Chapter 12—Part 2: Glacial Evidence/Snowball Earth

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

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

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

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

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 (?)

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)

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

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

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

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

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

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

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

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

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

Modern Earth Caldeira and Kasting, Nature (1992)

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

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

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)

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 http://LPDAAC.usgs.gov  Courtesy of Dale Andersen

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

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

One of the intrepid explorers Courtesy of Dale Andersen

Windows through the ice (McMurdo) Courtesy of Dale Andersen

“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

“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

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

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

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

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