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

新元古代雪球地球假说：来自纳米比亚盖碳酸盐岩的测试

基本信息

批准号：
11640414
负责人：
KAWAKAMI Shin-ichi
金额：
$ 1.98万
依托单位：
Gifu University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2000
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11640414/
关键词：
Earth History Neoproterozoic Snowball Earth hypothesis Ice Age Climate Change carbonate geochemisty Shima-shima gaku 現生代氷河期同位対比地球化学 Snowball Earth仮説ナミビア

项目摘要

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.

新元古代冰川沉积物和上覆的帽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合理地解释（霍夫曼等人，1998年）。对纳米比亚Otavi群中覆盖Chuos杂岩的新元古代Rasthof帽碳酸盐岩的基底部分（韵律岩;厚度为14.2 m）进行连续取样。通过使用X射线荧光显微镜（SXAM）的分析获得序列的化学轮廓。碳，氧，锶同位素配置文件也进行了测量。我们根据化学剖面的不同特征将其分为三个连续的区间。在较低的间隔I（1.6米的厚度），有没有波动的Ca和Mn的配置文件与相对较小的Fe含量。Ca、Mn的周期变化在中期显著。本文还讨论了环带的性质、可能的成因和旋回性。如果说亚毫米尺度的环带是年韵律，那么米尺度的环带则代表千年尺度的旋回。在I期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。