“海马”冷泉甲烷微渗漏范围、幅度及其与水合物稳定带的相互关系

The international scientific community has focused on understanding cold seeps in recent times. Methane is one of the most significant greenhouse gases, therefore understanding methane flux at cold seeps is a priority. However, that task is difficult, and only a few studies on active cold seeps have so far been undertaken in China. As one of the most important achievements of geological investigations in China during 2015, the “Haima” cold seep has potential to be a case study of domestic cold seeps. It has great potential to promote the development of multidisciplinary fundamental research involving marine geology, chemistry, geochemistry and biology. The proposed project intends to use a ROV equipped with a high-precision methane sensor and a Conductance-Temperature-Depth sensor to measure in situ methane concentrations at the submarine boundary layer of the “Haima” cold-seep locality. At the same time, accurate sampling will be conducted of surface sediments and bottom seawater aided by the ROV, to calculate the methane flux from the sulfate-methane transition zone in sediments and micro-seepage flux of the seawater-sediment interface at the cold seeps. Moreover, the mineralogical, geochemical, and geophysical information obtained from the sediments will be used to distinguish the range and extent of methane micro-seepage flux on the local marine carbon cycle at seeps, which will then be used to discuss the relationship between methane micro-seepage and the hydrate stability zone.

冷泉是近几十年来国际科学界的特别关注的热点之一。由于甲烷是一种重要的温室气体，使得冷泉甲烷渗漏通量成为冷泉研究的首要工作之一，也是冷泉研究的难点之一。目前对我国活动冷泉的发现和研究还非常少，“海马”冷泉作为2015年中国地质调查的最重大成果之一，有望成为我国冷泉研究的典型案例，对我国海洋地质、化学、地球化学、生物学等方面具有重大的推进作用。本研究拟利用无人遥控机器人携带高精度甲烷、温盐深等传感器，实时在线测量冷泉区海底边界层位海水甲烷含量，并对表层沉积物和底层海水进行精确取样，研究冷泉区海底沉积物硫酸盐-甲烷界面甲烷扩散通量，确定“海马”冷泉区海底沉积物甲烷扩散通量及沉积物—海水甲烷渗漏通量，结合矿物学、地球化学和地球物理等方法，判别冷泉区甲烷渗漏边界及空间分布，探讨冷泉甲烷微渗漏与水合物稳定带的相互关系。

本研究利用无人遥控机器人携带高精度甲烷、温盐深等传感器，实时在线测量“海马”冷泉区海底边界层位海水甲烷含量，并对表层沉积物和底层海水进行精确取样，初步确定了琼东南海域海水甲烷分布及海气界面甲烷交换通量、海底沉积物甲烷扩散通量，揭示了甲烷渗漏强度变化规律。利用矿物学、地球化学和地球物理等分析手段，针对“海马”冷泉区不同渗漏背景情况下沉积物柱状样品，以可能发生的铁锰驱动的甲烷厌氧氧化反应的沉积物元素和矿物组分分析为突破口，开展甲烷渗漏强烈与下伏水合物稳定带的关系研究，发现产甲烷带沉积物中富含铁锰等金属元素，其空间变异与物源紧密相关，且不同矿相的高活性铁在产甲烷带波动并富集，表明在硫酸盐匮乏的沉积层，大量埋藏的活性氧化铁可能作为甲烷厌氧氧化的电子受体存在于产甲烷带。针对形成冷泉和水合物气体来源不明问题，以南海北部天然气水合物勘查获取样品最多的神狐水合物试采区为例，开展水合物气源地球化学示踪分析，甲烷碳同位素结果表明水合物分解气成因虽为生物气及热解气混合成因，但是C2-C3碳、氢同位素以及CO2碳同位素结果显示，神狐试采矿体水合物甲烷主要为深部油气藏气体渗漏贡献，水合物气体烃源岩以陆源有机质为主，少量海相有机质，热解气中既有油型气也有煤型气，且以油型气为主，证实了浅层水合物与深部常规油气藏具有非常密切的耦合关系，为后续南海水合物资源评价及勘探开采提供了参考和借鉴。