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Abstract
This study presents the first attempts to synthesize calcareous nannofossil biostratigraphy of the Japan Sea, to evaluate the nature of the Tsushima Warm Current at high resolution using calcareous nannofossils, and to assess effects of oceanographic restriction in the Japan Sea on the distribution of calcareous nannofossils during the Pleistocene. Integrated Ocean Drilling Program Expedition 346 and International Ocean Discovery Program Expedition 350 provide a combination of lithostratigraphic, paleomagnetic, and paleontological data that are incorporated herein with high resolution calcareous nannofossil analyses, allowing for an unprecedented assessment of nannofossil assemblage dynamics in the Japan and Philippine seas during the Pleistocene. Integration of these data sets is of particular importance in interpreting paleoenvironmental history of the Japan Sea where regional environmental conditions played a large role in the distribution of calcareous nannofossil bioevents in the Pleistocene due to sensitivity to glacial-interglacial cyclicity resulting from shallow sill depths. The last occurrences (LO) of Helicosphaera sellii and Reticulofenestra asanoi occur later in the Japan Sea than in open ocean sites with the LO of H. sellii in Marine Isotope Stage (MIS) 21 and the LO of R. asanoi in MIS 37. This corresponds more closely to distribution patterns observed in the high latitude North Atlantic Ocean than in the adjacent Pacific Ocean. Calcareous nannofossil assemblage data were collected from Site U1427 in order to study evolution of the Tsushima Warm Current, which played a significant role in the paleoceanographic and paleoecological evolution of the Japan Sea. Nannofossil abundance data allow for the identification of three discrete intervals at Site U1427: I) MIS 31-27, II) MIS 27-21, and III) MIS 21-17. High-resolution quantitative analyses of samples recovered from Site U1437 allow for the determination of differences in the nannofossil assemblages between the marginal Japan Sea and the adjacent, open ocean Philippine Sea. Nannofossil assemblage data indicate warmer and more oligotrophic surface waters in the Philippine Sea throughout the middle Pleistocene. Additionally, the last occurrence of Reticulofenestra asanoi occurs stratigraphically higher at Site U1426 in the Japan Sea than in the Philippine Sea.
