Lithium isotopes in water and regolith in a deep weathering profile reveal imbalances in Critical Zone fluxes

To trace Critical Zone processes and to quantify Li fluxes from one Critical Zone compartment into another, we investigated the Li concentration and isotopic composition (δ7Li) of time-series water samples (including subsurface flow, groundwater and creek water), vegetation, bedrock (including separated minerals from bedrock), and regolith (including exchangeable fraction and clay-sized fraction of regolith) in a temperate forested headwater catchment in the Black Forest, Conventwald, Germany. Our estimation of the Li budget shows that atmospheric deposition and biological processes played minor roles in the Li cycle relative to chemical weathering. All water samples (δ 7Li value of 6.5 to 20.4‰) were enriched in 7Li compared to bedrock (-1.3‰) and regolith (∼-1.7‰), but δ7Li differed between water pathways: δ7Li variations in subsurface flow, creek water and groundwater were controlled by conservative mixing, exchangeable pool buffering and Li incorporation/adsorption, respectively. Fractionated heavy Li isotopes in water samples resulted from the formation of secondary solids which preferentially incorporated 6Li, with the separated clay-sized fraction of the regolith exhibiting more negative δ7Li values (-5.4 to -3.5‰) than the bulk regolith (∼-1.7‰). However, Li in secondary solids only accounted for 8 ± 6% of the total Li hosted in bulk regolith, and consequently δ7Li in soil did not differ significantly from δ 7Li in bedrock. This is unexpected considering water is continuously removing 7Li in preference over 6Li from regolith. Mass balance calculations applied at the catchment scale point to an irreconcilable imbalance with our data. On one hand, the regolith’s δ7Li values are not negative enough to balance the 7Li export by river water, and on the other hand Li in the riverine dissolved load only accounts for ∼30% of the Li solubilized from regolith. Therefore, we suggest that there might be a “hidden export pathway” for Li at our site, possibly subsurface removal of fine particles enriched in 6Li. In light of increasingly frequent observations of such isotopic imbalances in the Critical Zone this phenomenon deserves increased attention.