Snowball Earth Hoffman et al Science 1998 Evidence

Snowball Earth Hoffman et al. Science 1998

Evidence

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 CO 2 in the atmosphere (CO 2 stays in atm due to ice cover – reaches 350 x today’s level) • Sudden collapse of the ice sheet leads to extreme greenhouse earth • CO 2 dissolves in ocean, precipitates cap carbonates

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

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 CO 2 emission from volcano to bring CO 2 level to 0. 12 bar to get out of snow ball earth • If 0. 12 bar of CO 2 precipitate to present day level (0. 001 bar) 5 m layer thick of cap carbonate

Snowball Earth Hyde et al. Nature 1998

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 ocean less cloud more SW

Method • Glacial model: 4000 year depression time scale • Temperature and pressure independent rheology • Qo = 6% lower than today • CO 2 half of present day climate

Results • Model gives realistic present and LGM climate • With reduced Qo and CO 2 level Ice covered earth • Use the ice configuration as boundary condition for a GCM equatorial band of open water allows life to survive

Snowball Earth Bruce Runegar, Comments Nature 1998 If the equatorial ocean is open: how can the CO 2 in the atmosphere to extremely high level and end the snow ball earth? – It would simply dissolve in the ocean.

Snowball Earth Schrag and Hoffman, Comments Nature 1998 If the equatorial ocean is ice free, then the level of CO 2 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

Snowball Earth Hyde et al, Reply Nature 1998 CO 2 increase gradually, but the response of the ice Sheet is still a rapid melt back at come critical CO 2 level Life needs open ocean to survive. Multicellular cant live on Ridge along with chemosynthethic life (still needs oxygen) And ocean are anoxic.

Snowball Earth Mac. Kay, GRL 2000

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 CO 2 in the atm, and light for live.