Utility of Li and Li isotopes as tracers of continental weathering

Lithium is potentially an attractive tracer of continental weathering because its two isotopes have a large relative mass difference, it is unaffected by biological activity and it is only slightly incompatible during magmatic processes so tends to be relatively uniformly distributed in the Earth’s crust. Moreover, Li is conservative in the oceans, with a residence time of ca. 1 million years, and it is isotopically uniform on a global scale $({\delta}^{7}Li{\approx}31ppm$. Seminal work by Lui Chan and her co-workers has shown that the Li and Li isotope balance of the oceans is maintained by inputs of high-temperature hydrothermal fluids at oceanic ridges (with $({\delta}^{7}Li{\approx} +6.7ppm)$ and dissolved Li from rivers $(average {\delta}^{7}Li = +23ppm)$, and low-temperature removal of Li into oceanic basalts and marine sediments. Despite this potential, relatively little is known about the behaviour of Li during continental weathering. In this study, we will present an overview of the work that we have conducted on Li and Li isotopes in rivers, soils and estuaries from catchments in Greenland, Iceland and the Himalaya, as well as laboratory studies of mineral dissolution and precipitation. Each of these case studies illustrates the effects of weathering processes on the riverine isotope signal, and the estuarine data illustrate how this signal is transferred to the oceans. Our data suggest that variations in rock type have little effect on riverine $\delta^{7}Li$; the principal control is preferential removal of $^{6}Li$ into secondary minerals formed during weathering, leaving the residual waters enriched in $^{7}Li$. In subglacial environments, where weathering rates are very low, uptake of Li by ferric oxyhydroxides formed during sulphide oxidation is important. Our results clearly demonstrate that weathering processes can exert a significant effect on the Li isotope composition of natural waters. In order to understand whether changes in such processes with time are preserved, we have also generated records of the past variation in the Li and Li isotopic composition of the oceans. The challenges in the generation and interpretation of these records will be discussed