The mass independent fractionation of sulfur isotopes
(S-MIF) in geological samples would provide a record of the
past atmospheric composition, though the mechanism of
photochemical S-MIF is still poorly understood [1]. We
determined the high-precision UV absorption spectra of SO2
isotopologues [2] and calculated isotope fractionation factor of
SO2 photolysis as a function of wavelength. Using these
results, we show that the estimated fractionation factors give
mass independent distributions and are highly sensitive to the
atmospheric concentrations of O2, O3, CO2, H2O, CS2, NH4,
N2O, H2S, OCS and SO2 itself, because these molecules can
modify spectra of actinic UV flux. Although various UVshielding
scenarios can be considered, we found that the
negative ¿33S observed in all the Archean sulfate deposits
[1,3] could be expected only when OCS was present in the
atmosphere. This is because OCS uniquely absorbs >200 nm
region of solar UV flux. Further, we performed numerical
simulation of atmospheric reactions including OCS chemistry
and found that ppm-level OCS could be accumulated in a O2-
free reducing atmosphere when CO/CO2 ratio is greater than
1. Therefore, appreciable amount of OCS is likely to have
existed in the reducing Archean atmosphere. Such a high level
of OCS also absorbs infrared light from 8 to 13 µm, which is
not absorbed by water vapor. Hence, OCS could be an
alternative or even more efficient greenhouse gas than CO2 to
resolve the faint young Sun paradox [4]. Furthermore, OCS
also has absorption band in lethal UVC region like ozone, thus
could be an alternative UV-shielding molecule in an ozonefree
reducing atmosphere. The decline of OCS might have
coursed the late Archean Pongola glaciation (2.9 Ga) and
possibly resulted in UV crisis of terrestrial and shallow water
ecosystems until the rise of oxygen.