Sea surface temperature reconstructions using Mg/Ca of two mixed layer foraminifera species in the western Pacific warm pool for the last 4 Myr

Climate condition of the Plio-Pleistocene is characterized by global cooling and intensification of the Northern Hemisphere Icesheet. During this time period, sea surface temperatures (SSTs) both at mid and high latitudes showed long-term cooling, whereas SST at the western Pacific warm pool (WPWP) showed only minor cooling, resulting in a gradual increase of latitudinal SST gradient. However, SST reconstruction at the WPWP is controversial because the SST trends from different proxies are not consistent. The long-term continuous planktonic foraminiferal Mg/Ca-based SST record in WPWP has been only available from ODP Site 806 with Trilobatus sacculifer, whose calcification depth is slightly deeper than Globigerinoides ruber. Here, we conducted Mg/Ca-based temperature reconstruction using two mixed layer species of planktonic foraminifera, G. ruber and T. trilobus, at IODP Site U1488 for the last 4 Myr. The reconstructed temperature of T. trilobus shows consistent results with previously published one by T. sacculifer at ODP 806, whereas that of G. ruber showed slightly warmer SST and different pattern from T. trilobus. The temperature difference between two species decreased between 2.0 and 1.5 Ma, which coincides with decrease in SSTs in the subarctic, subantarctic, and east Pacific upwelling regions. Because of the limited seasonality in the WPWP, the temperature difference of these species at a single site probably reflects differences in their habitat depth. Our results suggest that the upper ocean stratification in the WPWP has been closely related to the meridional and zonal SST gradients, which are associated with the Northern Hemisphere Glaciation.