The Story of Saline Water in the Dead Sea Rift – The Role of Runoff and Relative Humidity

Abstract

Saline waters have been found in the subsurface of the Dead Sea basin along the area stretching from Lake Kinneret in the north to the Timna area in the south. The maximum salinity of the waters reaches 340 g L⁻¹, and their chemical composition is Ca-chloridic, like most subsurface brines in sedimentary basins elsewhere. It is now generally accepted that such waters evolved from ancient seawater via a three-stage mechanism, namely: (1) seawater evaporation in a marine lagoon; (2) modification of the resulting brine by water-rock interaction; (3) dilution of the brine by freshwater or mixing with other saline fluids.

The age of the parental seawater from which the salinity of the brines was derived is Late Miocene to Early Pliocene. All saline waters in the Dead Sea basin display Na/Cl ratios below that of seawater (0.86), indicating that their evolution should be tied to formation of rock salt bodies; indeed, such were actually found in the subsurface of the two tectonic depressions of the Dead Sea and Lake Kinneret (Sea of Galilee). It is likely that the ancient lagoon from which the seawater evaporated and in which the salt deposited was approximately 300 km long and 20 km wide.

The Na/Cl ratios in the brines and in fluid inclusions in halite crystals formed from these waters cover a wide range below the seawater value (0.86), reaching values as low as 0.1 eq/eq (All solute ratios in the paper are presented in equivalent units unless otherwise stated).

The maximum degree of evaporation that an aqueous solution may reach depends, amongst other things, on the relative humidity. Because evaporation may proceed only as long as the activity of water in the evaporating solution (a _H2O) is higher than the relative humidity above it, it is possible to estimate the maximum relative humidity that prevailed in the area during the deposition of the salt from the Na/Cl ratio in a given brine or in fluid inclusion enclosed in a halite sample.

The Pliocene lagoon waters that, at that stage, had high Mg/Ca ratios, started to migrate outwards from the basin towards the west, under the local hydraulic head, through the Judea group limestone layers of upper Cretaceous age which comprise in that area the margin of the basin. During their passage, the brines interacted with limestone beds resulting in discordant dolomite bodies. The resulting, brine accumulated at depth in the Northern Negev.

Upon decline of the regional hydraulic head, the waters that infiltrated out to the west reversed their flow backwards to the basin, now displaying a chemical composition significantly different than that on their way out. The newly acquired composition is thus characterized by low Mg/Ca and SO₄/Cl ratios, of ~0.5 and ~0.01, respectively, and is defined as R₁ water.

At some later time the lagoon was cut off from the sea, and its area was transformed to a lacustrine environment, allowing for other processes to take place which modified the composition of the lagoonal brines. As from the lagoon’s cut off from the sea, the contribution of dissolved marine salts to the basin was substituted by freshwater solutes carried in by runoff.

The freshwater that started to feed the saline lake(s) was saturated with respect to CaCO₃ minerals (calcite and aragonite), deposited its entire load of dissolved Ca²⁺ as CaCO₃ and CaSO₄ in the lake. Additional Ca²⁺ to compensate for the excess SO₄ + HCO₃ over Ca²⁺ was borrowed from the saline, Ca-chloridic brine in the lake, bringing about a marked increase in the Mg/Ca ratio therein. The present paper presents a model (Katz and Starinsky, Aquat Geochem 15:159–194, 2009) that describes the relation between the increase in the Mg/Ca ratio of the lake and the accumulated mass of CaCO₃ (calcite or aragonite) that was deposited in it. The evolving saline waters affected by this process, with Mg/Ca ratios >1, are defined as group R₂ waters.

An additional, significant modification of the brine inflicted by its passage from a marine lagoon to a lacustrine water body is reflected by the ⁸⁷Sr/⁸⁶Sr isotope ratio. We inspect two cases relevant to this question, the first in the Lake Kinneret area and the second in the Dead Sea basin.

The R₂ lacustrine waters on the eastern side of Lake Kinneret show ⁸⁷Sr/⁸⁶Sr isotope ratios around 0.706, contrasting in this regard with the R₁-type waters on the western side of the lake which are characterized by ⁸⁷Sr/⁸⁶Sr isotope ratios of ~0.708. We propose that the ⁸⁷Sr/⁸⁶Sr ratio transition 0.708 → 0.706 is driven by addition of freshwater with low ⁸⁷Sr/⁸⁶Sr ratios (0.704–0.707) originating in the runoff flowing to the lake over the basaltic terrain from the NE (⁸⁷Sr/⁸⁶Sr ~ 0.704).

In the southern, Dead Sea area, the lacustrine R₂ waters were fed by runoff with ⁸⁷Sr/⁸⁶Sr ~ 0.708 and, therefore, remained unchanged in this respect.

The Timna water composition is a result of interaction between diluted subsurface Ca chloride brines that originated as group R_S1 in the Dead Sea area with basic igneous rocks. The depletion of Mg²⁺ in the water is due to a reaction of destruction of Ca- plagioclase and formation of Mg²⁺ rich mixture of epidote and chlorite. The Sr isotope signature of ~0.706 was formed by exchange of high ⁸⁷Sr/⁸⁶Sr (~0.708) brines with low ⁸⁷Sr/⁸⁶Sr (~0.7045) igneous basic rocks, like olivine norite. The age of the water-rock interaction is estimated to be older than the R_s brine formation, i.e. <3–4 m.y.