[M-IS04] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS04-P05] Pliocene thermocline stability in the western Pacific warm pool inferred from the individual foraminifera analysis

Fumito Mizuno², *Takuya Sagawa¹ (1.Institute of Science and Engineering, Kanazawa University, 2.Graduate School of Natural Science and Technology, Kanazawa University)

Keywords:planktonic foraminifer, individual foraminifera analysis, Mg/Ca

Past behavior of the El Nino-Southern Oscillation (ENSO) has been investigated by the zonal sea surface temperature (SST) gradient in the tropical Pacific. Previous studies have shown that the zonal SST gradient was quite small during the Pliocene period, suggesting the permanent El Nino condition. However, since the SST reconstruction by “bulk” foraminiferal Mg/Ca analysis represents the average condition of each sample (a few centimeters thick), which generally includes several hundred years, the diminished zonal SST gradient does not necessarily mean El Nino condition in the year-to-year timescale. Here, we conducted the individual foraminiferal shell analysis (IFA) of Pulleniatina obliuqiloculata Mg/Ca picked from the sediment core IODP Site U1488. First, we compared Mg/Ca-based thermocline temperature (TT) variability of the late Holocene with SODA record and found that the TT variability is similar to that of SODA record at 150 m water depth. Then, we compared the Holocene variability with that of the mid-Pliocene warm period (MPWP). The TT variability during MPWP is approximately two-thirds of the Holocene, suggesting a more stable thermocline depth during MPWP. Considering a smaller difference between SST and TT, the stable thermocline is interpreted as a deeper and more stable condition during MPWP than the modern one.