Shoaling of the western equatorial Pacific thermocline during the last glacial maximum inferred from multispecies temperature reconstruction of planktonic foraminifera

Glacial–interglacial scale climate variability in the tropical Pacific is often discussed with reference to modern El Niño-Southern Oscillation (ENSO). However, proxy records show diverse results and result in inconsistent interpretations. To investigate changes in the thermocline depth of the western Pacific warm pool (WPWP), which is strongly related to the strength of the Walker circulation, this study conducted δ18O and Mg/Ca analyses of multispecies planktonic foraminifera for the last 25,000 years. Habitat depths and calcification temperatures of seven foraminiferal species were estimated by comparing measured δ18O values with the predicted values by assuming isotopic equilibrium. Reconstructed Mg/Ca-based temperatures, calculated with a newly calibrated multispecies equation, showed ~ 1–2 °C larger cooling at the thermocline depth than that at the surface during the last glacial maximum (LGM), suggesting shoaling of the thermocline. Furthermore, fresher surface water in the WPWP, which is attributed to the suppressed evaporation by low sea surface temperature, suggests the weakened deep atmospheric convection. These results suggest that the Walker circulation during the LGM was weaker than that during the late Holocene.