Middle to Late Pleistocene radiolarian biostratigraphy in the water-mixed region of the Kuroshio and Oyashio currents, northeastern margin of Japan (JAMSTEC Hole 902-C9001C)

Abstract. A continuous Quaternary sediment sequence was recovered from Hole 902-C9001C during the D/V Chikyu 2006 mission along the northeastern margin of Japan. The age and rate of deposition of this core were estimated using calcareous nannofossil biostratigraphy and oxygen isotope curves measured from benthic foraminifera (Uvigerina akitaensis) and dated from 740 ka to the present, a period that spanned the Brunhes normal polarity epoch. Sediment consisted of diatomaceous siltstone and contained an abundance of radiolarians. A total of 91 radiolarian species was found in the core, of which 74 were analysed. Of these radiolarian species, 36 demonstrated continuous stratigraphical distribution over the past 740 ka and 38 had shorter ranges of biostratigraphical interest. Three of the 38 species were determined to be novel and are described in the present study (Amphisphaera tanzhiyuani sp. nov., Schizodiscus japonicus sp. nov. and Siphonosphaera? paraphoros sp. nov.). Based on 17 radiolarian bioevents, including four datums which have been commonly used across a wide area of the North Pacific, the radiolarian sequence of this core was divided into 8 zones: Amphirhopalum virchowii Zone (613–740 ka), Spongaster tetras irregularis Zone (516–613 ka), Cyrtidosphaera reticulata Zone (357–516 ka), Spongurus cylindricus Zone (238–357 ka), Pterocanium depressum Zone (209–238 ka), Spongoliva ellipsoides Zone (131–209 ka), Ceratospyris problematica Zone (33–131 ka), and the Acanthodesmia vinculata Zone (0–33 ka).