Pliocene and Pleistocene paleoceanography in the western subarctic Pacific based on diatom analyses of ODP Leg 145 Hole 884B and IODP Expedition 323 Holes U1341B and U1343E

Abstract Late Pliocene–Pleistocene fossil diatom assemblages from Ocean Drilling Program (ODP) Leg 145 Hole 884B in the western subarctic Pacific were investigated to improve the diatom biostratigraphy. Moreover, the paleoceanographic records at Hole 884B were compared with those in Integrated Ocean Drilling Program (IODP) Expedition 323 Holes U1341B and U1343E in the Bering Sea for the time interval of 2.5–0 Ma in order to clarify the differences in the diatom assemblage fluctuations and the history of past ocean circulations around the subarctic Pacific. In Hole 884B, five diatom zones from the Neodenticula koizumii–Neodenticula kamtschatica Zone to the Neodenticula seminae Zone were identified. The decreases in the relative abundance of temperate-water species were recognized at ca. 1.9 Ma in Hole U1343E, ca. 2.1 Ma in Holes 884B and U1341B. The decreases of the temperate-water species at these sites indicate that a widespread cooling event must have occurred over the subarctic Pacific at ~2 Ma. In addition, the continuous low occurrences of the sea-ice related species from around 2 Ma onward, which indicate the presence of sea-ice, were observed at Sites 884 and U1341. We also point out that the age differences in decrease of temperate-water species at the three sites (i.e., delay in response of the temperate-water species at Site U1343) may be related to changes of seawater inflow from the North Pacific to Sites U1341 and U1343 and of the East Kamchatka Current from the Bering Sea to Site 884.