Development of the Western Pacific Warm Pool during the Late Miocene: A Probe of Indonesian Seaway Closure and Sea Level Change The Western Pacific Warm Pool (WPWP) is a dominant influence on tropical Pacific climate and ocean circulation. When did this oceanographic feature become established and did it influence the East Asian monsoon? Here we present preliminary results of an investigation into the early development of the WPWP as the Indonesian Seaway (IS) narrowed during late Miocene time. This study combines planktic foraminiferal population analyses with stable isotope data from ODP Sites 1146 (northern South China Sea; SCS), 1143 (southern SCS) and 806 (Ontong Java Plateau, OJP; western equatorial Pacific) at five time slices (0, 7, 9, 11 and 13 Ma). For example, at Site 1146 the modern assemblage, which is influenced by the monsoon, is 42% surface and 58% thermocline dwellers. At OJP, the modern surface dwellers dominate (81%) over the thermocline dwellers (19%), reflecting the thicker mixed layer and deeper thermocline observed at the WPWP today. At 7 Ma the population assemblages are reversed between the two sites while the isotopic analyses of OJP show a >1.3 per mil delta (super 18) O enrichment compared to modern values. Both the 7 Ma population structure and the isotopic data show that the WPWP was absent at OJP. At 11 Ma, the Site 1146 assemblage is nearly identical to the modern OJP, perhaps due to lessened influence of the East Asian monsoon and/or restricted flow because of low sea-level stand. The 11 Ma data at OJP suggests that a proto-WPWP may have developed as indicated by increased surface (74%) and decreased thermocline (23%) dwellers. We suggest that a proto-WPWP ( approximately 11-9.5 Ma) may have developed as a consequence of IS constriction and the major sea level fall at the middle/late Miocene transition. This constriction and the development of the WPWP ( approximately 11-9.5 Ma) coincides with a marked decrease in tropical carbonate mass accumulation rates. We suggest that the early development of the WPWP was related to the major sea level fall at the middle/late Miocene transition. Further, the subsequent late Miocene sea level rise increased IS throughflow, reduced the WPWP and increased tropical Indo-Pacific carbonate mass accumulation rates ( approximately 8-5 Ma).