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 (t1/2 = 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 isotopes2–4. 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.