The Cryogenian Period - 2

Dropstones

At a glacial terminus in quiet water, floating ice melts slowly and often drops exotic rocks and sediment far out on a lake bottom. These isolated rocks are referred to as dropstones. In characterizing the Snowball events as involving glaciation, most writers point to the abundance of dropstones as determinative. After all, any mass wasting can lead to an unconsolidated gemisch of unsorted rock that might be mistaken for glacial diamictite after a few millennia. However, dropstones are almost unique to glaciers, and they are abundant around Snowball events. See the following dropstone images from Halverson et al. (2002).

Sounds good, but why would we expect to see dropstones?  Snowball events are supposed to be dominated by sea glaciers. Sea ice doesn't cut through mountains and there is little reason to expect it to contain stones to drop.  Furthermore, the expectation is that a sea glacier would freeze additional water at the bottom, rather than melting. Hoffman & Schrag (1999); Pollard & Kasting (2005). Since a sea glacier is not melting, but growing, in open water, it would drop nothing which was encased in the ice. Sea glaciers would not be expected to drop much of anything until the very end, at the time of the global hothouse. But the Neoproterozoic dropstones are generally found in the middle of layered sedimentary deposits which clearly did not result from a sudden, chaotic thaw accompanied by giant waves, etc.

Workers in this area, without explicitly acknowledging this problem, generally assume that the dropstones are created by sea ice abrading the continental shelf at depth. Note that the sea glacier in most scenarios is expected to exceed 1000 m in height. Accordingly, the glacier would initially impact the continent in deep water, usually on the continental slope and at a considerable distance from shore, since most of the glacier would flow below the surface.  If so, the location of the dropstones might be dispositive. However, even assuming erosion of the continental shelf (which ought to leave some very distinctive signs), and even assuming that rock would be incorporated into "plucked" by) the glacier, a dropstone still has to be dropped, which normally requires the ice to melt. Under Snowball conditions, that shouldn't occur. In fact, Hoffman & Schrag (1999), while addressing another point, argued that material trapped in a Snowball glacier would generally be advected upward. The presence of numerous dropstones would appear to indicate either (a) that the glaciers were continental, not sea glaciers, or (b) that the sea glaciers were regularly melting at the base, suggesting a possible "slushball" scenario with thin or seasonal ice.

We are aware of only one study which attempts to work out the origin of the dropstones, although there are probably others. McMechan 2000) finds that the material transported by Neoproterozoic glaciers in British Columbia was of continental origin, i.e. that the ice sheets were continental, not sea glaciers. Since the material was dropped some distance from shore, it does not seem to have been constrained by massive sea ice and plainly was in water warm enough to cause melting.

Old Salts

Just one more. Numerous authors have mentioned, usually without much comment, the presence of halites or evaporites in association with Snowball events. Chakraborty et al. (2002); Holland (2003); Frimmel & Fölling (2004); Frimmel 2004); Schröder et al. 2004); Sumner & Grotzinger (2004). Since we already have an embarrassing number of unexplained phenomena, no one seems to have had time for this one yet.

Conclusion

You must be kidding!  Conclusions are for people who understand what's going on. ATW050926

Cryogenian Life

You might expect, with this frenetic cycle of freezing, thawing, steaming, crushing, and what-not, that life might have taken a considerable beating.  Perhaps it did, but the evidence is, at best, equivocal. As discussed elsewhere, a number of studies have looked at this issue. Knoll (1994); Corsetti et al. (2003); Porter 2004); Olcott et al. 2005); Huntley et al. (2006). The Tonian data seem to indicate a break from the billion-year stasis of the Mesoproterozoic. The Ediacaran included two (or even three) enormous bursts of diversification which we associate with the ediacaran fauna and the evolution of animals. What happened in between is less clear. The most sophisticated study currently is probably Huntley et al. (2006). They, and a number of older papers, claim a dip in diversity and imply a loss of abundance during the Cryogenian. However, the data are perhaps not good enough to reach either conclusion; and no one's results indicate any mass extinction or significant evolutionary bottleneck.   

Several important groups may even have first evolved, or undergone important development, during the Cryogenian, including the red and green algae, dinoflagellates, ciliates, stramenopiles, and testate amoebae. Porter & Knoll 2000). The amoebae are particularly important, as they represent the first fossil record of eukaryotic heterotrophs, the stem group of fungi and animals. All of this evolutionary novelty doesn't rule out slushball episodes, but it does seem inconsistent with the most extreme hard freeze scenarios.

Interestingly, testate amoebae are becoming important tools in Holocene paleoclimatology as extremely sensitive indicators of water level and environmental stability. See, e.g., Davis & Wilkinson (2004). The utility of this biomarker on the scale of megayears is unclear. However, given the scarcity of reliable climatic proxies in the Cryogenian, it might be worth looking into.

ATW061209.