Snowball Earth Hoffman et al. Science 1998

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Snowball EarthHoffman et al. Science 1998
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Evidence
delta 13C poor layer
ice rafter dropstones
iron rich deposits
delta 13C rich layer
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Hypothesis

• Ice buildup due to lower carbon dioxide in the
  atmosphere (Qo 6 lower)
• Biological productivity is at normal level prior
  to ice buildup
• Earth becomes ice covered, ocean become anoxic,
  and rich in ferrous iron
• Volcanic eruption raises the level of CO2 in the
  atmosphere (CO2 stays in atm due to ice cover
  reaches 350 x todays level)
• Sudden collapse of the ice sheet ? leads to
  extreme greenhouse earth
• CO2 dissolves in ocean, precipitates ? cap
  carbonates

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Method

• Studies rocks in Namibia (Ottavi group), measured
  delta 13C, thickness of carbonate layers, etc

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Mechanism

• Simple life would have survived in places where
  the ice sheet was thin and water (lakes was
  present)
• It would take 4 Myr at present day CO2 emission
  from volcano to bring CO2 level to 0.12 bar to
  get out of snow ball earth
• If 0.12 bar of CO2 precipitate to present day
  level (0.001bar) ? 5m layer thick of cap carbonate

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Snowball EarthHyde et al. Nature 1998
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Hypothesis

• Climate ice sheet models / GCM
• EBM can reproduce this fast transition from
  glacial to interglacial BUT is this scenario
  coherent with more complex climate model?
• Delays in melting, lag in ice growth ? filters
  high frequency changes in Qo
• Bedrock depression, lower sea level ? different
  land-ocean configuration ? albedo
• GCM Ice cover over ocean ? less cloud ? more SW

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Method

• Glacial model 4000 year depression time scale
• Temperature and pressure independent rheology
• Qo 6 lower than today
• CO2 half of present day climate

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Results

• Model gives realistic present and LGM climate
• With reduced Qo and CO2 level ? Ice covered earth
• Use the ice configuration as boundary condition
  for a GCM ? equatorial band of open water ?
  allows life to survive

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Snowball EarthBruce Runegar, Comments Nature 1998
If the equatorial ocean is open how can the CO2
in the atmosphere to extremely high level and
end the snow ball earth? It would simply
dissolve in the ocean.
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Snowball EarthSchrag and Hoffman, Comments
Nature 1998
If the equatorial ocean is ice free, then the
level of CO2 required to end the snow ball earth
are lower ? not consistent with cap carbonate
thickness
Also it implies a slow/gradual termination of
snow ball earth ? again not consistent with cap
carbonate
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Snowball EarthHyde et al, Reply Nature 1998
CO2 increase gradually, but the response of the
ice Sheet is still a rapid melt back at come
critical CO2 level
Life needs open ocean to survive. Multicellular
cant live on Ridge along with chemosynthethic
life (still needs oxygen) And ocean are anoxic.
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Snowball EarthMacKay, GRL 2000
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Results/Hypothesis

• Yes ice would be km thick if the only heat
  balance is geothermal heat flux
• BUT if you include light transmissivity, then it
  could be as thin as a few meters.
• Then you can have both ice covered ocean and
  large buildup of CO2 in the atm, and light for
  live.