Dinitrogen may have tracked O 2 levels due to an oxidative weathering and denitrification source of N 2, but pN 2 changes are debated. ( A) Uncertainties on gas concentrations are a factor of a few or more as detailed in Table 1, the text, and the other figures. The curve for ~0.4 Ga ago to present is from ( 265). Note that the suggestion that moderately high levels of methane may have contributed to greenhouse warming in the Proterozoic ( 260, 261) has been disputed ( 262, 263) and may depend on fluxes from sources on land ( 264). Orange shading is schematic but consistent with possible biological CH 4 fluxes into atmospheres of rising O 2 levels at the GOE and in the Neoproterozoic. The black curve is from a biogeochemical box model coupled to photochemistry ( 121). Constraints include a lower limit (blue) required for Archean S-MIF ( 44) and a tentative lower limit of ~3.5 Ga ago from a preliminary interpretation of xenon isotopes (black) ( 187). Various Precambrian pCO 2 proxy estimates are shown ( 42, 43, 168, 257– 259).
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( A) The black line is median CO 2 from a carbonate-silicate climate model, and yellow shading indicates its 95% confidence interval ( 34) this curve merges with a fit to CO 2 proxy estimates for 0.42 Ga ago to present from ( 256). Blue shading shows a schematic and speculative pN 2 range in different time intervals consistent with very sparse proxy data.
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( B) Constraints on surface atmospheric pressure (red) ( 56, 57) and the partial pressure of nitrogen, pN 2 (blue) ( 48, 49, 140). The post-Devonian black line for O 2 evolution approximately represents curves from calculations of C and S isotopic mass balance ( 254, 255). Charcoal since 0.4 Ga ago implies a lower bound of >0.15 bar (purple) ( 253). An Archean upper bound of 0.02-bar O 2 (dark green) is from plausible O 2 demands of macroscopic Ediacaran and Cambrian biota ( 120). ( A) Colored arrows faithfully represent known O 2 constraints, but the black line is speculative. Lack of mass-dependent sulfur isotope fractionation ( 75)īy analogy to the deep, anoxic Black Sea ( 272)įrom the N content of biotites (originally clays),ĭerived from adsorption of dissolved NH 4 +(aq) ( 148)īased on solubility constraints of Fe 2+ ( 139)įrom seawater fluid inclusions in quartz ( 50)
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Thermodynamic limit if microbes used available free energy Lower limit for sufficient reductant for S-MIF ( 44)Įnough methane to induce sufficiently rapid hydrogenĮscape to drag Xe + and fractionate Xe isotopes ( 187) Siderite weathering rinds on river gravel ( 257)Īlpine Lake paleosol, MN, United States ( 168)
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Lightning production of NO and HNO in a reducingĪtmosphere, dissolution of HNO, and evaporation Photochemistry if CH 4 was ~10 3 ppmv ( 166, 167) Modeled S 8 flux needed to create and carry S-MIF ( 44) Despite these advances, detailed understanding of the coevolving solid Earth, biosphere, and atmosphere remains elusive, however. These data imply that substantial loss of hydrogen oxidized the Earth. Isotopic mass fractionation of atmospheric xenon through the Archean until atmospheric oxygenation is best explained by drag of xenon ions by hydrogen escaping rapidly into space.
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Climate moderation by the carbon cycle suggests average surface temperatures between 0° and 40☌, consistent with occasional glaciations. The greenhouse gas concentrations were sufficient to offset a fainter Sun. They imply surface O 2 levels <10 −6 times present, N 2 levels that were similar to today or possibly a few times lower, and CO 2 and CH 4 levels ranging ~10 to 2500 and 10 2 to 10 4 times modern amounts, respectively. New geological proxies combined with models constrain atmospheric composition. The atmosphere of the Archean eon-one-third of Earth’s history-is important for understanding the evolution of our planet and Earth-like exoplanets.