Reconstruction of sub-bottom section of geologic methane-seep site - variety of seep activity indicated by fossil chemosynthetic communities

本研究明确了日本北部北海道始新世Poronai组和中新世Morai组中地质上过去的甲烷渗漏点的次底剖面，这两个组都是半深海粉砂岩，产出丰富的囊虫化石。 对它们的化石产状和渗漏碳酸盐岩块体（规模、形状、岩石学和内部结构）的详细观察表明，这两个渗漏点的渗漏活动方式截然不同。Poronai渗漏是以流体超压导致的破裂岩性为特征的沟道化聚焦流，而Morai渗漏是通过凝块泥间隙扩散和静态的，没有这种破裂结构。 这些结果表明，化能合成的囊蛤利用了这两种类型的渗流。 这与它们在半底栖生活状态下的高流动性有关，为解决它们在后白垩纪时期的成功和繁荣提供了一把钥匙。

This study made clear sub-bottom cross-sections of the geological past methane-seep sites in the Eocene Poronai Formation and the Miocene Morai Formation, both of which are bathyal siltstones yielding abundant vesicomyid fossils in Hokkaido, north Japan. Detail observations of their mode of fossil occurrences and seep-carbonate blocks (scale, shape, petrology, and internal textures) indicate that the two seep-sites are contrast in style of seepage activity.The Poronai seep is channelized focused-flow characterized by its fractured lithology due to fluid overpressure, whereas the Morai seep is diffusive and static through interspaces of clotted-mud, without such fracturing texture. These results show that chemosynthetic vesicomyid clams have utilized both types of seepage. This is related with their high mobility in semi-infaunal life position, which can provide a key to solve the question: why they have made success and flourish in the post-Cretaceous age.