Millennial-scale oscillations in the Kuroshio–Oyashio boundary during MIS 19 based on the radiolarian record from the Chiba composite section, central Japan

Abstract

Marine isotope stage (MIS) 19 is considered to be the best orbital analog for the present interglacial. Consequently, clarifying the climatic features of this period can provide us with insights regarding a natural baseline for assessing future climate changes. A high-resolution radiolarian record from 800 to 750 ka (MIS 20 to MIS 18) was examined from the Chiba composite section (CbCS) of the Kokumoto Formation, including the Global Boundary Stratotype Section and Point for the lower–middle Pleistocene boundary on the Boso Peninsula on the Pacific side of central Japan. Millennial-scale oscillations in the Kuroshio warm and Oyashio cold currents were revealed by the Tr index, which is estimated using a simple equation based on radiolarian assemblages. The estimated Tr values ranged between 0.1 and 0.8 for MIS 18 through MIS 19, with minimum and maximum values corresponding to values observed off present day Aomori (41°N) and the Boso Peninsula (35°N), respectively. The observed patterns tended to be synchronous with the total radiolarian abundance associated with their production. Multiple maxima in radiolarian abundance occurred during periods of the Oyashio expanded mode before 785 ka and during periods of Kuroshio extension after 785 ka in MIS 19. Such increases in radiolarian abundance with the Kuroshio extension during MIS 19 are likely related to improvements in nutrient and photic environments with the development of a two-layer structure along the Kuroshio–Oyashio boundary zone. A similar pattern of millennial-scale climatic changes was also recognized in a precipitation record from the Sulmona Basin in central Italy, suggesting a close relationship with the CbCS record as a result of a large-scale climate system similar to the Arctic Oscillation in the northern hemisphere.

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”

“…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).…”

“…Changes in marine microfossil assemblages and indicator taxa along with foraminiferal isotope data reveal millennial-scale fluctuations superimposed on glacial-interglacial orbital-scale variability, in which the phenomenon is generally coherent with detailed records from the North Atlantic and Mediterranean region (Haneda et al 2020b;Head 2021). 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).…”

“…Although the δ 13 C org and C/N records are generally modulated by glacial-interglacial orbital-scale variability, Izumi et al (2021) show that peaks of TOC at warmer intervals in MIS 19a represent enhanced organic matter preservation owing to water column stratification caused by the northward displacement of the Subarctic Front rather than increased surface water productivity. However, the radiolarian records infer that the warmer intervals are characterized by higher productivity, which may reasonably be explained by a stratified Kuroshio-Oyashio structure (Itaki et al 2022). 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.…”

“…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.…”

Evolution of Oceanography of the Central Northwest Pacific Over the Past 10 Million Years With Focus on Late Miocene Global Cooling

“…Thus, the distribution of Tetrapyle spp. is probably closely related to warm water and nutrients rather than KC water as suggested by Itaki et al (2022).…”