DEVELOPMENT OF THE WESTERN PACIFIC WARM POOL DURING THE LATE MIOCENE: LINKAGES BETWEEN TECTONIC GATEWAY CLOSURE AND SEA LEVEL
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).