The mechanisms controlling the release, transport, and isotopic composition of trace elements during weathering

The long-term carbon cycle of the Earth is dominated by the effects of mineral weathering, coupled with carbonate precipitation and the burial and oxidation of organic carbon. These processes are the key control on atmospheric concentrations of carbon dioxide and oxygen on million-year timescales (Holland, 1978). Long term climate records imply that chemical weathering acts as a negative feedback against metamorphism and volcanism, stabilising atmospheric CO2 to maintain a habitable environment (Walker, 1981) and much research has gone into quantifying the intensity of weathering processes and their associated fluxes of key cations and alkalinity through the Earth's surface environment.

The lithium isotopic system is a potentially powerful tracer of silicate weathering, with advantages over other elements such as strontium and osmium, as there are minimal effects on 7Li from carbonate weathering, the presence of evaporites and biological processes (Pistiner and Henderson, 2003). The isotopic fractionation between 6Li and 7Li is driven by the formation of secondary clay minerals, which preferentially incorporate 6Li and drive waters to be isotopically heavier than the rocks they drain (Huh, 2001). The balance between denudation of silicate rocks, dissolution of primary silicates, clay formation and eventual redissolution of secondary clays gives a characteristic combination of the concentration and isotopic state of Li in river systems (Pogge von Strandmann, Dellinger and West, 2021), and the ratio of the weathering rate to the denudation rate is considered to be the intensity of silicate weathering (Dellinger, 2015).

While trace element concentrations and isotopic compositions in river systems are readily measured, and a qualitative understanding of the link between these measurements and weathering intensity is well developed, there is insufficient understanding of the key mechanistic processes which generate these signatures. A fundamental complication of drawing environmental interpretations from this data is that chemical and isotopic trends are created by the processes which affect individual minerals and specific sites within their structures at the atomic level, which can behave in different ways (Hindshaw, 2019). A quantitative understanding of weathering in natural environments therefore requires improved knowledge of partitioning, isotope fractionation factors and the physical processes which distribute lithium (Pogge von Strandmann, Dellinger and West, 2021).