On January 17, 2020, 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, yielding 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 (Suganuma et al., 2021, Episodes, online).
On 30th January, 2020, the Executive Committee of the International Union of Geological Sciences ratified two proposals approved by the International Commission on Stratigraphy formalizing: 1) the Lower Pleistocene Subseries, comprising the Gelasian Stage and the superjacent Calabrian Stage, with a base defined by the GSSP for the Gelasian Stage, the Pleistocene Series, and the Quaternary System, and currently dated at 2.58 Ma; and 2) the term Upper Pleistocene, at the rank of subseries, with a base currently undefined but provisionally dated at ~129 ka (Head, Pillans & Zalasiewicz, 2021, Episodes, online). The Middle Pleistocene Subseries is defined by the recently ratified GSSP for the Chibanian Stage currently dated at 0.774 Ma (Suganuma et al., 2021, Episodes, online; and see above). These ratifications complete the official division of the Pleistocene into three subseries/ subepochs, in uniformity with the similarly subdivided Holocene Series/Epoch. The Upper Pleistocene Subseries is an official ratified term but its base is presently undefined. Historically, this base aligns approximately with the base of Marine Isotope Substage 5e. The definition of the base of Upper Pleistocene Subseries and its corresponding stage by GSSP is now a priority of the SQS.