High abundances of degraded triterpenoids in Miocene turbiditic sediments of south-central Hokkaido, Japan

Abstract

Aliphatic and aromatic degraded triterpenoids (TTs), including des-A and des-E TTs, were investigated in turbiditic and hemipelagic mudstones from the Miocene Kawabata Formation (Ishikari basin) and Abetsu and Nibutani formations (Hodaka basin) of south-central Hokkaido, Japan. des-A TTs with carbon skeletons of oleanane, ursane, and lupane are derived from angiosperms, while des-E TTs with hopane carbon skeletons are of bacterial origin. These compounds are thought to form through microbial degradation under dysoxic and anoxic conditions. We found that the concentrations and abundances of total degraded TTs, especially des-A TTs, relative to total organic carbon (TOC) were markedly higher in the Kawabata Formation despite significant levels in the Abetsu and Nibutani formations. These results clearly suggested that huge amounts of des-A TTs may have been transported to, and accumulated in, the Ishikari basin during the late Miocene. Degraded TT/TOC ratios are correlated with aquatic macrophyte n-alkane proxy (Paq) values in the Abetsu Formation. Higher Paq values are interpreted as indicating large contributions of aquatic, submerged or floating macrophytes, and are commonly observed in lake and pond environments. Thus, large amounts of degraded TTs were likely produced through the biodegradation of transported angiospermous TTs in dysoxic or anoxic environments, such as ponds and wetlands. Furthermore, we assumed that organic matter deposited in the Hidaka basin was transported from wetlands and marsh areas, especially floodplain lakes, of paleo-Hidaka Island. The class distributions of degraded TTs varied among samples from these formations. The higher relative abundances of des-A lupanes in the Ishikari basin (Kawabata Formation) suggest that TTs were supplied from mountainous forested areas, where lupenoid-producing woody plant taxa may occur. Meanwhile, less abundant des-A lupanes in the Hidaka basin (Abetsu and Nibutani formations) suggest little or no supply of TTs from mountainous forested areas.