Lithium isotopic features of Quaternary basaltic saprolite, Zhanjiang, China: Atmospheric input and clay-mineral adsorption

Abstract

Silicate weathering, in particular basalt, accounting only for one-sixth of the continental crust volume, accounts for one-third of atmospheric CO2 drawn down during weathering, as such, playing an important role in long-term carbon cycling. Li contents and isotopic compositions of a tropical regolith developed on young basalt (ca. 1 Ma) were investigated to elucidate Li isotopic fractionation during basalt weathering so as to trace basalt weathering. Li isotopic composition of sand from Asian dust-source regions and local dust near the basaltic saprolite weathering profile were studied as possible atmospheric contributors of Li to the saprolite. Saprolite has δ7Li values of −6.8‰ to −2.2‰, which is much lighter than that of fresh basalt (+3.5‰). Approximately 30% of the Li was lost from the upper saprolite profile, without significant isotopic fractionation. Lithium is enriched at the bottom of the profile, showing the lightest Li isotopic composition (δ7Li = −6.6‰ to −2.6‰). Calculations indicates that marine aerosols are the main source of atmospheric Li input to the profile. Adsorption of Li by secondary minerals results Li isotopic fractionation, accompanying desorption and resorption of Li by secondary minerals, resulting enriched Li and light Li isotopic composition in the regolith. A model established that Li isotopes showed sensitivity to continental weathering, supporting that Li isotopes can serve as an indicator of continental weathering. Continental weathering in the study area recorded the reasons of heavier Li isotopic composition of river water and seawater suggesting that Li isotopes can be used to study the circulation of materials on the Earth's surface. This has a guiding role in studying the Li isotopic composition of shale and rivers. The influence of temperature and precipitation on the fractionation behavior of lithium isotopes during silicate weathering warrants further investigation.