Freshening of the Dead Sea during the Last Glacial Revealed By Porewater Composition in ICDP Dead Sea Deep-Drill

Abstract:

The geological evolution of the brine lakes that filled the Dead Sea basin has been extensively studied on the sedimentary exposures and drill cores on the Sea marginal terraces of the modern Dead Sea. These geological sections documented the history of the epilimnion (upper brine) of the hypersaline lake during its high stands periods. The cores drilled during 2011 by ICDP in the deep basin of the Dead Sea at water depth of 300 m provided the first opportunity to study the history of the deepest part of the hypolimnion (deep brine) by measuring the chemical and isotopic composition of pore-fluids. The vertical profiles of chloride (Cl^-) sodium (Na⁺) and oxygen isotopes (δ¹⁸O) in the pore brines revealed a substantial decrease in salinity of the hypolimnion during the high stand of the last glacial Lake Lisan (the last glacial predecessor of the modern Dead Sea), particularly during MIS2 (~31-17 ka BP). Diffusion-deposition model indicated that Cl^- concentration of the deep hypolimnetic brine decreased gradually to less than 2/3 of its present value. The δ¹⁸O at the same time increased to maximum of ~7‰ (3‰ higher than today). Beforehand, during the interglacial and later during the post-glacial and the Holocene the Cl^- concentrations and δ¹⁸O values were similar to those of the modern Dead Sea. The slow dilution of the deep Ca-chloride brine was caused probably by continuous turbulent mixing of the hypolimnion with the less saline high δ¹⁸O epilimnetic brine, across the epilimnion/hypolimnion interface (EHI). The increase in δ¹⁸O during the salinity decrease of Lake Lisan was a result of evaporative fractionation of the less saline epilimnetic brine. The post-glacial δ¹⁸O decrease while salinity increased is attributed to the “reversed” δ¹⁸O fractionation during evaporation of very high salinity brine. Increase in the Na/Cl ratio due to dissolution of halite without reaching halite saturation was also observed during the freshening period.