Towards the Reconstruction of the Thermocline Ridge in the Western Tropical Pacific: Implications for LGM Tropical Atmospheric Circulation

Several studies have suggested that the intertropical convergence zone (ITCZ) shifts south in response to cooling the northern hemisphere during the Last Glacial Maximum (LGM). However, there have been few attempts to constrain the position of the ITCZ during the LGM with paleoclimate data. Since zonal wind patterns across the Pacific largely drive the approximately geostrophic currents near the equator, we would expect that a shift in the ITCZ would cause a shift in current positions, and therefore a shift in the seawater density gradients associated with the currents. The oxygen isotope ratio in planktonic foraminiferal calcite is sensitive mainly to the temperature and the oxygen isotope ratio of the seawater in which it was formed. Specifically, the species Globorotalia tumida calcifies at approximately 150 m depth, which makes it ideal for reconstructing the lateral density gradients in the upper ocean associated with the wind driven currents. We determine the oxygen isotope ratios in G. tumida for the recent and glacial sections of a series of sediment cores which transect the equator at approximately 145-150°E. We use this data to locate the thermocline ridge which characterizes the boundary between the Equatorial Countercurrent and the North Equatorial Current. Our transect currently runs from approximately 4°S to 11°N. The core top data appear to resolve the ridge well, matching expected oxygen isotope values calculated from instrumental salinity and temperature data. So far, we have identified the LGM based on planktonic foraminiferal isotope stratigraphies. The LGM data appear to show that the thermocline ridge is not significantly displaced southward relative to today, although our transect does not yet continue far enough north to show that the location of the LGM ridge is identical to the modern.