Molecular geochemical approach to the Paleoceanographic assessment of Neogene sediments of Yashima area, Akita Basin, Japan

Abstract

Specific source marker compounds were analyzed for solvent extracts from the Onnagawa, Funakawa and Tentokuji Formations, Yashima, Akita, NE Japan.
Relative abundances of higher plant markers, n-C₂₇ alkane, n-C₂₈ alkanoic acid and ω-hydroxy-C₂₂ alkanoic acid; to algal derived compounds increase upward through the Funakawa Formation, indicating the rapid increase of land plant input associated with the increase of detrital matter during the deposition of the Funakawa Formation. Dinosteranes as specific compounds of dinoflagellates are more abundant in the lower part of the Onnagawa and the Funakawa Formations than in the middle and upper parts of the Onnagawa and the Tentokuji Formations. C₂₅ highly branched isoprenoid (HBI) alkane as a specific compound of diatoms is abundant in the Onnagawa Formation, while less abundant in the Funakawa and Tentokuji Formations. The negative behavior of C₂₅ HBI alkane/n-C₂₁ alkane and dinosterane/4-desmethylsterane ratios through the Onnagawa Formation probably reflects the changes in primary producers likely due to the changes of nutrient silica concentration of surface water. The high dinosterane/4-desmethylsterane ratio in the Tentokuji Formation seems to reflect the increase of coastal dinoflagellates under the influence of riverine water inflow. An increase of hopane/sterane ratio in the Funakawa Formation probably reflects the increase of aerobic bacterial activity during early diagenesis resulted from bottom water oxygenation in this period.