Collaborative Research: Testing the Methane Hydrate Hypothesis for the Aftermath of Severe Neoproterozoic Ice Ages

摘要地质记录表明，天然气水合物失稳过程中甲烷的周期性释放可能是气候变化和海洋化学的一个重要因素。由于甲烷水合物存在于微妙的压力-温度平衡中，并且构成了地球表面最大的可交换碳库，因此海洋温度或海平面的细微变化可能导致失控的不稳定，对生物圈造成严重后果。这些事件最有可能发生在全球范围内快速变暖和/或海平面变化的时期。由于新元古代冰期（~850 Ma至600 Ma）的寒冷气候范围，这些事件的后果可以说是地球历史上研究这一现象的最佳间隔。此类事件的证据包括几乎覆盖所有新元古代冰川期的神秘盖层碳酸盐矿床的存在，以及与此类全球甲烷释放事件的预期范围和持续时间相似的强烈碳同位素异常。对这种甲烷水合物不稳定假说最令人信服的批评是，在当地推断与甲烷渗漏有关的盖层碳酸盐岩中，明显缺乏极端的碳同位素变化。在现代渗漏层和至少一些古代渗漏层中沉淀的碳酸盐通常与13C值小于-25%有关。在现代和古代渗漏中常见的其他观察结果包括硫同位素异常，以及消耗甲烷微生物的生物标志物证据。来自中国南方新黑古宙盖层碳酸盐岩组（陡山沱组）的初步数据表明，这些缺失的证据线元素确实存在于局部保存异常完好的类渗相中。本研究通过对中国盖层碳酸盐岩的详细研究，将为了解古渗漏相关过程提供一个窗口。这项研究的更广泛的目标是验证这样一个观点，即这些渗漏是全球范围内成岩蚀变程度更高的帽状碳酸盐岩中渗漏结构的代表，以及帽状碳酸盐岩和冰川后碳同位素漂移提供了新元古代雪球地球冰期后全球变暖的灾难性全球甲烷释放记录。

AbstractStudies of the geologic record suggest that episodic methane release from destabilization of gas hydrates may act as an important factor in climate change and marine chemistry. Because methane hydrates exist in delicate pressure-temperature equilibrium, and constitute the largest pool of exchangeable carbon at the Earth's surface, the possibility exists that subtle changes in ocean temperature or sea level could result in a runaway destabilization with dramatic consequences for the biosphere. These events are mostt likely during times of rapid global-scale warming and/or sealevel change. By virtue of the extent of cold climate during Neoproterozoic ice ages (from ~850 Ma to 600 Ma), the aftermath of such events is arguably amongst the best intervals in Earth history for investigating this phenomenon. Evidence for such events includes the presence of enigmatic capping carbonate deposits that overlie almost all Neoproterozoic glaciogenic and a strong carbon isotopic anomaly of similar nagmitude and duration to that anticipated for such a global methane-release event.The most compelling criticism of this methane hydrate destabilization hypothesis is the apparent lack of extreme carbon isotopic variation in cap carbonates inferred locally to be associated with methane seeps. Carbonate precipitated in modern seeps and in at least some ancient ones is typically associated with 13C values ranging from less than -25%. Other observations that are common in modern and ancient seeps include sulfur isotopic anomalies, and biomarker evidence for methane-consuming microbes. Preliminary data from a Neoporterozoic cap carbonate formation in southern China (the Doushantuo Formation) indicate that elements of these missing lines of evidence are, indeed, present locally in an exceptionally well preserved example of seep-like facies. This study will provide a window into ancient seep-related processes by studying the Chinese cap carbonates in detail. The broader objective of the proposed research is to test the proposition that these seeps are representative of seep like fabrics in more diagentically altered cap carbonates globally, and taht cap carbonates and post glacial carbon isotopic excursions provide a record of catastrophic global methane release coincident with global warming following Neoproterozoic snowball Earth ice ages.