1. Research School of Physical Sciences, Australian National University, Canberra ACT 2600, Australia

Abstract

It has been argued^1–2 that the early Precambrian atmosphere contained a negligible amount of oxygen and that the transition to the modern oxygen-rich atmosphere began some 2,000 Myr BP when photosynthetic oxygen produced by blue-green algae oxidized ferrous iron in solution and escaped from the ocean to the atmosphere. Others³ have argued for an even earlier occurrence of an oxygen-rich atmosphere. Although photosynthesis has provided the oxygen of the present atmosphere, the photosynthetic model for Precambrian oxygen production has difficulties. There is the problem of identifying the large deposits of reduced carbon needed to account for the red beds⁴ and the predominant role of photosynthesis as an oxygen source in the early Precambrian has been questioned on both biochemical and geochemical grounds⁵. Before photosynthesis the only possible abiotic source of atmospheric oxygen would have been the direct photodissociation of water vapour by solar UV photons. What atmospheric oxygen level could have been produced by this purely abiotic process? It has been argued⁶ that the abiotic oxygen level was entirely negligible, 10^-12 PAL (present atmospheric level), but this assumed that the water vapour content of the palaeoatmosphere was the same as at present with a constant H₂O mixing ratio of 3.8 p.p.m. above an altitude of 10 km. I argue here that this is too restrictive an assumption and calculate possible atmospheric oxygen levels for larger water vapour mixing ratios which cannot be ruled out by our limited knowledge of Precambrian conditions. Even a moderate oxygen content for the early Precambrian atmosphere could have had significant evolutionary implications because a biologically effective UV ozone screen would be established once the oxygen content exceeded 10_-2 PAL (refs 7–11).