Post on 02-Jan-2016
Gaia Revisited:The Interplay Between
Climate and Life on the Early Earth
James F. KastingDepartment of Geosciences
Penn State University
The Gaia Hypothesis
James Lovelock
1979 1988
• 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
Whitedaisycover(%)
Surface temperature (oC)20
P1
P2
Equilibrium States = points where lines cross
A
B
Systems Notation
= system component
= positive coupling
= negative coupling
Daisyworld feedback loop
() % whitedaisy cover
Surfacetemperature
(+)% white
daisy coverSurface
temperature
The feedback loop isnegative at point P1
The feedback loop ispositive at point P2
Whitedaisycover(%)
Surface temperature (oC)20
P1
P2
Equilibrium States = points where lines cross
Stable
UnstableA
B
PhanerozoicTime
First shelly fossils
Age of fish
First vascular plants on landIce age
Ice age
First dinosaurs
Dinosaurs go extinct
Ice age (Pleistocene)
Warm
Geologic time
Warm (?)
Rise of atmospheric O2 (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)
Surfacetemperature
AtmosphericH2O
Greenhouseeffect
Water vapor feedback
(+)
Positive feedback loops
Surfacetemperature
Snow and icecover
Planetaryalbedo
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 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
The Carbonate-Silicate Cycle
(metamorphism)
• Silicate weathering slows down as the Earth cools atmospheric CO2 should build up
Negative feedback loops(stabilizing)
The carbonate-silicate cycle feedback
(−)
Surfacetemperature
Rainfall
Silicateweathering
rate
AtmosphericCO2
Greenhouseeffect
• Thus, atmospheric CO2 levels may well have been higher in the distant past. However, there are good reasons for believing that CH4 was abundant as well
• The first is that atmospheric O2 levels were low…
Geologic O2 Indicators
H. D. Holland (1994)
(Detrital)
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
S isotopes in Archean sediments
Farquhar et al. (2001)
(FeS2)
(BaSO4)
33S versus time
Farquhar et al., Science, 2000
73 Phanerozoic samples
• Requires photochemical reactions (e.g., SO2 photolysis) in a low-O2 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 CH4 was abundant on the early Earth is that the biological source of (most)* methane is probably ancient
*Some methane comes from plants!
• Today, CH4 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:
CO2 + 4 H2 CH4 + 2 H2O
Methanogenicbacteria
Courtesy ofNorm Pace
“Universal”(rRNA) tree
of life
Root (?)
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
Pavlov et al., JGR (2000)
Late Archean Earth?
*This calculation was wrong. The CH4 absorption bands were inadvertently shifted
Old calculation*
• 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.
Window region
But, there may have been other IR-active hydrocarbon gases in addition to CH4
Low-O2 atmospheric model
• “Standard”, low-O2 model from Pavlov et al. (JGR, 2001)• 2500 ppmv CO2, 1000 ppmv CH4 8 ppmv C2H6
Ethane formation:
1) CH4 + h CH3 + Hor2) CH4 + OH CH3 + H2O
Then3) CH3 + CH3 + M C2H6 + M
• 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
Important band
Complete C2H6 Calculations
Limit of pCH4 > pCO2
Paleosol data
273.15 K
fCH4 = 0
10-5
10-4
10-3
10-2
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 O2 (Ice age)
First shelly fossils (Cambrian explosion)Snowball Earth ice ages
Warm
Ice age
Origin of life
Huronian Supergroup (2.2-2.45 Ga)
Redbeds
Detrital uraniniteand pyrite
Glaciations
S. Roscoe, 1969
Low O2
High O2
• The rise of O2 was caused by cyanobacteria, so the Huronian glaciation was triggered by a Gaian mechanism
Trichodesmium bloom
cyanobacteria
Interpretation (dueto Lyn Margulis):Chloroplasts resultedfrom endosymbiosis
• More recently, we have been trying to explain the climate of the Mid-Archean…
Geologic time
Warm (?)
Rise of atmospheric O2 (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)
Surfacetemperature
CH4
production
Hazeproduction
AtmosphericCH4/CO2
ratio
CO2 loss(weathering)
(–)
(–)
“Daisyworld” diagram for mid-Archean climate
Atmospheric CH4 concentration
Surf
ace
tem
pera
ture
Haze formationGreenhouseeffect
P1
P2
•
• Point P2 is stable
the mid-Archean Earth may have been covered in organic haze
(Credit Bill Moorefor 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 CH4 levels decreased as well the organic haze layer disappeared
Mid-Archean ocean(pre-oxygenic photosynthesis)
Fermentation and methanogenesis zone
[O2] 0
[SO4=] 1 mM
Sulfate reduction zone
sed
imen
ts
CH4
CH4
Organic C
Modern ocean*
(post-oxygenic photosynthesis)
Aerobic decay zone
Sulfate reduction zone
Fermentation and methanogenesis zone
[O2] 200 M
[SO4=] 30 mM
CH4 (little escapes)
sed
imen
ts
Organic C
*Late Archean ocean would be somewhere in between this and the mid-Archean ocean
“Daisyworld” diagram for Late Archean climate
Atmospheric CH4 concentration
Surf
ace
tem
pera
ture
Haze formationGreenhouseeffect
P1
P2
•
•
Point P1 is stable
the Late Archean Earth should have been haze-free
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