Vertical distribution of radionuclides in Lake Qinghai, Qinghai-Tibet Plateau, and its environmental implications

Radionuclides are widely used to perform geochronologic investigations of the trace sediment deposition process in aquatic environments on different temporal scales. This study presents the activities of radionuclides to select a reasonable geochronologic dating method and study the sediment depositional environment in Lake Qinghai on the Qinghai-Tibet Plateau. The range of activities (Bq.kg⁻¹) of ¹³⁷Cs, ²²⁶Ra, ²³⁸U, ²²⁸Ra, ²²⁸Th, and ⁴⁰K were 2–103, 24–49, 33–156, 20–49, 30–50, and 403–664, respectively. The sedimentation flux was calculated as 1.66 × 10⁶ ton.a⁻¹ based on the summary sedimentation rate in Lake Qinghai, and the sediment contributions from rivers, authigenic carbonates, lacustrine organisms, and atmospheric deposition were 45.18%, 22.44%, 3.02%, and 29.36%, respectively. The decreased ²²⁶Ra/²³⁸U radioactivity ratios from the catchment to the lake centre indicated that the removal of uranium relative to its daughter products occurs during erosion processes. The positive ²²⁸Th_ex (excess ²²⁸Th) values in the upper 1 cm of the sediment suggested that the sediment in the top 1 cm have been resuspended, mixed and/or transported. The distribution of ¹³⁷Cs in the sediments indicated that the maximum activity of ¹³⁷Cs in the sediments corresponds to the 1963 peak in the global fallout from atmospheric nuclear weapons testing; moreover, the 1986 Chernobyl accident signal was difficult to identify. The use of ¹³⁷Cs as a chronologic tool must be performed carefully due to its migration process in sediments.