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

Abstract

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.

Multiradionuclide evidence for the solar origin of the cosmic-ray events of AD 774/5 and 993/4

“…In addition, 36 Cl can be characterized by some mobility in the snowpack27 as it is deposited to ice caps in the form of gaseous HCl. Although this is not expected to be a major source of uncertainty at high accumulation sites27, this in turn implies that outgassing and migration of 36 Cl to upper snow layers can occur, further broadening the signal. The 36 Cl data from the GRIP ice core should consequently be regarded as more uncertain.…”

Year-round records of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) – Part 1: Fractionation of sea-salt particles

“…The existence of HCl re-emission from the snowpack was suggested by the observation of a trend of chloride levels in the upper meters of the snowpack, particularly at sites characterized by very low snow accumulating rates (less than 3 g H 2 O cm −2 yr −1 ) like Concordia and Vostok (Legrand and Delmas, 1988;Legrand et al, 1996), whereas the phenomenon is strongly reduced at sites with higher snow accumulation rates (5-10 g cm −2 yr −1 ) like Dronning Maud Land (Weller et al, 2004) or the South Pole (Legrand et al, 1996). A more direct evidence of the remobilization of HCl after its deposition in snow came from the observed presence of 36 Cl in the surface snow at Vostok (Delmas et al, 2004), due to a broad peak starting in 1940 and ending near to the surface instead of the expected peak in 1950-1960 related to atmospheric nuclear tests that took place in the late 1950s to the early 1960s. This process could result from HCl formed in the atmosphere being deposited and re-emitted, allowing HCl effectively to hop inland through several steps thus increasing its apparent lifetime.…”

A Comparison of 36Cl Nuclear Bomb Inputs Deposited in Snow From Vostok and Talos Dome, Antarctica, Using the 36Cl/Cl− ratio

“…The intensity of these marine nuclear tests permitted 36 Cl to reach the stratosphere, where it is thought to be in gaseous form (Zerle et al, ), then to be distributed across the globe before finally transferring to the troposphere. Consequently, the resulting 36 Cl pulse can be observed in both low‐ and high‐latitude ice cores from African (Thompson et al, ), Asian, Alpine, Andean (Heikkila, Beer, Feichter, Alfimov, et al, ), and North American (Green et al, ) glaciers, as well as in ice cores from Greenland and Antarctica (Delmas et al, ; Elmore et al, ; Heikkila, Beer, Feichter, Alfimov, et al, ; Synal et al, ). These observations have permitted to estimate the mean stratospheric residence time of anthropogenic 36 Cl at approximately 3 to 4 years (Heikkila, Beer, Feichter, Alfimov, et al, ).…”