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

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

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

White daisy cover (%) Surface temperature ( o C) 20 P1P1 P2P2 Equilibrium States = points where lines cross A B

Systems Notation = system component = positive coupling = negative coupling

Daisyworld feedback loop ()() % white daisy cover Surface temperature (+)(+) % white daisy cover Surface temperature The feedback loop is negative at point P 1 The feedback loop is positive at point P 2

White daisy cover (%) Surface temperature ( o C) 20 P1P1 P2P2 Equilibrium States = points where lines cross Stable Unstable A B

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

Geologic time Warm (?) Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Ice age Origin of life

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 Kasting et al., Scientific American (1988) The problem is amplified by water vapor feedback

Positive Feedback Loops (Destabilizing) Surface temperature Atmospheric H 2 O Greenhouse effect Water vapor feedback (+)

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

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

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 CO 2, H 2 O, and CH 4 –H 2 O, 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 CO 2, higher CH 4, or both 

The Carbonate-Silicate Cycle (metamorphism) Silicate weathering slows down as the Earth cools  atmospheric CO 2 should build up

Negative feedback loops (stabilizing) The carbonate-silicate cycle feedback (−)(−) Surface temperature Rainfall Silicate weathering rate Atmospheric CO 2 Greenhouse effect

Thus, atmospheric CO 2 levels may well have been higher in the distant past. However, there are good reasons for believing that CH 4 was abundant as well The first is that atmospheric O 2 levels were low…

Geologic O 2 Indicators H. D. Holland (1994) (Detrital)

S isotopes and the rise of O 2 Even stronger evidence for the rise of O 2 comes from sulfur isotopes in ancient rocks –Sulfur has 4 stable isotopes: 32 S, 33 S, 34 S, and 36 S –Normally, these separate along a standard mass fractionation line –In very old (Archean) sediments, the isotopes fall off this line

S isotopes in Archean sediments Farquhar et al. (2001) (FeS 2 ) (BaSO 4 )

 33 S versus time Farquhar et al., Science, Phanerozoic samples Requires photochemical reactions (e.g., SO 2 photolysis) in a low-O 2 atmosphere

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

Back to methane… A second reason for suspecting that CH 4 was abundant on the early Earth is that the biological source of (most) * methane is probably ancient * Some methane comes from plants!

Today, CH 4 is produced (mainly) in restricted, anaerobic environments, such as the intestines of cows and the water- logged soils underlying rice paddies Methanogenic bacteria are responsible for (most) methane production One common metabolic reaction: CO H 2  CH H 2 O 

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

Archean CH 4 concentrations It can be shown that CH 4 production rates in an anaerobic biosphere could have been comparable to today (P. Kharecha et al., Geobiology, 2005) Because O 2 was low, the lifetime of methane would have been long (~10,000 yrs), and CH 4 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 

Pavlov et al., JGR (2000) Late Archean Earth? * This calculation was wrong. The CH 4 absorption bands were inadvertently shifted  Old calculation *

The CH 4 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. Window region

But, there may have been other IR- active hydrocarbon gases in addition to CH 4 

Low-O 2 atmospheric model “Standard”, low-O 2 model from Pavlov et al. (JGR, 2001) 2500 ppmv CO 2, 1000 ppmv CH 4  8 ppmv C 2 H 6 Ethane formation: 1) CH 4 + h CH 3 + H or 2) CH 4 + OH  CH 3 + H 2 O Then 3) CH 3 + CH 3 + M  C 2 H 6 + M

Ethane (C 2 H 6 ) 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 CH 4 Important band

Complete C 2 H 6 Calculations Limit of pCH 4 > pCO 2 Paleosol data K fCH 4 = Calculations by Jacob Haqq-Misra

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

Geologic time Warm (?) Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Ice age Origin of life

Huronian Supergroup ( Ga) Redbeds Detrital uraninite and pyrite Glaciations S. Roscoe, 1969 Low O 2 High O 2

The rise of O 2 was caused by cyanobacteria, so the Huronian glaciation was triggered by a Gaian mechanism Trichodesmium bloom

cyanobacteria Interpretation (due to Lyn Margulis): Chloroplasts resulted from endosymbiosis

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

Geologic time Warm (?) Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Ice age Origin of life

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

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

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 effect on Titan, and perhaps on early Earth, as well S. Goldman et al., in revision

Possible Archean climate control loop (pre-oxygenic photosynthesis) Surface temperature CH 4 production Haze production Atmospheric CH 4 /CO 2 ratio CO 2 loss (weathering) (–)

“Daisyworld” diagram for mid- Archean climate Atmospheric CH 4 concentration Surface temperature Haze formation Greenhouse effect P1P1 P2P2 Point P 2 is stable  the mid-Archean Earth may have been covered in organic haze (Credit Bill Moore for the figure)

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 CH 4 levels decreased as well  the organic haze layer disappeared

Mid-Archean ocean (pre-oxygenic photosynthesis) Fermentation and methanogenesis zone [O 2 ]  0 [SO 4 = ]  1 mM Sulfate reduction zone sediments CH 4 Organic C

Modern ocean * (post-oxygenic photosynthesis) Aerobic decay zone Sulfate reduction zone Fermentation and methanogenesis zone [O 2 ]  200  M [SO 4 = ]  30 mM CH 4 (little escapes) sediments Organic C * Late Archean ocean would be somewhere in between this and the mid-Archean ocean

“Daisyworld” diagram for Late Archean climate Atmospheric CH 4 concentration Surface temperature Haze formation Greenhouse effect P1P1 P2P2 Point P 1 is stable  the Late Archean Earth should have been haze-free

Conclusions CH 4 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 O 2 and a corresponding decrease in CH 4 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