A continuous record of artificial tritium fallout at the South Pole (1954–1978)
Résumé
Snow samples were taken from a 5-m-deep pit located near the South Pole station in January 1975 and continuous deuterium, tritium and β activity profiles have been obtained from them. These three measurements and the stratigraphic observations allow us to deduce a precise chronology of the pit from 1950 to 1975, providing a continuous record of artificial tritium fallout in the southern hemisphere; this record has been extended to 1978 using samples from a second pit taken this last year. Taking advantage of the unusual 1957–1958 stable isotope content in the snow, we have demonstrated that an important part of the isotopic signal in the precipitation is retained in the snow in spite of the low accumulation rate (8.2 g cm−2 yr−1).
The first artificial tritium, due to the 1952 Ivy experiments was detected during 1954. A two-year delay between explosions and fallout is well established and allows us to relate the tritium fallout to the main nuclear tests from 1952 to 1960. This delay appears longer for the large 1961–1962 devices. A stratospheric half residence time equal to 20 months is deduced from the fallout decrease occurring after the 1966 peak. For the French southern hemisphere experiments, it is about one year. A sharp tritium decrease is observed after a high 1973 peak, providing a new tritium reference level for future glaciological studies in Antarctica.
The β and tritium peaks occur respectively during the Antarctic summer and the Antarctic winter, showing different injection mechanisms. This winter input and the high tritium values registered at the South Pole indicate a preferential tritium transfer over the pole area. Two mechanisms, stratospheric-tropospheric exchange and direct stratospheric cloud precipitation could account for this injection.