Geochemical evolution of the Dead Sea brine during the last glacial cycle from chemical compositions of interstitial soluble salts

Abstract

The chemical evolution of the brines that filled the Dead Sea Basin during the final phase of the last interglacial and throughout the last glacial period is constructed from the temporal variations in the Na/Cl, Mg/Cl, Br/Cl, and Br/Mg ratios of soluble salts that were extracted from cores that were drilled at the modern Dead Sea floor and from sediments of the last glacial Lisan Fm. that are exposed at the high terraces above the modern Dead Sea (covering the ∼ 100–14 ka period).

The main observations are: (1) Variations in the Na/Cl ratios reflect processes of halite precipitation/dissolution during arid/wet periods in the drainage basin, respectively, and exchanges between the upper and the lower brines; (2) Ions of Na⁺ and Cl^- were supplied to the brines by the dissolution of halite from the Mount Sedom salt diapir, and of halite deposits, which precipitated in the lake during the hyper-arid periods of the last interglacial; (3) The upper brine of Lake Lisan was sensitive to the hydroclimatic conditions in the lake’s drainage basin, since being directly exposed to the atmosphere, and to variations in flows of freshwater and groundwater brines, which had distinct influence on its chemical record. At the same time, the lower brine was isolated from the surroundings, by the overlying brine and received chemical inputs only by restrict mixing between the brines which buffered the intensity of regional hydro-climatic events.

Temporal patterns in the Na/Cl ratios indicate that between ∼ 100–30 ka, the deep brine evolved through a moderate but steady “enrichment” by Na + and Cl- ions due to continuous dissolution of halite by the upper brine of the lake. Towards the end of this period, between ∼ 43–30 ka, the Amiaz plain, a marginal basin, comprised a semi-isolated water-body that witnessed frequent episodes of halite precipitation/dissolution with temporal patterns that resemble millennial temperature (δ18O) variations in the Greenland ice core.

Between ∼ 30–18 ka (MIS 2), when Lake Lisan reached its highest stands and maximum spatial expansion, the soluble salts from the deepest basin and from the high sections of the Lisan Fm. indicate on centennial dissolution cycles of the Mount Sedom salt diapir that took place in the upper brine and sank down to the lower brine. During the ∼ 18–14 ka decline of the lake to its low levels of the Holocene, the elemental ratios reveal several episodes of enhanced supply of freshwater to the shrinking lake, causing intensified halite dissolution and supply of Na⁺ and Cl^- ions to the deep basin, setting the grounds for massive halite precipitation, which initiated after a sharp level decline at ∼ 14 ka.

The long-term (∼100–14 ka) temporal pattern in the elemental ratios of the lower brine resembles global CO2 concentrations and sea surface temperature trends, while the short-term fluctuations in the upper brine are correlated with short warm/cold cycles in the Greenland ice core δ18O data, indicating a strong impact of the global climate.