Past Oxygen Levels: Some Evidence from Uranium Deposits

Cloud (1965; p.33) wrote: “..... it is now generally agreed that the components of the present atmosphere came ultimately from within the earth, mainly by volcanism. Free oxygen, however, is not directly available from such sources; nor, in the absence of green plant synthesis, is it formed secondarily except in trivial and readily scavenged quantities from photolytic dissociation of CO₂ and H₂O. All who have considered the problem critically, therefore, agree that at an early stage in its history the terrestrial atmosphere was essentially anoxygenic, or reducing. Only at a later date, after the appearance of a photosynthetic source of oxygen could the atmosphere evolve towards its present oxygenic (and oxidizing) state.” It is fair to add that there is not universal agreement as to the nature of the earth’s atmosphere during past geological time. For example, Dimroth and Kimberley (1976) did not agree that an O₂-free atmosphere had existed at any time during the span of geological history recorded in well-preserved sedimentary rocks. However, such a stance is difficult to maintain in the face of abundant evidence which has been reviewed by workers such as McKirdy (1974), Biochenko et al. (1975) and Schidlowski et a1. (1975) all of whom, and others, have contributed to the hypothesis of a primordial atmosphere, possibly largely of methane, subsequently modified to being CO₂-rich by the addition of juvenile gases from the outgassing of the earth. Oxygen, generated both photolytically and photosynthetically, was probably a quite minor component of the atmosphere for a considerable period before oxidizable minerals became fully accessible to 02 in the atmosphere and in the hydrosphere.