扬子地台新元古代碳酸盐盖帽碳、氧同位素研究

Carbon and Oxygen Isotopic Variations of Neoproterozoic Cap Carbonates in Yangtze Platform

地质历史上最严重的冰期发生在新元古代末期（约635Ma），其结束的标志是沉积了一层薄薄的、横向连续展布的、δ^13C呈明显负异常的碳酸盐盖帽。地质学家提出了多种成因模型来解释碳酸盐盖帽δ^13C负漂移，最著名的有：生物产率下降说，滞留海水翻转说和甲烷渗漏分解说。对黔南、浙西新元古代碳酸盐盖帽碳氧同位素研究表明，δ^13C负异常是甲烷渗漏和海水翻腾共同作用的结果。理由如下：①贵州瓮安和朵丁剖面的岩石结构与现代和古代甲烷渗漏形成的碳酸盐沉积相似；②碳酸盐盖帽上覆的浅水相磷块岩沉积为海水倒转说提供了佐证，因为浅水区磷质来源于水体倒转时携带的深海磷。始于Marinoan冰期结束时海水倒转导致碳酸盐盖帽的形成，并促使磷块岩在浅水区域的沉积成矿。水体倒转的另一效应是将深海负^13C的碳酸盐带到浅水区域，碳酸盐盖帽和上覆磷块岩的负δ^13C值证实了这一点。

The end of the Earth＇s severe glaciations that occurred in late Neoproterozoic about 635 millions years ago is characterized by thin and laterally continuous cap carbonates associated with some of the largest known negative anomalies of δ^13C. To account for these strange geological phenomena, various models have been proposed, such as the shutdown of ecosystem productivity, the overturn of a stagnant glacial ocean, and the destabilization of permafrost gas hydrate. The author＇s study indicates that the negative excursions of carbon isotopic compositions of Neoproterozoic cap carbonates in southern Guizhou and western Zhejiang resulted from the combined action of destabilization of gas hydrate and overturning of sea water. Two pieces of evidence support this opinion： ① The lithologic structures of cap carbonates in Douding profile and Weng＇an profile in Guizhou are similar to those precipitated in modern or ancient seeps. ② The appearance of phosphorous deposits that overlie the cap carbonates in some shallow water areas supports the “oceanic overtum”hypothesis in that the resources of phosphorites were brought from the deep ocean in the process of overturn at the end of Marinoan glaciations. Another result of overturn was the transport of δ^13C depleted carbonate to the shallow water area. The negative excursion of δ^13C in cap carbonates and the overlying phosphorites can confirm this opinion.