evolution of earth’s atmosphere and climate
DESCRIPTION
Phanerozoic Time Ice age (Pleistocene) Dinosaurs go extinct First dinosaurs Ice age Age of fishes First vascular plants on land Ice age First shelly fossilsTRANSCRIPT
Evolution of Earth’s Atmosphere and ClimateJames Kasting
Department of GeosciencesPenn State University
PhanerozoicTime
First shelly fossils
Age of fishesFirst vascular plants on landIce age
Ice ageFirst dinosaurs
Dinosaurs goextinct
Ice age (Pleistocene)
Geologic time
Rise of atmospheric O2 (Ice age)
First shelly fossils (Cambrian explosion)Snowball Earth ice ages
Warm (The ‘Boring Billion’)
Ice ages
Warm (?) Origin of life‘Conventional’ interpretationof the Precambrian climaterecord
• The fact that most of the Precambrian appears to have been warm is remarkable, because the Sun is thought (by essentially everyone) to have been less luminous early in Earth’s history
The faint young Sun problem
Kasting et al., Scientific American (1988)
Te = effective radiating temperature = [S(1-A)/4]1/4
TS = average surface temperature
Greenhouse gases and CO2-climate feedbacks
• So, one needs more greenhouse gases, especially during the Archean
• CO2 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. AtmosphericCO2 increases when the climate cools because of slower rates of silicate weathering on land
Is CO2 the solution to the FYS problem?
• Thus, high CO2 levels could, in principle, have solved the FYS problem (see calculation at right)
• Unfortunately, geochemists have made this problem more difficult by attempting to measure paleo-CO2 concentrations…
J. F. Kasting, Science (1993)
Precambrian pCO2 from paleosols
• First estimate for Archean pCO2 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 CO2 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)
• 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)
K&M paleosol analysis: ancient soils
Kanzaki & Murakami, GCA (2015)
• If the new paleosol analysis is correct, then CO2 could have been high enough to solve the faint young Sun problem by itself
• Thus, high CO2 may or may not have been sufficient to offset the faint young Sun during the Archean, depending on whose paleosol interpretations are correct
• That said, there are additional reasons to think that other greenhouse gases (CH4) might have been important, the main one being that atmospheric O2 levels were low prior to ~2.5 Ga
Conventional geologic O2 indicators
H. D. Holland (1994)
(Detrital)
• Blue boxes indicate low O2• Red boxes indicate high O2• Dates have been revised; the initial rise of O2 is now placed at 2.45 Ga
• The conventional story about the rise of O2 has received strong support in recent years from studies of multiple sulfur isotopes…
S isotopes and the rise of O2
• 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
• Requires photochemical reactions in a low-O2 atmosphere
SO2 + h SO + O (photolysis: 190 nm < < 220 nm)SO2 + h SO2
* (photoexcitation: 240 nm < < 320 nm)
• This produces “MIF” (mass-independent fractionation)
“Normal” isotope mass fractionation
• Vibrational energy levels depend inversely on the reduced mass = (k/mR)1/2
En = (n+½) h• Increasing the mass
of one or both atoms decreases the vibrational frequency and energy, thereby strengthening chemical bonds
A simple harmonicoscillator
S isotopes in Archean sediments
• Sulfides (pyrite) fall above the mass fractionation line• Sulfates (barite) fall below it
Farquhar et al. (2001)
(FeS2)
(BaSO4)
33S
33S versus time
Farquhar et al., Science, 2000
73 Phanerozoic samples
High O2 Low O2
Sulfur MIF record• The Cloud/Holland
interpretation of the rise of O2 is strongly supported by the record of sulfur ‘mass-independent’ isotope fractionation, which shows that atmospheric O2 was low prior to ~2.45 Ga
• This does not preclude the possibility of ‘whiffs’ of O2 (Ariel Anbar’s term) during the Archean, for which there is geochemical evidence
Reinhard et al., Nature (2013)(Technique pioneered by Farquharet al., Science, 2000)
Question: What does the sulfur MIF tell us?
1. Must have had low enough O2 (and O3) to allow SO2 to be photolyzed
2. Must have had low enough O2 to prevent all volcanic SO2 from being oxidized to sulfate, as it is today
Archean sulfur cycle
Kasting, Science (2001) [Redrawn from Kasting et al., OLEB (1989)]
• In a low-O2 atmosphere, volcanic SO2 can be either oxidized or reduced (or it can exit the atmosphere as SO2)• By contrast, today, virtually all SO2 is oxidized to sulfate; thus, any MIF signal is eliminated by homogenization
Back to climate…
• Thus, methane could also have been an important greenhouse gas during the Archean– Its lifetime is long in a low-O2 atmosphere– It’s a moderately good greenhouse gas (but
not nearly as good as CO2, contrary to popular opinion)
– The methanogens that produce it are thought to be evolutionarily ancient..
Methanogenicbacteria
Courtesy ofNorm Pace
“Universal”(rRNA) tree
of life
Root (?)
Anoxic ecosystem modeling
• Coupled photochemical-ecosystem modeling of an methanogen- or H2-based anoxygenic photosynthetic ecosystem predicts Archean CH4 concentrations of 200-2000 ppm
• This is enough to produce 10-15 degrees of greenhouse warming
• Higher warming by CH4 is precluded by the formation of organic haze at CH4/CO2 ratios greater than ~0.1
Kharecha et al., Geobiology (2005)
Archean CH4-CO2 greenhouse
• Diagram shows a hypothetical Archean atmosphere at 2.8 Ga
• The black curves show predicted surface temperatures with zero and 1000 ppm of CH4
• The loss of much of this CH4 at ~2.5 Ga could plausibly have triggered the Paleoproterozoic glaciations
2.8 GaS/So = 0.8
J.F. Kasting, Science (2013)
Driese et al. (2011)
Geologic time
Rise of atmospheric O2 (Ice age)
First shelly fossils (Cambrian explosion)Snowball Earth ice ages
Warm (The ‘Boring Billion’)
Ice ages
Warm (?) Origin of life‘Conventional’ interpretationof the Precambrian climaterecord
Huronian Supergroup (2.2-2.45 Ga)
Redbeds
Detrital uraniniteand pyrite
Glaciations
S. Roscoe, 1969
Low O2
High O2
Conclusions• Earth’s early climate was probably kept warm
by a combination of higher CO2 and CH4– Interpretations of paleosols presently give conflicting
estimates of Precambrian CO2 levels– Atmospheric O2 was certainly low most of the time
prior to ~2.5 Ga, and CH4 levels were almost certainly high (200-2000 ppmv)
– Life plays a role in climate regulation, but Earth should remain habitable even without it
• The carbonate-silicate cycle plays a key role in Earth’s climate stability, especially in countering the faint young Sun problem