Calcareous nannofossil biostratigraphy of the Lower\u2013Middle Pleistocene boundary of the GSSP, Chiba composite section in the Kokumoto Formation, Kazusa Group, central Japan, and implications for sea-surface environmental changes

Koji Kameo,Yusuke Suganuma,Makoto Okada,Yuki Haneda,Yoshimi Kubota

doi:10.1186/s40645-020-00355-x

Abstract

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

Highlights

Calcareous nannofossils are major phytoplankton fossils that are used to determine the geologic age of strata and to infer past marine environmental changes
This study examined calcareous nannofossil assemblages of the Chiba composite section (CbCS) of the Kokumoto Formation of the Kazusa Group in order to extract the calcareous nannofossil index for the Lower–Middle Pleistocene boundary and to consider environmental changes in sea surface conditions along the southwest Pacific side of the Japanese islands around the M–B paleomagnetic polarity boundary
Local bioevents and floral changes associated with oceanographic conditions were found using high-resolution analysis of calcareous nannofossils: 1. Previous results of the calcareous nannofossil biostratigraphy of the Kazusa Group (e.g., Sato et al 1988, 1999) and resultant nannofossil biohorizons were summarized

Summary

Introduction

Calcareous nannofossils are major phytoplankton fossils that are used to determine the geologic age of strata and to infer past marine environmental changes. The marine Kazusa Group is a Japanese Pleistocene unit on the Boso Peninsula (central Pacific Japanese coast; Fig. 1) that yields abundant microfossils including pollen and spores, planktonic and benthic foraminifers, calcareous nannofossils, diatoms, and radiolarians (e.g., Oda 1977; Sato et al 1988). These microfossils have been used to determine the age of the Group and the paleoceanographic conditions.

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