[BPT06-P09] Calcareous nannofossil biostratigraphy of the upper part of the Kiwada Formation and estimated marine environment change
Keywords:Calcareous nannofossil, Paleoceanography, Quaternary, Kazusa Group, Kiwada Formation
In this study, we studied the Kiwada formation along the Yuki River, one of the branches of the Yoro River. Thirteen genera and 19 species of calcareous nannofossils were found in 31 samples in this formation. As a result, the last occurrences of large Gephyrocapsa spp. and Helicosphaera sellii were recognized slightly above the key-tephra Kd1 and the key-tephra Kd17, respectively. The last occurrence of large specimens of Gephyrocapsa spp. may be located near MIS 35 based on Pickering et al. (1999). However, it is difficult to determine exactly the stratigraphic horizon of the last occurrence of H. sellii due to sporadic occurrences of this species and the presence of slump deposits in the middle part of the Formation. According to stratigraphic changes in environmental indicators, past movements of the Kuroshio front during the deposition of the upper part of the Kiwada Formation can be estimated. Abundant occurrences of a cool water taxon, Coccolithus pelagicus (Tanaka, 1991), are recognized above Kd6 and just below Kd1. Thus, two boundaries called the Kuroshio and Oyashio fronts temporarily had moved southward and comparatively cool surface waters occupied areas around the Boso Peninsula in 1.33 Ma and 1.21 Ma. The Kuroshio front has repeatedly moved northward and southward in short period after 1.21 Ma because repetitions of abundant occurrences of both C. pelagicus and a warm Umbilicosphaera sibogae are observed above Kd1.
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