Foraminifera samples from the deep-sea sediment of an International Ocean Discovery Program (IODP) Site 363-U1489 (2º7.19'N, 141º1.67'E, water depth 3421 m), Western Equatorial Pacific, were measured for the Boron/Calcium ratios on the shells of benthic foraminifera (Cibicidoides wuellerstorfi), covering the time-interval from 4 to 2 Ma with an average time-resolution of ~20 ka. The measurements were performed by an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, Instrument type VG-X7) at the State Key Laboratory of Marine Geology, Tongji University, Shanghai, China. results are used to decipher changes in the carbonate-ion concentration of the deep Pacific during the Pliocene-Pleistocene transition.