Equilibrium line and mean annual mass balance of Finsterwalderbreen, Spitsbergen, determined by in situ and laboratory gamma-ray measurements of nuclear test deposits

Abstract

In order to determine the equilibrium line (EL) and the annual net mass balance over the accumulation area of Finsterwalderbreen, Spitsbergen, Svalbard (by detection of the 1962–63 radioactive peak from the 1961–62 atmospheric nuclear tests), we collected 14 ice cores, at elevations of 445–730 m, in the springs of 1994 and 1995. The corresponding samples were melted and filtered for laboratory gamma spectrometry. In the accumulation area, the 1962–63 radioactive layer is found well below the surface. The mean annual accumulation is not invariably related to altitude. The EL, averaged to 545 in a.s.l., leads to an accumulation area ratio of 0.3 and indicates a strong negative balance. The 584 Bq m−2 mean137 Cs deposition rate (1954–74 nuclear tests) for eight ice cores in the accumulation area is nearly twice the 340 Bq m−2 mean Svalbard value obtained from six other glaciers (at time of deposition). An in situ gamma-ray detector was lowered down each borehole, and137 Cs levels were recorded. The counting rate is proportional to the apparent deposition rate and the specific activity. The laboratory measurements perfectly match the in situ determinations. In the ablation area, a dust layer and the associated nuclear test deposits are concentrated close to the bare ice surface of the glacier, under the winter snow layer and present maximum 137 Cs and 210 Pb contents. The dust layer acts like a filter for radioactive materials removed from the glacier and its basin by melting and water flow. The original specific activities and deposition rates at a given location are enhanced by adsorption of additional radioactivity on the dust particles. A linear relationship exists between 137 Cs and 210 Pb deposition rates. This process is almost constant for all studied ice cores. The apparent 137 Cs deposition rate for seven ice cores in the ablation area is 465 Bq m−2 (at date of measurement: 1 July 1995).