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The article presents the comparative characteristic of lake water and pore water of bottom sediments of saline lakes located in the South of Western Siberia. Studied lakes are mostly Cl-Na composition and rarely Cl-HCO3-Na. The values of pH vary from 7.6 to 9.9, TDS from 5 to 300 g/L. It is shown that redox conditions change from oxic in the upper water layer to anoxic in pore water and the bottom layer. As a result, there is dissimilarity in interaction between water layer, bottom sediment and secondary minerals caused by differences in biological and geochemical processes in these two environments. According to thermodynamic calculations, lake water is in equilibrium with calcite, quartz, kaolinite, illite, Mg-montmorillonite, barely with respect to gypsum, thenardite and halite. Simultaneous formation of evaporite minerals and bacterial mats, CH4 and H2S accumulation lead to microbial sediment development.

Data are presented on detailed sampling of the surface and pore waters of soda Lake Doroninskoye (Eastern Transbaikalia, Russia). Particular attention has been paid to the possible REE precipitation species from both water types. Thermodynamic modeling at 25 °C and 1 bar total pressure was performed with the “HCh” code using a free-energy minimization algorithm. We modeled the heterophase 43-component system; both individual minerals and ideal solid solutions of the REE and numerous dissolved species were incorporated into the model. According to the calculations, redox conditions in the lake waters are established by the sulfur species. The main result is that the surface water is slightly supersaturated with respect to CaCO 3 -REE (65%) and F-apatite-REE (35%) solid solutions, as well as bastnasite Ce 0,5 La 0,25 Nd 0,2 Pr 0,05 CO 3 F. A trend of the HREE predominance in the solution during the process of early diagenesis has been confirmed. Nevertheless, the thermodynamic model does not entirely explain the REE fractionation in pore waters (pore solutions are enriched in the LREEs in situ but are drastically depleted according to the thermodynamic model). The results of this study suggest that further considerations about solid solutions should be taken into account to model and understand exact REE fractionation during water-rock interaction in salt lakes.