have caused the late
Archean glaciation.