Evolution of Earth’s Atmosphere and Climate James Kasting Department of Geosciences Penn State University

Phanerozoic Time Ice age (Pleistocene) Dinosaurs go extinct First dinosaurs Ice age Age of fishes First vascular plants on land Ice age First shelly fossils

Geologic time First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm (The ‘Boring Billion’) Rise of atmospheric O2 (Ice age) Ice ages ‘Conventional’ interpretation of the Precambrian climate record Origin of life Warm (?)

The fact that most of the Precambrian appears to have been warm is remarkable, because the Sun is thought (by essentially everyone) to have been less luminous early in Earth’s history 

The faint young Sun problem Te = effective radiating temperature = [S(1-A)/4]1/4 TS = average surface temperature Kasting et al., Scientific American (1988)

Greenhouse gases and CO2-climate feedbacks So, one needs more greenhouse gases, especially during the Archean CO2 is a prime candidate because it is part of a negative feedback loop (see panel at right) We should be cautious about over-interpreting this model, though, because land area may have been much smaller during the Archean Diagram illustrating the (modern) carbonate-silicate cycle. Atmospheric CO2 increases when the climate cools because of slower rates of silicate weathering on land

Is CO2 the solution to the FYS problem? Thus, high CO2 levels could, in principle, have solved the FYS problem (see calculation at right) Unfortunately, geochemists have made this problem more difficult by attempting to measure paleo-CO2 concentrations… J. F. Kasting, Science (1993)

Precambrian pCO2 from paleosols First estimate for Archean pCO2 was published by Rye et al. (1995) Criticized by Sheldon (2006) Can’t use thermodynamic arguments when the entire suite of minerals is not present He presented an alternative analysis of paleosols based on mass balance arguments (efficiency of weathering) If Sheldon and Driese are right about Precambrian CO2 levels, then other greenhouse gases would have been needed to keep the early Earth from freezing But, a new analysis method has recently been published.. Driese et al., 2011 (10-50 PAL) N. Sheldon, Precambrian Res. (2006)

Sheldon’s method New method Geochimica et Cosmoschimica Acta 159, 190 (June, 2015) Sheldon’s method Mass balance on soil silicates (following Holland and Zbinden, 1988) Involves assumptions about soil porosity, lifetime New method Detailed chemical modeling of porewater composition, pH. Involves multiple assumptions about soil and groundwater parameters

K&M paleosol analysis: ancient soils Kanzaki & Murakami, GCA (2015) If the new paleosol analysis is correct, then CO2 could have been high enough to solve the faint young Sun problem by itself

Thus, high CO2 may or may not have been sufficient to offset the faint young Sun during the Archean, depending on whose paleosol interpretations are correct That said, there are additional reasons to think that other greenhouse gases (CH4) might have been important, the main one being that atmospheric O2 levels were low prior to ~2.5 Ga 

Conventional geologic O2 indicators (Detrital) H. D. Holland (1994) Blue boxes indicate low O2 Red boxes indicate high O2 Dates have been revised; the initial rise of O2 is now placed at 2.45 Ga

The conventional story about the rise of O2 has received strong support in recent years from studies of multiple sulfur isotopes…

S isotopes and the rise of O2 Sulfur has 4 stable isotopes: 32S, 33S, 34S, and 36S Normally, these separate along a standard mass fractionation line In very old (Archean) sediments, the isotopes fall off this line Requires photochemical reactions in a low-O2 atmosphere SO2 + h  SO + O (photolysis: 190 nm <  < 220 nm) SO2 + h  SO2* (photoexcitation: 240 nm <  < 320 nm) This produces “MIF” (mass-independent fractionation)

“Normal” isotope mass fractionation Vibrational energy levels depend inversely on the reduced mass  = (k/mR)1/2 En = (n+½) h Increasing the mass of one or both atoms decreases the vibrational frequency and energy, thereby strengthening chemical bonds A simple harmonic oscillator

S isotopes in Archean sediments Farquhar et al. (2001) (FeS2) (BaSO4) 33S Sulfides (pyrite) fall above the mass fractionation line Sulfates (barite) fall below it

33S versus time High O2 Low O2 Farquhar et al., Science, 2000 73 Phanerozoic samples Farquhar et al., Science, 2000

Sulfur MIF record The Cloud/Holland interpretation of the rise of O2 is strongly supported by the record of sulfur ‘mass-independent’ isotope fractionation, which shows that atmospheric O2 was low prior to ~2.45 Ga This does not preclude the possibility of ‘whiffs’ of O2 (Ariel Anbar’s term) during the Archean, for which there is geochemical evidence Reinhard et al., Nature (2013) (Technique pioneered by Farquhar et al., Science, 2000)

Question: What does the sulfur MIF tell us? Must have had low enough O2 (and O3) to allow SO2 to be photolyzed Must have had low enough O2 to prevent all volcanic SO2 from being oxidized to sulfate, as it is today

Archean sulfur cycle In a low-O2 atmosphere, volcanic SO2 can be either oxidized or reduced (or it can exit the atmosphere as SO2) By contrast, today, virtually all SO2 is oxidized to sulfate; thus, any MIF signal is eliminated by homogenization Kasting, Science (2001) [Redrawn from Kasting et al., OLEB (1989)]

Back to climate… Thus, methane could also have been an important greenhouse gas during the Archean Its lifetime is long in a low-O2 atmosphere It’s a moderately good greenhouse gas (but not nearly as good as CO2, contrary to popular opinion) The methanogens that produce it are thought to be evolutionarily ancient..

(rRNA) tree of life Methanogenic bacteria “Universal” Courtesy of Root (?) “Universal” (rRNA) tree of life Courtesy of Norm Pace

Anoxic ecosystem modeling Coupled photochemical-ecosystem modeling of an methanogen- or H2-based anoxygenic photosynthetic ecosystem predicts Archean CH4 concentrations of 200-2000 ppm This is enough to produce 10-15 degrees of greenhouse warming Higher warming by CH4 is precluded by the formation of organic haze at CH4/CO2 ratios greater than ~0.1 Kharecha et al., Geobiology (2005)

Archean CH4-CO2 greenhouse Diagram shows a hypothetical Archean atmosphere at 2.8 Ga The black curves show predicted surface temperatures with zero and 1000 ppm of CH4 The loss of much of this CH4 at ~2.5 Ga could plausibly have triggered the Paleoproterozoic glaciations 2.8 Ga S/So = 0.8 Driese et al. (2011) J.F. Kasting, Science (2013)

Geologic time First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm (The ‘Boring Billion’) Rise of atmospheric O2 (Ice age) Ice ages ‘Conventional’ interpretation of the Precambrian climate record Origin of life Warm (?)

Huronian Supergroup (2.2-2.45 Ga) High O2 Redbeds Glaciations Detrital uraninite and pyrite Low O2 S. Roscoe, 1969

Conclusions Earth’s early climate was probably kept warm by a combination of higher CO2 and CH4 Interpretations of paleosols presently give conflicting estimates of Precambrian CO2 levels Atmospheric O2 was certainly low most of the time prior to ~2.5 Ga, and CH4 levels were almost certainly high (200-2000 ppmv) Life plays a role in climate regulation, but Earth should remain habitable even without it The carbonate-silicate cycle plays a key role in Earth’s climate stability, especially in countering the faint young Sun problem