When did the Earth's atmosphere become oxic? A Reply

In the fall 1997 issue of The Geochemical News, H. Ohmoto (1) outlined two very different views regarding the evolution of atmospheric oxygen: the Cloud-Walker-Kasting-Holland (C –W-K-H) model and the Dimroth-Kimberley-Ohmoto (D-KO) model. The first maintains that oxygen was absent or very low in the atmosphere prior to ca. 2.3 Ga, and that pO2 rose rapidly to values = 0.03 atm during the Great Oxidation Event (G.O.E.) between ca. 2.25 and ca. 2.05 Ga. The second model postulates an essentially constant atmospheric pO2 level (probably within ± 50% of the present atmospheric level [PAL]) since ca. 4 Ga. Ohmoto (1) concluded that “lines of evidence in favor of the D – K – O model appear to be accumulating”. This reply attempts to point out that these appearances may be misleading, and that the bulk of the available evidence, especially the newer evidence, bearing on the history of atmospheric oxygen strongly favors the C-W-K-H model. I will follow the same sequence of topics addressed by Ohmoto (1).