化学合成化石群集を伴う下部更新統小柴層中の冷湧水性炭酸塩岩の有機地球化学的特徴

抄録

Methane-derived carbonated rocks in Lower Pleistocene Koshiba Formation, Kazusa Group crop out in Sakae-ku, Yokohama City, are among the first described examples of carbonate rocks produced at fossil cold-seeps in Japan. The methane-related origin of these carbonates have been inferred from the occurrence of chemosymbiotic fossil community such as Lucinoma, Conchocele, Acharax and from the depletion of carbonate in ¹³C. The present work is focused on the distribution and δ¹³C values of biomarkers for Archaea associated with anaerobic methane oxidation. This carbonate rock contained ¹³C depleted PME (pentamethyleicosane), but was lacking in crocetane. Acyclic and cyclic C₄₀-isoprenoid (biphytanes) released upon the cleavage of ether bonds in polar compounds from the carbonate were not depleted in ¹³C. Carbon isotope variability among Archaeal biomarkers in this cold-seep carbonate suggests that Archaeal species were multiple and that only some were related to anaerobic methane oxidation.