Snowball Earth hypothesis challenged, again

Palaeomagnetic data from localities famed for their Neoproterozoic glaciogenic rocks point persuasively to several epochs between 750 and 550 Ma when widespread continental glaciation took place at low latitudes.  It is this evidence, along with theoretical consideration of drastic changes in the Earth’s albedo that would result from tropical land ice, that encouraged the idea of pole to pole ice cover.  Only a build-up of volcanogenic CO2 in the atmosphere could prevent such a “Snowball Earth” lasting indefinitely, and even with such relief it would have endured for millions of years.  Much of the geological evidence cited by those who support and promote this neo-catastrophic idea comes from excellent, but geographically quite limited occurrences of tillites or glaciomarine sediments, such as those of Namibia.  Some occurrences have never been seriously analysed, except as examples that superficially support the hypothesis.  One such sequence is that of Arabia, easily accessed in northern Oman and described by a British-Swiss team (Leather, J. et al. 2002.  Neoproterozoic snowball Earth under scrutiny: Evidence from the Fiq glaciation of Oman.  Geology, v. 30, p. 891-894).

Isotopic studies of carbonates from glaciogenic sediments (see Meltdown for Snowball Earth? in Earth Pages News for February 2002) seriously undermined several arguments by “Snowball Earth” supporters, but are open to various interpretations.  Hard geological evidence is less easy to rationalize.  A growing number of  Neoproterozoic glaciogenic sequences, such as the Port Askaig Tillite of the Scottish Dalradian Supergroup and others from the Congo and Kalahari cratons, and Laurentia, show dropstone-rich diamictites interbedded with sediments that show little if any sign of a glacial influence (Condon, D.J. et al. 2002.  Neoproterozoic glacial-rainout intervals: Observations and implications.  Geology, v. 30, p. 35-38).  Such evidence can be explained by climatic change and a fully functioning hydrological cycle.  The report on the Omani example by Leather and colleagues highlights splendid examples of sediments that mark cycles of glacial advance and retreat, reminiscent of those of the Pleistocene glacial epoch and more or less the same as in many Neoproterozoic occurrences.  It can only be a matter of time before Australian geologists enter the fray decisively, for glaciogenic sediments comprise up to 30% of the many-kilometres thick Umberatana Group in the Neoproterozoic of the Flinders Range in South Australia, and there are several other stratigraphically distinct diamictite sequences.

It seems likely that the “Snowball Earth” hypothesis is waning; an embarrassment for those geologists who have promoted it so assiduously over the last several years.  However, the enigma of low-latitude glaciation on a vast scale is likely to remain, unless, that is, all the diamictites can be shown to have non-glacial origins, which is not as unlikely as it might seem.  The Fiq sequence of Oman, like the Dalradian example in Scotland, formed in an actively extending basin.  Repeated seismicity on rift-bounding faults could have launched debris flows to deposit diamictites (a purely descriptive term for sediments containing a wide variety of clast sizes).  The most spectacular diamictite in the Dalradian Supergroup, and perhaps anywhere, is the Great Breccia of the Garvellachs.  Recent work suggests strongly that it is not glaciogenic, but the product of such a debris flow (Arnaud, E. & Eyles, C.H. 2002.  Catastrophic mass failure of a Neoproterozoic glacially influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland.  Sedimentary Geology, v. 151, p. 313-333).  The supposedly clinching evidence for diamictites’ origin from iceberg armadas is the way in which some clasts (“dropstones”) puncture underlying stratification.  All that is required is a means of puncturing, and sediment compaction around large, resistant clasts in a water saturated matrix is quite capable of doing that.  Even the long-held belief that glaciation is uniquely signified by polished and striated surfaces beneath diamictites containing similarly scratched clasts is coming into question.  Sites of large impacts, such as the Ries crater in Germany, include exactly similar features caused by ejecta blasted from the crater, cited by Vern Oberbeck, formerly of NASA, in a little-cited paper that proposed an impact origin for diamictites (Oberbeck, V.R. et al. 1993.  Impacts, tillites and the breakup of Gondwanaland.  Journal of Geology, v. 101, p. 1-19).

Post-apocalypse weathering in the Early Triassic

Environmental crises do not come bigger than that at the end of the Permian, when marine ecosystems virtually collapsed, and similar extinctions of terrestrial flora and fauna are becoming clear.  Whereas the Siberian Traps may indeed have been a triggering mechanism, there are carbon-isotope indicators that vast amounts of methane entered the atmosphere shortly afterwards, rapidly being oxidised to CO2.  The density of respiratory openings (stomata) in fossil leaves from the lowest Triassic is unusually low, indicating an abundance of CO2 in the atmosphere and probably enhanced “greenhouse” conditions.  Hot and humid conditions encourage weathering of the continental surface, and there are many Early Triassic palaeosols, some which mimic those in the tropics being found at unusually high palaeolatitudes.  Such soils harbour crucial evidence for surface conditions, and the high-latitude ones present a surprise (Sheldon, N.D. & Retallack, G.J. 2002.  Low oxygen levels in earliest Triassic soils. Geology, v. 30, p. 919-922).  Unlike tropical laterites, which are rich in kaolinite, high-latitude soils are dominated by illitic clays that signify incomplete breakdown of silicates.  The surprise comes in the form of an unusual mineral, berthierine; a green, serpentine-like mineral that is easily confused with chlorites in hand specimen.  It can form by reaction between clays and ferric oxy-hydroxides, but only under highly reducing conditions.  Because most soils since about 2000 Ma ago have formed in contact with an increasingly oxygen-rich atmosphere, achieving suitably reducing conditions demands input of a reductant to the soil “atmosphere”.  The most likely candidate is methane, whose oxidation would consume oxygen.  However, methane’s residence time in the air is around 10 years, because it is quickly oxidised to CO2, so methane release following the P-Tr boundary event seems as if it was sufficiently prolonged to influence considerably longer term soil formation.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s