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.

本数据集所包含的数据，来自一篇题为《镁同位素作为晚中新世湖相水体的水源与白云岩化过程示踪剂》的研究论文，该论文已投稿至《地球化学与宇宙化学学报》（Geochimica et Cosmochimica Acta）。本研究将镁同位素组成作为示踪剂，用以示踪现代基尼烈湖（Lake Kinneret，又名加利利海）及其晚中新世前身比拉湖与盖舍尔湖的水文湖沼系统中的淡水与咸水源，并监测地下水循环过程。研究团队对基尼烈湖流域内的淡水与卤水，以及形成于古湖体的晚中新世比拉组与盖舍尔组灰岩、白云岩样品，开展了镁、锶同位素比值与主量元素组成分析。箱式模型（box model）模拟结果显示，当海水蒸发至盐岩相阶段时，其对灰岩含水层的白云岩化作用可形成δ²⁶Mg值极高的卤水。该卤水形成了位于比拉湖东侧、构造运动形成的基纳罗特盆地内的高盐湖泽马赫湖。晚中新世时期泽马赫湖沉积了数千米厚的蒸发岩（盐岩）序列，这一沉积记录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，淡水与卤水的混合环境促使比拉组湖相灰岩的原生方解石沉积。该组灰岩的δ²⁶Mg值相对较高。在干旱期，比拉湖面积萎缩，泽马赫湖停止排放卤水，此时δ²⁶Mg值较低的地中海海水侵入并与比拉湖的半咸水混合，促使方解石泥发生白云岩化作用。这些湖相白云岩的δ²⁶Mg值低于其下伏的灰岩。随后在墨西拿期，该区域被盖舍尔组（Gesher Fm）的沼泽环境所覆盖。盖舍尔组白云岩的δ¹³C值偏低，这一证据表明此时的白云岩化作用由细菌活动驱动。氯化钙卤水的δ²⁶Mg值随时间呈下降趋势：从托尔顿期比拉湖的~1.2‰，降至墨西拿期盖舍尔沼泽的~0.5‰，再降至现代提比利亚温泉的0.15‰。这一趋势表明原始卤水持续被海水稀释（直至约530万年前的赞克尔阶海侵），之后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，或可适用于其他高盐卤水与低盐溶液发生镁交换的环境，例如晚新近纪地中海周缘典型的海相潟湖环境。此外，晚中新世地中海周缘边缘潟湖-湖相环境中不同盐度溶液的混合过程，可用以解释中生代海相浅海环境中的白云岩化作用条件。