Non-biological fractionation of stable Ca isotopes in soils of the Atacama Desert, Chile

Abstract

We measured Ca stable isotope ratios (δ 44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m −2) is present as sulfates (2.5 kmol m −2), and to a lesser extent carbonates (0.4 kmol m −2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ 44/40Ca E values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ 44/40Ca in the top six horizons resembles that of sulfate-δ 44/40Ca, but with values 0.1–0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ 44/40Ca values observed on Earth. Linear correlation among δ 44/40Ca, δ 34S and δ 18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ 34S values are positively correlated with sulfate-δ 18O values ( R 2 = 0.78) and negatively correlated with sulfate δ 44/40Ca E values ( R 2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ 44/40Ca fractionation factor of −0.4‰ in CaSO 4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO 4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.