CRONUS-Earth cosmogenic 36Cl calibration

Chlorine-36 production rates obtained from different geological calibration studies (e.g. Evans et al., 1997; Phillips et al., 2001; Schimmelpfennig et al., 2011; Stone et al., 1996; Swanson and Caffee, 2001) vary significantly, principally because of the many reactions contributing to the production of this nuclide. The CRONUS-Earth Project has provided high-quality geological calibration sites, including Lake Bonneville, Peru, and Scotland, for a large-scale calibration of 36Cl production rates. Three sites were used to calibrate the K and Ca spallation pathways for 36Cl production yielding production rates of 56.0 ± 4.1 at 36Cl (g Ca)−1 yr−1 and 155 ± 11 at 36Cl (g K)−1 yr−1 respectively, using Lifton-Sato-Dunai scaling (LSDn). The low-energy production parameter, Pf(0), was calibrated separately using CRONUS-Earth data from the Bonneville and Baboon Lakes sites where Cl concentrations were higher, and yielded a value of 759 ± 180 neutrons (g air)−1 yr−1. There is significant uncertainty associated with this pathway due to the sensitivity of this reaction to environmental conditions. The uncertainties associated with the calibrated production parameters were estimated based on the variance of calculated ages from independent ages for an independent secondary dataset.