Magnesium and strontium isotopes are used to trace water sources, brine evolution, and dolomitization processes in the hydrological system of modern Lake Kinneret and its late Miocene freshwater precursors, Lakes Bira and Gesher, along the eastern Mediterranean margin. We analyzed chemical composition, δ²⁶Mg, and ⁸⁷Sr/⁸⁶Sr in modern waters and in lacustrine limestones and dolostones deposited under freshwater to hypersaline conditions.
The isotopic composition of modern Lake Kinneret (δ²⁶Mg ≈ -0.9‰; ⁸⁷Sr/⁸⁶Sr ≈ 0.7075) indicates mixing of three principal sources: freshwater draining, carbonate (1) and basalt (2) terrains, and subsurface Ca-chloride brines (3). Late Miocene Lake Bira limestones and dolostones show δ²⁶Mg values of -1.0 to -3.5‰ and -2.8 to -1.8‰, respectively, decreasing systematically between ~9 and ~7 Ma.
Calculated lake-water δ²⁶Mg values range from -2‰ to +1‰, spanning typical carbonate waters to unusually heavy compositions.
Box-model calculations indicate that dolomitization of limestone by evaporated seawater, progressing to halite saturation, can generate hypersaline brines with elevated δ²⁶Mg via Rayleigh fractionation. We propose that such marine-derived brines filled the tectonic depression of Kinnarot Basin (e.g., the Zemach borehole). During humid periods, brine leakage into freshwater Lake Bira promoted limestone deposition with relatively high δ²⁶Mg. During arid intervals, brine inflow ceased, Mediterranean seawater mixed with brackish lake waters, and dolomitization of calcitic muds occurred, producing dolomites with lower δ²⁶Mg than the underlying limestones.
In the Messinian, deposition shifted to swampy environments of the Gesher Formation, where dolomitization was likely bacterially mediated, consistent with low δ¹³C values. The progressive decrease in δ²⁶Mg of Ca-chloride brines, from ~ +1.2‰ in Tortonian Lake Bira, to ~ -0.5‰ in Messinian swamps, to ~0.15‰ in the modern Tiberias spa, reflects long-term dilution by seawater and later by freshwater. These results suggest that similar brine-driven dolomitization processes may have operated in Neogene Mediterranean lagoons and ancient epeiric seas.