南海北部海底冷泉探测及其活动特征研究

The discovery of cold hydrocarbon seeps is one of the most important achievements of oceanography, which is same as hydrothermal vents, during last 30 years. Cold hydrocarbon seep, due to its close relationship to potential energy gas hydrate, global climate change and origin of life is now the focus of scientific community from many disciplines. However, questions remain about the occurrence, characterization, and causes of the cold hydrocarbon seeps on the northern South China Sea (SCS) continental slope. The kinetic models of gas hydrate crystallization, evolution (e.g., diversity and variability) of seep fluids over time-scales, and biogeochemical and ecological features of the life at cold hydrocarbon seeps, are also not clear. The answers to these questions, to a certain extent, are dependent on obtaining of direct deep seafloor observation data. This proposed project “Seafloor exploration and characterization of the cold hydrocarbon seeps of the northern South China Sea” will focus on seafloor exploration of the cold hydrocarbon seeps from the SCS. The scientific goals of the study is to 1) detect seepage of methane-rich fluids, determine the physical and chemical properties of bottom seawater and sediments at cold hydrocarbon seeps, and address how cold hydrocarbon seeps as well as gas hydrate evolve through time, and their effect on the biosphere, ocean, and atmosphere, 2) characterize the authigenic carbonates and processes involved, and 3) understand the evolution of cold hydrocarbon seeps and potential driving forces. The project is designed to assess the signature of methane at cold hydrocarbon seeps of the SCS and its role on the gas hydrate and global climate change. The study will advance our understanding of the mass and energy exchange between the open ocean and that of under the seafloor, and also will be essential for better understanding the deep processes in the South China Sea.

海底冷泉是上世纪海洋科学领域与热液并重的二大发现之一，与其相关的资源能源、环境效应和生命过程是近30年科学研究的焦点。然而对于南海北部冷泉的发育和活动特征、渗漏甲烷等流体向海底渗漏的过程及关键影响因素、冷泉活动区无光合作用生物特征及其资源利用、渗漏甲烷的微生物地球化学过程和沉淀为水合物的动力学机制等关键科学问题并不清楚。这些问题的解决均依赖于海底第一手资料的观测。申请项目“南海北部海底冷泉探测及其活动特征”拟通过对南海北部冷泉活动区海底探测，确定南海北部冷泉活动区海底水体和沉积物的物理和化学属性，研究南海北部沉积层中甲烷等流体向上渗漏及在海底形成冷泉的过程和冷泉活动特征、渗漏甲烷沉淀水合物的动力学过程和在海底氧化转变为固相碳酸盐的微生物地球化学过程，深化认识冷泉渗漏甲烷在海底的地球化学循环过程及其资源与环境效应，帮助理解海底之下的海洋与上覆海洋间物质和能量的交换，加深了解南海的深海过程。

南海北部40多个站位的冷泉碳酸盐岩大多具有明显富集重氧(18O)特征，指示渗漏流体来源于水合物分解释放的流体。冷泉碳酸盐岩U/Th年代学研究显示绝大多数冷泉活动发育于低海面时期或海平面下降期。. 南海北部陆坡东沙海域柱状沉积物DH-5中识别出了至少三次的甲烷释放事件（第Ⅰ段：400-550 cm，第Ⅱ段740-820 cm和第Ⅲ段1000-1150 cm）。有孔虫AMS碳14定年结果显示第Ⅰ段的甲烷释放发生在15.4-24.8 ka。. 对“蛟龙号”载人深潜在南海北部的东沙F站位冷泉活动区采集的冷泉碳酸盐岩和冷泉生物进行了研究，碳酸盐岩碳同位素（δ13C为-55.3‰～-34.3‰，δ18O为1.9‰～4.8‰，表明冷泉流体可能来源于水合物分解释放流体。冷泉贻贝B. platifron软体的δ13C为-77.6‰～-64.8‰，为渗漏甲烷的值，B. aduloides软体的δ13C为-37.4‰～-33.0‰，可能是海底界面孔隙水的DIC同位素特征。冷泉碳酸盐岩生物标志物δ13C低至-133‰ 到 -111‰，表明来源于生物成因甲烷。AMS 碳14 测年表明冷泉近1万年来呈多次幕式活动。. 东沙东北部水合物钻探区存在甲烷渗漏活动，AMS碳14测年确定约6ka BP以来发生过短暂的甲烷渗漏活动。通过数值模拟研究结果显示，D5和DF站位非常可能发育天然气水合物，而D-8站位可能不发育水合物。. 南海西北“海马冷泉”采集的自生碳酸盐岩的δ13C为-43.0‰～-27.5‰，δ18O为2.5‰～5.8‰，反映了天然气水合物的分解来源。AMS碳14测年主要发生于6.1-5.1 ka、3.9-2.9ka和现在三个阶段，与F站位冷泉活动的周期相似。. 西沙隆起西南部海底麻坑区可能已经停止活动处于休眠期，少量麻坑仍在发生微弱的甲烷渗漏，可能处于活动的衰落期。麻坑至少是在39 kyr前形成，且流体活动的中止可能与相对高海平面时期水合物的稳定或者水合物藏的消耗殆尽有关。在西沙西南部海底麻坑区一个麻坑中发现了沉积物孔隙水C离子浓度降低，并伴随有b孔隙水的δ18O值的升高，可能存在水合物发育。