冷湧水炭酸塩岩形成場における化学合成群集の進化

概要

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Methane-derived carbonate rocks can be found seven localities in Hokkaido, and represent good examples of carbonate rocks produced by cold-seepage. These carbonate rocks, Albian to Holocene age, contain fossil chemosynthetic biological communities. The special emphasis of this study is placed on the assemblage of biomarkers which indicate methane-oxidizing Archaea. Typical compounds include isoprenoid-based lipids, crocetane and PME from Archaea, and monomethyl branched carbon skeletons assigned to sulfate-reducing bacteria, consistent with syntrophic relationships in the anaerobic oxidation of methane (AOM) in all samples. These Archaeal biomarkers were strongly ^<13>C-depleted, which indicates that anaerobic oxidation of methane had occurred during carbonate precipitation. Biomarker analysis indicate that AOM in the age of Albian was performed by Archaea affiliated with the so called ANME-1 group, which characterized the presence of crocetane and PME, and the AOM from the age earlier than Cenomanian were performed by the mixture of ANME-2 group Archaea which characterized the presence of C_<40>-biphytanes and isoprenoid ketones, and ANME-1. Therefore, the assemblage of methane oxidizing Arhaea seems to be evolving between Upper Albian and Cenomanian. The massive expansion of ANME-2, which is strictly anaerobic Aarchaea, may have been occurred at the Mid Cretaceous Oceanic Anoxic Event (OAE1d).
2005-09-28