Estimation of sedimentation rates based on the excess of radium 228 in granitic reservoir sediments

Abstract

Knowledge of sedimentation rates in lakes is required to understand and quantify the geochemical processes involved in scavenging and remobilization of contaminants at the Sediment-Water Interface (SWI). The well-known ²¹⁰Pb excess (²¹⁰Pb_ex) method cannot be used for quantifying sedimentation rates in uranium-enriched catchments, as large amounts of ²¹⁰Pb produced by weathering and human activities may dilute the atmospheric ²¹⁰Pb. As an alternative dating method in these cases, we propose an original method based on ²³²Th decay series nuclides. This study focuses on an artificial lake located in a granitic catchment downstream from a former uranium mine site. The exponential decay of ²²⁸Ra excess (²²⁸Ra_ex) with depth in two long cores yields sedimentation rates of 2.4 and 5.2 cm yr^-1 respectively. These sedimentation rates lead to the attribution of the ¹³⁷Cs activity peak observed at depth to the Chernobyl fallout event of 1986. The ²²⁸Ra_ex method was also applied to two short cores which did not display the ¹³⁷Cs peak, and mean sedimentation rates of 2.1 and 4.0 cm y^-1 were deduced. The proposed method may replace the classical radiochronological methods (²¹⁰Pb_ex, ¹³⁷Cs) to determine sedimentation rates in granitic catchments.

Keywords: Dating; Granitic catchment; Radium 228; Recent sediments; Uranium mine.