The ³⁶ Cl Bomb‐Pulse in Ice at a High‐Accumulation Site in Antarctica

Abstract The radioisotope 36 Cl was produced prolifically by nuclear weapons testing, principally at Bikini and Enewetak Atolls in the 1950s. Much of this was injected into the stratosphere, from where it was dispersed worldwide. The fallout of this bomb‐produced 36 Cl has been measured as a function of time in an ice core from a high‐accumulation site in Antarctica. These measurements complement an earlier study in an Arctic ice core. A box model has been developed to describe simultaneously the data from both the two sites. Using an up‐to‐date catalog, the 36 Cl production of the individual tests has been estimated and used as input to the box model. Production is dominated by high‐yield thermonuclear tests carried out on barges moored in the lagoons of the atolls in the 1950s. Exchange times of 3.7 years for gaseous exchange between stratosphere and troposphere and 4.2 years for gaseous exchange between the stratospheres of the two hemispheres are deduced. The 36 Cl bomb pulse constitutes a global tracer with an input function that is well‐defined in space and time, and its exchange between stratosphere and troposphere is dominated by one‐way transport due to its rapid removal once in the troposphere. This latter behavior greatly simplifies the interpretation of the data in terms of an exchange time relative to the 14 C bomb pulse. The stratosphere‐troposphere exchange time derived here from the bomb‐produced 36 Cl offers an independent constraint on the flux of ozone between stratosphere and troposphere which agrees well with other more direct estimates.