High-resolution, well-preserved tritium record in the ice of Bortig Ice Cave, Bihor Mountains, Romania

Abstract

Two 2 m long ice cores (BA and BB) were extracted from the floor ice of Borştig Ice Cave in December 2005. Below a co-existing dust horizon (~13 cm underneath the 10 December 2005 ice surface) neither core presented any sign of hiatus, so the ice deposition is considered to be continuous. Tritium concentration of 94 samples from a 1.85 m long ice section were analysed by liquid scintillation counting technique. Samples from the lower 0.33 m of the sequence did not contain tritium above the critical level (7.2 TU). The highest value of tritium content (166.4±4.0 TU) was found at ~96 cm below the surface. This salient value is considered to be synchronous with the climax of tritium concentration in the Northern Hemisphere's atmospheric precipitation (1963). Beside this characteristic global radiochemical marker event, minor events were also detected, and dated (ie, 1954, 1958 and 1975) by corresponding peaks in the tritium concentration record of BB ice core to peaks of an estimation of tritium activity of past precipitation at Borştig Ice Cave location. The estimation was based on a data set from four nearby stations of the Global Network of Isotopes in Precipitation. The highest annual accumulation rate (6.74 cm/yr) was between 1958 and 1963 and gradually decreased to 0.54 cm/yr for the recent decades. The mean ice accumulation rate was 4.34 cm/yr over the 1954—1986 period. The estimated age at the bottom of the 21 m thick ice block assuming constant accumulation is roughly 500 years.