Hydroclimatic and geothermal controls on the salinity of Mbaka Lakes (SW Tanzania): Limnological and paleolimnological implications

The hydroclimatic and geothermal controls on the salinity of small tropical crater lakes Masoko, Katubwi, Kyambangunguru, Ilamba and Kingiri, aligned with the Mbaka fault line, north of Lake Malawi, are investigated by water stable isotopes which are used to trace evaporative processes as to establish lake water balances, and by chloride concentrations allowing to identify the main salinity sources. This region shows positive excess in the exchanges between atmosphere and lake surfaces (P – E > 0). With the exception of Lake Ilamba, the lakes are closed surface basins and their levels are relatively stable. As catchment inflows cannot be neglected, groundwater outflows have to compensate for this excess, the isotopic budgets show that these lakes constitute a series of windows on the local shallow aquifer. In addition, the estimated losses by evaporation cannot solely account for the lake salinity observed, as the most saline lake differs from not the most evaporated. Lake salinity is then investigated from chloride concentrations and seems to be controlled by inflows, which results from the mixing between (i) fresh and (ii) hydrothermal ground waters. The contrasts in lake salinity (almost one order of magnitude) are first related to the hydrothermal contribution, which increases with the proximity of the Mbaka fault. Second, due to positive exchange with the atmosphere, the lake salinity appears systematically diluted in regard to the respective inflows. This dilution effect increases as the fraction of total inputs lost by evaporation or the lake water residence time increases. This current hydrological study shows that local wetter conditions are not inconsistent with higher lake salinity and allows to conciliate the questioned and apparent contradictory Lake Masoko wetter and salty paleoenvironmental and paleoclimatic records for the last glacial maximum and Younger Dryas time intervals. Further, in a general manner, this work addresses the case of lake submitted to wet environment and highlights the potential contribution of hydrothermal water on lakes salinization, occurring in active regions, such as the East African Rift System.