Boron isotopes in precipitation: Experimental constraints and field evidence from French Guiana
Résumé
Precipitation samples collected during one year in French Guiana were analyzed both for boron concentrations and isotopic compositions (δ11B). B concentrations range from 3.5 ppb to 14 ppb and δ11B values range from +30.5‰ to +45‰. Mean annual values are 6 ppb and +41‰, respectively. A clear relationship is observed in precipitation samples between δ11B and NO3/B ratio, indicating that boron in rainwater originates from two sources. The first source is seawater. Experiments carried out in order to understand the volatilization of boron from solutions of seawater strongly suggest that the heavy component in rainwater samples is boron evaporated from seawater. We show that boric acid is preferentially evaporated and 11B preferentially lost. At seawater pH, we predict a seawater-derived atmosphere having a δ11B of +45–50‰ that corresponds to the 11B enriched end-member found in the rainwater samples of French Guiana. The second source of boron in French Guiana rainwater is less constrained but strong arguments favor a main contribution of a biomass-derived component. We propose the release of boron by biomass burning as the most likely second source. We estimate the contribution of the dissolution of marine gaseous B to French Guiana precipitation at about 80–90% of the total B in rainfall.
Finally, our results show that (i) most of the dissolved boron in the coastal rivers of French Guiana originates from atmospheric inputs and (ii) the δ11B values of rivers are consistent with δ11B values of yearly averaged rainwater. We demonstrate in this study the potential use of boron isotopes as tracers of the biomass burning and stress the importance of investigating boron isotopes in biomass.