Lithium and its isotopes in major world rivers: Implications for weathering and the oceanic budget

The outstanding problem in the lithium geochemical cycle is the lack of an isotopic mass balance in the ocean. The delta(6)Li compositions of fresh basalts (-4 parts per thousand), the hydrothermal fluids derived from them (average-9 parts per thousand), and seawater (significantly heavier at -32 parts per thousand) are well understood, but only very sparse river input data are available for Li mass balance calculations. In an attempt to rectify the situation we have measured the lithium concentrations and isotopic compositions of major world rivers draining representative geological terrains. This helps both to constrain the river endmember and to understand the behavior of lithium isotopes in the continental weathering environment. Fluvial isotopic compositions display a very large range, -6.0 to 32.3 parts per thousand. This is no definitive relationship between delta(6)Li and lithology but, in general, rivers draining marine evaporites are -20 to -22 parts per thousand, carbonates -26 to -32 parts per thousand, black shales -26 parts per thousand, shields -6.6 to 19 parts per thousand, and mixed siliceous terrains -6 to 28 parts per thousand. The flow-weighted mean concentration of the measured rivers is 215 nM at -23 parts per thousand. This updated riverine delta(6)Li value responsible for similar to 30% of the global riverine discharge, does not solve the isotopic imbalance if the measured Li concentration and isotopic compositions are representative of all rivers. The presence of a yet unidentified sink with a higher fractionation factor (alpha approximate to 1.023) than determined for low temperature basalt alteration (alpha = 1.019) is required for an isotopic steady-state of Li in the ocean. Authigenic clays are a possible candidate as clays are known to be enriched in both lithium and in the light isotope preferentially. Alternatively, the hydrothermal flux must be much less than half of that estimated by the He-3 inventory and the oceanic budgets of Sr-87/Sr-86 and Mg. The relationship of delta(6)Li to the major ions and Sr-87/Sr-86 suggests that the important processes affecting river dissolved lithium weathering. Copyright (C) 1998 Elsevier Science Ltd.