LITHIUM IN SEQUENTIAL EXTRACTIONS OF ALKALINE LAKE SEDIMENT CORES, WESTERN NEBRASKA; IMPLICATIONS FOR CHEMICAL WEATHERING

The hypersaline interdune lakes in the Sand Hills of western Nebraska are alkaline and rich in alkali elements, i.e. potassium (K) and sodium (Na), with solute origins that are not well understood. Lake deposits and waters cover ~ 266 km² and vary widely in geochemistry, both spatially (over <1 km) and temporally. Previous work has shown that anions are primarily CO₃/HCO₃ and SO₄ with lesser amounts of Cl. The fractionation of lithium (Li) isotopes during weathering in interdune environments could provide insight to solute origins. In preparation for future Li isotope ratio measurements, our study is using a five-step sequential extraction procedure (SEP) that is thought to differentiate among elements held by sorption and by specific mineral phases. The SEP steps release elements in each of these five steps: 1) exchangeable (ion exchange sites), 2) bound to carbonates (weak acid digestion), 3) bound to Fe and Mn oxide (reducible), 4) bound to organic matter (oxidizable), and 5) residual (alumino-silicates and other refractory minerals). Lake sediment cores taken from multiple lakes (to depths of ~ 1.5 m), previously collected in August 2015, have been sampled in 2-cm increments and at intervals where there were visible changes in color or texture. We applied the SEP on 27 samples from three lakes. Sequential extraction typically indicates that the highest amount of Li is in the residual fraction, while the smallest amount is in the exchangeable fraction. Preliminary Li concentration data from 2013 (drought year) and 2015 (wet year) in the lake water show the maximum concentrations of Li were 6.7 mg/L and 0.5 mg/L, respectively. The aqueous Na/Li and K/Li ratios in the three lakes ranged from 13,400 to 216,000 and 4,000 to 174,000, respectively, in samples collected in August of 2012, 2013, and 2015. During a dry year, evaporation causes the concentrations of all ions, but Na and K concentrate more than Li, causing the ratios to increase. This variation in ratios suggests either different contributions of local groundwater to the lakes during wet and dry periods, or relatively rapid water-sediment interactions that are also related to weather. If the variations result from water-sediment reactions, then these reactions remove Li preferentially as lake water evaporates during dry years.