2004 Volume 38 Issue 2 Pages 45-55
The biomarkers and their specific carbon isotopic compositions (δ13C) have been analyzed in Cretaceous carbonate rocks collected from four areas in Hokkaido, Japan. Anaerobic oxidation of methane (AOM) might occur in Tappu-Kanajirizawa and Teshionakagawa-Abeshinaigawa carbonate rocks, which have found to be related to cold-seep biological fossil communities. The biomarker distributions and their δ13C values showed that these carbonates were presumably linked to Archaea and bacteria associated with AOM in these cold-seep environments. These carbonate samples contained the "tail to tail" linked isoprenoid hydrocarbons, crocetane (2, 6, 11, 15-tetramethylhexadecane), PME (2, 6, 10, 15, 19-pentamethyleicosane), and their unsaturated derivatives. Squalane (2, 6, 10, 15, 19, 23-hexamethyltetracosane), C40 acyclic and cyclic isoprenoid hydrocarbons were also detected. The occurrence of these compounds indicates that Archaea clearly play an important role during carbonate precipitation in the cold-seep environments. Archeal biomarkers were depleted in 13C, indicating that archaeal isoprenoids were biosynthesized from isotopically depleted carbon such as that derived from methane. In addition to the archaeal markers, the isotopically 13C-depleted iso -and anteiso-alkanes, which were most likely derived from sulfate-reducing bacteria, were identified. These results support the model that the sulfate-reducing bacteria might be closely linked to Archaea responsible for the AOM, although these organisms were generally competitive. The Manji-Shikorozawa and Yubari-Utagoezawa carbonates, which were lacking the cold-seep fossil biological communities, showed similar biomarker distribution. Biomarker analyses indicated these carbonates could have potential as cold-seep carbonate, and biomarker is possibly a useful indicator of cold-seep carbonate.
