Vertical Distributions of Plutonium and ~(137)Cs in Lacustrine Sediments in Northwestern China: Quantifying Sediment Accumulation Rates and Source Identifications

摘要

We investigated the vertical distributions of ~(239+240)Pu and ~(137)Cs activities and ~(240)Pu/~(239)Pu atom ratios in two lakes in northwestern China. The ~(240)Pu/~(239)Pu atom ratio from the Chinese nuclear test (CNT) site at Lop Nor is too complicated to identify the regional fallout source. However, radionuclides from the test site may be resolved mainly by the vertical distribution of radionuclides because there is no overlap between the global fallout peak and the local fallout peak. We analyzed Pu isotopes and ~(137)Cs in sediments from two lakes (Sugan and Shuangta) located in northwestern China, near the CNT, and Lake Sihailongwan located in northeastern China to date recent lacustrine sediments and resolve global/local fallout sources. The apparently low ~(240)Pu/~(239)Pu atom ratio of 0.103 ± 0.010 at Lake Sugan was likely representative of the fallout from the Lop Nor site. Our results also demonstrated that the ~(239+240)Pu activity was more useful for recent chronology of lacustrine sediments, compared to ~(137)Cs. Sediment accumulation rates of 0.651, 0.058-0.061, and 0.015 g·cm~(-2)·a~(-1) for sediment cores of Lake Shuangta, Lake Sugan and Lake Sihailongwan, respectively, were obtained by using the ~(239+240)Pu fallout peak as a discrete time marker for 1964. The respective contributions of ~(137)Cs and ~(239+240)Pu from the CNT were estimated to be about 40 and 27% for Lake Sugan and 36 and 20% for Lake Shuangta. Plutonium isotopes in lacustrine sediments were proven to be useful for quantifying sediment accumulation rates and for source identification of the radioactive contamination.