RECONSTRUCTING LATE MIOCENE DEVELOPMENT OF THE WESTERN PACIFIC WARM POOL AND THE EAST ASIAN MONSOON USING STABLE ISOTOPE PALEOECOLOGY

Assemblage and stable isotope analyses (d13C and d18O) of several time slices provide a snapshot of changing marine paleoecology, an evolving water column, and the development of the Western Pacific Warm Pool, a major component of the El Niño-Southern Oscillation. Foraminifera were collected from deep sea sediments in the western equatorial Pacific (Ontong Java Plateau, ODP Site 806) and the South China Sea (ODP Sites 1143 and 1146). The full foraminiferal assemblages in five time slices (0, 7, 9, 11, and 13 Ma; four time slices at Site 1143) were isotopically analyzed from discrete size fractions to calibrate a long continuous stable isotope record of three foraminiferal species, each representing a key water mass.

The time slice analyses show individual foraminiferal species having three general isotopic trends: 1, sinking during ontogeny (e.g., deep thermocline species G. venezuelana); 2, maintaining position in the thermocline or mixed layer while metabolism (may have) decreased during ontogeny (e.g., upper thermocline species G. menardii and mixed layer species G. glutinata); and 3, increasing size and photosymbiont influence during ontogeny prior to sinking and gametogenesis (e.g., G. sacculifer). These isotopic trends distinguish mixed layer from thermocline depth habitats for extinct Miocene taxa. In turn, the habitats serve as proxies of changing water masses as the Warm Pool developed.

From the middle to late Miocene, the time slice analyses also show that the collective isotopic space occupied by each foraminiferal assemblage (from each of the three sites) remained mostly unchanged. Almost all planktic foraminifera at Ontong Java Plateau (Site 806) were isotopically heavier (in d18O values) than species from the South China Sea (Sites 1143 and 1146). However, by Holocene time (0 Ma time slice) the South China Sea assemblages are more dispersed and the lightest species at Ontong Java become lighter by >1.5‰.

Although onset of the East Asia Monsoon occurred at ~8 Ma, very little isotopic change occurs in the foraminiferal assemblages in the four Miocene time slices. The dramatic difference between the modern time slice (0 Ma) and the Miocene may imply a fundamental difference between the present day East Asian monsoon and/or the Western Pacific Warm Pool and their middle to late Miocene counterparts.