南海北部水合物分解释放的微体古生物和沉积记录及其环境效应

Methane hydrates present a potential hazard under anthropogenic climate change. The sensitivity of hydrate stability to changes in local pressure-temperature conditions and their existence beneath relatively shallow marine environments, mean that submarine hydrates are vulnerable to changes in bottom water conditions (i.e. warming). The potential climate impact of methane release following dissociation of hydrate has in the past been compared to climate feedbacks associated with the terrestrial biosphere and identified as a possible trigger of abrupt climate change. Support for the role of methane hydrate in terrestrial and marine records is extensive. Examples include the PETM thermal maximum and during Deglacial stages (including the termination of the Younger Dryas and limiting the extent of glaciation ), although further work is required to fully understand these records. The role of hydrate disassociation as a trigger for submarine landslides has also being questioned and investigated , with reports of known hydrate occurrences that coincide with slumping and submarine landslides are common . It is therefore imperative to improve our understanding of the global hydrate inventory and how it has changed in the past, so that we can begin to develop a picture of how it may change under anthropogenic climate change. Nowadays,gas hydrate dissolution and consequential effects have become an important sicentific topic in gas hydrate research. The northern South China Sea is one of the locations of gas hydrate occurrence worldwide. The objective of this proposal is to find the evidences of methane seeping induced by gas hydrate destabilization in the past in the northern South China Sea. By way of high resolution analysis of oxygen-carbon stable isotopic values of planktonic/benthic foraminifera, Mg/Ca ratio of benthic foraminifera and paleo-temperature,total carbonate, AMS14C chronology,sedimentology and geochemistry of sediments retrieved from the gas hydrate prospect and methane seeping area of the northern South China Sea, especially Dongsha area, we plan to discuss gas hydrate disassociation mechanism and its consequential effects,such as sea level change at late Quaternary glacial and interglacial stage,climate effects,etc. we also plan to discuss the coupling relationship between climate warming,seafloor landslide and gas hydrate destabilization base on the temporal and spacial records of gas hydrate decomposition,further to build the gas hydrate dynamic evolution model in the northern South China Sea.

天然气水合物作为气候、环境和生态灾变的潜在影响因子, 被认为可能是全球性地质灾变的主要促发机制。目前，水合物分解及其分解过程和结果是水合物当今及未来研究的一个重大科学目标。本项目拟通过对南海北部天然气渗漏活动及水合物发育有利区尤其是东沙海域获取的柱状样和钻孔样开展高分辨率底栖有孔虫碳氧同位素和Mg/Ca比古水温分析、钙质生物溶解作用和CaCO3测定、沉积物AMS14C 测年与沉积间断、碳酸盐岩沉积和地球化学特征等的分析研究，寻找南海地质历史时期天然气水合物分解释放甲烷的沉积证据，进一步探讨引发水合物分解的机制，尤其是晚第四纪冰期、间冰期交替出现导致的海平面升降、气候变化对水合物失稳从而分解释放的影响。并根据水合物分解释放的时间空间记录，探讨水合物分解释放与气候变暖、海底滑坡的耦合关系。建立适合南海北部陆坡的天然气水合物动态演化模式。

识别过去海底甲烷渗漏活动和天然气水合物分解事件对了解水合物成藏模式及甲烷在全球碳循环中的作用具有重要意义。为寻找地质历史时期水合物甲烷分解释放的沉积地球化学证据，对南海北部含水合物钻孔沉积物展开高分辨率底栖有孔虫碳氧同位素、碳酸盐岩和自生矿物同位素和地球化学元素、AMS14C和U/Th测年等分析研究。首次在南海发现底栖和浮游有孔虫δ13C值极负偏事件，底栖和浮游有孔虫δ13C最低值分别为-15.85‰和-9.06‰，初步研究表明有孔虫δ13C的异常负偏可能主要来源于水合物分解释放的甲烷缺氧氧化作用（AOM）的后期成岩作用，该发现对揭示影响有孔虫碳同位素负偏的因素有很好的研究意义；同时，还发现用于示踪过去海底强烈甲烷渗漏和水合物分解事件的新指标----钼（Mo）的强烈富集，最高富集系数达到273，而正常半深海沉积物中Mo的含量偏低，一般为亏损（富集系数＜1或＝1）。Mo元素的富集是高度还原硫化环境的指示，推断其富集与天然气水合物分解释放的甲烷厌氧氧化作用(anaerobic oxidation of methane, AOM)有关。结合Mo在海底富甲烷流体环境中的行为特征，研究指出只有在海底发生强烈的甲烷渗漏活动时，才能形成明显的Mo富集层。由于Mo在地质样品中所赋存的硫化物易于保存，因此，Mo地球化学研究有望能够更好地用于示踪地质历史时期中甲烷渗漏的强度和大规模的水合物分解事件；对自生黄铁矿硫同位素组成的研究表明不含水合物岩芯的黄铁矿硫同位素δ34SV-CDT变化范围为－40.488‰～－19.538‰，而含水合物岩芯的黄铁矿δ34SV-CDT变化范围为－38.9222‰～37.6608‰，尤其在水合物层的黄铁矿硫同位素组成偏重，δ34SV-CDT为22‰～27‰。含水合物层黄铁矿的硫同位素比值特点，是持续发生的AOM过程和水合物盖层效应共同作用的结果，属于水合物层中独特的黄铁矿硫同位素特征；AMS14C和U/Th测年结果，水合物分解的时间主要发生在末次冰期MIS3期（距今约36～26ka）和MIS6与MIS5的过渡期（倒数第二次冰消期）前后，距今约130～114ka期间。说明水合物分解与海平面变化的关系十分复杂。