Formal ratification of the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries of the Quaternary System: the Chiba Section, Japan^†

Abstract

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.

Paleoceanography of the northwestern Pacific across the Early–Middle Pleistocene boundary (Marine Isotope Stages 20–18)

“…5 ka as inferred from the chronologic uncertainty of 4 ka in the LR04 stack used as a target curve by Elderfield et al (2012) (Lisiecki and Raymo 2005) plus another ca. 1 ka of uncertainty in our tuning to the Elderfield et al (2012) record (Suganuma et al 2021).…”

“…The Chiba composite section (CbCS) comprises the Urajiro, Yanagawa, Yoro River, Yoro-Tabuchi, and Kokusabata sections (Suganuma et al 2021). We used the age model of Suganuma et al (2018), this being (See figure on previous page.)…”

“…The Chiba composite section (CbCS) including the Chiba section itself (35°17′ 39.6"N, 140°08′ 47.6" E to 35°17′ 36.9" N, 140°08′ 47.2" E; Suganuma et al 2021) exposed on the Boso Peninsula, central Japan, and occurring within the Kokumoto Formation of the Kazusa Group (Kazaoka et al 2015), represents a high-resolution continuous sedimentary succession deposited on the shelf edge to continental slope at depths greater than 200 m (Nishida et al 2016). This succession contains excellently preserved calcareous microfossils (Kameo et al 2020;Suganuma et al 2018), and its favorable nature has permitted the establishment of a centennial-to millennialscale benthic foraminiferal oxygen isotope (δ 18 O) stratigraphy (Okada et al 2017;Suganuma et al 2018;Haneda et al 2020a).…”

“…The combination of the M-B boundary, the marine isotope stratigraphy, and the Byk-E tephra bed allows paleoenvironmental proxies within the CbCS to be correlated at local, regional, and global scales. The Global boundary Stratotype Section and Point for the Middle Pleistocene Subseries and Chibanian Stage is placed at the base of the Byk-E tephra bed at the Chiba section (Head, 2019;Suganuma et al 2021).…”

Matuyama-brunhes Geomagnetic Reversal Record and Associated Key Tephra Layers in Boso Peninsula: Extraction of Primary Magnetization of Geomagnetic Fields From Mixed Magnetic Minerals of Depositional, Diagenesis, and Weathering Processes

“…In the Boso Peninsula, thick Pleistocene marine sediments, the Kazusa Group and the overlying Shimosa Group were deposited (Figure S1). The Kazusa Group is well exposed and contains a continuous stratigraphic succession with well-preserved marine microfossils, pollen, paleomagnetic reversal events, and many tephra layers Haneda et al, 2020;Nishida et al, 2016;Okada et al, 2017;Suganuma et al, 2018Suganuma et al, , 2021. The M-B boundary is widely distributed in the area (e.g., Niitsuma et al, 1971Niitsuma et al, , 1976Hyodo et al, 2016).…”