Riverine dissolved lithium isotopic signatures in low-relief central Africa and their link to weathering regimes

The isotopic composition of dissolved lithium (delta Li-7) near the Congo River mouth varied from 14% to 22% in 2010 and was negatively correlated to discharge. From the relationship between dissolved delta Li-7 and strontium isotopes, we suggest that this large variation is due to mixing of waters from two contrasting continental weathering regimes. One end-member (high delta Li-7 approximate to 25%) represents waters sourced from active lateritic soils covering the periphery of the basin (Li highly sequestered into secondary mineral products) and another representing blackwater rivers (low delta Li-7 approximate to 5.7%) derived from the swampy central depression where high organic matter content in water leads to congruent dissolution of the Tertiary sedimentary bedrock. This suggests that the lithium isotopic signature of tropical low-relief surfaces is not unique and traces the long-term, large-scale vertical motions of the continental crust that control geomorphological settings. This evolution should be recorded in the oceanic secular delta Li-7 curve.