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.