Influence of atmospheric deposits and secondary minerals on Li isotopes budget in a highly weathered catchment, Guadeloupe (Lesser Antilles)

Abstract

To better constrain Li dynamics in the tropics, we sampled critical zone compartments of a small forested andesitic catchment in Guadeloupe (soils, parent rock, atmospheric dust, plants, soil solutions, stream and rain waters). The aims of this study are to identify the origin of Li in the different compartments and to better characterize the behavior of Li and its isotopes during water–rock interaction in a highly cation-depleted soil. The Li isotope signature (δ7Li) of throughfall samples varies between +11.2‰ and +26.4‰. As this is lower than the seawater signature (31‰) and vegetation does not fractionate Li isotopes, our data indicate that Saharan dust (−0.7‰) significantly contributes to the throughfall signature.Li isotope composition measured in a 12.5m deep soil profile varies from +3.9‰ near the surface to −13.5‰ at 11m depth. Compared to unweathered andesite (+5‰), the deep soil signature is in agreement with preferential incorporation of light Li into secondary minerals. In the top soil however, our results also emphasized that atmospheric deposition (wet and dry) is a main source of Li to the soil. The decreasing δ7Li with increasing depth is consistent with a vertical gradient of incorporation of heavy atmospheric Li, this input being maximal near the surface. At the catchment scale, throughfall and total atmospheric inputs (sea salts+Saharan dust) provide 12.1 and 23.9gLiyr−1 respectively to the Quiock Creek catchment. These fluxes represent 34% and 67%, respectively, of Li exported at the outlet indicating that atmospheric deposition is one of the main Li inputs to the critical zone. Li concentration and isotopic mass balance at the catchment scale indicate that in addition to atmospheric deposition, secondary mineral phase dissolution is a major solute source and that andesite no longer participates in significant production of Li.