1. Chapter 12—Part 2: Glacial Evidence/Snowball Earth

2. Evidence for glaciations Brooks Range, Northern Alaska
(From Skinner & Porter,
The Blue Planet, p. 294)
Glacial till—pieces of rock picked up by glaciers as they move
across the landscape
Morraine—piles of glacial till deposited at the terminus (terminal
morraine) or sides (lateral moraine) of a glacier
Cirque—a steeply walled canyon carved into a mountain by
a glacier

3. Evidence for glaciations
Diamictite—a rock containing unconsolidated smaller
fragments
Tillite—a diamictite produced by burial of glacial till

4. Evidence for glaciations
Striations—parallel scratchings on rock surfaces caused by
the passage of glaciers (bearing rocks)

5. Evidence for glaciations
Dropstones—Isolated rocks found in smoothly laminated
marine sediments that are interpreted as having
fallen from melting icebergs

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

7. 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)

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

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

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

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

12. Possible explanations for low-latitude glaciation
High obliquity hypothesis
Earth’s spin axis was tilted at a high angle (> 54o to its orbital plane (currently 23.5o)
“Slushball Earth” model
The tropical oceans remained ice-free
Glaciers formed on high mountain tops and flowed down to the seashore faster than they could melt
“Snowball Earth” model
The Earth was completely ice-covered for as long as tens of millions of years
Joe Kirshvink and Paul Hoffman have been the champions

13. Strengths/weaknesses of the Snowball Earth model
Easy to explain from a climatic standpoint
Accounts for cap carbonates
Indeed, it predicts them!
Accounts for reappearance of BIFs
Weaknesses
“Hard Snowball” model (km-thick ice everywhere) poses significant problems for survival of photosynthetic organisms

14. Ghaub Glaciation (Namibia)
Maieberg
“cap”
Glacial
Tillite
Courtesy of Joe Kirshvink

15. Triggering a Snowball Earth
Need to reduce the concentrations of greenhouse gases (CO2 and/or CH4)
Possible ways to do this
Continental rifting created new shelf area, thereby promoting burial of organic carbon
Clustering of continents at low latitudes allows silicate weathering to proceed even as the global climate gets cold
Mid-Proterozoic CH4 levels were high because the ocean was sulfidic (no O2), then they dropped because of an increase in either O2 or sulfate

16. In any case, ice albedo feedback takes over
when the polar caps reach some critical
latitude (near 30o)
Surface
temperature
Snow and ice
cover
(+)
Planetary
albedo

17. Modern Earth
Caldeira and Kasting, Nature (1992)

18. Recovering from a Snowball Earth episode
Volcanic CO2 builds up in the atmosphere until the greenhouse effect becomes big enough to melt the ice
Once the meltback begins, the ice melts catastrophically (within a few thousand years)
Surface temperatures climb briefly to 50-60oC
CO2 is rapidly removed by silicate (and carbonate) weathering, forming cap carbonates

19. How did photosynthetic life survive the Snowball Earth?
Refugia such as Iceland?
Tidal cracks, meltwater ponds, tropical polynyas? (Hoffman and Schrag, Terra Nova, 2002)
“Jormungand” model (Abbot et al., JGR, 2011)
“Thin ice” model (C. McKay, GRL, 2000)
Tropical ice remains thin due to penetration of sunlight

20. Jormungand state
According to Abbot et al., liquid water remained present in a thin, wavy strip near the equator
This model looks like a mythical Norwegian sea serpent that was big enough to circle the Earth and grasp its tail in its mouth
The basic reason this state is possible is the same as for the thin ice model: the albedo of bare ice is much lower than that of snow
Abbot et al. (2011)

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

26. “Hard” Snowball Earth ice thickness
Ts  -27o C (Hyde et al., 2000) Fg z
Toc  -2oC
Let
k = thermal conductivity of ice
z = ice thickness
T = Toc – Ts
Fg = geothermal heat flux

27. “Hard Snowball” (cont.)
The diffusive heat flux is: Fg = kT / z
Solving for z gives:
z = kT / Fg
 (2.5 W/m/K)(29 K)/(9010-3 W/m2)
z  652 m
 No sunlight gets through

28. Thin ice model
Alternatively, if the ice were thin (1-2 m), then it would have let an appreciable amount of sunlight through
That energy would have had to escape by conduction through the ice
The ice could have been maintained in a thin state by this heat flux because the solar flux is much higher than the geothermal heat flux
Geothermal heat flux: Fg = 90 mW/m2
= 0.09 W/m2
Solar heat flux: FS = 1370 W/m2  (1/4)  (1-0.3)
 240 W/m2

29. Thin ice model (cont.)
The solar flux near the equator is about 20% higher than the global average, so ~300 W/m2
Suppose 1/10th of the sunlight makes it through the ice: 30 W/m2
This is higher than the geothermal heat flux by a factor of 30/0.09 = 333
So, the ice should be 333 times thinner than in the hard Snowball model: 652 m/333  2 m

30. Ice transmissivity (400-700 nm)
Possible
solution at
equator
Photosynthetic limit
C. McKay, GRL (2000)

31. Conclusions
A “Snowball Earth” of some sort probably did occur during both the early and late Proterozoic
An open-water (Jormungand) or thin-ice solution is possible under some circumstances. A more detailed physical model is required to say which of these models is correct.
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