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

Stable isotope data (Delta18O, Delta13C) 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. ?18OV-PDB values are between +3.17 and +5.91o/oo, which is lower than might be expected for evaporitic environments, recording reduced 18O enrichment at high salinity. ?13CV-PDB values are between ?1.32o/oo and ?4.79o/oo, showing an influence of carbon derived from the decomposition of organic matter rather than from exchange with atmospheric CO2. Covariance between ?18O and ?13C 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