MIOCENE STABLE ISOTOPE PALEOECOLOGY: SLICING AND DICING THE WATER COLUMN TO STUDY THE WESTERN PACIFIC WARM POOL AND EAST ASIAN MONSOON

A stable isotope (d13C and d18O), paleoecological study of planktic foraminifera was used to calibrate continuous, long stable isotope records at three deep sea sites in the western equatorial Pacific (Ontong Java Plateau, Site 806) and the South China Sea (ODP Sites 1143 and 1146). The full foraminiferal assemblages present in five time slices (0, 7, 9, 11, and 13 Ma; four time slices at Site 1143) were isotopically analyzed from discrete size fractions.

Individual foraminiferal species exhibited three general isotopic trends when d13C is plotted against d18O: 1, sinking during ontogeny (e.g., thermocline species G. baroemoenensis and G. venezuelana); 2, no change in temperature (in d18O) but possible decreasing metabolism during ontogeny while maintaining position within the thermocline or mixed layer (e.g., D. altispira, G. menardii, G. tumida group, G. glutinata, N. acostaensis, S. seminulina); and 3, increasing size and photosymbiont influence with ontogeny prior to sinking and gametogenesis (e.g., O. universa, G. extremus, G. obliquus, G. sacculifer). The trend lines defined by G. sacculifer and G. glutinata are very consistent in all time slices and at all sites, and are used to distinguish mixed layer versus thermocline depth habitats for the extinct Miocene taxa.

From ~13 Ma to ~7 Ma the collective isotopic space occupied by each assemblage (from each of the three sites) remained mostly unchanged. In addition, for the middle to late Miocene, almost all of the planktic foraminifera at Ontong Java Plateau (Site 806) were isotopically heavier than any species analyzed from the southern South China Sea (Site 1143); the northern South China Sea (Site 1146) had intermediate d18O values. However, by Holocene time (0 Ma time slice) the assemblages at Sites 1143 and 1146 are more dispersed and the lightest species at Ontong Java become lighter by >1.5‰. Though the onset of the East Asia Monsoon occurred at ~8 Ma (e.g., An et al., 2001), there is very little isotopic change in the foraminiferal assemblages in the four Miocene time slices. The radical difference between the assemblages in the modern time slice (0 Ma) and the Miocene may imply that the present day East Asian monsoon and/or the Western Pacific Warm Pool are fundamentally different from their middle to late Miocene counterparts.