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

The cosmogenic isotope Cl-36 is a valuable tool for understanding many Earth system processes, but an improved knowledge of its spatial distribution at the Earth's surface is critical for several applications. Meteoric Cl-36 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 Cl-36 on a continental scale within the European landmass using precipitation samples collected during spring 2007. Cl-36/Cl ratios increase with distance inland and the new data exhibit a strong exponential relationship with delta O-18 values of precipitation (R-S=-0.75), yielding a useful predictive framework for future studies. Precipitation events in central European regions are characterised by high Cl-36/Cl ratios, with a maximum measured in Lyon, France of 746 (+/- 134) x 10(-15). The new data confirm models for the dependence of Cl-36 fallout on latitude, with the highest mean springtime fallout (53.6 atoms m(-2) s(-1))occurring in the 40-50 degrees N latitudinal band, with sharp decreases in fallout in high latitude regions and more gradual decreases towards the lower latitudes. The Cl-36 bomb pulse, introduced by thermonuclear weapon testing, predominantly in the 1950's, has persisted in the environment for c. 50 years, but the new data indicate that Cl-36 fallout has now essentially returned to natural, pre-bomb values. (c) 2008 Elsevier B.V. All rights reserved.