The Chiba composite section (CbCS) in the Kokumoto Formation, Kazusa Group, central Japan is a thick and continuous marine succession that straddles the Lower–Middle Pleistocene boundary and the well-recognized Matuyama–Brunhes paleomagnetic polarity boundary. Although recent studies extensively investigated the CbCS, its chemostratigraphy, particularly around the Lower–Middle Pleistocene boundary, is poorly understood. Therefore, in this study, we performed multiproxy sedimentological and geochemical analyses of the CbCS, including the Chiba section, which is the Global Boundary Stratotype Section and Point for defining the base of the Middle Pleistocene Subseries. The aim of these analyses is to establish the high-resolution chemostratigraphy and to reconstruct the paleoenvironments of its sedimentary basin in detail. We used the K/Ti ratio as a broad proxy for the clastic material grain size of the sediments. Although the K/Ti ratio generally varies throughout the studied interval, the K/Ti ratio especially during Marine Isotope Stage (MIS) 19a shows a variation pattern like those of the foraminiferal oxygen isotope (δ¹⁸O) records. The records of the C/N ratio of bulk samples and carbon isotope ratio of the organic carbon (δ¹³C_org) suggest that the organic matter in the CbCS sediments during MIS 19c mostly originated from marine plankton, whereas the organic matter during MIS 18 and 19a was characterized by a mixture of marine plankton and terrestrial plants. These records are clearly indicative of changes in mixing ratio of marine vs. terrestrial organic matter in association with glacial–interglacial cycles from the late MIS 20 to the early MIS 18. In addition, we calculated the mass accumulation rates (MARs) of organic carbon, biogenic carbonate, and terrigenous material for quantitative interpretations on the paleoenvironmental changes. MAR calculations revealed that the contribution of marine organic carbon relative to terrestrial organic carbon increased during MIS 19c, and that the contribution of the terrigenous material relative to biogenic carbonate decreased during MIS 19c. Furthermore, we observed relatively large variations in the total organic carbon and total nitrogen contents during MIS 19a. These variations were probably caused by the relative decrease in bottom-water oxygen level, which is also supported by our trace-fossil data, although it is not certain whether the increase in organic-carbon flux at ~ 760 ka was due to the synchronous increase in biogenic productivity in surface water. Such a relative decrease in bottom-water oxygen level was partly due to the increased ocean stratification because of the northward displacement of the Kuroshio Extension Front.

日本中部和田集团国本组千叶组合区（CbCS）是厚而连续的海相演替，横跨下中更新世边界和公认的Matuyama-Brunhes古磁极性边界。尽管最近的研究广泛地研究了CbCS，但对其化学地层学，特别是下中更新世界线的了解很少。因此，在这项研究中，我们对CbCS进行了多代理沉积和地球化学分析，包括千叶剖面（即全球边界层型剖面和定义中更新世亚系列基础的点）。这些分析的目的是建立高分辨率的化学地层学，并详细重建其沉积盆地的古环境。我们使用K / Ti比作为沉积物碎屑物质粒度的广泛替代。尽管K / Ti比通常在整个研究区间内变化，但K / Ti比尤其是在海洋同位素阶段（MIS）19a期间显示出与有孔虫氧同位素（δ）相似的变化模式¹⁸ O）记录。散装样品和有机碳的碳同位素比率的C / N比的记录（δ ¹³ C ^_有机）表明MIS 19c期间CbCS沉积物中的有机物主要来自海洋浮游生物，而MIS 18和19a期间的有机物以海洋浮游生物和陆生植物的混合物为特征。这些记录清楚地表明了从MIS 20后期到MIS 18早期的冰与冰间周期的变化，海洋与陆地有机质混合比的变化。此外，我们计算了有机碳的质量累积率（MARs） ，生物碳酸盐和陆源物质，以定量解释古环境变化。MAR的计算表明，MIS 19c期间海洋有机碳相对于陆地有机碳的贡献增加了，MIS 19c期间陆源物质相对于生物碳酸盐的贡献降低。此外，我们观察到MIS 19a期间总有机碳和总氮含量的变化较大。这些变化可能是由于底部水氧含量的相对下降所致，这也得到了我们的痕迹化石数据的支持，尽管尚不确定〜760 ka处有机碳通量的增加是否是由于同步增加所致。在地表水中的生物生产力。底部水氧含量的这种相对下降部分是由于黑潮延伸锋的北移导致海洋分层增加。这些变化可能是由于底部水氧含量的相对下降所致，这也得到了我们的痕迹化石数据的支持，尽管尚不确定〜760 ka处有机碳通量的增加是否是由于同步增加所致。在地表水中的生物生产力。底部水氧含量的这种相对下降部分是由于黑潮延伸锋的北移导致海洋分层增加。这些变化可能是由于底部水氧含量的相对下降所致，这也得到了我们的痕迹化石数据的支持，尽管尚不确定〜760 ka处有机碳通量的增加是否是由于同步增加所致。在地表水中的生物生产力。底部水氧含量的这种相对下降部分是由于黑潮延伸锋的北移导致海洋分层增加。