Pliocene integrated chronostratigraphy from the Anno Formation, Awa Group, Boso Peninsula, central Japan, and its paleoceanographic implications

摘要

The Pliocene climate is one of the best analogs for the climate of a globally warmer future. Here, we present a new Pliocene?integrated chronostratigraphy from the Anno Formation in the uppermost Awa Group, which is distributed throughout the Boso Peninsula, central Japan, based on paleomagnetic and benthic foraminiferal oxygen isotope records. This new chronostratigraphy provides valuable constraints for paleoceanographic and paleoclimatic studies in the northwestern Pacific Ocean, where the number of paleoceanographic records is limited due to the lack of calcareous microfossils from deep-sea sediment cores, with the exception of some plateaus at water depths above the calcite compensation depth (CCD). Paleomagnetic results indicate that the Anno Formation corresponds to the period extending from the Nunivak normal polarity subchronozone (4.493–4.631?Ma) to Chron C2An.2n (3.116–3.207?Ma), which is just above the Mammoth reversed polarity subchronozone. Although foraminifera are not found in the middle Anno Formation, our oxygen isotope records from the upper and lower Anno Formation demonstrate the recording of glacial–interglacial cycles. However, the amplitude of our δ_(18)O profile is much larger than that of the LR04 stack, with similar to slightly lower glacial values and much lower interglacial values. This observation implies that the bottom water had lower δ_(18)O values and/or a warmer water mass during interglacials compared with global average deep-water regions.