Archean Atmosphere

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Archean Atmosphere

• Faint young Sun paradox presents dilemma
• 1) What is the source for high levels of
  greenhouse gases in Earths earliest atmosphere?
• 2) How were those gases removed with time?
• Models indicate Suns 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)

2
Source of Greenhouse Gases

• Input of CO2 and other greenhouse gases from
  volcanic emissions
• Most likely cause of high levels in Archean

3
Is Volcanic CO2 Earths 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

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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?

5
Removal of Atmospheric CO2

• Slow chemical weathering of continental rocks
  balances input of CO2 to atmosphere
• Chemical weathering reactions important
• Hydrolysis and Dissolution

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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

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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

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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
• Earths long term habitably requires only that
  the two are reasonably well balanced

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What Controls Weathering Reactions?

• Chemical weathering influenced by
• Temperature
• Weathering rates double with 10C 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

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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

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Chemical Weathering Earths Thermostat?

• Chemical weathering can provide negative feedback
  that reduces the intensity of climate warming

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Chemical Weathering Earths Thermostat?

• Chemical weathering can provide negative feedback
  that reduces the intensity of climate cooling

13
Greenhouse vs. Faint Young Sun

• Cold surface temperatures created by the faint
  young Sun compensated by stronger atmospheric CO2
  greenhouse effect

14
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

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Greenhouse vs. Faint Young Sun

• When solar luminosity strengthen, chemical
  weathering increased and helped transfer
  atmospheric CO2 into sediments

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Phanerozoic Volcanism Weathering

• As solar luminosity increased
• Earth warmed and became wetter
• Chemical weathering increased
• CO2 levels dropped
• Continental crust grew during Pre-Cambrian
• Became more siliceous (granitic)
• Slow warming of Earth
• Caused changes in chemical weathering
• Moderated Earths climate

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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

18
Gaia Hypothesis

• Biology affects geochemical processes that
  influence climate
• Gaia hypothesis
• Life has regulated Earths 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

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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

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Gaia

• Hypothesis unproven
• Extent to which life regulated climate unknown
• Life plays active roles in biogeochemical
  processes
• Must contribute to the thermostat that regulates
  Earths climate

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Plate Tectonics and Climate

• Position of continents, volcanic CO2 emissions
  and continental elevation

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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?

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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

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Test of Polar Position Hypothesis

• Assembly of Gondwana carried large continental
  masses across the South Pole
• Were ice sheets present?

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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

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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

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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

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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

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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

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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

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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

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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

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Why carbon Isotopes?

• Carbon isotopes tell us when carbon cycle not in
  balance

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Burial of Organic Matter and d13C

• Burial of 13C-depleted 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

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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

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Model Evaluation

• Model works pretty well

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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

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Tectonic Uplift and Weathering

• Uplift causes several tectonic and climatic
  effects that affects weathering by fragmenting
  fresh rock

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Testing the Hypothesis

• Times of continental collision coincide with
  times of glaciations
• Uplift weathering hypothesis consistent with
  geologic record

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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

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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

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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