Pu-239,Pu-240 and U-236 records of an ice core from the eastern Tien Shan (Central Asia)

摘要

Radioisotopes (Pu-239, Pu-240, U-236) formed during atmospheric nuclear weapons testing (NWT) can be used for dating and therefore be applied as markers of the beginning of the Anthropocene Epoch. Moreover, Pu-240/Pu-239 ratios enable source identification (general fallout, local emission sources). Ice core segments from a 57.6 m ice core to bedrock from eastern Tien Shan, China were selected for Pu-239,Pu-240 and U-236 analyses by using compact low-energy accelerator mass spectrometry. The observed Pu-240/Pu-239 atom ratios were 0.18 +/- 0.02, with one exception, indicating global fallout. No evidence for emissions from the nearby local sources Lop Nor was observed. The total deposition rates during NWT for Pu-239 and Pu-240 amount to 1.55 x 10(9) atoms.cm(-2) and 3.1 x 10(8) atoms.cm(-2), respectively. With the higher undisturbed fallout of Pu-239 compared with values reported for glaciers from European Alps at Col du Dome and Colle Gnifetti as well as from the polar region, the eastern Tien Shan glacier would be an ideal site for defining the start of the Anthropocene. U-236 is a rather new environmental tracer, while little is known about its global fallout from NWT. The observed deposition flux of U-236 during NWT amounts to 3.5 x 10(8) atoms.cm(-2) at the drill site. The average U-236/Pu-239 ratio of 0.27 +/- 0.09 is in good agreement with literature values from global fallout.