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 o f an isotopic mass balance in the ocean. The delta(6)Li compositions o f fresh basalts (-4 parts per thousand), the hydrothermal fluids deriv ed from them (average-9 parts per thousand), and seawater (significant ly heavier at -32 parts per thousand) are well understood, but only ve ry sparse river input data are available for Li mass balance calculati ons. In an attempt to rectify the situation we have measured the lithi um concentrations and isotopic compositions of major world rivers drai ning 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 compositi ons 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 t housand, carbonates -26 to -32 parts per thousand, black shales -26 pa rts per thousand, shields -6.6 to 19 parts per thousand, and mixed sil iceous terrains -6 to 28 parts per thousand. The flow-weighted mean co ncentration 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 imbala nce if the measured Li concentration and isotopic compositions are rep resentative of all rivers. The presence of a yet unidentified sink wit h a higher fractionation factor (alpha approximate to 1.023) than dete rmined for low temperature basalt alteration (alpha = 1.019) is requir ed for an isotopic steady-state of Li in the ocean. Authigenic clays a re a possible candidate as clays are known to be enriched in both lith ium and in the light isotope preferentially. Alternatively, the hydrot hermal 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 relations hip of delta(6)Li to the major ions and Sr-87/Sr-86 suggests that the important processes affecting river dissolved lithium weathering. Copy right (C) 1998 Elsevier Science Ltd.