Geochemical History of the Dead Sea

Abstract

A Southward view of the Dead Sea western coast. The steep western escarpment of the Dead Sea basin, composed mainly of Upper Cretaceous limestone and dolomite, can be seen on the right. Beach terraces left by the shrinking lake run parallel to the shore. The larger part of the area between the present water line and the mountains was still under Dead Sea water just 50–60 years ago. The current fall of the lake’s stand is around 1 m year−1. Three on-shore sinkholes can be seen in the front of the photo, as well as two submerged ones near its lower left corner. These were caused by dissolution of a Holocene salt layer located tens of meters below the surface, resulting in the collapse of the overlying sediments. The retreat of the Dead Sea in recent years was followed by eastward migration of the freshwater–brine interface. This in turn brought diluted groundwater in contact with the subsurface salt layer, triggering its dissolution, and is considered as the culprit of the spreading phenomenon. Many sinkholes contain brine that was left over by the receding Dead Sea and was trapped within the surrounding sediments. Once inside a sinkhole, these brines evolve chemically by evaporation to various degrees. The difference in color between the brine in the onshore sinkholes reflects salinity-related differences in their biology. The evolution of the Dead Sea basin (DSB) brines from their birth in a Pliocene lagoon to their accommodation in the Dead Sea is described and discussed. The history of the brines is divided into two periods, corresponding to the successive depositional environments that prevailed in the DSB, namely a marine lagoon and an inland saline lake. Ancient Mediterranean seawater, supplied into the DSB lagoon through an inland channel from the north, was concentrated by evaporation into the halite field. The resulting Mg-enriched solution dolomitized surrounding, upper Cretaceous limestone, losing most of its Mg2+ to the limestone in exchange for Ca2+. Cretaceous marine Sr2+, concurrently released from the limestone into the brine, lowered its (Pliocene time) 87Sr/86Sr ratios. Consequently, a Ca-chloridic solution with lowered 87Sr/86Sr ratios was formed. Frequently changing conditions along the active Dead Sea rift enabled back-flow of the Ca-chloridic brines to the DSB, where they mixed with fresh seawater, unloading into the mixture their limestone-Sr. This process is reflected by the 87Sr/86Sr ratios (0.7082–0.7087) in the lagoon’s gypsum, dolomite, aragonite, and halite, which is intermediate between that of Pliocene seawater (0.709) and that in the upper Cretaceous limestone (0.7074–0.7077). Disconnection of the lagoon from the ancient Mediterranean brought about its end, and opened the (ongoing) lacustrine chapter of the DSB, without interrupting the reflux of Ca-chloride brine back into the basin. By that time, the chloridic brines processed by the lagoon became locked in a large, almost closed system reservoir in the DSB and vicinity. We propose that a Ca-chloridic lake, the DSB Lake, which was recharged by fresh runoff and by the returning brine, was born and existed in the DSB as of that time. Based on oxygen isotope data, the origin of the freshwater H2O was in Mediterranean rain. Geological evidence and theoretical calculations constrain the frequent fluctuations of the DSB Lake stand within a minimum of 430–450 and a maximum of 165 mbsl, and its corresponding salinity shifts to within 90 and 340 g l−1, relating these extremes to specific points in time and in the rock sequence. Stratification of the water column, brought about by increase in the inflow/evaporation ratio of the DSB Lake, was a frequent and normal situation, and accounts for the abundant, fine aragonite–detritus lamination in large parts of the DSB section. It is shown that aragonite laminae thicker than 0.1–0.2 mm are not annual deposits but accumulated during several years. Dissolved bicarbonate accumulated in the upper part of the rising lake water column, and was used up for aragonite crystallization upon the subsequent lake decline. Detritus laminae were formed during the respective winter seasons. While remaining Ca-chloridic throughout, the DSB Lake’s composition changed in time as a result of: (a) mixing with fresh waters; (b) removal of the SO4 2− and HCO3 − imported into the lake in CaCO3 and CaSO4 minerals. A mass-balance model, explaining the change in the Mg/Ca ratio in the DSB Lake from its initial value (~0.16) is proposed, and its results are compatible with the composition of the saline springs. It demonstrates that the change in the Mg/Ca ratio in the DSB Lake is mainly caused by removal of Ca from the lake, required to compensate for the Ca < (SO4 + HCO3) relationship in the inflow freshwaters. The Mg/Ca ratio in the Dead Sea fluctuated between 4.2 and 4.6 during the last 50 years or so. Insufficient resolution makes it impossible to determine whether the observed changes vary systematically in time. Saline spring waters flow into and mix with the lake, evaporating together, thereby contributing their salts and changing the Dead Sea’s composition. The change is expressed in depletion of Ca, and in the enrichment of the lake in Mg, K, and Br, which do not form independent minerals therein. An attempt to predict the future of the Dead Sea is presented based on the chemical composition of brine in recent sinkholes developing along the Dead Sea coast, as well as on thermodynamic modeling of the Dead Sea brine evolution. The sinkhole brines are compatible with evaporative evolution of Dead Sea water and display salinities up to ~550 g l−1, within the bischofite range. Thermodynamic simulation predicts as well, that under current conditions Dead Sea water may evaporate to a level of not much below 550 mbsl. Simulated evaporation of Dead Sea water to a concentration factor of 50 yielded a 90 m thick column of chloride minerals containing halite, carnallite, bischofite, tachyhydrite, and CaCl2·4H2O (by that order).