Characteristics of artificial radionuclides in sedimentary soil cores from a volcanic crater lake

Abstract

The distributions of ¹³⁷Cs, ²³⁷Np, and ^239+240Pu activity concentrations in sedimentary soil cores of the volcanic crater-lake have been studied. The ²⁴⁰Pu/²³⁹Pu atom ratios measured by MC-ICP-MS and mutual activity ratios were examined. These results were used to evaluate the sedimentation characteristics of the crater-lake (Baengnokdam of Mt. Halla, Korea). The ¹³⁷Cs, ²³⁷Np, and ^239+240Pu activity concentrations showed similar distribution patterns and one maximum peak was observed in each sediment core, except at St.10. For all sediment cores, the activity concentrations were in the range 1.03 × 10⁰-1.92 × 10² Bq·kg^-1 dw for ¹³⁷Cs, 7.56 × 10^-3 - 7.15 × 10⁰ mBq·kg^-1 dw for ²³⁷Np, and 5.20 × 10^-3 - 5.13 × 10⁰ Bq·kg^-1 dw for ^239+240Pu, respectively. The averaged ²⁴⁰Pu/²³⁹Pu atomic ratio (0.159) was slightly less than the global fallout ratio (0.176). The averaged inventories were estimated to be 9.21 × 10³±5.34 × 10³ Bq·m^-2 for ¹³⁷Cs, 2.27 × 10²±1.58 × 10² Bq·m^-2 for ^239+240Pu, and 3.22 × 10^-1±1.78 × 10^-1 Bq·m^-2 for ²³⁷Np. The averaged ^239+240Pu/¹³⁷Cs and ²³⁷Np/^239+240Pu activity ratios were 2.21 × 10^-2 and 2.21 × 10^-3, respectively. The mean sedimentation rates calculated using ^239+240Pu activity concentrations at the central area (St.30 - St.45) and at all stations (St.5 - St.75) were estimated to be 0.844 cm yr^-1, and 0.767 cm yr^-1, respectively. In addition, the sedimentation rates calculated using ²¹⁰Pb and ²²⁶Ra were 0.856 cm yr^-1 at depths of 0-35 cm and 0.204 cm yr^-1 at depths of 35-55 cm. These results imply that the sedimentation in Baengnokdam was relatively slow (0.204 cm yr^-1) until about 44 years ago and then became faster (0.856 cm yr^-1) to the present. The excess ²¹⁰Pb dating is consistent with the sedimentation rate calculated from the vertical ^239+240Pu profile.

Keywords: Artificial radionuclide; Crater-lake; Sedimentation.