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.

本文提供的数据隶属于一篇投稿于《Geochimica et Cosmochimica Acta》（地球化学与宇宙化学学报）的研究论文，题为《镁同位素作为晚中新世湖相水体的水源示踪剂与白云岩化过程监测指标》。本研究以现代加利利湖（Lake Kinneret，又名基尼烈海）及其晚中新世前身比拉湖（Lake Bira）与盖谢尔湖（Lake Gesher）组成的水文湖沼系统为研究对象，将镁同位素组成作为淡水与咸水水源的示踪剂，以及地下水循环的监测指标。研究人员对基尼烈湖流域内的淡水与卤水、以及沉积于古湖体中的晚中新世比拉组（Bira Formation）与盖谢尔组（Gesher Formation）岩层中的灰岩和白云岩，开展了镁、锶同位素比值与主量元素组成分析。箱型模型模拟结果显示，当海水蒸发至盐岩相（halite facies）阶段时，其对灰岩含水层的白云岩化作用可形成具有极高δ26Mg值的卤水。该卤水形成了位于比拉湖东侧构造性基尼罗特盆地内的高盐湖泽马赫湖（Lake Zemah）。晚中新世时期泽马赫湖沉积的数千米厚蒸发岩（evaporite，盐岩）序列，证实了该高盐湖的存在。在湿润期，泽马赫湖的卤水侵入淡水比拉湖，卤水与淡水的混合环境促成了比拉组湖相灰岩的原生方解石组分沉积，此类灰岩具有较高的δ26Mg值。在干旱期，比拉湖面积萎缩，泽马赫湖的卤水补给停止，低δ26Mg值的地中海海水侵入并与比拉湖的半咸水混合，进而导致方解石泥发生白云岩化。此类湖相白云岩的δ26Mg值低于其下伏的灰岩。随后的墨西拿期（Messinian），该区域被盖谢尔组的沼泽环境所覆盖。盖谢尔组白云岩中极低的δ13C值表明，此阶段的白云岩化作用由细菌活动驱动。氯化钙卤水的δ26Mg值随时间呈现下降趋势：托尔顿阶（Tortonian）时期比拉湖的卤水δ26Mg值约为1.2‰，墨西拿期盖谢尔沼泽的卤水约为0.5‰，现代蒂比瑞斯温泉的卤水则为0.15‰。这一趋势表明原始卤水持续被海水稀释（直至约530万年前的赞克尔海侵（Zanclean transgression）），后续又被淡水进一步稀释。本文所阐述的晚中新世湖相碳酸盐岩白云岩化过程，可推广至其他高盐卤水与低盐溶液发生镁元素交换的地质环境，例如晚新近纪地中海周边典型的海相泻湖环境。此外，晚中新世地中海周边边缘泻湖-湖相环境中不同盐度溶液的混合过程，可为中生代海相浅海环境的白云岩化作用提供类比参考。