Surface water hydrography of the Kuroshio Extension during the Pliocene–Pleistocene climate transition

The Pliocene–Pleistocene climate transition offers an opportunity to study the effect of glaciation on the ocean–climate system. We present a Globigerinoides ruber δ18O record from Ocean Drilling Program Site 1208 (Kuroshio Current Extension; KCE). This exclusively (sub)tropical foraminifer, a summer/fall mixed-layer dweller at the KCE, affords the first long (3.0 Ma to 1.8 Ma) orbital-scale (2.5-kyr time step) account of the sea surface in this area. The sectionʹs temperature-corrected benthic foraminiferal δ18O record constrains global changes in ice volume, yielding a Δδ18O record that primarily reflects summer/fall KCE hydrography (temperature and salinity). A 0.3‰ decrease in Δδ18O values at 2.7 Ma coincides with the onset of Northern Hemisphere glaciation, indicating as much as 1.5 °C warming during the summer/fall to suggest that the subtropical North Pacific sea surface provided heat and moisture for expanding ice sheets. On the orbital scale, the 41-kyr cycle that dominates high-latitude climate is absent from the Δδ18O record, indicating a stable surface water hydrographic regime on this time scale. Rather, the Δδ18O record varies at and is coherent with the 19-kyr precessional component of the regional insolation curve, supporting a direct response to subtropical insolation and insensitivity to extra-regional forcing factors, such as ice sheets.