Lithium Isotopes as a Tracer of Watershed Weathering Characteristics
Collaborators: Oleg Pokrovsky (CNRS GETS, Toulouse) and Philip Pogge Von Strandmann (University College London)
Understanding the rate and type of continental weathering is important to determining the long-term effects on the global carbon system. Specifically, chemical weathering of silicates involves the drawdown of CO2, changing carbon concentrations in the atmosphere. Furthermore, chemical weathering provides a significant amount of material to the oceans affecting ocean chemistry. Since weathering is linked with the carbon cycle, and that is linked with climate change, developing a better picture of continental weathering provides further insight into changing climates in the past, present, and future.
A variety of isotope tracers can be used to understand weathering. Strontium, osmium, and calcium isotope tracers are useful and have been used to tell us about the age and type of rock being weathered. Lithium isotopes, however, can inform us about weathering processes that are at work now and in the past. Previous research has established that lithium isotopes do not fractionate during the dissolution of primary minerals, but do fractionate as 6Li is preferentially adsorbed or incorporated in secondary minerals. This fact tells us that lithium isotope fractionation is dominated by silicate weathering, and when lithium isotope measurements are applied, can be used to determine weathering regimes.
In May 2015, I started my research by applying lithium isotopes, a non-traditional stable isotope system, to understand continental weathering in three major watersheds in Siberia: the Ob, Yenisei, and Lena Rivers. These rivers drain a range of lithologies and topographies, while the watersheds draining into the rivers experience varying levels of permafrost cover throughout the year. This research will allow us to study lithium isotope flux in a region dominated by permafrost, which has not been thoroughly studied. Using MC-ICP-MS, we hope to better understand the weathering characteristics of specific watersheds in Siberia, at multiple scales; pinpoint the variables that affect lithium isotope fractionation, such as permafrost cover, lithology, topography, vegetation, and river location; and with all of these pieces of information, we hope to be able to reconstruct the paleoenvironment based on the interplay between weathering, the carbon cycle, and climate change.