Lithium isotopes as a probe of weathering processes: Orinoco River

Lithium isotopes have the potential to be effective tracers of weathering processes due to their large relative mass difference and therefore fractionation. In this study an attempt is made to fill a major gap in the knowledge of Li isotope fractionation during continental weathering and of the mechanisms involved. Finally the relationship between the suspended and dissolved material is made on a basin-wide scale. The Orinoco basin provides a clear contrast in reaction-limited and transport-limited weathering regimes that has already been documented by a comprehensive study on its fluvial geochemistry (Edmond et al., Geochim. Cosmochim. Acta 60 (1996) 2949-2976; Edmond et al., Geochim. Cosmochim. Acta 59 (1995) 3301-3325). Conspicuous in our new results is the difference in delta(6)Li of the dissolved load between the Andean (-30 to -22parts per thousand) and Shield (-22 to -7parts per thousand) tributaries, while the delta(6)Li of the suspended load is similar between the two. To a first approximation, during superficial weathering in high-relief, tectonically active terrains the dissolved load is high in Li and isotopically heavy (more negative delta(6)Li), whereas in stable Shield regions the concentrations are low and isotopically light in proportion to the increasing degree of weathering. (C) 2001 Elsevier Science B.V. All rights reserved.