After a short discussion on the Pb210 dating method, the results of Pb210 measurements on snow and firn samples from four Antarctic stations are reported as well as the experimental procedure used. Pb210 was also measured in the air at ground level, at Base Roi Baudouin, the average Pb210 activity of aerosols in 1958 being 1.3 × 10−3 dpm/kg of air. In samples of surface snow accumulated during the last two to five years, the average Pb210 activity is of the order of 100 dph/kg of snow. This activity seems to be independent of the local rate of accumulation. The variations of Pb210 activity as a function of depth were studied on two vertical firn profiles spanning an interval of more than 200 years, at Base Roi Baudouin (70°S, 24°E) and at Amundsen Scott Station (South Pole). Within experimental errors, the Pb210 activity was found to decrease exponentially with depth at both stations. The most plausible interpretation is that the rate of water accumulation and the initial Pb210 concentration in the snow have remained constant at both places over the last 100 years. The annual rates of water accumulation calculated from the decay curves are 6±1 cm at the South Pole and 45±3 cm at Base Roi Baudouin, in good agreement (to better than 10 percent) with accumulation rates so far obtained by other methods.

In the area around King Baudouin Station (700S, 24°E), it is shown that a refer ence horizon, easy to identify, wa" formed by the stratospheric fallout of radioactive debris from thermonuclear bomb tests. Gross fj activity and Sr" have bee n measured in snow and tim 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 fj activi ty can be taken as a measure of the fission products concentration. The followi ng average values were founci: from 1955 to 1960, 14 dpm/kg of snow; beginning of 1955, 22 dpm/kg (SrOG = 4.5) ; 1953 and 1954, 2 dpm/kg; and from 1935 to 1952: 0.5 dpm/kg (SrOO < 0.1). The sharp tenfold increase in thefj activity at the beginning of 1955 is attributed to the sudden release in the antarctic troposphere of fission prociucts from the CasUe thermonue!ear tests series (March 1954). The previous and first thermonuclear test (Ivy, November 1952) is less marked. The fj activity before 1952 is essentially due to Pb"'O and K". 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 fim profiles.

I N an earlier communication', variations in the isotopic composition of the oxygen in falls of snow collected during 1958 at the Belgian Ai^tarctic Base (King Baudouin Base, 70° 26' S., 24° 19' E.) were Bhown.

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