Paleoceanography and dinoflagellate cyst stratigraphy across the Lower–Middle Pleistocene Subseries (Calabrian–Chibanian Stage) boundary at the Chiba composite section, Japan

Abstract

A dinoflagellate cyst record from the highly resolved Chiba composite section in Japan has been used to reconstruct sea-surface paleoceanographic changes across the Lower–Middle Pleistocene Subseries (Calabrian–Chibanian Stage) boundary at the global stratotype, constituting the first detailed study of this microfossil group from the Pleistocene of the Japanese Pacific margin. Cold, subarctic water masses from 794.2 ka gave way to warming and rapid retreat of the Subpolar Front at 789.3 ka, ~ 2000 years before the end of Marine Isotope Stage (MIS) 20. Throughout the fully interglacial conditions of MIS 19c, assemblages are consistent with warm sea surface temperatures but also reveal instability and latitudinal shifts in the Kuroshio Extension system. The abrupt dominance of Protoceratium reticulatum cysts between 772.9 and 770.4 ka (MIS 19b) registers the influence of cooler, mixed, nutrient-rich waters of the Kuroshio–Oyashio Interfrontal Zone resulting from a southward shift of the Kuroshio Extension. Its onset at 772.9 ka serves as a local ecostratigraphic marker for the Chibanian Stage Global Boundary Stratotype Section and Point (GSSP) which occurs just 1.15 m (= 1300 years) below it. An interval from 770.1 ka to the top of the examined succession at 765.8 ka (MIS 19a) represents warm, presumably stratified but still nutrient-elevated surface waters, indicating a northward shift of the Kuroshio Extension ~ 5 kyrs after the termination of full interglacial conditions on land.

Preface for the article collection “Stratigraphy and paleoclimatic/paleoenvironmental evolution across the Early–Middle Pleistocene transition in the Chiba composite section, Japan, and other reference sections in East Asia”

“…Based on the abundant occurrences of a warm-water calcareous nannofossil and radiolarian species, Kameo et al (2020) and Itaki et al (2022) respectively show periodic intensifications of the Kuroshio Current following the end of MIS 19c and continuing into MIS 18, indicating repeated northward shifts of the Subarctic Front. In addition, the dinoflagellate cyst record reveals instability in the Kuroshio Extension system throughout MIS 19c and the abrupt influence of the Kuroshio-Oyashio Interfrontal Zone in MIS 19b, signaling the first stadial of the latter part of MIS 19 (Balota et al 2021). This instability and similar rapid shifts are also seen in the cyclic appearance of cold-water calcareous nannofossil and radiolarian taxa from MIS 19b to MIS 18 (Kameo et al 2020;Itaki et al 2022).…”

“…This composite section comprises five individual exposures cropping out along a 7.4 km transect, all securely linked by a detailed framework of tephra beds. Deposited on the continental slope and receiving substantial organic input from both marine and terrestrial sources (Balota et al 2021;Izumi et al 2021;Suganuma et al 2018), it therefore provides a rare opportunity to capture both terrestrial and marine environmental variability continuously across MIS 19 and hence the Lower-Middle Pleistocene boundary.…”

“…The Chiba composite section has yielded a diverse array of well-preserved marine and terrestrial microfossils and provides an ultra-high-resolution benthic and planktonic foraminiferal isotope record that captures centennial-to millennial-scale paleoceanographic and paleoclimatic changes from late MIS 20 to early MIS 18 (Suganuma et al 2021). In this issue, Kameo et al (2020), Balota et al (2021), andItaki et al (2022) document the assemblage changes of calcareous nannofossils, dinoflagellate cysts, and radiolarians, respectively, to explore variations in the position of the Subarctic Front between the subtropical Kuroshio and subarctic Oyashio currents. Because the Subarctic Front is presently situated just east of the Boso Peninsula (Qiu 2001), the Chiba composite section is located at an ideal position to record the past displacement of the front.…”

“…The dinoflagellate cyst record, which might represent warm-season conditions rather than the cold-season signal reflected by most of the other proxies, includes increased cyst concentrations during the initial stadial following MIS 19c. This implies increased surface water productivity resulting from a southward shift of the Subarctic Front and increased influence of the nutrientrich waters of the Oyashio Current (Balota et al 2021).…”

Depositional process of the Byk‐E tephra bed in the Chiba section, central Japan: A marker bed defining the Global Boundary Stratotype Section and Point for the Chibanian Stage

“…Based on several stratigraphic studies (e.g., Mitsunashi et al, 1959), the chronostratigraphic framework of the Kazusa Group has been established based on the biostratigraphy of the foraminiferal fossils (Oda, 1977) and nannofossils (Sato & Takayama, 1988), magnetostratigraphy (Niitsuma, 1976), the oxygen isotopic stratigraphy (Pickering et al, 1999), and combinations of these stratigraphic information (Okada & Niitsuma, 1989;Tsuji et al, 2005). In the case of the Kazusa Group, various paleoceanographic studies have been conducted for the chronostratigraphic framework (Balota et al, 2021;Horikawa et al, 2001;Igarashi, 1994;Izumi et al, 2021;Kameo et al, 2020;Kubota et al, 2021;Suganuma et al, 2018).…”