The Eocene–Oligocene Transition (EOT: ∼34.4–33.7 Ma) marks a major climatic shift from a warmhouse to a coolhouse state, characterized by the extinction of rosette-shaped calcareous nannofossils (Discoaster spp.) and the expansion of the first large-scale Antarctic ice sheet. While changes in ocean circulation and declining atmospheric CO2 levels have been proposed as primary drivers, recent studies emphasize the role of the carbon cycle. This study investigates paleoceanographic changes in the Western Pacific using a multi-proxy approach at IODP Site U1438 (Hole U1438D), analyzing a 334-m sediment core. We integrate calcareous nannofossil assemblages, X-ray fluorescence, and bulk-sediment oxygen (δ18O) and carbon (δ13C) isotopes to assess environmental shifts across the EOT. Our findings reveal a decline in Discoaster spp. and Reticulofenestra umbilicus, alongside an increase in Coccolithus pelagicus, indicating cooler sea-surface temperatures and enhanced nutrient availability. Concurrent, shifts in carbon and oxygen isotopic values reflect significant cooling and the onset of Antarctic ice sheet expansion. These results provide new insights into the interactions between the carbonate compensation depth, nannofossil preservation, and broader climate dynamics during the EOT.