Keiji Horikawa

Millennial- to decadal-scale fluctuation in the paleo-Kuroshio Current documented in the Middle Pleistocene shelf succession on the Boso Peninsula, Japan

Abstract Millennial-, century-, and decadal-scale cyclicities were identified in a transgressive shelf succession of the Middle Pleistocene (ca. 0.7xa0Ma) on the Boso Peninsula, Japan. The shelf succession consists mainly of a sandridge complex and associated outer shelf deposits. The deposition was interpreted to have been controlled by fluctuation in speeds and paths of the paleo-Kuroshio Current. In general, the modern Kuroshio Current approaches the southern coasts of the Boso Peninsula in response to its large meander that is interpreted to respond to the El Nino episodes. Depositional cycles in the Kuroshio-Current-controlled shelf succession are interpreted to document high-frequency paleoclimatic oscillation in the equatorial Pacific region during the Middle Pleistocene time. The finding indicates that high-frequency paleoclimatic instability, that has been strongly expressed in the Upper Pleistocene successions in the North Atlantic region, was pervasive in time and space back to the Middle Pleistocene.

Long-term ENSO-like events represented in the Middle Pleistocene shelf successions, Boso Peninsula, Japan

Abstract Millennial-, century-, and decadal-scale stratigraphic cyclicities are described from transgressive shelf successions of the Middle Pleistocene Ichijiku and Kakinokidai Formations (ca. 0.7 Ma) in the Boso Peninsula, Japan. The Ichijiku Formation (ca. 400 m thick) represents a shelf sandridge complex developed under the influence of the large meanders of the paleo-Kuroshio Current, and intertongues with outer shelf muddy deposits of the Kakinokidai Formation (ca. 170 m thick) to the northeast. Most of the two formations are classified as a transgressive systems tract formed during the rise in glacioeustasy from oxygen isotope stages 18 to 17. Three different types of cyclicities are identified in the transgressive shelf deposits. The first cyclicity (ca. 3000 years duration) is represented by coarsening-upward units of the Kakinokidai Formation associated with the northeastward extension of the sandridge complex. Lower parts of each coarsening-upward unit are characterized by intercalations of tempestites more than the upper parts. The second cyclicity (ca. 250 years duration) is defined by master bedding surfaces within sandridge deposits, interpreted as formed in response to lateral migration of foresets of the sandridge complex. The third cyclicity (ca. a few decades or shorter duration) is defined as the units bounded by reactivation surfaces within large-scale trough cross-bedded sandstone units. The three different types of cyclicities are interpreted to document fluctuations in the paths and speeds of the paleo-Kuroshio Current in response to the large meanders. Oscillations of the large meanders of the modern Kuroshio Current every few to 10 years are interpreted to be in response to ENSO (El Nino-Southern Oscillation) events, and are also associated with the decrease in frequency of typhoons in the western Pacific. If our stratigraphic interpretation is true, the paleo-Kuroshio-Current-controlled shelf successions document high-frequency oscillations in paleoclimates responding to the long-term ENSO-like events in the western Pacific during the Middle Pleistocene.