Lithium isotopes and implications on chemical weathering in the catchment of Lake Donggi Cona, northeastern Tibetan Plateau

This study presents lithium (Li) isotope ratios (δ7Li) for rocks, sediments, suspended particulate material, and dissolved Li from the Lake Donggi Cona catchment, located on the northeastern Tibetan Plateau, China. The average δ7Li = +1.9‰ of the bedrocks is estimated from local loess. δ7Li values decrease progressively within the sediment cascade from loess, to river and lake floor sediments. The lake floor sediments average at −0.7‰. The difference between bedrock and lake sediments reflects the preferential fractionation of dissolved 6Li into clay minerals (mostly illite) in the weathering zone and grain-size sorting during fluvial sediment transport. The δ7Li values of stream and lake water samples range from +13.6 to +20.8‰, whereas thermal waters fall between +5.9 and +11.6‰. The δ7Li values of lake water samples are close to +17‰ and reflect mixing of waters from two perennial inflows and thermal waters. Dissolved Li in streams represents an integrated isotopic signal derived from soil solutions in the weathering zone. An apparent isotopic fractionation of −17.8 ± 1.6‰ (αsec-sol ∼ 0.982) between secondary minerals and solution was determined. An inflow that drains a sub-catchment in the north carries a high proportion of thermal waters. Despite of the high proportion of admixed thermal waters with high Li concentrations and low δ7Li, this stream has the highest δ7Li values of about +21‰. This is consistent with admixing of thermal waters to solutions in the weathering zone and subsequent fractionation by preferential uptake of isotopically light dissolved Li into secondary phases. Based on Li isotope ratios of the dissolved and solid export flux from the weathering zone we calculated that around five times more Li is exported in particles than dissolved in streams. An average δ7Li value of about +17‰ of most streams and the lake is reflecting a low weathering intensity and chemical weathering rate of about 4 t/km2/a. Low weathering rates and an erosion dominated weathering system are consistent with moderate precipitations, the cold climate, and the high relief of the study area.