Review of fossil chemosynthetic assemblages in Japan

摘要

Seventy-five fossil chemosynthetic assemblages of Cretaceous to Holocene age have been discovered in Japan; many are interpreted to have been cold seep dependent and one is associated with a whale-fall. Fossil hydrothermal-vent assemblages have not been found in Japan. Non-seep assemblages of solemyids, lucinids, and/or thyasirids are not included in this study. Various combinations of five diagnostic features characterize cold seep assemblages: 1) the abundant and often exclusive occurrence of large invertebrate taxa suspected of harboring chemosynthetic bacteria, 2) associated authigenic carbonates greatly depleted in C-13, 3) a three-dimensional distribution of the assemblage that is laterally restricted, 4) associated pockmark-or diapir-related sedimentary structures, and 5) the presence of conduit-related tectonic or sedimentological structures such as faults, slumps or debris flows. Excluding the whale-fall assemblage, we can divide Cenozoic fossil chemosynthetic assemblages in Japan into three types regardless of geochronologic age. Type I is dominated by vesiconryid bivalves and/or tube worms, characterized by its allochthorrous or parautochthonous mode of occurrence in deep-water (greater than 1000 m) settings, along with tectonically or sedimentologically induced structures such as faults, slumps, diapirs, or debris flows, likely dependent on seepage controlled by subduction-related tectonism, and/or on methane coming from the drastic decomposition of gas hydrate. Type 11 is dominated by vesicomyid bivalves (in most cases the species differ from those in Type 1) and is characterized by autochthonous occurrences in muddy sediments from less than 1000 m water depth. Type III is dominated by Lucinoma and/or Conchocele and characterised by autochthonous occurrences in muddy sediments from depths of less than 300 m. A living community similar to the Type 11 and Ill assemblages has not yet been observed in the seas around Japan. It is noteworthy that no tectonically or sedimentologically induced structures have been observed in and around the Type 11 and III fossil localities, although the muddy sediments sometimes show brecciated facies that suggest an explosive effusion of methane.