WESTERN PACIFIC PALAEOCEANOGRAPHY ACROSS THE EARLY–MIDDLE PLEISTOCENE BOUNDARY (~773 KA): DINOFLAGELLATE CYSTS OF THE CHIBA COMPOSITE SECTION, JAPAN. KEYWORDS: PALEOCEANOGRAPHY, DINOFLAGELLATES, PLEISTOCENE, JAPAN, MIS 19

The Chiba composite section, central Boso Peninsula, Japan is one of three candidate Global Boundary Stratotype Sections and Points for the Early–Middle Pleistocene boundary, and the only candidate for the Pacific realm. The primary guide for this boundary is the Matuyama–Brunhes polarity reversal, which coincides with Marine Isotope Stage (MIS) 19 at ~773 ka. MIS 19 also serves as a close analogue to our present interglacial. The Early–Middle Pleistocene boundary occurs at the midpoint of the Early–Middle Pleistocene transition, an interval marked by a fundamental reorganization of the Earth’s climate system. To understand this dynamic event more fully, and to elucidate the structure of MIS 19 and paleoceanographic changes across the Early–Middle Pleistocene boundary in the western Pacific, 71 samples from the Tabuchi section of the Chiba composite section were studied for dinoflagellate cysts. The sampled interval spans ~791–763 ka (upper MIS 20 through lower MIS 18) and has already been documented for paleomagnetism and benthic foraminiferal oxygen isotopes. The succession was deposited on the continental slope and consists of continuously deposited siltstone with no evidence of turbidites. Dinoflagellate cysts of moderate preservation have been recorded in all samples. More than 27 taxa are present including cysts of Protoceratium reticulatum, Spiniferites spp. indet., Lingulodinium machaerophorum, cysts of Protoperidinium nudum, and Spiniferites mirabilis + hyperacanthus. Dinoflagellate cyst assemblages closely track the oxygen isotope record from the Tabuchi section. Those from the plateau of MIS 19 (full interglacial conditions) reflect warm surface waters throughout, suggesting a stable climate during this part of MIS 19. A significant decrease in cyst concentrations is observed at the MIS 20/19 transition (Termination IX). The MIS 19/18 transition is marked by a sharp decrease in species richness and shift to dominance of Protoceratium reticulatum cysts, suggesting cooler conditions (MIS 18). Work is in progress to assess the lead-lag relationship between the dinoflagellate cyst and benthic δ¹⁸O records, to characterize MIS 19 in detail, and to evaluate the position of the Matuyama–Brunhes paleomagnetic boundary relative to the internal structure (climate dynamics) of MIS 19.