³⁶Cl bomb pulse measured in a shallow ice core from Dye 3, Greenland

Abstract

Nuclear weapons tests at oceanic sites during the 1950s produced a large amount of ³⁶Cl (t_1/2 = 3.0 × 10⁵ yr) through neutron capture of ³⁵Cl in seawater. Part of this anthropogenic ³⁶Cl was injected into the stratosphere from where it was redistributed throughout the Earth. This pulse of ³⁶Cl was first detected in rainfall by Schaeffer et al.¹. The global deposition rates are several orders of magnitude larger than the natural pre-and post-bomb ³⁶Cl production rate that reflects cosmic-ray spallation of atmospheric ⁴⁰Ar. Because ³⁶Cl is not a fission product, the anthropogenic ³⁶Cl is produced mainly in marine tests carried out on small islands and barges where a large amount of seawater chlorine is present to serve as a target. This target element requirement leads to a temporal dependence different from the other bomb produced isotopes^2–4. We have determined the temporal dependence of ³⁶Cl fallout by measuring the depth profile of ³⁶Cl in an ice core from Dye 3 Greenland (65° 11′ N, 43° 50′ W) using tandem accelerator mass spectrometry⁵. We show here that the results agree well with a calculation of the ³⁶Cl produced and injected into the stratosphere by the tests.