Assessing hydrological controls on the lithium isotope weathering tracer

To investigate the impact of riverine discharge and weathering intensity on lithium isotopes (delta 7Li) in a mono-lithological terrain, this study examines the dissolved load and leached suspended load (exchangeable, oxide, and clay fractions) from Icelandic rivers spanning a wide range of discharge, weathering rates, and weathering intensity. The delta 7Lidissolved co-varies inversely with the discharge, confirming that water-rock interaction time is a primary control on the secondary mineral formation that fractionates Li isotopes. The boomerang shape observed in global rivers between the weathering intensity (i.e. W/D = weathering rate/denudation rate) and delta 7Lidissolved also exists for these basaltic rivers at low to medium W/D. However, these rivers do not extend to such low delta 7Lidissolved values as seen in the global compilation at low W/D, indicating that there is a lithological control on this relationship arising from the type of the lithology-specific secondary minerals forming and their precipitation rates. In addition, the Delta 7Lix-dissolved between each leached solid phase and the dissolved load also co-varies with discharge. At low discharge (long water-rock interaction times), Delta 7Lix-dissolved values agree with experimentally-determined equilibrium values, whereas less fractionated values are observed at higher discharge (shorter water-rock interaction times). As a result, there is a different relationship between W/D and Delta 7Liclay-source in this basaltic terrain than previously reported from global multi-lithological river sediment samples, with clay leachates from Iceland more closely mimicking the boomerang shape of the dissolved load. However, the relationship between delta 7Li and weathering processes is complicated because the fractionation between the clay fraction and the dissolved load is not constant but varies with both W/D and discharge. Overall, this study confirms the utility of Li isotopes as a tracer of modern and palaeo-weathering processes, and also has important implications for the specific interpretations of detrital delta 7Li values, which may be more sensitive to weathering parameters than previously thought.

Automated summary

This study investigates the impact of riverine discharge and weathering intensity on lithium isotopes in a mono-lithological terrain using Icelandic rivers as the case study. The results show that water-rock interaction time is a primary control on the fractionation of Li isotopes. The relationship between weathering intensity and delta 7Li is observed in these basaltic rivers, but with different values compared to global rivers due to lithological control. The study also highlights the varying fractionation between the clay fraction and the dissolved load with both W/D and discharge. Overall, this study confirms the utility of Li isotopes as a tracer of weathering processes and provides important insights for interpreting detrital delta 7Li values.