Abstract
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 (δ13Corg) 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.
Highlights
The Lower to Middle Pleistocene is an important geological period that is characterized by gradual changes in the climate systems that affected the evolution of biota (Maasch 1988; Mudelsee and Schulz 1997; Mudelsee and Stattegger 1997; Raymo et al 1997; Head and Gibbard 2005; Clark et al 2006; Head et al 2008; Elderfield et al 2012; Head and Gibbard 2015)
4.1 Concentrations of major inorganic elements Data obtained by TATSCAN-F2 and Magix PRO analyses are summarized in Table S1
Based on the discussion above, we propose the hypothesis that the two peaks in the total organic carbon (TOC) and total nitrogen (TN) values at 764 and 760 ka were caused by enhanced organicmatter preservation due to relative decrease in the bottom-water oxygen level, which were partly attributable to increased ocean stratifications at 764 and 760 ka because of the northward displacement of the Kuroshio Extension Front
Summary
The Lower to Middle Pleistocene is an important geological period that is characterized by gradual changes in the climate systems that affected the evolution of biota (Maasch 1988; Mudelsee and Schulz 1997; Mudelsee and Stattegger 1997; Raymo et al 1997; Head and Gibbard 2005; Clark et al 2006; Head et al 2008; Elderfield et al 2012; Head and Gibbard 2015). The Chiba composite section (CbCS) in the Kokumoto Formation of central Japan is a thick and continuous marine succession that straddles the Lower–Middle Pleistocene boundary and the wellrecognized Matuyama–Brunhes paleomagnetic polarity boundary (M–B boundary) (Niitsuma 1976; Okada and Niitsuma 1989; Suganuma et al 2015; Nishida et al 2016; Okada et al 2017; Suganuma et al 2018; Haneda et al 2020a). Because the M–B boundary serves as the primary guide for the Lower–Middle Pleistocene stage boundary (Head and Gibbard 2005), the Chiba section, which is one of the main sections of the CbCS (see Section 2 for a detailed description) that contains many key tephra layers and chronostratigraphic tie-points, has recently been determined as the Global Boundary Stratotype Section and Point (GSSP) for the base of the Middle Pleistocene Subseries (Suganuma et al 2021). The detailed chemostratigraphy (i.e., organic carbon isotope ratio, the contents of organic elements such as carbon and nitrogen, and major inorganic elements) of the CbCS, near the Lower–Middle Pleistocene boundary, remains poorly understood
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