No abstract

No abstract

A continuous, well-preserved core was obtained from the Choshi area, on the Pacific side of Japan, to investigate paleoceanographic and paleoclimatic changes around the northwestern Pacific region during the middle Pleistocene. Siliciclastic sequences in the core are divided into five formations -the Obama, Yokone, Kurahashi and Toyosato Formations in the Inubo Group and the Katori Formation, in ascending order. Examination of calcareous nannofossils and magnetic polarities detected four datums in the core sediments of the Inubo Group: the top of Reticulofenestra asanoi, the base of Helicosphaera inversa, the top of Pseudoemiliania lacunosa and the Brunhes-Matuyama boundary. Fourteen marine isotope stages (MIS24-MIS11) were identified in the δ 18 O and δ 13 C records based on detected datums and the graphic correlation with the standard stack oxygen isotope curve. Magnetic susceptibility and gamma-ray attenuation porosity evaluator density were also measured and low values characterize the glacial intervals. Biogenic sedimentation by primary production may be larger during the glacial periods because of invasions of nutrient-rich northern surface-waters related to the southward shift of the Kuroshio front in the Choshi area.

The Executive Committee of the International Union of Geological Sciences on January 17, 2020 ratified the Global Boundary Stratotype Section and Point (GSSP) defining the base of the Chibanian Stage/Age and Middle Pleistocene Subseries/Subepoch at the Chiba section of the Chiba composite section, Japan. The Chiba composite section is a continuous and expanded marine sedimentary succession in the east-central Japanese archipelago facing the Pacific Ocean. It contains well-preserved pollen, marine micro-and macrofossils, a tightly-defined Matuyama-Brunhes (M-B) paleomagnetic polarity boundary, two geomagnetic field paleointensity proxies, and numerous tephra beds, allowing the establishment of a robust and precise chronostratigraphic framework. Its open-ocean continental slope setting has captured both terrestrial and marine environmental signals from upper Marine Isotope Stage (MIS) 20 to lower MIS 18. The M-B reversal serves as the primary guide for the Lower-Middle Pleistocene boundary, yield-ing an astronomical age of 772.9 ka. The GSSP is positioned 1.1 m below the directional midpoint of the reversal, at the base of a regional lithostratigraphic marker, the Ontake-Byakubi-E (Byk-E) tephra bed, in the Chiba section. The GSSP has an astronomical age of 774.1 ka and occurs immediately below the top of Marine Isotope Substage 19c.

No abstract