High-resolution diatom record of paleoceanographic variations across the Early–Middle Pleistocene boundary in the Chiba Section, central Japan

Abstract

We conducted diatom analyses of a core from the Pleistocene marine sequence drilled at the Chiba Section, central Japan, to reveal detailed paleoceanographic variations across the Early–Middle Pleistocene boundary. Diatom valves are most abundant at 4.5 m below the Matuyama–Brunhes magnetic polarity boundary, followed by the second highest abundance at 18.6 m above the boundary, which is consistent with the sea-level changes inferred from planktonic δ18O data. The highest abundance is correlated with the Marine Isotope Stage (MIS) 19.3 sea-level highstand, and the second-highest with the MIS 19.1 highstand. The abundance of the extinct species Actinocyclus ingens is quite low around these sea-level highstands, whereas it is greatest at the core depth with the minimum diatom total valves, which is correlated to the MIS 19.2 lowstand. Cold-water diatom species that are characteristic of the Oyashio Current are dominant but coexist with warm-water species of the Kuroshio Current below a core depth just postdating MIS 19.2. Above this depth, warm diatom species of the Kuroshio Current become dominant. The change in dominant diatoms suggests that the Kuroshio Front shifted northward just after MIS 19.2 and remained in the northern area until at least early MIS 18. This shift caused a rapid increase in sea-surface temperature by 4–5 °C within a sediment thickness of 1.3 m (representing ∼300 years).