Atmospheric Composition and Climate on the Early Earth James F. Kasting Department of Geosciences Penn State University

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

Geologic time First shelly fossils (Cambrian explosion) Snowball Earth ice ages “Snowball Earth” episodes may have occurred during both the early and late Proterozoic Eon, based on evidence for low-latitude glaciation. Warm Rise of atmospheric O2 (Ice age) Ice age Origin of life Warm (?)

From a theoretical standpoint, it is curious that the early Earth was warm, because the Sun is thought to have been less bright 

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)

The faint young Sun problem The best solution to this problem is higher concentrations of greenhouse gases in the distant past Kasting et al., Scientific American (1988)

Greenhouse gases Greenhouse gases are gases that let most of the incoming visible solar radiation in, but absorb and re-radiate much of the outgoing infrared radiation Important greenhouse gases on Earth are CO2, H2O, and CH4 H2O, though, is always near its condensation temperature; hence, it acts as a feedback on climate rather than as a forcing mechanism The decrease in solar luminosity in the distant past could have been offset either by higher CO2 or by reduced gases, such as NH3 and CH4

Sagan and Mullen liked ammonia (NH3) as an Archean greenhouse gas Sagan and Mullen, Science (1972) Sagan and Mullen liked ammonia (NH3) as an Archean greenhouse gas They were aware that atmospheric O2 was low on the early Earth 

Mass-independently fractionated S isotopes strongly “Conventional” geologic indicators show that atmospheric O2 was low prior to ~2.2 Ga Detrital H.D. Holland (1994) Mass-independently fractionated S isotopes strongly support this conclusion

Problems with Sagan and Mullen’s hypothesis Ammonia is photochemically unstable with respect to conversion to N2 and H2 (Kuhn and Atreya, 1979)

Owen, Cess, and Ramanathan, Nature (1979) A few years later, Owen et al. used an evolutionary sequence suggested by Michael Hart to show that CO2 (+ H2O) was capable of solving the faint young Sun problem -- Was this solution ad hoc, though, or are there reasons for thinking that CO2 levels were once much higher?

The carbonate-silicate cycle (metamorphism) Silicate weathering slows down as the Earth cools  atmospheric CO2 should build up This is probably at least part of the solution to the faint young Sun problem

CO2 vs. time if no other greenhouse gases (besides H2O) J. F. Kasting, Science (1993) In the simplest story, atmospheric CO2 levels should have declined monotonically with time as solar luminosity increased But, there are reasons to believe that this simple story may not be correct!

pCO2 from paleosols (2.8 Ga) Absence of siderite (FeCO3) places upper bound on pCO2 May need other green- house gases (CH4?) CO2 upper limit Rye et al., Nature (1995) Today’s CO2 level (310-4 atm)

Rosing et al. place constraints on pCO2 based on the mineralogy of banded iron-formations, or BIFs Siderite (FeCO3) and magnetite (Fe3O4) are found within the same units  pCO2 should lie near the phase boundary Hematite is found, as well, but they don’t talk about this! The implied atmospheric CO2 concentrations are really low! 

Rosing et al.: CO2 from BIFs Rye et al. Ohmoto Sheldon von Paris et al. If the new CO2 constraints are correct, then other warming mechanisms are clearly needed Rosing et al. suggest a reduced cloud albedo caused by the absence of biogenic sulfur gases, but this doesn’t work if the climate was as warm or warmer than today J. F. Kasting, Nature, Apr. 1

Let’s think a little more about how the BIFs might have formed...

BIF depositional environment (Early Proterozoic, Kuruman Iron Formation, South Africa) Klein & Beukes, Econ. Geol. (1989) Siderite forms in the near-shore environment Hematite forms off-shore Magnetite forms somewhere in-between

Possible Archean BIF depositional environment pH2  10-4 atm 4 Fe++ + CO2 +11 H2O  4 Fe(OH)3 + CH2O + 8 H+ Iron-oxidizing bacteria (photosynthetic) FeCO3 Iron-reducing bacteria pH2  310-6 atm 3 Fe(OH)3 + 0.5 H2  Fe3O4 + 5 H2O Fe3O4 Hydrogen minimum zone Fe2O3

Rosing et al., Nature (2010) Quoted upper limit on pCO2 Limit at magnetite- hematite boundary Rosing et al., Nature (2010)

pCO2 estimates from BIFs Bottom line: pCO2  210-3 atm (6 PAL) at 25oC Roughly 2.5 times that (15 PAL) at 35oC  Not nearly enough to solve the faint young Sun problem by itself!

So, we could really use another greenhouse gas or two This brings us back to methane…

CH4 has a strong absorption band at 7.7 m, right in the edge of the Window region CH4 has a strong absorption band at 7.7 m, right in the edge of the 8-12 m “window” region where H2O and CO2 have weak absorption So, methane is a reasonably strong greenhouse gas Figure courtesy of Abe Lerman, Northwestern Univ.

Today, CH4 is produced mainly in restricted, anaerobic environments, such as the intestines of cows and the water-logged soils underlying rice paddies Methanogenic bacteria (methanogens) are responsible for most methane production Methanogens are probably evolutionarily ancient  

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

Additional evidence for ancient biogenic methane? Ueno et al. (Nature, Mar., 2006) have found isotopically light (56‰) CH4 in fluid inclusions in 3.5-b.y.-old rocks from the Pilbara craton in Australia Carbon has 2 stable isotopes: 12C and 13C “Isotopically light” means depleted in 13C There are conflicting reports as to whether such isotopically light CH4 could be produced by abiogenic (Fischer-Tropsch type) synthesis

But, its lifetime in a low-O2 atmosphere would be ~1000 times longer Furthermore… Photochemical lifetime of CH4 is relatively short (~10 years) today, because it reacts with the hydroxyl radical, OH: O3 + h  O2 + O(1D) ( < 310 nm) O(1D) + H2O  2 OH OH + CH4  CH3 + H2O But, its lifetime in a low-O2 atmosphere would be ~1000 times longer For all of these reasons, we think that methane was probably abundant in the Archean atmosphere  1000 ppmv, or more

If methane was abundant in the early atmosphere, it should have affected climate…

CH4-CO2 greenhouse Paleosol data (Rye et al.) 10-2 10-3 10-4 10-5 fCH4 = 0 273.15 K 2.8 Ga S/So = 0.8 Still, the implied abundance of CO2 during the Late Archean may exceed observational constraints J. Haqq-Misra et al., Astrobiol. (2008)

But, there should also have been other, higher hydrocarbons in the low-O2 Archean atmosphere…

Low-O2 atmospheric model Ethane formation: 1) CH4 + h  CH3 + H or 2) CH4 + OH  CH3 + H2O Then 3) CH3 + CH3 + M  C2H6 + M “Standard”, low-O2 model from Pavlov et al. (JGR, 2001) 2500 ppmv CO2, 1000 ppmv CH4  8 ppmv C2H6

Important ethane band Ethane (C2H6) absorbs within the 8-12 m “window” region When ethane is included in the climate model, most of the original greenhouse warming is recovered

CH4/CO2/C2H6 greenhouse No C2H6 J. Haqq-Misra et al., Astrobiology (2008)

CH4/CO2/C2H6 greenhouse No C2H6 C2H6 included If one includes ethane, then one predicts large amounts of warming, particularly at high CH4/CO2 ratios J. Haqq-Misra et al., Astrobiology (2008)

CH4/CO2/C2H6 greenhouse No C2H6 C2H6 included If one includes ethane, then one predicts large amounts of warming, particularly at high CH4/CO2 ratios But, high CH4/CO2 ratios also trigger the formation of organic haze J. Haqq-Misra et al., Astrobiology (2008)

Titan’s organic haze layer Haze is thought to form from photolysis (and charged particle irradiation) of CH4 It can produce an anti-greenhouse effect (Picture from Voyager 2)

Anti-greenhouse effect Incoming visible/near-IR radiation mostly makes its way through the atmosphere Outgoing thermal-IR radiation is absorbed and re-radiated Net effect is to warm the surface Anti-greenhouse effect Incoming visible/near-IR radiation is absorbed and re-radiated high in the atmosphere Net effect is to cool the surface

CH4/CO2/C2H6 greenhouse with haze Paleosols Water freezes When cooling by the haze is included, the required CO2 partial pressure still exceeds the published paleosol limit

CH4/CO2/C2H6 greenhouse with haze Late Archean Earth? Paleosols Water freezes Present CO2 100  Present If the Archean climate was as warm as today, and with no significant albedo feedback, we still need roughly 100 PAL of CO2 for this mechanism to work (way above what is allowed by Rosing et al.)

New idea (E. Wolf and O.B. Toon, in press) Shield the lower atmosphere with fractal organic haze particles Haze particles can be treated as large spheres composed of thousands of much smaller spheres The smaller spheres block out short wavelengths of light, i.e., UV light! This has the potential to make ammonia stable in the lower atmosphere, so maybe Carl Sagan was right after all!

Best evidence for the reduced greenhouse gas hypothesis: When atmospheric O2 levels rose, most of the CH4 (and NH3) was lost, and the greenhouse effect decreased, triggering glaciation 

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

Conclusions Both methane and ethane, and possibly ammonia, may have contributed to the greenhouse effect back when atmospheric O2 levels were low High atmospheric CH4/CO2 ratios can trigger the formation of organic haze. This has a cooling effect. But it can also block out solar UV radiation very efficiently! The Paleoproterozoic glaciation at ~2.4 Ga may have been triggered by the rise of O2 and loss of the methane component of the atmospheric greenhouse

Backup slides

33S versus time 73 Phanerozoic samples Requires photochemical reactions (e.g., SO2 photolysis) in a low-O2 atmosphere Farquhar et al., Science, 2000

Updated sulfur MIF data (courtesy of James Farquhar) Includes new data at 2.8 Ga and 3.0 Ga from Ohmoto et al., Nature (2006) and Farquhar et al., Nature (2007) New low- MIF data glaciations

Mechanism for producing sulfur MIF The SO2 absorption spectrum exhibits strong ro-vibronic structure between 190 and 220 nm The bands shift in wavelength when different SO2 isotopologues are present, leading to a phenomenon referred to as self-shielding J. Lyons, GRL (2007)

slope of their absorption x-sections in the 190-220 nm region PNAS, 2009 190 200 210 220 Wavelength (nm) Shielding by OCS (or O3) produces the right sign for 33S because of the slope of their absorption x-sections in the 190-220 nm region