Pliocene to Earliest Pleistocene (5–2.5 Ma) Reconstruction of the Kuroshio Current Extension Reveals a Dynamic Current

The Kuroshio Current Extension (KCE) is the major western boundary current of the North Pacific Subtropical Gyre. To better understand how the KCE behaved under elevated CO₂ conditions and how it came into its modern configuration, we use stable isotopic analyses from mixed-layer planktic foraminifera from three Ocean Drilling Program sites that lie to the north, underneath of, and south of the present day KCE. Our data support previous hypotheses suggesting that the KCE played a significant role in delivering moisture to the high latitudes and contributed to extensive ice growth. Samples span from 5 to 2.5 Ma and cover three tectonic and climate events. In response to constriction of the Central American Seaway, the KCE region warmed then cooled, with a potential equatorward shift of the current and intensification of the North Pacific Subtropical Gyre. During the mid-Piacenzian Warm Period, we define the mid-Piacenzian 1 and 2 events, which were times when the KCE warmed and subsequently developed a strong temperature/salinity gradient between the two northernmost sites. Northern Hemisphere ice growth at ∼2.9 Ma brought the KCE into its modern-day configuration. Paradoxically, the southernmost Hole 1209A appears coolest through the study interval. This paradox is resolved by invoking seasonal preferences of the species analyzed, a finding that highlights the need to consider seasonal and ecological preferences of plankton when interpreting geochemical records in midlatitude regions. We propose several hypotheses for drivers of KCE behavior that require further testing using higher-resolution proxy data tied to additional regional and global records.