Calcareous nannofossil biostratigraphy of the Lower–Middle Pleistocene boundary of the GSSP, Chiba composite section in the Kokumoto Formation, Kazusa Group, central Japan, and implications for sea-surface environmental changes

Progress in Earth and Planetary Science

Koji Kameo¹  Makoto Okada²  Yoshimi Kubota³  Yuki Haneda⁴  Yusuke Suganuma⁵ 

[1] Department of Earth Sciences, Faculty of Science, Chiba University, 1-33, Yayoi, Inage, 263-8522, Chiba, Chiba, Japan;Department of Earth Sciences, Ibaraki University, 2-2-1, Bunkyo, Mito, 310-8512, Ibaraki, Japan;Department of Geology and Paleontology, National Museum of Nature and Science, 4-1-1, Amakubo, Tsukuba, 305-0005, Ibaraki, Japan;Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Central 7, Higashi, Tsukuba, 305-8567, Ibaraki, Japan;National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, 190-8518, Tokyo, Japan;Department of Polar Science, School of Multidisciplinary Sciences, The Graduate University for Advanced Studies (SOKENDAI), 190-8518, 10-3, Midori-cho, Tachikawa, Tokyo, Japan;

关键词: Calcareous nannofossils;    Chiba composite section;    Lower–Middle Pleistocene boundary;    M–B paleomagnetic polarity boundary;    Gephyrocapsa;    Kuroshio Current;   

DOI  :  10.1186/s40645-020-00355-x

来源: Springer

【 摘 要 】

The Chiba composite section (CbCS), in the middle of the Boso Peninsula in central Japan, was ratified as the Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene boundary, accompanied by the Matuyama–Brunhes (M–B) paleomagnetic polarity boundary in January 2020. This study examined the calcareous nannofossil biostratigraphy of the CbCS to describe potential nannofossil events and discuss sea-surface environments around the M–B paleomagnetic polarity boundary. There are no clear biohorizons at the M–B paleomagnetic polarity boundary, although a temporary disappearance of Gephyrocapsa specimens (≥ 5 μm in diameter), an important calcareous nannofossil genus in the Pleistocene, occurs just above the Lower–Middle Pleistocene boundary. Although this is a characteristic event around the M–B paleomagnetic polarity boundary, it is unclear whether the event is globally traceable.Changes in the environmental proxy taxa of calcareous nannofossils in the CbCS revealed that sea-surface environments were driven by glacial-interglacial and millennial-scale climate forces. The time-transgressive change of the Tn value, a calcareous nannofossil temperature index, is mostly concordant with the planktonic foraminiferal oxygen isotope fluctuation. Abundant occurrences of a warm-water species, Umbilicosphaera spp., indicate that the Kuroshio Current was strong after ~ 783 ka. Even the strong influence of the Kuroshio Current, cooling events related to southward movements of the Kuroshio Front occurred every 10,000 years based on the presence of a cold-water taxon, Coccolithus pelagicus. Additionally, the inflow of coastal waters strengthened after ~ 778 ka because of abundant occurrences of Helicosphaera spp. Millennial-scale sea-surface changes were also inferred from detected floral fluctuations of less than 10,000 years.Graphical abstract

【 授权许可】

CC BY