The role of the Kuroshio Extension in the Pliocene-Pleistocene climate transition

Despite the North Pacific's potential role as a connection between high and low latitudes during the Pliocene-Pleistocene climate transition, the region remains largely unexplored. Here, from Ocean Drilling Program Site 1208 in the Kuroshio Current Extension (KCE), I present the region's first long, orbital-scale (2500-year time step) surface (3.00–1.76 Ma) and deep (3.70–1.76 Ma) marine records to span this cooling episode. A benthic foraminifer (Planulina wuellerstorfi) δ18O record provides orbital-scale age control and confirms continuous stratigraphy at the single-hole site. The δ18O time series records ice-volume changes, following a 41-kyr cycle that intensifies in amplitude with stepwise advances of northern hemisphere glaciers (NHG) toward mid-latitudes at 3.3 and 2.7 Ma. Accordingly, alkenone-based sea surface temperature (SST) estimates reveal cooling at 2.7 Ma with the onset of widespread NHG, and at 2.2 Ma with the appearance of cool waters in the eastern equatorial Pacific (EEP). On the orbital scale, SSTs follow a 41-kyr cycle that doubles in amplitude at 2.7 Ma, low temperatures corresponding to glacial intervals. In contrast, summer surface hydrography (mostly temperature) as reconstructed through Globigerinoides (Gs.) ruber δ18O measurements suggest warming of the KCE at 2.7 Ma, coincident with the intensification of NHG, supporting the idea that a warm North Pacific provided moisture for expanding glaciers. At the orbital scale, the summer hydrographic reconstruction varies predominantly on precesional (∼1/20-kyr frequency) rhythms, reflecting the influence of subtropical insolation. The contrast between KCE cooling (alkenones) with the advance of NHG to mid-latitudes and the insensitivity of summer hydrography (Gs. ruber δ18O) to ice volume changes implies winter heat loss to the atmosphere, and thus the emergence of the region as a major locus of ocean-atmosphere heat transfer at 2.7 Ma. Shifts in the timing of the 41-kyr KCE cooling cycle suggests the competing influence of glacial climate and insolation over the subtropical sea surface. After evolving in rhythm from 3.0 to 2.1 Ma, a timing offset between KCE and EEP SST cycles develops to reveal a decoupling of the two ends of the subtropical gyre after the upwelling of cool waters in the equatorial Pacific that ended permanent El Niño