Reconstructing evolution of the western Equatorial Pacific and its thermocline dynamics since the latest Miocene

The Western Equatorial Pacific (WEP) plays a critical role in modulating global climate by regulating heat transport and driving El Niño-Southern Oscillation dynamics through thermocline-mixed layer changes. Unlike Eastern Equatorial Pacific, where a shallow, nutrient-rich thermocline drives productivity, factors governing thermocline dynamics and productivity shifts in the WEP remain less understood. This study presents planktic foraminiferal faunal and stable isotope data from ODP Site 803D to investigate the paleoceanographic evolution of the WEP since ca. 6.5 Ma. During the latest Miocene, the WEP experienced cooling of the upper water column and a decrease in productivity. This trend reversed in the early Pliocene when a warming phase initiated, marking the onset of permanent El Niño conditions in the Pacific. Afterward, the WEP experienced cooling concomitant with thermocline shoaling and increased productivity owing to the influence of southern-sourced intermediate waters. Such incursions of southern-sourced waters established a link between high latitude processes and tropical dynamics. Subsequently, the mixed layer in this region thickened due to the constriction of the Indonesian Gateways. We suggest that the modern-day Western Pacific Warm Pool began to emerge between 2.2 and 2.0 Ma, as simultaneous mixed layer warming and upper thermocline cooling initiated a new vertical thermal stratification. By 2.0 Ma, the study region attained present-day oligotrophic setting, driven by low input from the Southern Ocean and increased upper ocean thermal stratification. Our findings highlight the dynamic interplay between tropical ocean gateways, high-latitude influences, and ocean-atmosphere feedbacks in shaping the long-term evolution of thermocline structure and productivity in the WEP.