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

新元古代冰川沉积和上覆盖层碳酸盐的密切联系长期以来一直被认为是一个谜，但现在可以用雪球地球假说来合理解释(Hoffman等人，1998)。连续采集了纳米比亚奥塔维群新元古代覆盖卓斯斜长石盖层的碳酸盐岩底部(韵律岩，厚度14.2米)。利用X射线荧光显微镜(SXAM)进行分析，获得了该序列的化学图谱。还测量了碳、氧和锶的同位素分布。根据化学剖面的不同特征，我们将其划分为三个连续的层段。在较低的I段(厚1.6m)，Fe含量相对较低的Ca、Mn剖面没有起伏。钙、锰的旋回变化在中段显著。讨论了条带的性质及其可能的起源和周期性。如果亚毫米尺度的条带是年节律，那么米尺度的条带代表千年尺度的周期。在间隔I中观测到的碳同位素漂移的时间尺度估计为103-104年，比LPTM事件小1~2个数量级。

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.