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Sulfur isotope fractionation between aqueous andcarbonate-associated sulfate in abiotic calcite and aragonite

Barkan, Y.. Paris, G. ; Webb, S.M. ; Adkins, J.F. ; Halevy, I., GCA

Sulfur isotope fractionation between aqueous andcarbonate-associated sulfate in abiotic calcite and aragonite

Barkan, Y.. Paris, G. ; Webb, S.M. ; Adkins, J.F. ; Halevy, I.

Geochimica et Cosmochimica Acta, 2020, 280, 317-339

Abstract :

Sulfate (SO42-) incorporated into calcium carbonate minerals enables measurements of sulfur (S) isotope ratios in carbon-ate rocks. This Carbonate Associated Sulfate (CAS) in marine carbonate minerals is thought to faithfully represent the S iso-tope composition of the seawater sulfate incorporated into the mineral, with little or no S isotope fractionation in the process.However, comparison between different calcifying species reveals both positive and negative S isotope fractionation betweenCAS and seawater sulfate, and a large range of S isotope ratios can be found within a single rock sample, depending on thecomponent measured. To better understand the isotopic effects associated with sulfate incorporation into carbonate minerals,we precipitated inorganic calcite and aragonite over a range covering more than two orders of magnitude of sulfate concen-tration and precipitation rate. Coupled measurements of CAS concentration, S isotope composition and X-ray absorptionnear-edge spectra (XANES) permit characterization and explanation of the observed dependence of S isotope fractionationbetween CAS and aqueous sulfate (CAS-SO42-isotope fractionation) on sulfate concentration and precipitation rate. In arag-onite, the CAS-SO42-isotope fractionation is 1.0 ± 0.3‰and independent of the sulfate (and CAS) concentration. In contrast,the CAS-SO42-isotope fractionation in calcite covaries strongly with the sulfate concentration and weakly with the precipi-tation rate, between values of 1.3 ± 0.1 and 3.1 ± 0.6‰. We suggest that the correlation between aqueous sulfate concentra-tion and CAS-SO42-isotope fractionation in calcite reflects a dependence of the equilibrium S isotope fractionation on theconcentration of CAS, through the effect of the sulfate impurity on the carbonate mineral’s energetic state.Ó2020 Elsevier Ltd. All rights reserved

Voir en ligne : https://doi.org/10.1016/j.gca.2020....




publié mercredi 20 mai 2020