Bomb-test 36Cl measurements in Vostok snow (Antarctica) and the use of 36Cl as a dating tool for deep ice cores

A large pulse of atmospheric 36Cl generated by a limited number of nuclear tests peaked in the late 1950s to early 1960s. The corresponding enhanced 36Cl deposition is seen in various glaciological archives in the Northern Hemisphere. The profile of the bomb spike recorded in firn layers at Vostok Station, central East Antarctica, has been measured by employing accelerator mass spectrometry (AMS). The records obtained from two well-dated data sets collected in snow pits in 1997 and 1998 show a broad 36Cl peak, beginning as early as the 1940s and reaching its maximum in the 1960s. The signal is followed by a long-lasting tail up to the surface. This pattern is totally unexpected. We show that the results, unlike the Greenland data, can be explained by a mobility of HCl in the Antarctic firn. This experiment demonstrates the instability of gaseous Cl− deposits, a phenomenon which has important implications for the use of natural cosmogenic 36Cl radionuclides as a reliable dating tool for deep ice cores from low-accumulation areas. However, during glacial times, under favourable atmospheric chemistry conditions this dating method may still be applicable. Snow metamorphism and ventilation are assumed to be the two main physical processes responsible for the observed patterns.