Mg isotopes and chemical composition of waters and Late Miocene carbonate rocks

The data provided here are part of a research article entitled "Mg isotopes as monitors of water sources and dolomitization process in late Miocene lacustrine bodies", submitted to Geochimica et Cosmichimica Acta. The Mg isotope compositions 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, lakes Bira and Gesher. The Mg and Sr isotope ratios and major elements compositions were analyzed in freshwaters and brines in the watershed of Lake Kinneret and limestones and dolostones that comprise the late Miocene Bira and Gesher Formations, which were deposited in the ancient water-bodies. A box model shows that dolomitization of limestone aquifers by seawater evaporated up to the halite facies, can produce a brine with exceptionally high d26Mg. This brine formed the hypersaline Lake Zemah, located in the tectonic Kinnarot Basin east of Lake Bira. The existence of this hypersaline lake is revealed by the kilometers thick of evaporite (halite) sequences that were deposited in Lake Zemah during the late Miocene. During wet periods, the Zemah brine penetrated the freshwater Lake Bira and the freshwater-brine mixture accommodated the deposition of primary calcites comprising the lacustrine limestones of Bira Formation. These limestones have rather high d26Mg. During arid periods, Lake Bira contracted, the brine discharge from Lake Zemah stopped and low d26Mg Mediterranean Sea waters penetrated and mixed with Lake Bira’s brackish waters leading to dolomitization of the calcite muds. These lacustrine dolomites have lower d26Mg than their underlying limestones. Later, during the Messinian the area was dominated by swamps of the Gesher Fm. Dolomitization proceeded by bacterial activity, as evident from the low d13C values in the Gesher Fm. dolomites. The temporal decrease in the d26Mg 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 ~5.3 Ma) and later with freshwaters. The dolomitization process described in the manuscript for the late Miocene lacustrine carbonates, may be relevant to other environments where the exchange of Mg occurs between hypersaline brines and less-saline solutions, e.g., marine lagoons that characterized the late Neogene circum-Mediterranean. Moreover, the mixing processes between solutions of distinct salinities in the marginal lagoonal-lacustrine environments of late Miocene circum-Mediterranean may illustrate the conditions of dolomitization in the shallow marine environments of the Mesozoic seas.

所提供的数据系收录于题为《镁同位素作为晚中新世湖泊水体水源及白云石化过程的监测指标》的研究论文之中，该论文已提交至《地球化学与宇宙化学学报》。镁同位素组成被用作淡水和咸水水源的示踪剂，以及现代基尼烈特湖（加利利海）及其晚中新世前身的湖泊比拉和盖舍湖泊水文-湖泊学系统中地下水循环的监测工具。在基尼烈特湖流域的淡水和盐湖中，以及构成晚中新世比拉和盖舍沉积的古代水体中的石灰岩和白云岩中，对镁和锶同位素比以及主要元素组成进行了分析。箱式模型表明，海水蒸发至石盐相的石灰岩含水层白云石化，可以产生具有异常高d26Mg的卤水。这种卤水形成了位于比拉湖东部构造基纳罗特盆地的超咸水泽马湖。在晚中新世，泽马湖中沉积了数千米的蒸发岩（石盐）层序，揭示了该超咸水湖的存在。在湿润时期，泽马湖的卤水渗透进淡水湖比拉，淡水与卤水的混合物为比拉湖石灰岩层中的原生方解石的沉积提供了场所。这些石灰岩具有较高的d26Mg。在干旱时期，比拉湖收缩，泽马湖的卤水排放停止，低d26Mg的地中海海水渗透并与比拉湖的咸淡水混合，导致方解石泥的白云石化。这些湖泊白云岩的d26Mg值低于其下伏的石灰岩。随后，在墨西拿期，该区域被盖舍组的湿地所主导。白云石化过程由细菌活动驱动，这在盖舍组白云岩中低d13C值中得到证实。从托尔托尼亚湖比拉中的约1.2‰（托尔托尼亚湖比拉）到墨西拿期盖舍湿地的约0.5‰，再到现代提伯利亚温泉的0.15‰，d26Mg值的时序下降表明原始卤水与海水（直至赞克莱安海侵约5.3 Ma）的持续稀释，以及随后与淡水的稀释。论文中描述的晚中新世湖泊碳酸盐的白云石化过程，可能与镁在超咸卤水和较不咸溶液之间发生交换的其他环境相关，例如，晚新生代地中海周边的特征性海洋泻湖。此外，在晚中新世地中海周边的边缘泻湖-湖泊环境中，不同盐度溶液的混合过程，可能说明了中生代海洋浅水环境中白云石化的条件。