Gas emissions and ROV survey tracks of the Regab pockmark (Northern Congo Fan)

Pockmarks are seafloor depressions commonly associated with fluid escape from the seabed and are believed to contribute noticeably to the transfer of methane into the ocean and ultimately into the atmosphere. They occur in many different areas and geological contexts, and vary greatly in size and shape. Nevertheless, the mechanisms of pockmark growth are still largely unclear. Still, seabed methane emissions contribute to the global carbon budget, and understanding such processes is critical to constrain future quantifications of seabed methane release at local and global scales. The giant Regab pockmark (9°42.6' E, 5°47.8' S), located at 3160 m water depth near the Congo deep-sea channel (offshore southwestern Africa), was investigated with state-of-the-art mapping devices mounted on IFREMER's (French Research Institute for Exploitation of the Sea) remotely operated vehicle (ROV) Victor 6000. ROV-borne micro-bathymetry and backscatter data of the entire structure, a high-resolution photo-mosaic covering 105,000 m2 of the most active area, sidescan mapping of gas emissions, and maps of faunal distribution as well as of carbonate crust occurrence are combined to provide an unprecedented detailed view of a giant pockmark. All data sets suggest that the pockmark is composed of two very distinctive zones in terms of seepage intensity. We postulate that these zones are the surface expression of two fluid flow regimes in the subsurface: focused flow through a fractured medium and diffuse flow through a porous medium. We conclude that the growth of giant pockmarks is controlled by self-sealing processes and lateral spreading of rising fluids. In particular, partial redirection of fluids through fractures in the sediments can drive the pockmark growth in preferential directions.

海底麻坑（pockmark）是一类常见的海底凹陷，通常与海底流体逸出相关联，被认为在甲烷向海洋乃至最终向大气的迁移过程中发挥显著作用。这类麻坑广泛分布于各类区域与地质环境中，其尺寸与形态差异悬殊。然而，海底麻坑的形成机制目前仍未被充分阐明。尽管如此，海底甲烷排放会对全球碳收支产生影响，因此厘清相关过程对于限制局地与全球尺度下海底甲烷释放的未来量化工作至关重要。位于非洲西南近海刚果深海水道附近、水深3160米的巨型雷加布海底麻坑（Regab pockmark，坐标：东经9°42.6′，南纬5°47.8′），借助法国海洋开发研究院（French Research Institute for Exploitation of the Sea, IFREMER）遥控无人潜水器（remotely operated vehicle, ROV）Victor 6000搭载的最先进测绘装置开展了调查。研究整合了该完整构造的潜水器搭载微地形测深与反向散射数据、覆盖最活跃区域105000平方米的高分辨率摄影镶嵌图、气体排放侧扫测绘图，以及动物群落分布与碳酸盐岩壳分布的相关地图，从而实现了对巨型海底麻坑前所未有的精细化全景刻画。所有数据集均表明，该麻坑依据渗流强度可划分为两个特征迥异的区域。研究推测，这两个区域是地下两种流体流态的地表表现：一是通过裂隙介质的集中流，二是通过多孔介质的弥散流。研究最终得出结论：巨型海底麻坑的生长受控于自密封过程与上升流体的侧向扩散；尤为关键的是，流体经沉积物裂隙发生的部分改向，可驱动麻坑沿特定方向扩张。