The lithium isotopic composition of waters of the Mono Basin, California

Mono Lake, a major closed-basin alkaline salt lake in eastern California, derives its water from a mixture of creeks and springs, with the former providing in excess of 75% of the total. The Li isotopic composition of lake water has not varied significantly over a 4 year meromictic period (δ 7Li ∼ +19.5). Springs are isotopically distinct: groundwater springs and seeps carry water enriched in isotopically heavy Li whereas thermal springs supply isotopically light (δ 7Li < lake), but 10 times more Li-rich, water. Isotopic fractionation during crystallization of carbonate tufa and evaporitic salt appears to be insignificant, and thus cannot be called on as a principal control of the isotopic balance of Li of the lake. Isotopic differences between the end-member source components permit a water budget to be calculated, suggesting (1) springs provide > 50% of the Li to the lake; (2) the Li budget is sensitively balanced on small thermal spring contributions, < 3% of the total spring inflow; and (3) the residence time of Li in the lake is 28 ka. Other Great Basin closed lakes have variable Li isotopic compositions (δ 7Li from +16.7 to +23.7), all of which differ significantly from those of several major lakes and seawater (homogeneously ∼ +32).