Multiproxy sedimentological and geochemical analyses across the Lower–Middle Pleistocene boundary: chemostratigraphy and paleoenvironment of the Chiba composite section, central Japan

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.

Paleoceanography and dinoflagellate cyst stratigraphy across the Lower–Middle Pleistocene Subseries (Calabrian–Chibanian Stage) boundary at the Chiba composite section, Japan

“…The palynofacies is characterized by common to dominant terrestrial material, and it is therefore assumed that dinoflagellate cyst assemblages reflect changes across the continental shelf and not simply oceanic waters above the continental slope on which the sediments were deposited. The dominance of marine AOM particularly between 786.60 and 772.85 ka, representing most of MIS 19c and the earliest part of MIS 19b, closely follows the high-resolution records for δ 13 C org and C/N (Suganuma et al, in press;Izumi et al 2021;Fig. 19) and corroborates the interpretation based on elevated concentrations of radiolarians (Fig.…”

“…17). The dominance of terrestrially derived organic matter can be explained by transport from a partially exposed shelf owing to the lowered global sea level, but also apparently by relatively low marine productivity as discussed above and inferred from other marine indicators including low radiolarian concentrations and more negative δ 13 C org and higher Ca/Ti ratios (Suganuma et al, in press;Izumi et al 2021;Fig. 19).…”

“…All marine proxies are consistent with sea-surface temperatures that are among the warmest for the Chiba composite section. The general co-dominance or alternating dominance of marine AOM with terrestrial organic matter throughout this zone is reflected also in the δ 13 C org and C/N values (Suganuma et al, in press;Izumi et al 2021;Fig. 19a, b) and presumably relates to high biological productivity judging from the radiolarian concentrations (Fig.…”

“…17). Changes in palynofacies, with marine AOM becoming more common from~787.28 ka and then mostly abundant to dominant from 786.60 ka, precisely follow increases in δ 13 C org and decreases in C/N values (Suganuma et al, in press;Izumi et al 2021;Fig. 19).…”

“…19 Dinoflagellate cyst record (this study) compared with paleoclimatic and paleoceanographic records through MIS 20 to MIS 18 from the Chiba composite section (CbCS) (from Suganuma et al 2018, in press). a Organic carbon stable isotope (δ 13 C org ) and b C/N ratio (= total organic carbon [TOC]/total nitrogen [TN]) for bulk samples (Izumi et al 2021). c Relative abundance of nannofossil F. profunda (Kameo et al 2020).…”