新元古代晚期盖帽碳酸盐岩的成因与“雪球地球”的终结机制

Formation of Late Neoproterozoic Cap Carbonates and Termination Mechanism of “Snowball Earth”

新元古代晚期约635Ma的地球发育了到达赤道附近的冰川作用,地质记录上表现为代表寒冷气候的冰期沉积杂砾岩,直接被代表温暖环境的碳酸盐岩层(常称盖帽碳酸盐岩)覆盖。由于盖帽碳酸盐岩奇特的岩石学和地球化学特征,引起了对其成因认识的巨大争论,提出了“雪球地球”和“甲烷渗漏”等假说。“雪球地球”假设可以解释一些令人困惑的地学现象,如低纬度和低海拔冰川沉积、盖帽碳酸盐岩、碳酸盐δ13C负漂移和条带状铁矿层等,但许多科学家对此提出了质疑。最近对盖帽碳酸盐岩的δ13C分析结果(最低达-41‰)、盖帽碳酸盐岩发育的类似现代冷泉碳酸盐岩沉积组构等似乎支持“甲烷渗漏”假说。

The Earth＇s most severe glaciation is considered to have occurred about 635 million years ago, in the late Neoproterozoic era, and is believed to have engulfed the entire earth under ice to create what came to be described as ＂Snowball Earth＂. Carbonate rocks directly and ubiquitously overlie Neoproterozoic glacial diamicrites on almost every continent, and are commonly referred to as ＂cap carbonates＂. Their unusual facies and strongly negative carbon isotopic signature stirred up considerable debates in the academic community, as they wonder how cap carbonates could have formed, thus ＂Snowball Earth＂ and ＂Methane Seep＂ were hypothesized. The ＂Snowball earth＂ hypothesis can explain many phenomena related to the Neoproterozoic glaciations, cap carbonates have been controversially ascribed to the aftermath of almost complete shutdown of the ocean ecosystems for millions of years during such ice ages. Conversely, it has also been suggested that these carbonate rocks were the result of destabilization of methane hydrates during deglaciation and concomitant flooding of continental shelves and interior basins. Recent carbon isotopic data of cap carbonates （δ^13C as low as -41‰） provides direct evidence for methane -influenced process during deglaciation, strongly supported ＂Methane Seep＂ hypothesis.