Archean Atmosphere

Faint young Sun paradox presents dilemma
 1) What is the source for high levels of
greenhouse gases in Earth’s earliest
atmosphere?
 2) How were those gases removed with time?
 Models indicate Sun’s strength increased
slowly with time
 Geologic record strongly suggests Earth
maintained a moderate climate throughout
Earth history (i.e., no runaway greenhouse
like on Venus)
Source of Greenhouse Gases

Input of CO2 and other greenhouse gases from
volcanic emissions
 Most likely cause of high levels in Archean
Is Volcanic CO2 Earth’s Thermostat?



If volcanic CO2 emissions provided Archean
greenhouse, has volcanic activity continuously
slowed through geologic time? No, but…
Carbon input balanced by removal
 Near surface carbon reservoirs
Stop all volcanic input of CO2
 Take 270,000 years to deplete atmospheric
CO2
 Surface carbon reservoirs (41,700 gt)
divided by volcanic carbon input (0.15 gt y-1)
 Rate of volcanic CO2 emissions have potential
to strongly affect atmospheric CO2 levels on
billion-year timescale
Volcanic CO2 inputs?

No geologic, geophysical or geochemical
evidence indicates that rates of tectonism
decreased slowly through Earth history
 Rates of volcanic CO2 input did not
change slowly with time
 Volcanic CO2 emissions did not moderate
Earth climate through geologic time
 If not inputs, what about a change in
removal rate of atmospheric CO2?


Removal of Atmospheric CO2
Slow chemical weathering of continental rocks
balances input of CO2 to atmosphere
Chemical weathering reactions important
 Hydrolysis and Dissolution
Hydrolysis



Main mechanism of chemical weathering that
removes atmospheric CO2
Reaction of silicate minerals with carbonic acid
to form clay minerals and dissolved ions
Summarized by the Urey reaction
 CaSiO3 + H2CO3  CaCO3 + SiO2 + H2O
 Atmospheric CO2 is carbon source for
carbonic acid in groundwater
 Urey reaction summarizes atmospheric CO2
removal and burial in marine sediments
 Accounts for 80% of CO2 removal
Dissolution
Kinetics of dissolution reactions faster
than hydrolysis
 Dissolution reaction neither efficient nor
long term
 Dissolution of exposed limestone and
dolostone on continents and precipitation
of calcareous skeletons in ocean
 CaCO3 + H2CO3  CaCO3 + H2O + CO2
 Although no net removal of CO2
 Temporary removal from atmosphere

Atmospheric CO2 Balance

Slow silicate rock weathering balances
long-term build-up of atmospheric CO2
 On the 1-100 million-year time scale
 Rate of chemical hydrolysis balance rate
of volcanic emissions of CO2
 Neither rate was constant with time
 Earth’s long term habitably requires only
that the two are reasonably well
balanced
What Controls Weathering Reactions?

Chemical weathering influenced by
 Temperature
 Weathering rates double with 10°C rise
 Precipitation
 H2O is required for hydrolysis
• Increased rainfall increases soil
saturation
 H2O and CO2 form carbonic acid
 Vegetation
 Respiration in soils produces CO2
• CO2 in soils 100-1000x higher than
atmospheric CO2
Climate Controls Chemical Weathering


Precipitation closely linked
with temperature
 Warm air holds more
water than cold air
Vegetation closely linked
with precipitation and
temperature
 Plants need water
 Rates of photosynthesis
correlated with
temperature
Chemical Weathering: Earth’s Thermostat?

Chemical weathering can provide negative feedback
that reduces the intensity of climate warming
Chemical Weathering: Earth’s Thermostat?

Chemical weathering can provide negative feedback
that reduces the intensity of climate cooling
Greenhouse vs. Faint Young Sun

Cold surface
temperatures
created by the
faint young Sun
compensated by
stronger
atmospheric CO2
greenhouse effect
Archean Volcanism & Weathering


Early Archean volcanism probably produced more
atmospheric CO2
 Counteracted lower radiant energy and
warmed our planet
 Volcanism did not slow at same rate as Sun
increase in strength
Earth Earth probably still cold
 Weathering slow
 Continents small
 Continental crustal rocks silica-poor (basaltic)
 Stoichiometry of Urey reaction different
 Less efficient CO2 removal from
atmosphere
Greenhouse vs. Faint Young Sun

When solar
luminosity
strengthen,
chemical
weathering
increased and
helped transfer
atmospheric CO2
into sediments
Phanerozoic Volcanism & Weathering
As solar luminosity increased
 Earth warmed and became wetter
 Chemical weathering increased
 CO2 levels dropped
 Continental crust grew during PreCambrian
 Became more siliceous (granitic)
 Slow warming of Earth
 Caused changes in chemical weathering
 Moderated Earth’s climate

Other Greenhouse Gases?

Why not other greenhouse gases?
 CH4 and NH3
 Oxidize rapidly in atmosphere
 Are biologically utilized
 H2O
 Detritial sediments indicate liquid
water present on Earth for last 4 by
 H2O(v) in atmosphere provides
positive climate feedback
Gaia Hypothesis


Biology affects geochemical processes that
influence climate
Gaia hypothesis
 Life has regulated Earth’s climate
 All evolution occurred to keep Earth
habitable (extreme interpretation)
 Life affected atmospheric O2 evolution
 Plants can affect chemical weathering
 Marine carbonate organisms sink for carbon
 Photosynthesis and burial of organic matter
can affect atmospheric CO2
Record of life


Critics of Gaia
 Life evolved late in Earth history
 Early life forms too primitive to
affect geochemical cycles
 CaCO3 shells appeared only 0.6
bya
Supporters of Gaia
 Antiquity of bacteria
 Development of atmospheric O2
 Life became more complex when
Earth needed it
 Countered the faint young Sun
Gaia

Hypothesis unproven
 Extent to which life
regulated climate
unknown
 Life plays active roles
in biogeochemical
processes
 Must contribute to the
thermostat that
regulates Earth’s
climate
Plate Tectonics and Climate

Position of continents, volcanic CO2
emissions and continental elevation
Evidence for Climate Change


Geologic record reveals record
of long-term climate change
Is the timing of “ice house”
intervals on Earth related to
 Continental configuration and
position?
 Related to a tectonic control
on atmospheric CO2?
 Change in CO2 supply?
 Changes in weathering?
Polar Position Hypothesis
Ice sheets appear on continents when they
are in polar positions
 No ice should appear on Earth if
continental landmasses are equatorial
 No world-wide change in climate only on
the slow tectonic movement of
continents
 Testable hypothesis

Test of Polar Position Hypothesis

Assembly of Gondwana carried large
continental masses across the South Pole
 Were ice sheets present?
Polar Positions and Ice Sheets
Parts of Gondwana
lay over the South
Pole for ~100 my
 Evidence for
glaciations exist
 Ordovician (~430
my) glaciations
lasted less than 10
my and probably
less than 1 my

Polar Position Hypothesis


Presence of continents in polar positions does
guarantee glaciations (question of preservation)
Another factor is required to regulate climate
on tectonic time scales
Changing Atmospheric CO2
Polar position alone does not explain
climate variations over last 500 my
 Change in atmospheric CO2 important
 BLAG model
 Driven by changes in CO2 input that
result from sea floor spreading
 T. C. Chamberlain or Raymo/Ruddiman
Model
 Driven by changes in the rate of uplift
and weathering

BLAG

The rate of global
average seafloor
spreading
 Controls delivery of
CO2 to atmosphere
 Direct injection
from rock reservoir
 Changes in
atmospheric CO2
control climate
Carbon Cycle Model

Seafloor spreading the driver of change
 Model relies on feedback through
chemical weathering
 Transport of carbon to oceans
 Burial of carbon in sediments
 Return of carbon from mantle through
volcanism
Carbon Cycling

Carbon cycles continuously between rock
reservoir and atmosphere
 CO2 removed from atmosphere by chemical
weathering, deposited in ocean sediments,
subducted and returned by volcanism
Organic Carbon Burial Affect CO2

If the rate of organic carbon burial increases,
less organic matter available for decomposition
and less carbon returned to the atmosphere as
CO2
 Atmospheric CO2 reservoir shrinks
Organic Carbon Burial Affect O2

If the rate of organic carbon burial increases,
less organic matter available for decomposition
and less oxygen is used during decomposition
 Atmospheric O2 reservoir grows
Why carbon Isotopes?

Carbon isotopes tell us when carbon cycle not in balance
Burial of Organic Matter and d13C


Burial of 13Cdepleted organic
matter leaves
remaining DIC
enriched in 13C
Increases in d13C
of marine
carbonates
indicates an
increase in the
rate of burial of
organic matter in
ocean or on land
BLAG Input and Output


Input to model
 Record of d13C variations in marine carbonates
 Proxy for rate of organic carbon burial
Output from model
 Variation in atmospheric CO2 and O2
 Weathering rates through time
 Atmospheric CO2 controls temperature
• Precipitation and reaction rates
 Atmospheric O2 can affect weathering
Model Evaluation

Model works pretty well
Competing Hypothesis?

Uplift Weathering Hypothesis
 Chemical weathering is the active driver
of climate change
 Rate of supply of CO2 constant, rate
of removal changes
 Global mean rate of chemical weathering
depends on availability of fresh rock and
mineral surfaces
 Rate of tectonic uplift
controls/enhances exposure of fresh
rock surfaces
Tectonic Uplift and Weathering

Uplift causes
several
tectonic and
climatic
effects that
affects
weathering by
fragmenting
fresh rock
Testing the Hypothesis

Times of continental collision coincide with
times of glaciations
 Uplift weathering hypothesis consistent
with geologic record
What is the Difference?

Key factors controlling weathering differ
 BLAG – chemical weathering is a
negative feedback
 Moderates climate change driven by
volcanic CO2 inputs
 Uplift weathering – chemical weathering
is the driver of climate change
 Physical fragmentation and exposure
of fresh material during uplift
• Removes atmospheric CO2
Weathering in Amazon Basin

Chemical weathering is
more intense in the
Andes Mountains
 80% of the ions that
reach the Atlantic
Ocean from eastern
Andes
 20% from the Amazon
basin lowlands
 Lowlands intensely
weathered quickly
Academic Arguments?

Processes of uplift and exposure are linked to
volcanic CO2 emissions
 Plate tectonics
 Both processes are important factors
affecting global geochemical cycles
 One or the other may be more important at
any given time
 Explain better geologic observations
 Neither explanation fully incorporates
biological influences
 Life plays active roles in biogeochemical
processes