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Calcium isotope fractionation between aqueous compounds relevant to low-temperature geochemistry, biology and medicine

Abstract : Stable Ca isotopes are fractionated between bones, urine and blood of animals and between soils, roots and leaves of plants by >1000 ppm for the 44 Ca/ 40 Ca ratio. These isotopic variations have important implications to understand Ca transport and fluxes in living organisms; however, the mechanisms of isotopic fractionation are unclear. Here we present ab initio calculations for the isotopic fractionation between various aqueous species of Ca and show that this fractionation can be up to 3000 ppm. We show that the Ca isotopic fractionation between soil solutions and plant roots can be explained by the difference of isotopic fractionation between the different first shell hydration degree of Ca 2+ and that the isotopic fractionation between roots and leaves is controlled by the precipitation of Ca-oxalates. The isotopic fractionation between blood and urine is due to the complexation of heavy Ca with citrate and oxalates in urine. Calculations are presented for additional Ca species that may be useful to interpret future Ca isotopic measurements. Calcium is the fifth most abundant element in the Earth and meteorites 1. It plays a central role in cosmochemical, geochemical and biological processes. It is the most refractory major element (temperature of 50% condensation of 1505 K, ref. 2) and it is therefore one of the main constituents of the first solids formed in the solar system, the so called calcium-aluminium rich inclusions. Because of its refractory behavior, calcium is generally supposed not to be lost by evaporation during planetary formation and therefore its isotopic composition has been used as a tracer of the materials that have accreted to form the Earth 2,3. In surfaces environments, Ca is ubiquitous in living organisms and many minerals (e.g. calcite), and it plays a central role in the regulation of the carbon cycle and climate evolution 4. Calcium has 6 stable isotopes (
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Frédéric Moynier, Toshiyuki Fujii. Calcium isotope fractionation between aqueous compounds relevant to low-temperature geochemistry, biology and medicine. Scientific Reports, Nature Publishing Group, 2017, 7, ⟨10.1038/srep44255⟩. ⟨insu-02611247⟩

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