1. Sustained Habitability on a Dynamic Early Earth James Kasting Department of Geosciences Penn State University

2. Talk Outline Part 1: Precambrian climate evolution— constraints on surface temperature and greenhouse gas concentrations Part 2: The rise of atmospheric O 2 and the enigma of low Proterozoic O 2 concentrations

3. after Bassinot et al. 1994 O isotopes—the last 900 k.y. O isotopes in carbonate rocks are routinely interpreted as measurements of paleotemperature during the Pleistocene glacial epoch High  18 O means low temperature, and vice versa Part of this is an ice volume effect, and part is thermodynamic. The two effects add (PDB)

4. O isotopes and surface temperatures When we apply this technique to rocks older than about 200 Ma, we get screwy results – Carbonates get lighter and lighter throughout the Phanerozoic, and earlier, implying warm temperatures in the distant past – Cherts, which are less able to exchange their O atoms with water, show this same trend on geologic time scales Carbonates Cherts (time reversed) Shields & Veizer, G 3 (2002) P. Knauth Paleo 3 (2005)

5. Face value temperatures from O isotopes Chert data: Mean surface temperature was 70  15 o C at 3.3 Ga –Ref: Knauth and Lowe, GSA Bull. (2003) Carbonate data: Surface temperatures remain significantly elevated (enough to cook the trilobites) until as recently as the early Devonian (~400 Ma) It is hard to believe any of this, though, because there is evidence for glaciation at several different times in the Precambrian 

6. Geologic time Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm (The ‘Boring Billion’) Ice ages Warm (?) Origin of life ‘Conventional’ interpretation of the Precambrian climate record

7. Question: What assumptions are implicit in the ‘conventional’ interpretation of the climate record? 1.The glacial evidence trumps the oxygen isotope record in cherts (because O isotopes suggest that it was hot all the time and we know that it wasn’t)  Did seawater isotopic composition change with time? 2.Weak evidence for Mesoproterozoic glaciation is ignored 3.The standard model of solar evolution is correct and the young Sun was ~30% dimmer than today 

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

9. Greenhouse gases and CO 2 - climate feedbacks So, one needs more greenhouse gases, especially during the Archean CO 2 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 CO 2 increases when the climate cools because of slower rates of silicate weathering on land

10. What can we say empirically about CO 2 levels in the distant past? Some controversial constraints on Archean CO 2 can be derived from paleosols (ancient soils)

11. Precambrian pCO 2 from paleosols First estimate for Archean pCO 2 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 CO 2 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.. N. Sheldon, Precambrian Res. (2006) Driese et al., 2011 (10-50 PAL)

12. 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 Geochimica et Cosmoschimica Acta 159, 190 (June, 2015)

13. K&M paleosol analysis: modern soils Filled symbols: measured soil pCO 2 –This is much higher than atmospheric CO 2 because of root respiration Empty symbols: calculated pCO 2 Get reasonable agreement, although calculated pCO 2 is sometimes significantly lower than observed Kanzaki & Murakami, GCA (2015)

14. K&M paleosol analysis: ancient soils Kanzaki & Murakami, GCA (2015) If the new paleosol analysis is correct, then CO 2 could have been high enough to solve the faint young Sun problem by itself Driese et al. (2011) Som et al. (2012) – upper limit from raindrops

15. That said, methane should also have been an important greenhouse gas during the Archean –Its lifetime is long in a low-O 2 atmosphere –It’s a moderately good greenhouse gas (but not nearly as good as CO 2, contrary to popular opinion) –The methanogens that produce it are thought to be evolutionarily ancient..

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

17. Anoxic ecosystem modeling Coupled photochemical- ecosystem modeling of an methanogen- or H 2 -based anoxygenic photosynthetic ecosystem predicts Archean CH 4 concentrations of 200- 2000 ppm This is enough to produce 10-15 degrees of greenhouse warming Higher warming by CH 4 is precluded by the formation of organic haze at CH 4 /CO 2 ratios greater than ~0.1 Kharecha et al., Geobiology (2005)

18. Archean CH 4 -CO 2 greenhouse Diagram shows a hypothetical Archean atmosphere at 2.8 Ga The black curves show predicted surface temperatures with zero and 1000 ppm of CH 4 The loss of much of this CH 4 at ~2.5 Ga could plausibly have triggered the Paleoproterozoic glaciations 2.8 Ga S/S o = 0.8 J.F. Kasting, Science (2013) Driese et al. (2011)

19. Geologic time Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm (The ‘Boring Billion’) Ice ages Warm (?) Origin of life ‘Conventional’ interpretation of the Precambrian climate record

20. This brings us up to the Proterozoic and the question: Was the “Boring Billion” really so boring? – Some recent authors have argued for glaciations during the Mesoproterozoic – We can return to this question in the Discussion, if we like (I have slides), but for now I’ll move on to other more interesting questions

21. Part 2: The rise of atmospheric O 2 and the enigma of low Proterozoic O 2 concentrations

22. Conventional geologic O 2 indicators H. D. Holland (1994) (Detrital) Blue boxes indicate low O 2 Red boxes indicate high O 2 Dates have been revised; the initial rise of O 2 is now placed at 2.45 Ga

23. Sulfur MIF record The Cloud/Holland interpretation of the rise of O 2 is strongly supported by the record of sulfur ‘mass- independent’ isotope fractionation, which shows that atmospheric O 2 was low prior to ~2.45 Ga This does not preclude the possibility of ‘whiffs’ of O 2 (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)

24. Cr isotopes and pO 2 Cr has two accessible oxidation states, Cr +3 and Cr +6. As with U, the oxidized state is soluble, while the reduced state is insoluble 53 Cr is enriched in the +6 state relative to 52 Cr when Cr is oxidized If O 2 is present during weathering, then 53 Cr is preferentially removed from soils and deposited in sediments, such as banded iron-formations

25. A ‘whiff’ of O 2 at 3.0 Ga Crowe et al. (Nature, 2013) argued for small amounts of atmospheric O 2 (3  10 -4 PAL) at 3.0 Ga The analysis is based on 53 Cr depletion in the Nsuze paleosol (top panel) and enrichment of 53 Cr and U in the contemporaneous Ijzermyn iron formation (bottom panel) Blue bar is the range of mantle values This suggests that O 2 was being produced by cyanobacteria at 3.0 Ga Paleosol Iron formation  53 Cr

26. Question: If O 2 was being produced at 3.0 Ga, why did it not rise to significant levels until about 2.45 Ga? –I won’t talk about this, but we can return to this question in the discussion

27. The conventional view of the rise of atmospheric O 2 A widely copied chart for the history of atmospheric O 2 is that created by Lee Kump in 2008 The Archean value (< 10 -5 PAL) is an upper limit from sulfur MIF – Actual surface O 2 concentrations were likely much smaller except in the vicinity of localized oxygen oases L.R. Kump, Nature (2008)

28. The conventional wisdom on Proterozoic O 2 levels was challenged last year by Planavsky et al., who argued that the actual values were much lower than previously assumed  Science (2014)

29. Evidence for low Proterozoic O 2 The evidence comes from Cr isotopes in ironstones, which fractionate during oxidative cycling The lack of departure of Proterozoic Cr isotopes from the mantle value (gray bar) suggest that pO 2 was < 10 -3 PAL Phanerozoic Proterozoic

30. The new view of the rise of atmospheric O 2 The new estimate for Proterozoic O 2 is 100 times lower than the previous best guess This may have made it difficult for animals to evolve until the end of the Neoproterozoic It also raises questions about surface UV fluxes.. L.R. Kump, Nature (2008) Planavsky et al.

31. Proterozoic UV fluxes Proterozoic O 2 levels of 0.1 percent PAL or below would have allowed much more solar UV radiation to penetrate to ground level, although some protection would still be provided This could have posed a problem for planktonic organisms, both algae and cyanobacteria Planavsky et al. limit J.F. Kasting, Precambrian Res. (1987)

32. Some possible questions for discussion 1.Were Proterozoic O 2 concentrations really 0.1% PAL or below? If so, what was holding them down? 2.What accounts for ‘whiffs’ of O 2 during the Archean? 3.Can we believe any of the Precambrian atmospheric CO 2 estimates from paleosols? Are better pCO 2 indicators available? 4.What do the oxygen isotope data from cherts and carbonates really tell us? Not discussed: 5.Is the published evidence for mid-Proterozoic glaciation believable, or was it truly the boring billion? 6.What caused O 2 to increase sharply at the beginning of the Proterozoic, i.e., the GOE? 7.What role, if any, does the seafloor play in controlling atmospheric O 2 and CO 2 concentrations over long time scales?

33. Backup slides…

34. See also Williams, J. Geol. Soc. London 162, 111 (2005) The authors argue that these rocks contain evidence for Mid-Proterozoic glaciation

35. 1.8-Ga King Leopold glacial markers(?) Subglacial Nye channels Glaciofluvial pebble conglomerate (overlying) Schmidt and Williams (2008)

36. Gerrit Kuipers, Frank F. Beunk & Frederik M. van der Wateren Cryoturbation and slump fold-like sedimentary structures in ca. 1.9 Ga old dacitic metavolcanic sediments in West Bergslagen, Central Sweden, are recognized as a lowland periglacial environment. This type of environment is comparable with present day tundra in Siberia. …This discovery is corroborated by a previous report of glacial sediments and structures from NW Australia of ca. 1.8 Ga age. Both occurrences developed at low geographical latitudes, at locations far apart in the Late Palaeoproterozoic supercontinent Columbia. Either suggest the existence of a ca. 100 Ma long epoch of extreme, though possibly Intermittent glaciations during the ca. 1.4 Ga long ‘Proterozoic gap’ (~2.2–0.77 Ga) From which no convincing glacial deposits were previously known. Geology Today 29, 218 (2013)

37. Periglacial evidence (1.91-1.89 Ga) Fig. 2. Looking eastward, on to section through ice-wedge cast and cryoturbate structures in well-bedded metadacitic tuffites in their present state, partially overgrown by lichen. Stratigraphical younging is to the top. Fig. 3. The ice-wedge cast in Fig. 2, published as ‘fold-like synsedimentary structure’ by Lundström (1995). Kuipers et al. (2013)

38. These formations, which include various types of glacial deposits, including diamictites and dropstones, were originally thought to be Neoproterozoic in age The authors have redated the rocks and now place the glaciation between 1.1 Ga and 1.3 Ga

39. Late Mesoproterozoic Glaciation (?) (1.1-1.3 Ga) Geboy et al., Precambrian Res. (2013) 1.1 Ga 1.3 Ga Vazante Group