Chemistry of rainwater in the Massif Central (France): a strontium isotope and major element study

Abstract

Atmospheric aerosols (sea salt, crustal dust, and biogenic aerosols) are the primary source of dissolved species in rainwater as well as one of the sources of dissolved species in river water. Chemical weathering studies require quantification of this atmospheric input. The crustal component of atmospheric input can have various origins, both distant and local. The proportions of the various inputs (marine, distant or local) are determined in this study. Strontium isotope ratios and Ca, Na, K, Mg, Al, Cl, SO 4, NO 3 and Sr concentrations were measured in rainwater samples collected in the Massif Central (France) over a period of one year. Each sample, collected automatically, represents a monthly series of rain events. Chemical composition of the rainwater samples varied considerably and the 87Sr/ 86Sr ratios ranged between 0.709198 and 0.713143. Using Na as an indicator of marine origin, and Al for the crustal input in rain samples, the proportion of marine and crustal elements was estimated from elemental ratios. A marine origin of 4 to 100% of Cl, of 0.6 to 20% of the SO 4, of <1 to 10% of Ca, <1 to 40% of K, 4 to 100% of Mg and 1 to 44% of Sr was determined. Strontium isotopes were used to characterize the crustal sources. The 87Sr/ 86Sr ratios of the crustal sources varied considerably from 0.7092 to 0.71625 and indicate the occurrence of multiple sources for the crustal component in the analysed rainwaters.