High resolution paleo-climatological data from IMAGES core MD97-2141 (8.80 N, 121.31 E, 3633 m) located in the Sulu Sea within the western tropical Pacific reveal the first evidence of continuous millennial-scale variability in surface ocean conditions over the last 150,000 years. The East Asian Monsoon and Western Pacific Warm Pool both influence the Sulu Sea, have a significant influence on global climatic conditions today, and thus the past behavior of these systems is of interest. The millennial-scale planktonic foraminiferal oxygen isotope (del18O) oscillations of Globigerinoides ruber (G. ruber) between 30,000-65,000 years (MIS3) are apparently in-phase with the Greenland ice core record and have amplitudes 1/3 to 2/3 the size of the Sulu Sea glacial-interglacial del18O amplitude of 1.3 ‰. In the same interval variations in planktonic foraminifera Mg/Ca suggest that millennial-scale sea surface temperature (SST) variations were small (0.6-1°C) and out-of-phase with del18O indicating that del18O variability was mainly driven by changes in surface water salinity. This result implies that the linked East Asian monsoon and the western Pacific Intertropical Convergence Zones (ITCZ) have fluctuated on the same millennial time scale as higher latitude climatic systems.

To further investigate the origin of the MIS3 del18O G.ruber variations, the relative abundance of all planktonic foraminifer species and the del18O values of four species (G. ruber and sacculifer, mixed-layer dwellers; Neogloboquadrina dutertrei, a thermocline dweller; and Globorotalia crassaformis, a deep dweller) was determined in 35 samples between 35,000 and 55,000 years ago at Sulu Sea Site MD97-2141. Combined, these data provide a detailed reconstruction of changes in the western tropical Pacific thermocline structure between 35,000 and 55,000 years ago. The del18O composition of the mixed-layer planktonic foraminifera, (G. ruber and Globigerinoides sacculifer), and upper thermocline species (N. dutertrei), which displays poor similarity with the del18O of the sub-thermocline dweller Globorotalia crassaformis during MIS3. del18O G.crassaformis shows larger del18O variations (~1 ‰) than the surface dwellers which is interpreted to indicate past fluctuations in the influence of high salinity North Pacific Tropical Waters (NPTW) that currently enter the Sulu Sea across the 420 m deep sill, called Mindoro Strait, during the months of the winter monsoon. The faunal and isotopic data in this study suggest that an switch from winter to summer monsoon predominance after 55 kyr, however this predominance is interrupted by at least three episodes of increased winter monsoon between 42-46 kyr.

Comparison of the proxy SST and planktonic foraminiferal del18O profiles for the last glacial/interglacial sequence from fourteen cores in tropical and subtropical oceanic settings indicates that termination I in of del18O coincides with SST change at some sites, while del18O lags SST by 3,000 years at other locations. A comparison of SST and del18O shows a linear increase in SST from glacial to interglacial conditions. Sites where SST is leading the del18O record indicate fresher conditions during the LGM. It has been observed that these sites with SST leading the del18O record are located in areas that are influenced by increased atmospheric water vapor during times of today’s La Niña.