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 O 2 needed to be met. Here, we argue that the second condition was the oxidation of the surface and crust to the point where free O 2 became more stable than competing reduced gases such as CH 4 , 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 O 2 became favored. We speculate that hydrogen escape determined the history of continental growth, and we are confident that hydrogen escape provided a progressive bias to biological evolution.

Models for noble gases in the Earth's mantle are evaluated against a number of observational constraints: (1) high 3 He/ 4 He ratios do not correlate with high (initial) 3 He concentrations; (2) the 3 He/ 4 He data for mid-ocean ridge basalts and ocean island basalts do not represent two different distributions (Anderson 2001); (3) globally robust correlations between 3 He/ 4 He ratios and lithophile isotopic systems are not observed; (4) diverse local correlations exist that are broadly linear; (5) large, local geographical 3 He/ 4 He variations are observed, which are inconsistent with a strongly localized (i.e., plume-stem) flux of high- 3 He/ 4 He material; and (6) dramatic temporal 3 He/ 4 He variations are observed on very short time scales (10 2 years). Layered (reservoir) models for noble gases, in which a deep and radially constrained region of the Earth's mantle preserves unradiogenic He and Ne isotopic compositions because of a high noble gas concentration, do not seem consistent with these observations. Heterogeneous (nonlayered) mantle models for noble gases, in which the carrier of unradiogenic He is a relatively noble gas–poor component scattered in the (upper) mantle, appear more consistent with the constraints. We propose that the carrier of unradiogenic noble gases is primarily olivine. Olivine-rich lithologies, produced in previous partial melting events, are a natural part of the statistical upper mantle assemblage (SUMA), a highly heterogeneous assemblage of small- to moderate-scale (∼1–100 km) enriched and depleted lithologies with a wide range of chemical composition, fertility, age, and isotopic signatures (Meibom and Anderson, 2004). The isotopic signatures of oceanic basalts, including noble gases, are obtained by partial melting of the SUMA under slightly different pressure and temperature ( P-T ) conditions, i.e., different degrees of partial melting and different degrees of homogenization prior to eruption (Morgan and Morgan, 1999; Meibom and Anderson, 2004; Rudge et al., 2005; Ito and Mahoney, 2005). Unradiogenic noble gas isotopic compositions are not tracers of deep-mantle components in the source materials of oceanic basalts. Noble gas isotopic compositions may, however, indirectly indicate potential temperature, because the order in which different upper-mantle lithologies melt depends on the P-T conditions.

Abstract The continental cycle of silicate weathering and metamorphism dynamically buffers atmospheric CO 2 and climate. Feedback is provided by the temperature dependence of silicate weathering. Here we argue that hydrothermal alteration of oceanic basalts also dynamically buffers CO 2 . The oceanic cycle is linked to the mantle via subduction of carbonatized basalts and degassing of CO 2 at the mid-ocean ridges. Feedback is provided by the dependence of carbonatization on the amount of dissolved carbonate in sea water. Unlike the continental cycle, the oceanic cycle has no thermostat. Hence surface temperatures can become very low if CO 2 is the only greenhouse gas apart from water. Currently the continental cycle is more important, but early in Earth’s history the oceanic cycle was probably dominant. We argue that CO 2 greenhouses thick enough to defeat the faint early Sun are implausible and that, if no other greenhouse gases are invoked, very cold climates are expected for much of Proterozoic and Archaean time. We echo current fashion and favour biogenic methane as the chief supplement to CO 2 . Fast weathering and probable subduction of abundant impact ejecta would have reduced CO 2 levels still further in Hadean time. Despite its name, the Hadean Eon might have been the coldest era in the history of the Earth.