Radioactivity levels in a firn core from the Yukon Territory, Canada

Abstract

Gross β-activity and tritium concentration measurements made on a 26.8m length of consolidated snow (firn) core retrieved from an altitude of 5340 m on Mt. Logan (Yukon Territory, Canada) have revealed a detailed chronology of natural and anthropogenic radioactivity levels extending from about 1943 to 1980. Oxygen isotope measurements were used to assist in the dating of the core and a reliable time scale spanning 1950–1980 has been established with an accuracy of at least ± 0.5 y, and in some cases ± 0.25y. An immediate result is the establishment of mean annual accumulation rates, which are, for the years 1963–1980: 0.36m y −1 and for 1951–1980: 0.39m y − water equivalent. Another result is the estimation of the ‘stratospheric residence time’ for the fission fallout products (mainly 90Sr and 137Cs). A value of the stratospheric residence time of about 1 y is derived from data spanning 1963–1967. Reference to the history of atmospheric thermonuclear device testing shows that individual or specific groups of tests by different countries can be identified. Comparison of the gross β-activity data with a similar set from Station Centrale (Greenland) shows that although the same trends exist in the two data sets, the amplitude of most corresponding major peaks in the Greenland data is significantly lower. Since the accumulation rates at the two sites are almost identical and since the deposition of radioactive aerosols in any case is expected to be dominated by dry fall-out, the differences are assumed to be related to the trajectory path and to the diffusion rates of the radionuclide clouds.