The Snowball Earth hypothesis explains sea-level glaciation near the late Neoproterozoic equator by invoking ice-albedo runaway to completely entomb the oceans beneath sea-ice. Such an "ultimate" ice age should last for millions of years while sufficient atmospheric greenhouse gas accumulates via volcanic degassing to force melting. This long, cold, ice age would be followed by rapid, hot deglaciation. Hypothesized rapid deglaciation appears supported by ubiquitous presence of presumed ∼635 Ma "cap carbonates" containing unusual sedimentary features and exceptional geochemical signatures.
This thesis challenges simple interpretation of cap carbonate features, suggesting a need for new, detailed versions of the Snowball Earth hypothesis. In particular: (1) In the moat of an active salt-withdrawal syncline, interpreted Snowball-glacial strata of South Australia's Elatina Formation are conformably overlain by Nuccaleena cap dolostone, negating conventional inference of initial deglacial unconformity. Considering the full range of Elatina Formation facies variation, it is not clear whether the proper position of initial Elatina–Nuccaleena deglaciation lies at the base of the Nuccaleena Formation or at the base of Elatina Formation itself. (2) Three correlatable geomagnetic reversals punctuate Nuccaleena cap dolostone near its base, in its middle, and in its upper, mixed transition to postglacial siltstone. This magnetostratigraphic signal, carried by detrital hematite grains, offers potential for global chronostratigraphy. If modern reversal duration statistics extrapolate to the late Neoproterozoic, ultimate deglaciation of Snowball Earth was prolonged over ∼50,000–500,000 years. The apparent contradiction of this timescale with anomalous cap carbonate physical sedimentology and the characteristic timescale for recent ice sheet decay might be explained by pervasive microbial binding and aerosol feedbacks. (3) Elatina glaciation reached a paleolatitude of 14.3 ± 2.0° - lower than the critical latitude for ice-albedo runaway, but higher than previously considered.
An alternative explanation to these paleomagnetic challenges to conventional Snowball Earth interpretation could invoke an itinerantly nonuniformitarian (anomalously weak and/or non-dipole dominated) geomagnetic field. In such a case, however, conventional paleolatitude calculations which motivated the Snowball Earth hypothesis in the first place must also be reconsidered. Anomalous geomagnetic field geometry might be supported by greater-than-expected magnetization dispersion recorded by Nuccaleena cap dolostone.
|Advisor:||Evans, David A. D.|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 69/06, Dissertation Abstracts International|
|Keywords:||Cap carbonates, Deglaciation, Elatina Formation, Paleomagnetism, Snowball Earth|
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