The Neoproterozoic Snowball Earth hypothesis : Tests from the cap carbonates of Namibia

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The close association of Neoproterozoic glacial deposits and overlying cap carbonates has long been regarded as enigma, but now can be reasonably explained by the snowball Earth hypothesis (Hoffman et al., 1998). The basal part (rhythmite ; 14.2 m in thickness) of the Neoproterozoic Rasthof cap carbonate overlying Chuos diamixtite, in the Otavi Group, Namibia was continuously sampled. Chemical profiles of the sequence were obtained by the analysis using the X-ray fluorescence microscope (SXAM). Carbon, oxygen, and strontium isotopic profiles were also measured. We divided it into three successive intervals based on the different characteristics in the chemical profiles. In the lower interval I (1.6 m in thickness), there are no fluctuations in Ca and Mn profiles with relatively small Fe contents. Cyclic changes of Ca and Mn are remarkable in the middle interval. The nature of banding and their possible origin, and cyclicities have been discussed. If the sub-mm scale banding is annual rhythm, the meter-scale banding represent millennial-scale cycles. The time scale of carbon isotopic excursions observed in interval I is estimated to be 103-104 years, which is about 1 or 2 order of magnitude smaller than the LPTM event.