Mg isotopes as monitors of water sources and dolomitization process in late Miocene lacustrine bodies of the Eastern Mediterranean margins

Mg (and Sr) isotopes are applied as tracers of fresh and saline water sources and monitors of groundwater circulation in the hydrological-limnological system of the modern Lake Kinneret (the Sea of Galilee) and its late Miocene predecessors. The chemical and isotope compositions were measured in various water sources of modern Lake Kinneret and in limestones and dolostones that were deposited in fresh to saline lacustrine environments of the late Miocene Lakes Bira and Gesher. The δ²⁶Mg values and ⁸⁷Sr/⁸⁶Sr ratios of modern Lake Kinneret (−0.89 ‰, 0.7075) show that it receives waters and salts from three primary sources: freshwaters draining carbonate and basaltic rocks and subsurface Ca-chloride brines. The δ²⁶Mg values of Lake Bira limestones and dolostones range from ‑1.0 to −3.5 ‰ and −2.8 to −1.8 ‰, respectively, and decline between ∼9 and ∼7 Ma. The calculated δ²⁶Mg values of the relevant waters lie between −2 ‰ to 1 ‰. While the low δ²⁶Mg values are typical of carbonate waters, the high values lie at the edge of the documented range. A box model shows that dolomitization of limestone by evaporated seawater (up to the halite facies) could produce a brine with an exceptionally high δ²⁶Mg by Rayleigh fractionation process. This marine-derived brine comprised a hypersaline solution that filled Lake Zemah, at the tectonic Kinnarot Basin east of Lake Bira. During wet periods, Lake Zemah brine penetrated the freshwater Lake Bira, and the freshwater-brine mixture facilitated the deposition of limestones, characterized by high δ²⁶Mg. During arid periods, Lake Bira contracted, the brine discharge from Lake Zemah ceased, and Mediterranean Sea waters with low δ²⁶Mg mixed with Lake Bira's brackish waters, leading to dolomitization of the calcitic muds. These lacustrine dolomites have lower δ²⁶Mg than their underlying limestones. Later, during the Messinian period, the area was dominated by isolated swamps (Gesher Formation). Dolomitization was promoted by bacterial activity, as evident from the low δ¹³C values of the Gesher Formation dolomites. The temporal decrease in the δ²⁶Mg values of the Ca-chloride brines from ∼1.2 ‰ (in the Tortonian Lake Bira) to ∼0.5 ‰ (in the Messinian Gesher swamps) to 0.15 ‰ (in the modern Tiberias spa) indicates a continuous dilution of the original brine with seawater (until the Zanclean transgression at ∼5.3 Ma) and later with freshwaters. We suggest that a similar dolomitization process could have been active in other lagoonal environments, e.g., the circum-Mediterranean marine lagoons during the late Neogene. Similar conditions may have existed and induced dolomitization in the shallow margins of the Tethys Ocean and other epeiric seas during the Mesozoic.