4:30 PM - 4:45 PM
[HQR06-05] Assessing the plausibility of beta dose rate calculation in luminescence dating of sand matrices within gravelly deposits
Keywords:Luminescence dating, Sand matrix, Beta dose rate, Fluvial terrace, Pleistocene
Fluvial and alluvial deposits consisting of gravelly deposits are common in and around mountainous landscapes. These deposits typically lack both organic materials suitable for radiocarbon dating and the sand lenses that are generally targeted in luminescence dating. This problem has not attracted much attention, yet is critical for establishing chronologies of fluvial and alluvial deposits. Establishing high-resolution luminescence chronologies of gravelly fluvial and alluvial deposits or obtaining ages at specific stratigraphic levels from such deposits is essential to understand fluvial responses to climate changes and tectonics (e.g. Zondervan et al., 2022). In Japan, estimating the uplift rate based on the luminescence ages of fluvial terraces composed of gravelly deposits could be utilized to assess the safety of radioactive waste disposal.
Luminescence dating of sand within the matrix between coarser clasts can be applicable where quartz or feldspar grains suitable for luminescence dating are present. Although very few studies have conducted luminescence dating of sand within the matrix between coarser clasts, this method has underutilized potential for establishing chronologies of gravelly fluvial and alluvial deposits. When dating sand within the matrix between coarser clasts, estimation of dose rate to dosimeter grains is complicated by the heterogeneity of the matrix, which might have limited the application of dating sand matrices within gravelly deposits. Although some previous studies have demonstrated the potential of dating sand preserved between coarser clasts, methods for estimating the dose rate are not well established. In this study, a recently proposed model for calculating average beta dose rates in granular matrices was modified to be applicable to sand-sized dosimeter grains within gravelly deposits. We applied the modified model to sand-sized K-rich feldspars within sand matrices obtained from gravelly fluvial deposits in the Tokachi Plain, northern Japan, and compared the ages of samples obtained from sand matrices with those from sand lenses.
Although the weight of <2 mm grains accounted for only 20%–35% of the bulk sediment, these grains were estimated to contribute approximately 70% of the external beta dose rate according to the model, because larger grains have a larger self-dose. Taking into account that the beta dose to dosimeter grains is mainly derived from smaller matrices (e.g. <2 mm), beta dose rates were also calculated based on the infinite matrix dose rate of the <2 mm fraction, along with the conventional water correction method. The ages of sand matrices calculated based on the beta dose rates derived from both the model and the infinite matrix dose rate of the <2 mm fraction were generally consistent with those of sand lenses. The dose rate calculated based on the model might be more accurate than that calculated using the infinite matrix dose rate of <2 mm fraction, but calculating the beta dose rate using the infinite matrix dose rate of <2 mm fraction is useful as a simple approach.
Acknowledgments
The main part of this research project was conducted as regulatory-supporting research funded by the Secretariat of the Nuclear Regulation Authority, Japan.
Luminescence dating of sand within the matrix between coarser clasts can be applicable where quartz or feldspar grains suitable for luminescence dating are present. Although very few studies have conducted luminescence dating of sand within the matrix between coarser clasts, this method has underutilized potential for establishing chronologies of gravelly fluvial and alluvial deposits. When dating sand within the matrix between coarser clasts, estimation of dose rate to dosimeter grains is complicated by the heterogeneity of the matrix, which might have limited the application of dating sand matrices within gravelly deposits. Although some previous studies have demonstrated the potential of dating sand preserved between coarser clasts, methods for estimating the dose rate are not well established. In this study, a recently proposed model for calculating average beta dose rates in granular matrices was modified to be applicable to sand-sized dosimeter grains within gravelly deposits. We applied the modified model to sand-sized K-rich feldspars within sand matrices obtained from gravelly fluvial deposits in the Tokachi Plain, northern Japan, and compared the ages of samples obtained from sand matrices with those from sand lenses.
Although the weight of <2 mm grains accounted for only 20%–35% of the bulk sediment, these grains were estimated to contribute approximately 70% of the external beta dose rate according to the model, because larger grains have a larger self-dose. Taking into account that the beta dose to dosimeter grains is mainly derived from smaller matrices (e.g. <2 mm), beta dose rates were also calculated based on the infinite matrix dose rate of the <2 mm fraction, along with the conventional water correction method. The ages of sand matrices calculated based on the beta dose rates derived from both the model and the infinite matrix dose rate of the <2 mm fraction were generally consistent with those of sand lenses. The dose rate calculated based on the model might be more accurate than that calculated using the infinite matrix dose rate of <2 mm fraction, but calculating the beta dose rate using the infinite matrix dose rate of <2 mm fraction is useful as a simple approach.
Acknowledgments
The main part of this research project was conducted as regulatory-supporting research funded by the Secretariat of the Nuclear Regulation Authority, Japan.