Acknowledgements
We gratefully acknowledge the Research Institute for Humanity and Nature
staff for the use of the TC/EA-IRMS. This study was conducted by the
support of Joint Research Grant for the Environmental Isotope Study of
Research Institute for Humanity and Nature. We thank Prof. Tsubasa Otake
for financial and administrative support. This work was supported by
JSPS KAKENHI Grant Numbers JP20H00184. We thank Dr. Masakazu Ito for
providing the bentonite ore.