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Snowball Earth Hoffman et al. Science 1998

Snowball Earth Hoffman et al. Science 1998. Evidence. delta 13C poor layer. ice rafter dropstones. iron rich deposits. delta 13C rich layer. Hypothesis. Ice buildup due to lower carbon dioxide in the atmosphere (Qo: 6% lower) Biological productivity is at normal level prior to ice buildup

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Snowball Earth Hoffman et al. Science 1998

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  1. Snowball EarthHoffman et al. Science 1998

  2. Evidence delta 13C poor layer ice rafter dropstones iron rich deposits delta 13C rich layer

  3. Hypothesis • Icebuildup 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 today’s level) • Sudden collapse of the ice sheet  leads to extreme greenhouse earth • CO2 dissolves in ocean, precipitates  cap carbonates

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

  5. 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

  6. Snowball EarthHyde et al. Nature 1998

  7. 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

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

  9. 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

  10. 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.

  11. SnowballEarthSchrag 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

  12. SnowballEarthHyde 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.

  13. Snowball EarthMacKay, GRL 2000

  14. 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.

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