The middle–late Miocene siliceous mudstones and siltstones of the Akita and Aomori regions in Northern Honshu, Japan, provide an excellent geological record of the paleoenvironment of the Japan Sea. In this paper, we present results of sedimentological and geochemical analyses of the Akaishi and Odoji Formations from the Tsugaru Basin in the Aomori region and of the Onnagawa Formation from the Sugisawa area in the Akita region. Lithofacies descriptions, geochemical compositions (including major elements, total organic carbon [TOC], total nitrogen [TN], and total sulfur [TS]), and petrographic observations of the framboidal pyrite were used to investigate the origin of the detrital fraction, the paleoproductivity, and the paleoredox conditions of the bottom waters in the Japan Sea. Moreover, through interpretation of these data, the relationship between organic carbon preservation and the paleoenvironmental evolution was clarified. The data indicate that paleoweathering was moderate to high in the detrital fraction source terrain; the detrital fraction originated from a felsic source and was probably transported from Asia to the Japan Sea by monsoons and from the Japanese islands by riverine input; the high concentrations of biogenic SiO₂ indicate high primary productivity in the surface water, probably induced by intense upwelling of nutrients; the Tsugaru Basin area was characterized by oxygenated bottom water, whereas the Sugisawa area was characterized by oxygenated to low-oxygenated bottom water based on the lithofacies variation and the size of the framboidal pyrite; and the organic carbon accumulation in the Odoji and Onnagawa Formations was mainly controlled by paleoproductivity and dilution from the detrital fraction rather than the paleoredox conditions. Alternatively, the organic carbon accumulation in the Akaishi Formation was likely controlled by the paleoredox conditions.

日本北部本州秋田和青森地区的中晚期中新世硅质泥岩和粉砂岩为日本海的古环境提供了极好的地质记录。在本文中，我们介绍了青森县津轻流域的赤石组和大通寺组以及秋田县杉泽地区的恩纳川组的沉积学和地球化学分析结果。岩相描述，地球化学组成（包括主要元素，总有机碳[TOC]，总氮[TN]和总硫[TS]）以及黄铁矿黄铁矿的岩石学观测资料用于研究碎屑级分的起源，古生产力，以及日本海底水的古氧化还原条件。此外，通过解释这些数据，阐明了有机碳保存与古环境演化之间的关系。数据表明，碎屑级分源地带的古风化程度为中等至较高。碎屑级分起源于长英质，可能通过季风从亚洲运到日本海，并通过河流输入从日本诸岛运到；高浓度的生物成因SiO₂表示地表水中的初级生产力较高，这可能是由于养分的强烈上升所引起的；津轻盆地地区的特征是含氧的底水，而杉泽地区的特征是根据岩相变化和黄铁矿黄铁矿的大小将其氧化成低氧的底水。Odoji和Onnagawa组中的有机碳积累主要受古生产力和来自碎屑组分的稀释控制，而不是受旧的氧化还原条件控制。另外，赤石组中有机碳的积累很可能受古氧化还原条件的控制。