Snowball Earth Hypothesis

Paleomagnetism and Neoproterozoic glacial deposition

Paleomagnetism takes advantage of the alignment of magnetic minerals in rock deposits (“termed natural remnant magnetization”) to establish where the deposits were formed.  If the magnetic minerals are aligned horizontally, the deposit formed at an equatorial region; conversely, if the magnetic minerals are vertical, they formed at a polar region (Global Climate Change Course).

Neoproterozoic glacial features have been recorded around the world. But what is even more striking is evidence pointing to glacial deposits in very low latitude, near the equator. Harland with his team have shown evidence of glacial deposits at a number of sites near the equator by utilizing the paleomagnetism technique to estimate the latitudes of a number of Neoproterozoic sites (Hoffman, 2002). Linda Sohl at Lamont-Doherty Earth Observatory confirmed Harland's study, by doing a finer paleomagnetism revision of six Neoproterozoic glacial deposits, showing polarity reversals in glacial deposits, evidence that the glacial deposits have been around for 100 of thousands of years, or even longer (Hoffman, 2002). The Neoproterozoic glacial deposit paleomagnetic studies are irrefutable, they point to prolonged glacial depositions at near equator latitudes, this is consistent with snowball Earth phenomena (Hoffman, 2002).

Figure 3, Neoproterozoic glacial features. Striated pavement (a) overlain by the Smalfjord diamictite at Bigganjargga, Varangerfjord, north Norway (Reusch, 1891) (G.P. Halverson photo) (Hoffman, 2002)

Banded iron-formations

Several examples of Neoproterozoic glacial deposition in marine waters are abnormally rich in iron oxides and sulfides. This type of sedimentary ore deposit is called banded iron-formation or BIF, which is otherwise restricted to much earlier time in Earth history (Huffman, 1999). Most BIF transpire in rocks older than 1850 million years and are supposed to have formed at a time when the atmosphere had little free oxygen and seawater in the deep ocean contained abundant iron. This iron precipitated in upwelling zones when it encountered more oxidizing surface waters (Huffman, 2002). The transitory return of BIF, habitually associated with glacial deposits, after a gap of over a billion years is puzzling (Huffman, 1999).

Figure 4, Dolomite dropstone in banded iron formation (d) within the Rapitan diamictite at Snake River, Mackenzie Mountains (G.A. Gross photo) (Hoffman, 2002).

In a snowball Earth scenario the deep ocean would quickly become anoxic, allowing iron to buildup to high concentrations (Huffman, 2002). But once the glaciation ended, the ocean would quickly become oxidized, allowing the iron to precipitate out in close association with the deposits of sediment-laden icebergs (Huffman, 1999).

“Cap” dolostones

Neoproterozoic "cap" dolostones are a world-wide occurrence, particularly unusual in regions where carbonate rocks are otherwise lacking.

The alteration from glacial deposits to "cap" dolostone is sudden and lacks evidence of significant interval (Huffman, 1999). In the thawing period of snowball Earth, the concentrated chemical weathering of silicate rocks and termination of carbonate rocks would be a consequence of a strong hydrologic cycle, the low pH of carbonic acid rainfall, and the large surface area of frost-shattered rock and fine grained rock particles produced by the grinding action of glaciers. The products of chemical weathering reactions and bicarbonate would be distributed by rivers to the ocean, where they would counteract the acidity of the surface waters and force massive precipitation of inorganic carbonate sediment in the quickly warming surface ocean (Huffman, 1999). "Cap" dolostones are no contradiction; they are the probable outcome of the "ultra-greenhouse" environment unique to the momentary consequences of a "snowball" Earth.

Figure 5, Rapitan diamictite sharply bounded (c) by carbonate strata, without transitional facies or intercalation at Stone Knife River, Mackenzie Mountains, north-west Canada (Hoffman, 2002).

Life, at peak snowball Earth conditions

On a completely ice-covered snowball Earth the thickness of ice in the tropical regions would be limited by the sunlight piercing into the ice cover and by the “latent heat flux generated by freezing at the ice bottom”, the freezing speed would balance the “sublimation rate from the top of the ice cover” (Space Science Division, 2000). Heat transport models of the permanently ice-covered Antarctic dry valley lakes applied to the snowball Earth indicate that the tropical ice cover would have a thickness of 10 meters. At this thickness of ice, more than 0.1% of light will still penetrate the ice cover. This light level is adequate for photosynthesis and could explain the survival of the eukaryotic algae (Space Science Division, 2000).

Conclusion