Waiting for O 2

Oxygenic photosynthesis appears to be necessary for an oxygen-rich atmosphere like Earth’s. However, available geological and geochemical evidence suggests that at least 200 m.y., and possibly as many as 700 m.y., elapsed between the advent of oxygenic photosynthesis and the establishment of an oxygen atmosphere. The interregnum implies that at least one other necessary condition for O2 needed to be met. Here, we argue that the second condition was the oxidation of the surface and crust to the point where free O2 became more stable than competing reduced gases such as CH4, and that the cause of Earth’s surface oxidation was the same cause as it is for other planets with oxidized surfaces: hydrogen escape to space. The duration of the interregnum was determined by the rate of hydrogen escape and by the size of the reduced reservoir that needed to be oxidized before O2 became favored. We speculate that hydrogen escape determined the history of continental growth, and we are confi dent that hydrogen escape provided a progressive bias to biological evolution. *[email protected] Zahnle, K., and Catling, D., 2014, Waiting for O 2 , in Shaw, G.H., ed., Earth’s Early Atmosphere and Surface Environment: Geological Society of America Special Paper 504, p. 37–48, doi:10.1130/2014.2504(07). For permission to copy, contact [email protected]. © 2014 The Geological Society of America. All rights reserved. INTRODUCTION This volume addresses Earth from its beginnings in the Hadean ca. 4.4 Ga to the rise of oxygen in the Paleoproterozoic a mere 2.2 b.y. ago. Two very interesting things happened on Earth during the fi rst half of its history: (1) life began, and (2) later, and perhaps a bit less important, the atmosphere began to fi ll up with oxygen. From the perspective of its inhabitants, these may be the two most important events in Earth’s history. Neither is well understood. Oxygen raises two issues that are usefully separated. One is the matter of abundant O 2 , which is the distinctive feature of Earth’s atmosphere. The other is the oxidation of the surface and atmosphere, a state that is more widespread in the solar system. Oxygen and oxidation are different things and refl ect different processes acting on different time scales, although it is plausible that one is prerequisite to the other. It could be that it was free oxygen in the atmosphere that oxidized the surface, or it could be that oxidation of the surface allowed free oxygen to endure. Here, we presume that surface oxidation is a prerequisite to O 2 . It has long been considered probable from hints in the geological record that oxygenic photosynthesis appeared much earlier than widespread crustal oxidation (Holland, 1962; Buick, 2008), and thus that surface oxidation played a role in the rise of oxygen (Berkner and Marshall, 1965). Where we go beyond Berkner and Marshall is that we give a reason: Oxygenation is caused by the on May 11, 2015 specialpapers.gsapubs.org Downloaded from