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