THE PROOF IS IN THE PLANKTON: WARMING OF THE KUROSHIO CURRENT EXTENSION ACROSS THE MID-PIACENZIAN WARM PERIOD

Western boundary currents, like the Kuroshio Current Extension (KCE), are currently warming 2–3 times faster than other regions of the world ocean. Observational sea surface temperature data from the past century indicate the KCE has warmed by approximately 1–2°C. Previous Pliocene studies that have reconstructed the KCE during the mid-Piacenzian Warm Period (mPWP) at a lower resolution determined that the current may have warmed by 3–4°C across MIS KM5. This study uses stable isotopic data from two species of thermocline-dwelling planktic foraminifera, Globoconella inflata and Neogloboquadrina incompta, from Ocean Drilling Program Hole 1207A, northwest Pacific Ocean, to characterize the surface ocean behavior within the KCE from 3.5–3.1 Ma. We also investigate how the paleoecology of species that grow during the winter (G. inflata) versus year-round (N. incompta) affect geochemical interpretations. Isotopic data and analyses based on 409 total samples with replicates indicate that even though both species are thermocline dwellers, their δ18O and δ13C values differ, namely across MIS KM5. Within MIS KM5, warming of the KCE is indicated by a 2.0‰ decrease in the G. inflata δ18O stable isotopic data whereas the N. incompta data only exhibit a ~0.30‰ increase across MIS KM5. Data from both species indicate relative current temperature stability across MIS K1, the second pulse of warming within the mPWP. This finding indicates that N. incompta and G. inflata records reveal differences in their isotopic signals, and care must be taken to address and understand the seasons in which species are most abundant in order to more precisely reconstruct ancient ocean behavior within mid-latitude systems with high seasonality. Finally, if the δ18O signals correspond dominantly to temperature, the KCE could warm another 6–7°C in addition to the 1–2°C it has already warmed in the past century.