240Pu/239Pu Ratios in Sun Moon Lake Sediments: Implications for Sources of Nuclear Fallout in Taiwan

Abstract

The isotopic composition of plutonium in dated Sun Moon Lake sediments was determined by mass spectrometry in effort to trace the source(s) and timing of nuclear fallout in Taiwan. The 240Pu/239Pu ratios in the sediment core vary by a factor of >5, with the highest ratio, 0.22, corresponding to the maximum level of nuclear fallout in 1963 and the lowest ratios, 0.04-0.06, being identified at two time horizons: 1982-83 and 1992-93. At other depth intervals, the 240Pu/239Pu ratios fall within 0.15-0.19. The data indicate that global fallout from the stratosphere is the dominant source of Pu in the sediments, where the tropospheric transport of low burn-up debris from Lop Nor in NW China is mostly responsible for the unusually low 240Pu/239Pu ratios.

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