1. Snowball Earth
Earth 202

2. Before talking about Snowball Earth, let’s look briefly at Earth’s long-term climate history 

3. Phanerozoic Time Ice age (Pleistocene) Dinosaurs go extinct
First dinosaurs
Ice age
First vascular plants on land
Ice age
Age of fish
First shelly fossils 3

4. Geologic time First shelly fossils (Cambrian explosion)
Snowball Earth ice ages
Warm
Rise of atmospheric O2 (Ice age)
Ice age
Warm (?) 4

5. Low Latitude Glaciations
Paleomagnetic data indicate low-latitude glaciation at 2.3 Ga, 0.75 Ga, and 0.6 Ga
Paleoproterozoic glaciations (~2.3 Ga) may be triggered by the rise of O2 and the corresponding loss of CH4
Late Precambrian glaciations studied by Hoffman et al., Science 281, 1342 (1998)

6. The Great Infra-Cambrian Ice Age W. Brian Harland & M. J. S
The Great Infra-Cambrian Ice Age W. Brian Harland & M.J.S. Rudwick, Scientific American 211 (2), 28-36, 1964
Courtesy of Joe Kirschvink

7. Using Magnetic Data to Determine Paleolatitudes Polar Equatorial
Courtesy of Adam Maloof

8. Periglacial Outwash Varves From the Elatina Formation, South Australia
Courtesy of Joe Kirschvink

9. Late Precambrian Geography* (according to Scotese)
* glacial deposits
Hyde et al., Nature, 2000

10. Possible Explanations for Low-Latitude Glaciation
High obliquity hypothesis (George Williams, papers since1970’s)
Equatorial glaciation predicted for obliquities exceeding 54o
Explains how the photosynthetic algae and other life forms survived (polar regions remain ice-free)
Difficult to explain dynamically
Inconsistent with evidence (just shown) for high-latitude glaciation

11. Possible Explanations for Low-Latitude Glaciation (cont.)
2. “Slushball Earth” model (Hyde et al., Nature, 2000)
Tropics remain ice-free (+25o to 25o latitude)  photosynthetic algae survive
Climate state is metastable and, hence, improbable
Requires large mountain ranges on paleo-Australia that do not appear to have existed
Has difficulty explaining cap carbonates…

12. Ghaub Glaciation (Namibia)
Maieberg
“cap”
Glacial
tillite
Courtesy of Joe Kirshvink

13. Possible explanations for low-latitude glaciation (cont.)
“Snowball Earth” model (Joe Kirshvink, 1990)
Easy to explain from a climatic standpoint
Accounts for cap carbonates (indeed, it predicts them!)
Accounts for reappearance of BIFs
“Hard Snowball” model (km-thick ice everywhere) poses significant problems for the photosynthetic biota

14. Triggering a Snowball Earth
Need to get CO2 levels below ~2.5 PAL at 600 Ma, for S/S0 = 0.95 (Hyde et al.)
Possible ways to do this
Continental rifting created new shelf area, thereby promoting burial of organic carbon (Hoffman et al., Science, 1998)
Clustering of continents at low latitudes allows silicate weathering to proceed even as the global climate gets cold (Marshall et al., JGR, 1988)
Various triggers involving CH4

15. In any case, ice albedo feedback takes over
when the polar caps reach some critical
latitude (near 30o) 

16. Increasing CO2
Modern Earth
Caldeira and Kasting, Nature (1992)

17. Recovering from a Snowball Earth episode
Volcanic CO2 builds up to ~0.1 bar over a time span of ~30 m.y. (old result—Caldeira and Kasting)
This estimate may be too low (Pierrehumbert, Nature, 2004)
Our “thin-ice” model recovers at much lower CO2 levels, which it reaches more quickly

18. The carbonate-silicate cycle
(metamorphism)
Silicate weathering slows down as the Earth cools
 atmospheric CO2 should build up 18

19. Recovering from a Snowball Earth episode
Once the meltback begins, the ice melts catastrophically (within a few thousand years)
Surface temperatures climb briefly to 50-60oC
Post-Snowball temperatures would be 10-15o lower in the thin-ice model
CO2 is rapidly removed by silicate (and carbonate) weathering, forming cap carbonates

20. How did photosynthetic life survive the Snowball Earth?
Refugia such as Iceland?
Tidal cracks, meltwater ponds, tropical polynyas? (Hoffman and Schrag, Terra Nova, 2002)
“Thin ice” model (C. McKay, GRL, 2000)
Tropical ice remains thin due to penetration of sunlight

21. Antarctic Dry Valleys McKay’s “thin ice” model was inspired
by his visits to the
Antarctic lakes
Image from: Land Processes
Distributed Active Archive
Center, USGS
Courtesy of Dale Andersen

22. Lake Bonney (Taylor Valley)
Photosynthetic life thrives beneath ~5 m of ice
Courtesy of Dale Andersen

23. McMurdo Sound dive hole
Ice thickness
2.5-3 m
Courtesy of Dale Andersen

24. One of the intrepid explorers
Courtesy of Dale Andersen

25. Photograph by Norbert Wu Life Magazine, Dec. (2004)
Jellyfish photographed
beneath 2 m of Antarctic
sea ice
Photograph by
Norbert Wu
Life Magazine, Dec. (2004)

26. Windows through the ice (McMurdo)
Courtesy of Dale Andersen

27. “Hard” Snowball Earth Ice Thickness
Ts  -27o C (Hyde et al., 2000) Fg z
Toc  0oC
Let
k = thermal conductivity of ice
z = ice thickness
T = Toc – Ts
Fg = geothermal heat flux
Diffusive heat flux:
Fg = kT / z

28. Ice transmissivity (400-700 nm)
Photosynthetic limit
But, if the ice was thin, then sunlight would have penetrated,
and this energy would also have had to exit via thermal
conduction
C. McKay, GRL (2000)

29. Problems with McKay’s Model
Unrealistic treatment of radiative transfer within the ice
Treated ice as being a pure absorber, whereas it mostly scatters radiation in the visible
Did not account for equatorward flow of sea glaciers (Goodman and Pierrehumbert, 2003)

30. Absorption Spectrum of Ice
Visible IR
Warren et al., JGR 107, 3167 (2002)

31. Sea Glacier Schematic Diagram
Ocean
Ice
Atmosphere
90oN or S
Latitude
Equator To Ta qa v h hs
Snow Ts
Ice is snow-covered at high latitudes where P-E > 0
Sea glacier flow follows Weertman (1957), Morland (1987),
and MacAyeal (1996), as modified for global 2-D symmetry
by Goodman and Pierrehumbert (2003)

32. Sea ice flow included Bubbly ice (0 = 0.999) Clear ice (0 = 0.994)
Pollard and Kasting, JGR (2005)

33. Whether or not flow of sea glaciers will preclude the thin-ice solution depends on the details of the calculation, especially: How dirty is the ice?
In any case, there may have been protected basins in the tropics on which thin ice might have formed 

34. The Mediterranean—a possible Snowball refuge

35. Conclusions
A thin-ice Snowball Earth solution is possible under some circumstances. A more detailed physical model is required to say whether or not it is expected.
Such a model does a good job of explaining cap carbonates (unlike the Slushball model)
Photosynthetic life survives this catastrophe much more easily than in the hard Snowball model  the paleontologists should like it