南海北部莺歌海岭头岬近岸烃类渗漏甲烷的归宿

Hydrocarbon gas seeps is a significant source of methane conveying to the oceans and the atmosphere. Understanding the methane migration process and dynamics of these systems in the ocean is of great scientific significance to assess the influence on global climate change and global carbon cycle. There are several active seep fields such as the Yinggehai Rivulet Mouth and the Lingtou Promontory distributing on the near-shore Yinggehai, however, the process of methane in the seawater is not yet systematically studied. The Lingtou Promontory seep field provides an ideal place to study on the methane migration and transformation process and dynamics of these systems. This project intends to measure the seep bubbles and marine environment features, to obtain the methane content in bubbles, the dissolved methane content and bottom atmosphere methane content at three dimensional space (different stations in different depth) respectively. The aim of this project is to determine the spatial distribution of methane in the seawater and discuss its control factors, to quantify the fluxes of methane dissolved and oxidized by microbe and the sea-air methane flux, to reveal the migration and transformation processes of methane and the fate of methane in the Lingtou Promontory seep field. It can more deeply understand the biogeochemical cycle mechanisms of methane in shallow water seep fields.

烃类气体渗漏是向海洋和大气中输送甲烷的一个重要途径。弄清楚海洋中甲烷输送过程及其机制对评估全球气候变化及全球碳循环有着重要的科学意义。莺歌海近岸分布着如莺歌海海口、岭头岬等活动渗漏区，然而，对此区域海底渗漏释放的甲烷在水体中的过程并不清楚。岭头岬村近岸作为其中一个典型渗漏区，是开展海洋中甲烷迁移转化过程及其机制研究的理想场所。本申请项目拟选取岭头岬近岸渗漏区为研究对象，对渗漏气泡与海洋环境特征进行原位观测，获得该渗漏区在三维空间上（不同站位不同水深中）气泡甲烷含量、溶解甲烷含量与底层大气甲烷含量，确定该渗漏区甲烷在水体中的分布特征并探讨其控制因素，估算该渗漏区甲烷在其水体中的溶解和微生物氧化的量及海-气界面的甲烷释放通量，揭示该渗漏区甲烷的迁移转化过程及其归宿，以期更加深入的理解浅海渗漏甲烷的生物地球化学循环机制。

甲烷是大气中最丰富的烃类气体，在全球气候变化中起着重要作用。因此，定量烃类渗漏区中释放到海水中甲烷气泡及溶解甲烷含量及其海气通量具有重要意义。在南海北部F站位和海马冷泉区及莺歌海岭头岬渗漏区分布着众多的烃类渗漏喷口，大量的甲烷以气泡形式或溶解态从海底进入海洋水体中。准确量化冷渗过程中的甲烷泡通量，对于评估其对全球甲烷收支和气候变化的影响至关重要。本项目对岭头岬烃类渗漏区与F站位和海马冷泉区开展了研究，我们在岭头岬渗漏区采集了48个观测点的表层和底层水体，确定了表层和底层海水中溶解甲烷浓度及其分布特征，表层海水和底层海水中溶解甲烷浓度的显著差异表明从海底扩散的大部分甲烷在水体中发生了溶解。溶解甲烷的碳同位素比值表明为生物成因和热成因混合气。此外，我们采用图像处理技术和半自动气泡计数算法确定了F站位和海马冷泉区甲烷泡的气泡直径、大小分布和气泡释放频率。平均气泡直径为2.54-6.17mm与气泡释放频率为4.8-26.1 s-1。F站位冷泉区气泡的直径和频率低于海马冷泉渗漏区,这可能由多种因素导致的如天然气储层的性质,静水压力,沉积物中迁移途径和孔隙大小。南海海马冷泉渗漏区(12.6 L h-1)和F站位冷泉区(4.9 L h-1)释放的甲烷通量以及莺歌海浅海渗漏区甲烷海气通量（672.57 μmol m−2 d−1）表明海马冷泉区和F站位及莺歌海盆地浅海渗漏区很可能是海洋和大气中甲烷的重要来源甲烷进入海洋的一个源。