Organic geochemistry of the Japan Sea sediments-1: Bulk organic matter and hydrocarbon analyses of Core KH-79-3, C-3 from the Oki Ridge for paleoenvironment assessments

摘要

Organic geochemical study of bulk organic matter (OM), hopanoid hydrocarbon and normal hydrocarbon (C23～C35) was conducted for a 936-cm-long sediment core sample from the Oki Ridge of the Japan Sea (Core KH-79-3, C-3; 37°03.5′ N, 134°42.6′E, water depth 935 m). Stable carbon isotopic ratios were also measured for both bulk OM and individual hydrocarbons. The following results were obtained: (1) The weight ratios of total organic carbon to total nitrogen range from 6.2 to 9.4 in the core. The δ13C values of bulk OM range from ?25.1～?20.7%.. The δ13C values of OM in the sections of 140～190 cm are lower (?25～?24‰) than those in the other sections (?23～?21‰). This result indicates that OM in the core except for the 140～190 cm sections is essentially of marine origin. (2) The δ13C value of diploptene (a hopanoid hydrocarbon) in the last glacial maximum (LGM), is ?66.3‰ (vs. PDB), which indicates it originating in methanotrophic bacteria. This result provides evidence to support for the previous ideas (Oba et al., 1980, 1984; Masuzawa and Kitano, 1984) that the bottom waters in the Japan Sea were anoxic in LGM. (3) Long chain (C23～C35) n-alkanes of higher-plant wax origin were found throughout the core. Their concentration is high in 140～190 cm in depth, suggesting that eolian dust load was high in LGM. (4) The n-alkane/TOC ratio increases with decreasing δ13C values of bulk OM. This result indicates that the load of terrestrial (probably eolian dust-derived) OM to the Japan Sea became higher in colder climates. (5) The CPI values of long-chain n-alkanes are different in different δ13O stages of paleoclimate, probably reflecting variations in species of terrestrial higher plants as a result of climatological adaptations.