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Stable isotope geochemistry of magnesite from Holocene salt lake deposits, Taoudenni, Mali



Stable isotope data (δ18O, δ13C) were obtained for magnesite (MgCO3) from Holocene salt lake deposits of the Taoudenni–Agorgott basin, Mali, in which it is the only carbonate mineral present. The deposits have a high glauberite content (Na2Ca(SO4)2), representing a type of continental evaporite formation that is commonly magnesite-bearing. Samples from seven different levels in a 5 m long sequence were analysed. δ18OV-PDB values are between +3.17 and +5.91‰, which is lower than might be expected for evaporitic environments, recording reduced 18O enrichment at high salinity. δ13CV-PDB values are between −1.32‰ and −4.79‰, showing an influence of carbon derived from the decomposition of organic matter rather than from exchange with atmospheric CO2. Covariance between δ18O and δ13C is strong for the lower part of the sequence, recording coupled 18O and 13C enrichment as commonly observed for closed basins. Covariance is lacking for the upper part of the sequence, with only periodic massive magnesite deposition in a more shallow lake. Copyright © 2012 John Wiley & Sons, Ltd.

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