Reassessing calcareous microfossil biostratigraphy and their paleoceanographic implications for Plio-Pleistocene formations in Miyagi-jima and adjacent islands, east of Okinawa-jima

抄録

Planktonic foraminiferal zones PL4 and PL5 were correlated to the siltstone facies of the Shimajiri Group distributed in Miyagi-jima and adjacent islands, east of Okinawa-jima. The base of the Chinen Formation has yielded Pleistocene CN13b calcareous nannofossils and N22 planktonic foraminiferal zones. The Yonabaru and Shinzato formations in southern Okinawa-jima are demarcated by a distinct tuff bed near the base of Zone PL5 defined by the last occurrence of Dentoglobigerina altispira. However, this boundary key bed is absent in Miyagi-jima and adjacent islands, likely due to limited channel-fill sediment distribution. Neogene deposits of Okinawa-jima and Miyagi-jima have incorrectly been correlated with each other in previous studies, and should be merged into the Shimajiri Formation as their lithofacies are indistinguishable. An analysis of planktonic foraminiferal assemblages suggests seawater cooling after the mid-Pliocene warm period or intensification of upwelling due to the enhanced Asian monsoon during PL5.