Chapter 12—Part 1: The faint young Sun problem

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

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

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 Te = effective radiating temperature = [S(1-A)/4]1/4 TS = average surface temperature Kasting et al., Scientific American (1988)

Positive feedback loops (destabilizing) Water vapor feedback Surface temperature Atmospheric H2O (+) Greenhouse effect This feedback makes the faint young Sun problem worse

The faint young Sun problem More H2O Less H2O Kasting et al., Scientific American (1988)

Positive feedback loops (destabilizing) Snow/ice albedo feedback Surface temperature Snow and ice cover (+) Planetary albedo This feedback, which is not included in 1-D climate models, would have further destabilized the early climate

Possible solutions to the FYS problem Think back to the planetary energy balance equation Te4 = S (1 – A) 4 Albedo changes? A possible feedback involving biogenic sulfur gases and cloud condensation nuclei has been suggested (Rosing et al., Nature, 2010), but it is probably not strong enough to solve the problem Geothermal heat? Too small (0.09 W/m2 vs. ~240 W/m2 from absorbed sunlight

Possible solutions to the FYS problem Increasing the greenhouse effect also works Ts = Te + Tg Possible greenhouse gases NH3: Doesn’t work very well (photolyzes rapidly) CO2: Works! (supplied by volcanoes) CH4: Also works (probably requires life) Sagan and Mullen (1972) liked methane (CH4) and ammonia (NH3) because they were aware that O2 levels were low on the early Earth…

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

Witwatersrand gold ore with detrital pyrite (~3.0 Ga) • Pyrite = FeS2 • Detrital pyrite is pyrite that was not oxidized during weathering  Atmospheric O2 was low when this deposit formed P. Cloud, Oasis in Space (1988)

Caprock Canyon (Permian and Triassic) redbeds Redbeds contain oxidized, or ferric iron (Fe+3) Fe2O3 (Hematite) Their formation requires the presence of atmospheric O2 Reduced, or ferrous iron, (Fe+2) is found in sandstones older than ~2.4 b.y. of age http://www.utpb.edu/ceed/GeologicalResources/West_Texas_Geology/links/permo_triassiac.htm

Banded iron- formation or BIF (>1.8 b.y ago) • Alternating layers of Fe- rich and Fe-poor silica Fe+2 is soluble, while Fe+3 is not • BIFs require long- range transport of iron  The deep ocean was anoxic when BIFs formed (H.D. Holland, 1973)

What caused the rise of O2? We are almost certain that the rise of O2 was caused by cyanobacteria, formerly known as blue-green algae They’re not algae, though, because algae are eukaryotes (which have cell nuclei), whereas cyanobacteria are prokaryotes (lacking cell nuclei) http://www.primalscience.com/?p=424

Different forms of cyanobacteria a) Chroococcus b) Oscillatoria c) Nostoc (coccoid) (filamentous) (heterocystic) Nitrogen-fixing

Trichodesmium bloom Cyanobacteria are important in the ocean today because they can fix nitrogen Trichodesmium fixes N in the morning and makes O2 in the afternoon Eukaryotic algae and higher plants all learned to photosynthesize from cyanobacteria  Berman-Frank et al., Science (2001)

Ribosomal RNA Ribosomes are organelles (inclusions) within cells in which proteins are made Surprisingly, ribosomes contain their own RNA (ribonucleic acid) The RNA is also the catalyst for protein synthesis, indicating that life may have passed through an “RNA World” stage The RNA in ribosomes evolves very slowly, so looking at differences in the RNA of different organisms allows biologists to look far back into evolution http://www.scilogs.eu/en/blog/lindaunobel/ 2010-06-29/mountains-beyond-mountains

(rRNA) tree of life cyanobacteria “Universal” Chloroplasts in algae and higher plants contain their own DNA Cyanobacteria form part of the same branch on the rRNA tree (ribosomal RNA) Interpretation (due to Lynn Margulis): Chloroplasts resulted from endosymbiosis

They can’t do the whole thing, though Back to climate… So, reduced gases like CH4 and NH3 probably played some role in solving the faint young Sun problem They can’t do the whole thing, though NH3 is photolyzed (broken apart) by solar UV radiation and turns into N2 and H2 CH4 is also photolyzed and forms organic haze particles, which can create an anti-greenhouse effect This brings us back to CO2…

The carbonate-silicate cycle Atmospheric CO2 builds up as the climate cools, so higher atmospheric CO2 is a natural solution to the FYS problem

Negative feedback loops (stabilizing) The CO2-climate feedback Rainfall Surface temperature Silicate weathering rate (−) Greenhouse effect Atmospheric CO2

CO2 vs. time if no other greenhouse gases (besides H2O) One can easily solve the FYS problem with CO2 (and H2O), if one is allowed enough CO2 J. F. Kasting, Science (1993)

CO2 from paleosols Some geochemists have attempted to place limits on CO2 based on geochemical indicators such as paleosols (ancient soils) The latest estimates (Kanzaki & Murakami, 2015) are consistent with the climate calculations That said, there are still reasons to think that other greenhouse gases were present Catling & Kasting, in prep.

Why CH4 was important on the early Earth Atmospheric O2 levels were low Substrates for methanogenesis should have been widely available, e.g.: CO2 + 4 H2  CH4 + 2 H2O Methanogens are thought to be evolutionarily ancient 

(rRNA) tree of life Methanogenic bacteria Universal Root? Methanogens are all found within one branch of the Archaea, not far from the presumed root of the tree of life Courtesy of Norm Pace

CH4 is a good greenhouse gas because it absorbs in the 8-12 m Window region CH4 is a good greenhouse gas because it absorbs in the 8-12 m “window” region where H2O and CO2 absorption is weak Figure courtesy of Abe Lerman, Northwestern Univ.

Feedbacks in the methane cycle Furthermore, there are strong feedbacks in the methane cycle that would have helped methane become abundant Doubling times for thermophilic methan-ogens are shorter than for mesophiles Thermophiles will therefore tend to outcompete mesophiles, producing more CH4 and further warming the climate

CH4-climate positive feedback loop Surface temperature CH4 production rate (+) Greenhouse effect

But If CH4 becomes more abundant than CO2, organic haze begins to form, as it does on Saturn’s moon, Titan 

Titan’s organic haze layer The haze is formed from UV photolysis of CH4 It creates an anti-greenhouse effect by absorbing sunlight up in the stratosphere and re-radiating the energy back to space This cools Titan’s surface Image from Voyager 2

Possible Archean climate control loop production (–) Surface temperature Haze production Atmospheric CH4/CO2 ratio (–) CO2 loss (weathering)

“Daisyworld” diagram for Archean climate Greenhouse effect Haze formation Effect of CH4 on surface temperature Surface temperature Atmospheric CH4 concentration

“Daisyworld” diagram for Archean climate Greenhouse effect Haze formation Effect of CH4 on surface temperature Effect of surface on CH4 • P2 Surface temperature • P1 Atmospheric CH4 concentration

“Daisyworld” diagram for Archean climate Greenhouse effect Haze formation • P2 Point P2 is stable the Archean Earth should have been covered in organic haze Surface temperature • P1 Atmospheric CH4 concentration

CH4/CO2/C2H6 greenhouse with haze Late Archean Earth? Paleosols Water freezes When one puts all of this together, one comes up with this complicated figure (Fig. 12-4 in ed. 3 of The Earth System) The methane greenhouse ended, though, when atmospheric O2 went up at ~2.4 Ga, possibly triggering the Paleoproterozoic glaciations J. Haqq-Misra et al., Astrobiology (2008)

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

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

Conclusions So, reduced gases like CH4 probably played some role in solving the faint young Sun problem during the Archean The CH4 went away when O2 levels rose, possibly triggering the Paleoproterozoic glaciations High atmospheric CO2 was also involved, though The high CO2 was a natural consequence of the negative feedback provided by the carbonate-silicate cycle