Ca isotope fractionation in a high-alkalinity lake system: Mono Lake, California

The behavior of the Mono Lake Ca isotope system is similar in some ways to that of the global oceans, in that the average δ44/40Ca of lake water is positive (estimated average of +1) and both riverine inputs and precipitated carbonates are isotopically light (δ44/40Ca between −0.5 and 0). We present a calcium isotope budget of the lake to constrain the long-term average lake water Ca isotope composition. Archived water samples indicate that the lake δ44/40Ca varied by over 2‰ between 1995 and 2010. The most extreme excursions are toward higher δ44/40Ca, and are probably caused by carbonate precipitation events induced by breakdown of chemostratification. This variability indicates that the lake is out of steady state with respect to calcium isotopes, and that unlike the ocean, calcium isotopes in Mono Basin carbonate sediments likely do not record the balance between weathering and carbonate mineralization fluxes to and from the lake.