Gaia Revisited: The Interplay Between Climate and Life on the Early Earth

1. Gaia Revisited:The Interplay Between Climate and Life on the Early Earth James F. Kasting Department of Geosciences Penn State University

2. 1979 1988 The Gaia Hypothesis • Earth’s climate is regulated • by (and for?) the biota James Lovelock Lynn Margulis

3. Hypothetical gray world partly covered by white daisies Surface temperature controlled by albedo feedback loop  Daisyworld

4. Equilibrium States = points where lines cross B White daisy cover (%) P1 P2 A 20 Surface temperature (oC)

5. Systems Notation = system component = positive coupling = negative coupling

6. % white daisy cover % white daisy cover Surface temperature Surface temperature Daisyworld feedback loop The feedback loop is negative at point P1 () The feedback loop is positive at point P2 (+)

7. Equilibrium States = points where lines cross B Stable White daisy cover (%) P1 Unstable P2 A 20 Surface temperature (oC)

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

9. Origin of life Geologic time First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Rise of atmospheric O2(Ice age) Ice age Warm (?)

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

11. The Faint Young Sun Problem Kasting et al., Scientific American (1988) • The problem is amplified by water vapor feedback

12. Positive Feedback Loops(Destabilizing) Water vapor feedback Surface temperature Atmospheric H2O (+) Greenhouse effect

13. Snow/ice albedo feedback Surface temperature Snow and ice cover (+) Planetary albedo Positive feedback loops • This feedback loop was not included, but it makes • the actual problem even worse and can lead to • Snowball Earths under some circumstances

14. The Faint Young Sun Problem Kasting et al., Scientific American (1988) • The problem is amplified by water vapor feedback • The best solution to this problem is higher concentrations • of greenhouse gases in the distant past

15. 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, higher CH4, or both 

16. The Carbonate-Silicate Cycle (metamorphism) • Silicate weathering slows down as the Earth cools •  atmospheric CO2 should build up

17. Negative feedback loops(stabilizing) The carbonate-silicate cycle feedback Rainfall Surface temperature Silicate weathering rate (−) Greenhouse effect Atmospheric CO2

18. Thus,atmospheric CO2 levels may well have been higher in the distant past. However, there are good reasons for believing that CH4was abundant as well • The first is that atmospheric O2levels were low…

19. Geologic O2 Indicators (Detrital) H. D. Holland (1994)

20. S isotopes and the rise of O2 • Even stronger evidence for the rise of O2 comes from sulfur isotopes in ancient rocks • 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

21. S isotopes in Archean sediments (FeS2) (BaSO4) Farquhar et al. (2001)

22. 33S versus time 73 Phanerozoic samples • Requires photochemical reactions (e.g., SO2 photolysis) • in a low-O2 atmosphere Farquhar et al., Science, 2000

23. Archean Sulfur Cycle Kasting, Science (2001) Redrawn from Kasting et al., OLEB (1989) Original figure drafted by Kevin Zahnle

24. Back to methane… • A second reason for suspecting that CH4 was abundant on the early Earth is that the biological source of (most)* methane is probably ancient *Some methane comes from plants!

25. Today, CH4 is produced (mainly) in restricted, anaerobic environments, such as the intestines of cows and the water-logged soils underlying rice paddies • Methanogenic bacteriaare responsible for (most) methane production • One common metabolic reaction: CO2 + 4 H2  CH4 + 2 H2O 

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

27. Archean CH4 concentrations • It can be shown that CH4 production rates in an anaerobic biosphere could have been comparable to today (P. Kharecha et al., Geobiology, 2005) • Because O2 was low, the lifetime of methane would have been long (~10,000 yrs), and CH4 could have accumulated to 1000 ppmv or more (as compared to 1.7 ppmv today) • This is enough to produce a substantial modest greenhouse effect 

28. Late Archean Earth? Old calculation* *This calculation was wrong. The CH4 absorption bands were inadvertently shifted  Pavlov et al., JGR (2000)

29. Window region • The CH4 absorption spectrum was inadvertently shifted longward • in our earlier model, moving the 7.7-m band further into the • 8-12 m “window” region Figure courtesy of Abe Lerman, Northwestern Univ.

30. But, there may have been other IR-active hydrocarbon gases in addition to CH4

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 band • Ethane (C2H6) absorbs within the 8-12 m “window” region • This may provide a way to recover some of the warming • that we’re not getting from CH4

33. Complete C2H6 Calculations Paleosol data Limit of pCH4 > pCO2 10-2 273.15 K 10-3 10-4 10-5 fCH4 = 0 Calculations by Jacob Haqq-Misra

34. One of the most attractive features of the methane greenhouse model is that it correlates well with the glacial record 

35. Origin of life Geologic time First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Rise of atmospheric O2(Ice age) Ice age Warm (?)

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

37. The rise of O2 was caused by cyanobacteria, so the Huronian glaciation was triggered by a Gaian mechanism Trichodesmium bloom

38. cyanobacteria Interpretation (due to Lyn Margulis): Chloroplasts resulted fromendosymbiosis

39. More recently, we have been trying to explain the climate of the Mid-Archean…

40. Origin of life Geologic time First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Rise of atmospheric O2(Ice age) Ice age Warm (?)

41. Interestingly, the mid-Archean glaciation appears to coincide with an anomaly in the sulfur MIF data 

42. 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) New low- MIF data glaciations

43. Titan’s organic haze layer • Both the low-MIF values and the glaciation at 2.9 Ga may have been caused by the production of photochemical haze • Absorption of incident solar radiation by the haze gives rise to an anti-greenhouse effecton Titan, and perhaps on early Earth, as well S. Goldman et al., in revision

44. Possible Archean climate control loop(pre-oxygenic photosynthesis) CH4 production (–) Surface temperature Haze production Atmospheric CH4/CO2 ratio (–) CO2 loss (weathering)

45. “Daisyworld” diagram for mid-Archean climate Greenhouse effect Haze formation • P2 • Point P2 is stable • the mid-Archean Earth may have been covered in organic haze Surface temperature • P1 (Credit Bill Moore for the figure) Atmospheric CH4 concentration

46. Explaining Archean climate trends • Then, speculatively, oxygenic photosynthesis was invented around 2.8 Ga • Parts of the surface ocean became oxygenated • Marine sulfate levels increased • The methane source from marine sediments decreased • Atmospheric CH4 levels decreased as well  the organic haze layer disappeared

47. Organic C CH4 CH4 Sulfate reduction zone Mid-Archean ocean(pre-oxygenic photosynthesis) [O2]  0 [SO4=]  1 mM sediments Fermentation and methanogenesis zone

48. Organic C Modern ocean*(post-oxygenic photosynthesis) *Late Archean ocean would be somewhere in between this and the mid-Archean ocean [O2]  200 M [SO4=]  30 mM Aerobic decay zone Sulfate reduction zone CH4(little escapes) sediments Fermentation and methanogenesis zone

49. “Daisyworld” diagram for Late Archean climate Greenhouse effect Haze formation • • Point P1 is stable • the Late Archean Earth should have been haze-free P1 Surface temperature • P2 Atmospheric CH4 concentration

50. Conclusions • CH4 was probably an important greenhouse gas during the Archean (1000 ppmv or greater) • The Paleoproterozoic glaciations at ~2.3 Ga were likely triggered by the rise of O2 and a corresponding decrease in CH4 • The Mid-Archean glaciations at ~2.9 Ga may have been caused by the development of a thick organic haze layer • Understanding climate feedbacks is essential to understand Earth’s history • Gaia was alive and well on the early Earth