New benthic foraminiferal δ18O and δ13C data from Ocean Drilling Program (ODP) Site 884 (northwestern Pacific) add significant structure to a previous lower resolution record; our higher resolution detail and comparisons with published isotopic records provide new insights into paleoceanographic changes in the northwestern Pacific from the middle Eocene to the early Oligocene.

From the early-middle until the mid-middle Eocene (ca. 49–43 Ma), a comparison of the Site 884 δ18O values with published ODP δ18O records from the Pacific (Site 1218), Atlantic (Site 1260), and Southern Ocean (Site 689) reveals that these basins were bathed by a common water mass, probably originating from the Southern Ocean, or by different water masses having a similar δ18O signature. From the mid-middle until the early-late Eocene (ca. 42–37 Ma), the Site 884 record reveals increasing complexity in deep-water circulation. The relatively low Site 884 δ18O values suggest that the northwestern Pacific was bathed by a water mass that was warmer than at the other Pacific, Atlantic, and Southern Ocean sites used in this study. Based on the relatively low δ13C values at Site 884, there are two possible scenarios for the origin of this northwestern Pacific water mass: (1) nutrient-rich surface waters downwelling at the higher latitudes of the North Pacific (Bering Sea); or (2) warm saline deep waters originating at low latitudes (possibly from the Tethys). In the late Eocene (ca. 36.5 Ma), the Site 884 δ18O and δ13C values increased, becoming more similar to Sites 1218 (Pacific), 1053 (Atlantic), and 689 (Southern Ocean), indicating an additional change in deep-water circulation, with a deep-water mass originating from the Southern Ocean bathing the northwestern Pacific.

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