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Hardwater lake; Redox cycling; Coprecipitation of REE and Y with calcite; Sorption of REE and Y by hydrous ferric oxide and MnO2; Seasonal changes; Authigenic calcite
Abstract:
The variation of major and rare earth elements and yttrium (REY) in the monomictic hardwater Lake Tiberias during the wet and dry seasons of the hydrological year was studied in two profiles. The average volume and Cl concentration of the known and unknown saline inflows of 1.6 × 107 m3 and 1.2 × 109 mol are derived by closing both balances. This brine corresponds to a mixture of 83% of groundwater from Cretaceous aquifers and 17% of very saline deep brine. Taking cycling of calcite in the hypolimnion into account, the settling rate of authigenic calcite is estimated to be 3.3 mol m−2 a−1.
In the stratified lake of the dry season dissolved inorganic carbon increases by 490 μM at the thermo-/chemocline due to microbial reduction of SO42−, NO3−, chemical reduction of Fe(III) and MnO2 colloids, and cycling of calcite in the hypolimnion. REY distribution in the stratified water column is dominantly controlled by coprecipitation with calcite, hydrous ferric oxides and MnO2 in the epilimnion and cycling of these compounds in the hypolimnion. The positive Ce anomaly in the hypolimnetic water is produced by cycling of MnO2. The simulation of the increase of REY in the hypolimnion reveals that hydrous ferric and manganese oxides only play a negligible role except Ce. Only about 10% of REY from cycled matter enhance REY in solution. Most of the released REY are adsorbed by particular matter and thus settling on the floor of the lake.
Different from Na, U, SO42− and SiO2, the other elements, in particular REY, increase in the mixed water column from the top to the lower third and mostly decrease thereafter toward the bottom in the mixed lake during the wet season. The behavior of REY is caused by some cycling of calcite and pH-dependent re-equilibration of REY bound to hydrous ferric and manganese oxides adsorbed by particular matter.
