The distribution of meteoric Cl-36 in precipitation across Europe in spring 2007

The cosmogenic isotope 36Cl is a valuable tool for understanding many Earth system processes, but an improved knowledge of its spatial distribution at the Earthʹs surface is critical for several applications. Meteoric 36Cl fallout reflects complex interactions between atmospheric production, transport, and deposition processes, and predictive models require experimental validation. This study investigates, for the first time in a systematic way, the spatial distribution of meteoric 36Cl on a continental scale within the European landmass using precipitation samples collected during spring 2007. 36Cl/Cl ratios increase with distance inland and the new data exhibit a strong exponential relationship with δ18O values of precipitation (RS = − 0.75), yielding a useful predictive framework for future studies. Precipitation events in central European regions are characterised by high 36Cl/Cl ratios, with a maximum measured in Lyon, France of 746 (± 134) × 10− 15. The new data confirm models for the dependence of 36Cl fallout on latitude, with the highest mean springtime fallout (53.6 atoms m− 2 s− 1) occurring in the 40–50°N latitudinal band, with sharp decreases in fallout in high latitude regions and more gradual decreases towards the lower latitudes. The 36Cl bomb pulse, introduced by thermonuclear weapon testing, predominantly in the 1950ʹs, has persisted in the environment for c. 50 years, but the new data indicate that 36Cl fallout has now essentially returned to natural, pre-bomb values.