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

2. 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

3. 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 (?)

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

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. 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)

7. 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

8. 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 

9. 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

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

11. 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?

12. 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

13. 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!

14. 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)

15. 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! 

16. 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

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

18. 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

19. 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) H2  Fe3O4 + 5 H2O
Fe3O4
Hydrogen minimum zone
Fe2O3

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

21. 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!

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

23. 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.

24. 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  

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

26. 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

27. 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

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

29. CH4-CO2 greenhouse
Paleosol data
(Rye et al.)
10-2
10-3
10-4
10-5
fCH4 = 0 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)

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

31. 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

32. 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

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

34. 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)

35. 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)

36. 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)

37. 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

38. 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

39. 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.)

40. 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!

41. 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 

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

43. 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

44. Backup slides

45. 33S versus time
73 Phanerozoic samples
Requires photochemical reactions (e.g., SO2 photolysis)
in a low-O2 atmosphere
Farquhar et al., Science, 2000

46. 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

47. 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)

48. 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 nm region