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

Abstract

Precipitation of calcium carbonate minerals from aqueous solutions causes surface-controlled kinetic stable Ca isotope fractionation. The magnitude of fractionation depends on the relative rates of ion attachment to and detachment from the mineral surface, which in turn is predicted to depend on both the saturation state and the solution stoichiometry or the Ca2+:CO32- activity ratio. Experimental studies have not directly investigated the effects of varying solution stoichiometry on calcium isotope partitioning during calcite or aragonite growth, but natural alkaline lake systems such as Mono Lake, California provide a test bed for the hypothesized stoichiometry dependence. Mono Lake has a Ca2+:CO32- activity ratio of about 0.0001, seven orders of magnitude lower than ocean water and typical terrestrial freshwater. We present chemical and isotopic measurements of streams, springs, lake water, and precipitated carbonates from the Mono Basin that yield evidence of stoichiometry-dependent Ca isotope fractionation during calcite, aragonite and Mg-calcite precipitation from the alkaline lake water. To estimate the Ca isotope fractionation factors, it is necessary to characterize the lake Ca balance and constrain the variability of lake water chemistry both spatially and temporally. Streams and springs supply Ca to the lake, and a substantial fraction of this supply is precipitated along the lake shore to form tufa towers. Lake water is significantly supersaturated with respect to carbonate minerals, so CaCO3 also precipitates directly from the water column to form carbonate-rich bottom sediments. Growth rate inhibition by orthophosphate likely preserves the high degree of supersaturation in the lake. Strontium isotope ratios are used to estimate the proportions of fresh and alkaline lake water from which each solid carbonate sample precipitated. Carbonate minerals that precipitate directly from lake water (low Ca2+:CO32-) experience relatively large Ca isotope fractionation during growth. Tufa and shoreline carbonates that precipitate from lake water with a significant fraction of spring water (higher Ca2+:CO32-) are considerably less fractionated, as predicted from theory.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.