Early to Middle Pleistocene paleoceanographic history of southern Japan based on radiolarian data from IODP Exp. 314/315 Sites C0001 and C0002

During the mid-Pleistocene Transition (MPT), spanning from 0.8 to 1.2. Ma, the prevailing 41. kyr glacial cycles were replaced by longer cycles with a period of ca. 100. kyr. This event was also accompanied by important climate changes, such as the expansion of the Northern Hemisphere Ice Sheet. The impact of the MPT on the Kuroshio Current off southern Japan remains unknown, however, in the absence of a long core covering the period of the MPT. In this area, preservation of calcareous microfossils is poor, but polycystine radiolarians are abundant and well preserved. Therefore, they are an excellent alternative proxy for reconstructing past climate change. Indeed, because numerous studies have examined the geographic distribution of modern radiolarian species, the relationships between several radiolarian assemblage groups (e.g., equatorial to sub-arctic assemblage groups) and climatic conditions are well understood. We examined the impact of the MPT on the Kuroshio Current off the Kii Peninsula, southern Japan, by investigating radiolarian assemblages in deep-sea cores collected during Integrated Ocean Drilling Program Expedition 314/315. We studied the radiolarian assemblages in 293 samples from cores obtained at Site C0001 Holes E and F and Site C0002 Hole D. The age models for these cores, which are based on biostratigraphy and chemostratigraphy, showed that sediments from the period between Marine Isotope Stage (MIS) 15 and MIS 8, which includes the Mid-Brunhes Event, are missing from these cores. However, continuous data are available between MIS 63 (1.82. Ma) and MIS 15 (0.5. Ma). Therefore, using the ecological properties of 11 key radiolarian species, we reconstructed regional paleoceanographic changes between 1.82 and 0.5. Ma. Radiolarian fauna revealed that the relative abundance of Kuroshio and the Central Pacific water taxa (e.g., Didymocyrtis tetrathalamus and the Polysolenia spinosa/lappacea group, respectively) increased drastically during the MPT, possibly indicating a northward shift of the Kuroshio at that time. The relative abundance of Rhizosphaera mediana, an Oyashio/Kuroshio mixed-water species, also increased during this interval. Thus, a southward shift of the Oyashio Current may have occurred synchronously with a northward shift of the Kuroshio during the MPT.