Calcareous Nannofossil Response to Climate Variability During the Middle Pleistocene Transition in the Northwest Pacific Ocean (Ocean Drilling Program Leg 198 Site 1209)

The Middle Pleistocene Transition represents an important reorganization of climate that switched from a 41‐kyr periodicity to a quasiperiodic 100‐kyr cycle. Here, we have investigated this time interval using calcareous nannofossils from a midlatitude well‐preserved sedimentary record from the Ocean Drilling Program Site 1209 in the North West Pacific Ocean. We studied samples from 1.45 to 0.45 Ma, looking for significant changes in the nannofossil assemblages. Micropaleontological data were processed using an information theoretic model comparison that deconvolutes variations in nannofossil assemblages using different predictors. The major findings are that calcareous nannofossils permit the detection of discrete events during the Middle Pleistocene Transition. They display three different associations belonging, respectively, to the 41‐kyr world, the transition, and the 100‐kyr world. The statistical approach demonstrated that nannofossils responded abruptly at the boundaries of the transition developing in two rapid steps superimposed on a more gradual process. Moreover, a strong decrease in the genus Reticulofenestra and the beginning of the Gephyrocapsa caribbeanica acme permitted us to pinpoint climate events within the Middle Pleistocene Transition. Finally, through the nannofossil paleoecology, we documented an anomalous long‐term warming trend that could be related to an intensification of the Walker circulation and the stabilization of long‐term conditions like La Niña at the midlatitudes of the NW Pacific during the Middle Pleistocene Transition.