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The interval spanning the uppermost Eocene to the lower Oligocene was a time of major climatic and oceanographic change, including major changes in sediment composition and faunal assemblages. Here we examine the radiolarian fauna during the Eocene-Oligocene transition (40-30 Ma) from three sites: Ocean Drilling Program Site 1218 and Integrated Ocean Drilling Program Sites U1333 and U1334, all of which appear to contain complete sections of the Eocene/Oligocene boundary and to have been located near the Pacific Equator during this time interval. Using previous work that interrelated the stratigraphic position of samples from all three sites, the abundance and first and last appearances of 76 stratigraphically important radiolarian species, species groups, and variant forms are documented in >600 samples. This quantitative analysis of species abundance has allowed us to accurately define 109 first and last appearance datums and to document the reworking of older microfossils into younger sections.

Previous studies showed that the evolution of the Japan Sea paleoceanography since the Miocene has been influenced by the regional tectonism (e.g., opening/closing of the connecting seaways) and regional/global climate. In the Japan Sea, Expedition 346 of the Integrated Ocean Drilling Program (IODP) retrieved core sediments dating back to the Miocene at two sites (U1425 and U1430). In this study, we reconstruct shallow-to-deep-water hydrography of the Japan Sea during the Mio-Pliocene based on radiolarian assemblages at Sites U1425 and U1430 considering the local tectonism and changes in global/regional climate. Our data suggest that glacioeustatic sea-level changes have probably had an influence on the local paleoceanography between 9.5 and 7.0 Ma. Indeed, warm water probably flowed from the North Pacific into the Japan Sea when sea level was high via shallow central and eastern seaways. In addition, the sill depth of the northern seaway was probably close to 1000 m between 9.5 and 7.8 Ma and had probably allowed inflow of oxygen minimum zone water from the North Pacific to the Japan Sea when sea level was high. In contrast, our data imply that Cycladophora nakasekoi, an endemic species to the Japan Sea, dominated between 9.5 and 7.3 Ma when sea level was low. Our data also suggest a progressive shoaling of the sill for the period since 7.8 Ma and that global climatic events such as such the late Miocene cooling (7.5-5.5 Ma) and the early Pliocene warmth have had a sustained influence on the Japan Sea. During the mid-Pliocene, a deep cooling of the subsurface to intermediate water of the Japan Sea likely occurred because species related to subarctic subsurface to intermediate waters were dominant between 5 and 3.8 Ma. The Northern Hemisphere Glaciation (ca. 3.0-2.7 Ma) and Mid-Pleistocene Transition (1.2-0.8 Ma) have both likely intensified the cooling of the Japan Sea.

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