莺歌海岭头岬近岸海底烃类气体渗漏原位在线观测研究

Nature hydrocarbon seep in marine environment is an important source of methane and other gases to atmosphere. Accurate quantification of methane flux from seafloor seeps is necessary to evaluate the influence on global methane budget and global climate change. There are hundreds of hydrocarbon seeps distributing on the seafloor along the near west coast of Hainan Island. However, few studies on the quantitative value of methane flux, activity characteristics and influence factors of hydrocarbon seeps have been conducted until now. The Lingtou promontory seep field, which is located at 300 meters away from seashore and at a water depth of 3-20 meters, provides an ideal place for long-time in situ and on-line observation of activity characteristics and mechanisms of hydrocarbon seeps. This project plans to carry out in situ and on-line obversation of 3-4 hydrocarbon seeps at seabed in 1-2 years and acquire gas flux, bubble property and temperature, salinity, depth and dissolved methane concentration of bottom seawater. By comparatively studying the space-time and correlation among each parameter, the activity characteristics and mechanism of hydrocarbon seeps will be revealed and the gas flux from hadrocarbon seeps will be quantified in the Lingtou Promontory seep field. It provides the theoretical foundation for understanding the influence of hydrocarbon seeps on marine environment and carbon cycle in the continental margin of northern South China Sea.

烃类气体渗漏是海洋环境中甲烷等气体进入大气的重要途径。准确测定烃类渗漏释放甲烷的量对评估全球甲烷预算及其对全球气候变化的影响有重要的科学意义。海南岛西部近岸海底发育几百个烃类渗漏喷口，关于其释放甲烷的量、活动特征和影响因素至今尚未开展研究。莺歌海岭头岬烃类渗漏区是其中的一个典型区，它位于距岸约300m、水深约3-20m的海底，是开展长期、原位在线观测及其活动特征和机制研究的理想场所。本项目拟对该渗漏区内3-4个烃类渗漏喷口开展长期（1-2年）的原位在线观测，获得天然气渗漏通量、渗漏气泡特性和底部水的温盐深及溶解甲烷浓度，对比研究各观测参数的时空性和相关性，揭示岭头岬近岸烃类气体渗漏的特征与规律，估算岭头岬渗漏区天然气释放的量，为深入理解南海北部烃类气体渗漏对海洋环境的影响和碳循环过程提供理论依据。

项目原位测定了莺歌海岭头岬渗漏区渗漏喷口释放天然气的量和底部海水溶解甲烷浓度以及分析解释了该渗漏区渗漏喷口的气体组成、浅层剖面与地震资料。观测结果显示该渗漏区天然气渗漏通量为22-77 L/h，平均为53.4L/h，水深每增加1m导致天然气流速降低20-30L/h或35-56%，即高潮时天然气渗漏流速低，反之亦然。渗漏喷口底部海水溶解甲烷浓度约为970-1005nm/L，与海底温度，水深明显的负相关，与天然气渗漏通量呈正相关。其主要原因是水深变化引起的静水压力变化可能影响了沉积物孔隙道或裂隙渗漏天然气活动，从而控制烃类气体渗漏的通量变化。通过原位测定单个渗漏喷口释放天然气的量获知岭头岬渗漏区约1km2内释放天然气通量约为12×103m3/yr。.岭头岬渗漏区渗漏的天然气主要由CO2、CH4、N2和少量的非甲烷碳烃化合物组成，且天然气气泡在上升过程中发生了气体交换作用。气泡中二氧化碳含量高(67.35%)与其δ13CPDB比值(-0.49~0.86)表明二氧化碳是无机成因来源的。而气泡中甲烷含量(23%)及重δ13CPDB比值(-34.43~-37.53)和高干燥指数(0.98 ~0.99)表明甲烷为热成因来源，来自于中央坳陷区梅山组烃源岩。从浅层剖面上可见岭头岬渗漏区烃类渗漏喷口呈线性排列分布且近平行于1号断层，其形成可能与薄弱带或小断裂有关，且烃类气体沿着侧向运移通道运移至近岸海底。.通过三年的研究，项目在冷泉原位观测及流体渗漏活动做了大量的有益尝试，揭示了近岸烃类气体渗漏的特征与规律并估算了渗漏区释放天然气的量，有效的加深了对南海北部烃类气体渗漏对海洋环境的影响和碳循环过程的理解。