Sulfur, ultraviolet radiation, and the early evolution of life

Abstract

The present biosphere is shielded from harmful solar near ultraviolet (UV) radiation by atmospheric ozone. We suggest here that elemental sulfur vapor could have played a similar role in an anoxic, ozone-free, primitive atmosphere. Sulfur vapor would have been produced photochemically from volcanogenic SO2 and H2S. It is composed of ring molecules, primarily S8, that absorb strongly throughout the near UV, yet are expected to be relatively stable against photolysis and chemical attack. It is also insoluble in water and would thus have been immune to rainout or surface deposition over the oceans. The concentration of S8 in the primitive atmosphere would have been limited by its saturation vapor pressure, which is a strong function of temperature. Hence, it would have depended on the magnitude of the atmospheric greenhouse effect. Surface temperatures of 45 degrees C or higher, corresponding to carbon dioxide partial pressures exceeding 2 bars, are required to sustain an effective UV screen. Two additional requirements are that the ocean was saturated with sulfite and bisulfite, and that linear S8 chains must tend to reform rings faster than they are destroyed by photolysis. A warm, sulfur-rich, primitive atmosphere is consistent with inferences drawn from molecular phylogeny, which suggest that some of the earliest organisms were thermophilic bacteria that metabolized elemental sulfur.

Bolide impacts and the oxidation state of carbon in the Earth's early atmosphere

“…This produces an atmospheric H2 mixing ratio of ~t0 3, near the upper end of the range (10 -s to 10 -3) used in previous studies (e.g. Yung and McElroy, t979;Pinto et al, 1980;Levine and Augustsson, 1985;Kasting et al, 1984Kasting et al, , 1990. This outgassing rate is near the upper limit of the flux that might reasonably have been produced by early volcanos (Walker, 1977) and by photostimulated reduction of ferrous iron in the surface ocean (Braterman et at., 1983).…”

Oceanic protection of prebiotic organic compounds from UV radiation

“…although the photochemistry is much more complex than indicated in the equation (35,36). The quantum yield (37) for the above reaction is approximately 1.…”

UV shielding of NH3and O2by organic hazes in the Archean atmosphere

“…Walker et al [1983] and Kasting [1993] predict negligible ground level 02 concentrations, whereas Holland [1994] [2000] presented evidence for mass-independent sulfur isotope fractionation in sediments deposited prior to •2.1 Ga. Although the details of the process remain to be elucidated, these data are consistent with fractionation by photochemical reactions, followed by removal of sulfur from the atmosphere in a variety of different oxidation states [see, e.g., Kasting et al, 1989]. The data thus support the notion that the Archean atmosphere was almost completely anoxic.…”