The inference of ozone and the timing of this so-called “great oxidation event” (GOE) at 2.4 Ga comes primarily from analyses of sulfur isotopes in the rock record. The analyses and the interpretation, described by Farquhar et al. (2010) is based on the mass independent isotopic fractionation of sulfur. Basically, there are four stable sulfur isotopes, 32S, 33S, 34S and 36S. Virtually all reactions that involve formation or the breaking of this website chemical check details bonds among these isotopes

is mass-dependent, that is the isotope with the smaller mass is reactive (has a higher zero point kinetic energy) and the resulting products are predicted from first principles to be enriched in the lighter isotope. However, up until ~2.4 Ga, the isotopic fractionations in the geologic record are mass independent. SO2 has a UV absorption cross section, peaking at ~200 nm. Breaking of bonds by high energy photons does not lead to mass dependent isotopic fractionation. Hence, one interpretation of the mass independent fractionation is that short wave UV radiation reached the Earth’s surface prior to ~2.4 Ga, but subsequently that radiation was quenched. Stratospheric ozone absorbs short wave UV radiation on the contemporary Earth, and the source of ozone is O2. Hence, the loss of the mass independent isotopic fractionation of sulfur at 2.4 Ga suggests a change in

the oxidation state of Earth’s atmosphere. The mass independent fractionation signal Vistusertib order for S never returned, and hence, it is concluded that the transition from an anaerobic world to an oxidized world occurred once, and only Methane monooxygenase once, in Earth’s history. It should be noted that the concentration of oxygen that arose during the GOE is extremely poorly constrained. Formation of stratospheric ozone is not limited by O2 above ca.

0.1% of the present atmospheric level. Geochemists use other proxies, including N isotopes (Godfrey and Falkowski 2009), transition metal composition and isotopic values (Kaufman et al. 2007) and even mineral composition (Hazen et al. 2008) to further attempt to constrain the concentration of oxygen during the GOE and to understand what controlled the net accumulation of the gas over the ensuing 2.3 billion years. Geological contingencies High concentrations of free molecular oxygen in a planetary atmosphere cannot come about simply by high energy photolysis of water; that reaction is self quenching as UV becomes increasingly blocked. Further, as in all redox reactions, a reductant (the equivalent of hydrogen) is formed. To bring about a change in the oxidation state of the atmosphere, the redox reactions cannot be at equilibrium, but rather the reductant has to be removed and stored for long periods of geological time. Hence, the evolution of oxygenic photosynthesis was a necessary, but not sufficient condition for the oxidation of the planetary surface. In a simple geochemical sense, net production of oxygen on Earth implies the burial and sequestration of reductant.