Stable isotope chemistry of gaylussite and nahcolite from the deposits of the crater lake at Malha, northern Sudan

Gaylussite (Na2CO3·CaCO3·5H2O) and nahcolite (NaHCO3) in deposits of the crater lake at Malha are predictably characterized by a strong 18O enrichment that is typical of evaporitic environments. Both minerals also yield high δ13C values, which, in the case of gaylussite, are covariant with the δ18O values. The fact that the covariant trend, which is a feature that is commonly recognized for the deposits of closed basins, pertains both to the synsedimentary and pedogenic gaylussite deposits of the Malha basin demonstrates that 13C enrichment was mainly controlled by inorganic processes. An important biological control on the carbon isotope composition is also contradicted by the occurrence of a covariance for synsedimentary gaylussite that mainly formed by bottom growth in a well-mixed water mass. Degassing of carbon dioxide is the most likely cause of 13C enrichment, although this implies that CO2 loss is a process that occurs concurrently with the evaporation of water, without, as could be expected, being driven by changes in the relative abundance of compounds within the CO2–H2O system. The lack of a covariance of δ18O and δ13C for nahcolite seems to be partly related to bacterial activity, which did play an important role during lake stages with nahcolite formation, and partly to the lack of a further increase of δ18O with continued evaporation at advanced stages of the evaporitic concentration of brines.