The Western Pacific Warm Pool (WPWP) is a dominant influence on tropical Pacific climate and ocean circulation. When did this prominent oceanographic feature become established and did it influence the intensity of 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. Planktic foraminiferal population analyses were combined with multi-species, multi-size fraction 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 three time slices (0, 7, and 11 Ma). The development of the WPWP was examined by comparing the changing water column structure and productivity between the seasonally dynamic SCS and the seasonally stable OJP.

At Site 1146 the modern assemblage, which is influenced by the East Asian monsoon, is 42% surface and 56% thermocline dwellers. In contrast, OJP surface dwellers dominate (81%) over thermocline dwellers (17%) reflecting the thicker mixed layer and deeper thermocline in the core of the WPWP. At 7 Ma the population assemblages are reversed between the two sites while the isotopic analyses of OJP show a >1.3‰ d¹⁸O enrichment compared to modern values. Both the population structure and the isotopic data show that the WPWP was absent over OJP at 7 Ma. At 11Ma, 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 11Ma data at OJP suggests that a proto-WPWP may have developed as indicated by increased surface (74%) and decreased thermocline (23%) dwellers.

We believe that Indonesian Seaway constriction may have greatly modulated the tropical ocean-climate system during the late Miocene. This constriction and the early development of the WPWP (~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 stimulated increased carbonate mass accumulation rates across the tropical Indo-Pacific (~8-5 Ma).