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

Stable isotope data (δ 18 O, δ 13 C) were obtained for magnesite (MgCO 3 ) 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 (Na 2 Ca(SO 4 ) 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. δ 18 O V‐PDB values are between +3.17 and +5.91‰, which is lower than might be expected for evaporitic environments, recording reduced 18 O enrichment at high salinity. δ 13 C V‐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 CO 2 . Covariance between δ 18 O and δ 13 C is strong for the lower part of the sequence, recording coupled 18 O and 13 C 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.