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

2. 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

3. 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 (?)

4. 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 

5. 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)

6. 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

7. 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)

8. 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)

9. 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

10. 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

11. 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 

12. 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

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

14. 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)

15. “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

16. 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

17. 33S versus time High O2 Low O2 Farquhar et al., Science, 2000
73 Phanerozoic samples
Farquhar et al., Science, 2000

18. 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)

19. 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

20. 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)]

21. 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..

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

23. Anoxic ecosystem modeling
Coupled photochemical-ecosystem modeling of an methanogen- or H2-based anoxygenic photosynthetic ecosystem predicts Archean CH4 concentrations of ppm
This is enough to produce 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)

24. 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)

25. 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 (?)

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

27. 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 ( 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