The main formation processes for different types of salt lakes: Evidence from isotopic composition with case studies of lakes in Transbaikalia, Russia

Highlights: • Formation of the salt lakes' composition • Soda lakes are formed resulting from the water-rock interaction. • Secondary minerals and bacterial mats inherit the isotopic composition of water. • The water of soda lakes is more enriched in ¹⁸. The isotopic composition of the natural waters of Transbaikalia was studied. The composition of δ¹⁸ and δ¹³ in the carbonates and bacterial mats that have dissolved and precipitated into the bottom sediments, as well as δ¹⁸ of the aluminosilicate fraction of the sediments and rocks of the catchment areas were analysed. All secondary minerals inherit oxygen from water, but their oxygen atoms are much heavier than the oxygen of water because when it interacts with rock, oxygen fractionation occurs, and there is a transition of heavy isotopes to secondary carbonate and aluminosilicate formations. Bacterial mats are isotopically heavier in oxygen, and they use oxygen from carbon dioxide ions for their life activity. Thermodynamic calculations have confirmed the possibility of the formation of chemogenic carbonates and secondary aluminosilicates in lakes. Water has been found to be more enriched in ¹⁸O in soda salt lakes than in chloride and sulphate lakes. Our studies indicate that the heavy isotope enrichment of salt lake waters is due not only to the evaporation process but also to the interaction of water with rock. The participation of aluminosilicate hydrolysis reactions in oxygen fractionation is confirmed by the direct dependence of the "oxygen shift" on pH, the value of which is determined by the degree of such interaction.