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

摘要对地质记录的研究表明，天然气水合物失稳的幕式甲烷释放可能是气候变化和海洋化学的一个重要因素。由于甲烷水合物以微妙的压力-温度平衡状态存在，并构成了地球表面最大的可交换碳库，因此存在这样一种可能性，即海洋温度或海平面的细微变化可能会导致失控的不稳定，从而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迅速变暖和/或海平面变化的时期。由于新元古代冰期(约850 Ma至600 Ma)气候寒冷的程度，这类事件的余波可以说是地球历史上研究这一现象的最佳时段之一。这类事件的证据包括存在神秘的盖层碳酸盐沉积，覆盖了几乎所有的新元古代冰川成因，以及与预期的全球甲烷释放事件类似的强烈的碳同位素异常。对这种甲烷水合物失稳假说最令人信服的批评是，在当地推断与甲烷渗漏有关的碳酸盐盖层中显然缺乏极端的碳同位素变化。现代渗漏和至少一些古代渗漏中沉淀的碳酸盐通常与~(13)C值有关，范围在-25%以下。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，以及消耗甲烷的微生物的生物标记证据。中国南部新元古代盖层碳酸盐岩建造(陡山沱组)的初步数据表明，这些遗失的证据线中的元素确实存在于当地保存异常完好的渗流相实例中。这项研究将通过对中国盖层碳酸盐的详细研究，为了解古代与渗漏有关的过程提供一个窗口。拟议研究的更广泛目标是测试这样一种命题，即这些渗漏代表着全球更具对角蚀变的盖状碳酸盐中的类似渗漏的织物，而冰盖碳酸盐和冰川后的碳同位素漂移提供了新元古代雪球地球冰期之后与全球变暖一致的灾难性全球甲烷释放的记录。

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.