Multi-isotopic composition (delta Li-7-delta B-11-delta D-delta O-18) of rainwaters in France: Origin and spatio-temporal characterization

Romain Millot (1) , Emmanuelle Petelet-Giraud (1) , Catherine Guerrot (1) , Philippe Négrel (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

In the present work, the first results are reported for both Li and B isotope ratios in rainwater samples collected over a long time period (i.e. monthly rainfall events over 1 a) at a national scale (from coastal and inland locations). In addition, the stable isotopes of the water molecule (delta D and delta O-18) are also reported here for the same locations so that the Li and B isotope data can be discussed in the same context. The range of Li and B isotopic variations in these rainwaters were measured to enable the determination of the origin of these elements in rainwaters and the characterization of both the seasonal and spatio-temporal effects for delta Li-7 and delta B-11 signatures in rainwaters. Lithium and B concentrations are low in rainwater samples, ranging from 0.004 to 0.292 mu mol/L and from 0.029 to 6.184 mu mol/L, respectively. delta Li-7 and delta B-11 values in rainwaters also show a great range of variation between +3.2 parts per thousand and +95.6 parts per thousand and between -3.3 parts per thousand and +40.6 parts per thousand over a period of 1 a, respectively, clearly different from the signature of seawater. Seasonal effects (i.e. rainfall amount and month) are not the main factors controlling element concentrations and isotopic variations. delta Li-7 and delta B-11 values in rainwaters are clearly different from one site to another, indicating the variable contribution of sea salts in the rainwater depending on the sampling site (coastal vs. inland: also called the distance-from-the-coast-effect). This is well illustrated when wind direction data (origin of air masses) is included. It was found that seawater is not the main supplier of dissolved atmospheric Li and B, and non-sea-salt sources (i.e. crustal, anthropogenic, biogenic) should also be taken into account when Li and B isotopes are considered in hydrogeochemistry as an input to surface waters and groundwater bodies as recharge. In parallel, the isotopic variations of the water molecule, vector of the dissolved B and Li, are also investigated and reported as a contour map for delta O-18 values based on compiled data including more than 400 delta O-18 values from throughout France. This delta O-18 map could be used as a reference for future studies dealing with delta O-18 recharge signature in relation to the characterization of surface waters and/or groundwater bodies

Domaines: Géochimie; Milieux et Changements globaux

Romain Millot, Emmanuelle Petelet-Giraud, Catherine Guerrot, Philippe Négrel. Multi-isotopic composition (delta Li-7-delta B-11-delta D-delta O-18) of rainwaters in France: Origin and spatio-temporal characterization. Applied Geochemistry, 2010, 25 (10), p.1510-1524. ⟨10.1016/j.apgeochem.2010.08.002⟩. ⟨hal-00553187⟩