This study investigates the timing of development of oligotrophic conditions and thickening of the West Pacific Warm Pool (WPWP) during the Pliocene. It has been hypothesized that the evolution of the WPWP and the establishment of strong equatorial Pacific zonal gradients are closely related to the narrowing of the Indonesian Gateway (IG) as well as the closure of the Central American Seaway (CAS). However, the timing of these events remains unclear. Here we report Pliocene-to-recent relative abundances of planktic foraminifera at DSDP Site 214, in the eastern Indian Ocean, and at ODP Site 807, in the western Pacific, along with stable isotope values at the latter site. A comparison of the abundance of mixed-layer species (MLS) from both sites indicates a pronounced increase in their population between ~3.15 and 1.6 Ma. On the contrary, there is a contemporaneous decrease in the Globigerinita glutinata population during this time, which together suggest the development of oligotrophic conditions in the western tropical Pacific. Our data suggest that the oligotrophic WPWP resembling present-day conditions developed around ~3.15 Ma, and was closely linked to the gradual constriction of the IG
