Western Pacific paleoceanography across the Early–Middle Pleistocene boundary (~773 ka, Marine Isotope Stage 19): Dinoflagellate cysts of the Chiba composite section, Japan.
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The Chiba composite section, central Boso Peninsular, Japan, is a candidate Global Boundary Stratotype Section and Point for the Early–Middle Pleistocene Subseries boundary. This well-exposed, continuous and expanded marine silty sedimentary succession has a detailed paleomagnetic record and an ultra-high-resolution oxygen isotope stratigraphy supported by U-Pb zircon dating of the Byk-E tephra bed which occurs ~1 m below the Matuyama– Brunhes boundary. A detailed dinoflagellate cyst (dinocyst) study from late Marine Isotope Stage (MIS) 20 to late MIS 19 provides information on paleoceanographic changes in the Western Pacific Ocean between 791 and 770 ka. During Termination IX, a decrease in heterotrophic species (e.g. Brigantidinium spp., Selenopemphix nephroides) and corresponding increase in warm-water autotrophs (e.g. Spiniferites hyperacanthus/mirabilis, Spiniferites sp. 2) indicate a progressive decline in productivity corresponding to deglaciation and a transition from cold nutrient-rich waters of MIS 20 to warm conditions in MIS 19. This gradual warming is interrupted by a sudden but brief cooling at 790 ka which is also reflected in the MAT-based dinocyst sea-surface temperature (SST) for February and August. The dinocyst record shows a gradational response to the onset of MIS 19c. Abundant warm-water autotrophic dinocysts during MIS 19c reflects warm conditions and a northward shift of the warm Kuroshio current from 786 to 773 ka. The end of MIS 19c is marked by a sharp decrease in species richness and evenness, an abrupt and sustained rise to dominance of Protoceratium reticulatum, and a corresponding increase in dinocyst concentrations, indicating an increase in productivity and presaging the onset of the MIS 18 glaciation. Pollen and spore concentrations show a strong terrestrial influence during MIS 19. During the peak of MIS 19, Tsuga has low abundance while angiosperm pollen (deciduous broadleaved trees) increase significantly, which is similar to vegetation presently dominant in the northeastern part of the Japanese archipelago.