Fission products in Antarctic snow, A reference level for measuring accumulation

Abstract

In the area around King Baudouin Station (70°S, 24°E), it is shown that a reference horizon, easy to identify, was formed by the stratospheric fallout of radioactive debris from thermonuclear bomb tests. Gross β activity and Sreo have been measured in snow and firn samples from 1935 to 1960. The samples were dated by stratigraphy and by oxygen isotope-ratio measurements. The contribution of natural radionuclides is negligible, and the gross β activity can be taken as a measure of the fission products concentration. The following average values were found: from 1955 to 1960, 14 dpm/kg of snow; beginning of 1955, 22 dpm/kg (Sr90 = 4.5); 1953 and 1954, 2 dpm/kg; and from 1935 to 1952: 0.5 dpm/kg (Sr90<0.1). The sharp tenfold increase in the β activity at the beginning of 1955 is attributed to the sudden release in the antarctic troposphere of fission products from the Castle thermonuclear tests series (March 1954). The previous and first thermonuclear test (Ivy, November 1952) is less marked. The β activity before 1952 is essentially due to Pb210 and K40. There are indications that this radio-active horizon has been formed at the same time over the whole ice cap, but direct checks are needed on more, well-dated firn profiles.