Gondwana Research 11 (2007) 573–574www.elsevier.com/locate/gr

Correspondence

Testing the Neoproterozoic glacial models

The interval of time immediately preceding the EdiacaranPeriod (N635 Ma to ∼800 Ma) is punctuated by extremeglaciations that appear to extend to low latitudes (Hoffman et al.,1998a; Evans, 2000). Two main models have been forwarded toexplain these glaciations. Williams and Schmidt (2004) call on adrastic change in the Earth's obliquity in order to elucidate theselow latitude glaciations. Hoffman et al. (1998a) called upon acomplete freezing of the Earth dubbed “Snowball Earth”.

The high obliquity model is testable. First, the distributionglacial rocks should be constrained to latitudes less than ∼45°(Evans, 2000). Secondly, the deposition of latitudinallydependent facies (evaporites, redbeds) should show a spatialdistribution consistent with the model.

At present, no reliable high latitude paleomagnetic poleshave been obtained from Neoproterozoic glacial facies andtherefore the high obliquity model cannot be rejected on thisbasis. In his analysis on 2.5 billion years worth of evaporitefacies, Evans (2006) found them to be restricted to latitudesbetween 10°–35°. Because a high obliquity earth would shiftevaporite facies equator-ward, the analysis of Evans (2006)falsifies the high obliquity model.

Fig. 1. 13C curve modified from Halvorsen et al. (2005). The dashed lines follow thecurve along with the extent and length of glaciations follows Halvorsen et al. (2005).(Sturtian and Marinoan A). The darker dashed line shows the time-shifted 13C curve fcurves are identical for the pre ∼790 Ma and post 636 Ma periods. The ‘outlier’ geocnegative 13C anomalies are thought to mark the onset of the Marinoan and Gaskiers

1342-937X/$ - see front matter © 2006 International Association for Gondwana Rdoi:10.1016/j.gr.2006.11.002

The snowball earth model is also falsifiable. These tests canbe broken down into two groups— conceptual models andground truth. Models may examine mechanisms for plungingthe earth into a snowball state (e.g. Schrag et al., 2002;Donnadieu et al., 2004), coming out of the snowball state(Hoffman et al., 1998b) or the extent of the glaciations (Hydeet al., 2000). Paleomagnetic tests of the snowball earth are a bittrickier since, in principle, any paleolatitude would fit thesnowball earth model. Hoffman et al. (1998a) claim that theinitial stages of cap carbonate deposition will be extremely rapid(20 m in only 50 years), thus paleomagnetic directions in capcarbonates should exhibit little secular variation and noreversals (Trindade et al., 2003). The problem with testing theconceptual models of the snowball earth hypothesis is that anyfailures can be accommodated in a new model.

The snowball earth hypothesis implies that the glaciationsshould be synchronous events. Hoffman et al. (1998a,b) explainthat synchroneity is demonstrated by the variation in stableisotopes (13C) before, during and after the glacial events. Knoll(2000) and Halvorsen et al. (2005) argued that the 13C signal isa robust and near instantaneous recorder of seawater chemistryand therefore useful as a relative geochronometer. Thenecessary absolute age controls on isotopic curves/glacial

centerline of the highest density of observations. The ages along the top of theThe lighter dashed lines represent the 13C curves for the light-grey lined epochsor the second estimate of the glacial episodes (Sturtian and Marinoan B). The 13Chronologic ages are listed along the base of the graph. The Trezona and Wonokaglacial epochs respectively.

esearch. Published by Elsevier B.V. All rights reserved.

574 Correspondence

deposits have yet to be resolved in most cases. If the 13C curvesand the glacial events can be independently correlated withprecise geochronometry, then one of the major criticismsagainst the Snowball hypothesis can be eliminated.

The number of absolute age determinations on theNeoproterozoic glacial deposits has recently increased. Halvor-sen et al. (2005) used many of these ages to develop a global13C curve for the Neoproterozoic glaciations. The growth of thegeochronologic database resulted in questions regarding thenumber and synchroneity of the glacial events. Fig. 1 shows theglobal 13C curve proposed by Halvorsen et al. (2005).Halvorsen et al. (2005) used geochronologic data to definethe limits of the three main glaciations (Sturtian, Marinoan andGaskiers). They suggest that the Sturtian glaciations occurredbetween 700–746 Ma and give two possible ranges for theMarinoan, both ending at 636 Ma. The Gaskiers is thought to bea non-global glaciation that took place ∼580 Ma.

Recent geochronologic data illustrate the difficulties in using13C trends in global correlation. Lund et al. (2003) report an age of∼685 Ma for the Edwardsburg Formation (Idaho) placing theSturtian glaciation∼15Ma younger than shown in Fig. 1. Kendallet al. (2006) report a 657±5 Ma Re–Os age from the AralkaFormation (∼7–9 m above the Areyonga diamictite, Australia)and 643±2.4 Ma from the Tindelpina Formation (Australia).These Re–Os ages are supported by recent 658 Ma U–Pb agesfrom the Merinjina Formation (Australia, Fanning, 2006). Theseages imply that the archetypal ‘Sturtian’ glaciations are eitherdiachronous, have been miscorrelated or that there are more short-lived glacial epochs (Kendall et al., 2006; Fanning, 2006).

The ‘Snowball Earth’ hypothesis assumes that globalglaciations in the Neoproterozoic are represented by synchro-nous and distinct episodes. Correlations between these eventsare based primarily upon features in the 13C record. Theassumption that the features of the 13C curves are distinctenough to allow correlation in the absence of absolute ages ischallenged by a growing geochronologic database. Futuregeochronologic work on these critical sequences may resolvethe current conundrum and alternative explanations for theseenigmatic glaciations should be considered (e.g. Leather et al.,2002; Eyles and Januszczak, 2004).

References

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Eyles, N., Januszczak, N., 2004. ‘‘Zipper-rift’’: a tectonic model forNeoproterozoic glaciations during the breakup of Rodinia after 750 Ma.Earth-Science Reviews 65, 1–73.

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Schrag, D.P., Berner, R.A., Hoffman, P.F., Halverson, G.P., 2002. On theinitiation of a snowball Earth. Geochemistry, Geophysics and Geosystems,3. doi:10.1029/2001GC000219.

Trindade, R.I.F., Font, E., D'Agrella-Filho,M.S., Nogueira, A.C.R., Riccomini, C.,2003. Low-latitude and multiple geomagnetic reversals in the NeoproterozoicPuga cap carbonate, Amazon craton. Terra Nova 15, 441–446.

Williams, G.E., Schmidt, P.W., 2004. Neoproterozoic glaciation: reconcilinglow paleolatitudes and the geologic record. In: Jenkins, G.S., et al. (Ed.),The Extreme Proterozoic: Geology, Geochemistry, and Climate. AmericanGeophysical Union Geophysical Monograph, vol. 146, pp. 145–159.

Joseph G. MeertDepartment of Geological Sciences,

241 Williamson Hall, Gainesville, FL 32611 USAE-mail address: jmeert@ufl.edu.

Tel.: +1 352 846 2414; fax: +1 352 392 9294.

6 November 2006