[S-GL19] Frontier research on chronostratigraphic unit boundaries

[SGL19-P04] A magneto-oxygen isotopic stratigraphy using an oriented boring core drilled through the Quaternary base horizon in the southernmost Boso Peninsula, Japan

Ryoka Yuhara¹, *Makoto Okada¹, Daiki Nagatomo¹, Yusuke Suganuma² (1.Department of Earth Sciences, College of Science, Ibaraki University, 2.National Institute of Polar Research)

Keywords:Quaternary base boundary, Gauss-Matuyama geomagnetic reversal boundary, Boso Peninsula

To obtain detailed paleomagnetic records of the Gauss-Matsuyama geomagnetic reversal boundary (G-M boundary), which is a reference for the basement boundary of the Quaternary, an oriented drilling was conducted near the basement of the Quaternary in the Chikura Group from May to June 2021. As a result, we obtained a 51-m core consisting of nearly flawless sandstone-siltstone alternation. After cutting and nondestructive measurements of the core at the Kochi Core Center, we collected samples for foraminiferal extraction at a layer thickness interval of approximately 1 m. We attempted continuous sampling of cubic specimens of 2 cm per side for paleomagnetic measurement. In order to effectively remove secondary magnetization and to reconstruct detailed geomagnetic field variations at the G-M boundary, hybrid demagnetization using a combination of a thermal demagnetization and a progressive AF demagnetization, which was confirmed to be effective in the Chikura Group by Konishi and Okada (2020), was performed using a pass-through superconducting magnetometer and an electric furnace for thermal demagnetization at the same laboratory.
According to preliminary paleomagnetic measurements obtained so far, a transgression near the equator of the VGP (virtual geomagnetic pole) associated with an RPI (relative paleointensity) minimum at a depth of about 10 m, indicating the G-M boundary. Simultaneous 10Be measurements confirmed the position of the 10Be/9Be peak, which indicates the maximum intensity of cosmic rays in the vicinity of the RPI minimum. Oxygen isotope measurements of benthic foraminifera confirmed that the core covers MIS 102-G3, and the G-M boundary is located in MIS 103. Further studies on the continuous paleomagnetic and oxygen isotopic measurements in the vicinity of the G-M boundary are expected to contribute to establishing an international chronostratigraphy in the vicinity of the Quaternary basement boundary.
References
Konishi and Okada (2020): doi.org/10.1186/s40645-020-00352-0