1. School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
2. Centre for Ecology and Hydrology, Edinburgh Research Station, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK

Correspondence to: Colin Goldblatt^1,2 Correspondence and requests for materials should be addressed to C.G. (Email: c.goldblatt@uea.ac.uk).

Abstract

The history of the Earth has been characterized by a series of major transitions separated by long periods of relative stability¹. The largest chemical transition was the 'Great Oxidation', approximately 2.4 billion years ago, when atmospheric oxygen concentrations rose from less than 10^-5 of the present atmospheric level (PAL) to more than 0.01 PAL, and possibly² to more than 0.1 PAL. This transition took place long after oxygenic photosynthesis is thought to have evolved^{3, 4, 5}, but the causes of this delay and of the Great Oxidation itself remain uncertain^{6, 7, 8, 9, 10, 11}. Here we show that the origin of oxygenic photosynthesis gave rise to two simultaneously stable steady states for atmospheric oxygen. The existence of a low-oxygen (less than 10^-5 PAL) steady state explains how a reducing atmosphere persisted for at least 300 million years after the onset of oxygenic photosynthesis. The Great Oxidation can be understood as a switch to the high-oxygen (more than 5 10^-3 PAL) steady state. The bistability arises because ultraviolet shielding of the troposphere by ozone becomes effective once oxygen levels exceed 10^-5 PAL, causing a nonlinear increase in the lifetime of atmospheric oxygen. Our results indicate that the existence of oxygenic photosynthesis is not a sufficient condition for either an oxygen-rich atmosphere or the presence of an ozone layer, which has implications for detecting life on other planets using atmospheric analysis^{12, 13} and for the evolution of multicellular life.

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