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Waiting for O2 Downloaded from specialpapers.gsapubs.org on May 11, 2015 The Geological Society of America Special Paper 504 2014 Waiting for O2 Kevin Zahnle* Space Science Division, National Aeronautics and Space Administration Ames Research Center, MS 245-3, Moffett Field, California 94035, USA David Catling Department of Earth and Space Sciences/Astrobiology Program, University of Washington, Seattle, Washington 98195, USA ABSTRACT 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 inter- regnum 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. INTRODUCTION atmosphere, a state that is more widespread in the solar system. Oxygen and oxidation are different things and refl ect different This volume addresses Earth from its beginnings in the processes acting on different time scales, although it is plausible Hadean ca. 4.4 Ga to the rise of oxygen in the Paleoproterozoic a that one is prerequisite to the other. It could be that it was free mere 2.2 b.y. ago. Two very interesting things happened on Earth oxygen in the atmosphere that oxidized the surface, or it could during the fi rst half of its history: (1) life began, and (2) later, be that oxidation of the surface allowed free oxygen to endure. and perhaps a bit less important, the atmosphere began to fi ll up Here, we presume that surface oxidation is a prerequisite to O2. with oxygen. From the perspective of its inhabitants, these may It has long been considered probable from hints in the geological be the two most important events in Earth’s history. Neither is record that oxygenic photosynthesis appeared much earlier than well understood. widespread crustal oxidation (Holland, 1962; Buick, 2008), and Oxygen raises two issues that are usefully separated. One thus that surface oxidation played a role in the rise of oxygen is the matter of abundant O2, which is the distinctive feature of (Berkner and Marshall, 1965). Where we go beyond Berkner and Earth’s atmosphere. The other is the oxidation of the surface and Marshall is that we give a reason: Oxygenation is caused by the *[email protected] Zahnle, K., and Catling, D., 2014, Waiting for O2, 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. 37 Downloaded from specialpapers.gsapubs.org on May 11, 2015 38 Zahnle and Catling steady loss of hydrogen to space. In our story, the apparent delay indicators of the fi rst appearance of an oxic atmosphere (Hol- between the origin of oxygenic photosynthesis and the establish- land, 1999; Kump, 2008; Guo et al., 2009; Bekker and Holland, ment of an oxygenated atmosphere is explained by how long it 2012). Mars’s surface is locally characterized by the presence of took to oxidize the surface through the steady loss of hydrogen to strong oxidants such as peroxides (seen by Viking; Hunten, 1979) space (Catling et al., 2001; Claire et al., 2006). and perchlorates (seen by Phoenix; Hecht et al., 2009), and on a This chapter is not intended to give an even-handed over- global scale, there are extensive and locally thick sulfate depos- view of the history of oxygen, if such were possible. The intent its (encountered by the Mars exploration rovers on the ground of the October 2011 Pardee symposium on which this volume is and mapped by satellites from above). Hydrogen escape is fast based was for the speakers to take clear and opposing points of enough on Mars to generate its modest atmospheric reservoir 5 view. This essay holds to the original intent. Our point of view is of O2 in just 10 yr (Nair et al., 1994; Zahnle et al., 2008). On as stated earlier, but it harms nothing to state it again more force- Mars, the oxidation appears to be quite shallow, with no evidence fully: We think that oxygenation of Earth was caused by hydro- that oxidation extends to the mantle or even to the deeper crust. gen escape. Hydrogen escape stepped Earth through a series of If so, planetary oxidation would have been quick (Hartman and titratable reservoirs and provided the bias that drove biological McKay, 1995). evolution to develop the O2 atmosphere. Readers seeking a mod- In contrast to the other worlds of the solar system, most ern, broadly encompassing review of oxygen that is relatively accounts of the rise of oxygen on Earth either marginalize or free of overt biases would be well served to read Farquhar and ignore the role of hydrogen escape in oxidizing the surface. Johnston (2008). Rather, oxidation of the surface is usually ascribed to a shrink- ing infl uence of reduced volcanic gases from a reduced mantle OXIDATION compared to the accumulation of reduced carbon (from CO2) in continents, or to an enhanced role of continents versus the mantle More than 60 years ago, Harold C. Urey (1952, p. 352) in weathering, or both, although how the mechanism(s) might wrote that the “highly oxidized condition is rare in the cosmos work is debated (Holland, 1962; Kasting et al., 1993; Kump et and exists in the surface regions of the Earth and probably only al., 2001; Holland, 2002, 2009; Kump and Barley, 2007; Gaillard in the surface regions of Venus and Mars. Beyond these we know et al., 2011). A variant that is easier to understand posits the pref- of no highly oxidized regions at all, although undoubtedly other erential subduction of reduced matter by the mantle (Hayes and localized regions of this kind exist.” His underlying interest in the Waldbauer, 2006). The variant is easier to understand because, matter was in the conditions pertinent to the spontaneous origin like H escape, it actually oxidizes the surface. of life. Urey was greatly infl uenced by Oparin’s arguments in What these theories have in common is the primacy they favor of an anaerobic origin of life. Oparin regarded a prebiotic assign to secular cooling in Earth’s evolution. Secular cooling source of organic molecules as essential to the phenomenon, and is undeniable and has been viewed as a major driver of plan- therefore concluded that Earth’s fi rst atmosphere was strongly etary evolution since the nineteenth century, if not before. For reducing (Urey, 1952). Urey argued that, because hydrogen is the example, Lowell (1908) provides a good general overview of the most abundant element in the cosmos, a reduced atmosphere is hypothesis. Secular cooling is obviously a major factor setting to be expected at early times relevant to the origin of life, while the direction and pace of planetary evolution. Its consequences the “highly oxidized condition of planetary surfaces” is an evolu- are easy to see throughout the solar system. However, secular tionary result of hydrogen escape. In addition, Urey regarded the cooling does not by itself change the surface’s oxidation state. tension between the reduced interior and the oxidized surface as Hence, theories of oxygenation that start with secular cooling contributing to life’s subsequent evolution. This too is an impor- depend on second-order effects to produce oxidation. These are tant insight into the boundary conditions imposed by a planet on usually effects that decrease the infl uence of the mantle on the life and its evolution. surface over time, either through changes in the rate or style of It turns out that oxidation of planetary surfaces is not rare plate tectonics or in the quantity or quality of volcanic gases, or in the solar system. In all cases apart from Earth, the oxidation more indirectly through the consequences of continental growth is clearly caused by hydrogen escape. For example, several icy (which, if a fact, might have something to do with cooling). satellites (Ganymede, Europa, Rhea) have extremely thin O2 On the face of it, the idea that the mantle oxidizes the sur- atmospheres derived from splitting water molecules in ice by face is rather puzzling. The gases that come from the mantle are ultraviolet (UV) photons or energetic particle bombardment reducing and have always been reducing, because the mantle is (Cruikshank, 2010). Hydrogen escapes easily, but oxygen, which reduced. That they might have been more or less reducing in the is much heavier, does not. As a consequence, over time, the sur- past does not make them oxidizing. The reasonable expectation face ice and any contaminants in the ice become highly oxidized. is that the surface would be reduced by these gases. Why the Mars provides a more Earth-like example. Mars is red sum of second-order effects should favor oxidizing the surface because much of the iron at its surface is oxidized.
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