Chlorine-36 in seawater

Abstract

Natural cosmogenic 36Cl found in seawater originates from spallation of atmospheric 40Ar, capture of secondary cosmic-ray neutrons by dissolved 35Cl, and river runoff which contains 36Cl produced in situ over the surface of the continents. The long residence time of chloride in the ocean and long half-life of 36Cl compared to the oceanic mixing time should result in a homogenous 36Cl/Cl ratio throughout the ocean. Production by neutron capture in the course of nuclear weapons testing should be insignificant averaged over the oceans as a whole, but may have led to regions of elevated 36Cl concentration. Previous attempts to measure the 36Cl/Cl ratio of seawater have been hindered by interferences, contamination, or insufficient analytic sensitivity. Here we report preliminary measurements on seawater samples, which demonstrate that the 36Cl/Cl ratio is 0.5 ± 0.3 × 10-15, in reasonable agreement with calculated contributions from the sources listed above.