Continuous Dilution of the Ca-Chloride Dead Sea Bottom Brine during the Last Glacial – Evidence from Porewater from the ICDP Core

Abstract:

The major ions composition of porewater extracted from core catchers collected along the ~450 m of the ICDP Dead Sea Deep core represents the chemical composition of the deepest parts of the terminal-hypersaline lakes that occupied the basin over the last 220 ka. Profiles of the conservative elements Br^- and Mg²⁺ show that the deep brine underwent a steady long-term dilution during the time interval starting from the thick halite lithological section deposited at 123Kya (MIS 5e) and ending at the halite section deposited at <11Kya (MIS 1). The halite layers were deposited during extreme arid conditions during which lake level dropped dramatically and deep brine salinity rose substantially. The secular variation in Br^- and Mg²⁺, the occurrences of major halite deposition events correlates extremely well with the 100Ky Milankovich cycle and the global benthic δ¹⁸O LR04 stack paleoclimate proxy (Lisiecki and Raymo, 2005). The freshening of the lake between the two extreme arid events must have been accompanied by dramatic water level rise and the establishment of long term stratification of the water column. The dilution of the brine was also accompanied by increase in the Na/Cl ratio, indicative of halite dissolution, most likely from the salt diaper of Mt. Sedom, which at the high lake stand was in contact with the brine and therefore susceptible to dissolution. The halite dissolved during the high stand of the last interglacial supplied the Na and Cl for the precipitation of halite during the post-glacial extreme arid event. Freshening of the hypolimnion and rise in Na/Cl took place mostly through turbulent mixing between the epilimnion and the deep hypolimnic water. The turbulent mixing with fresher epilimnetic brine, dropped the density of the hypolimnion from 1.25 g•cm^-3 at 123Kya (MIS 5e) to 1.18 g•cm^-3 at 16Kya, suggesting that the density of the epilimnion was even lower. A sharp lake level drop sometime after 16Kya lead to salinity rise of the epilimnion which upon reaching ca. 1.18 g•cm^-3 resulted in complete overturn and mixing of the water column as evident from a sudden increase in the hypolimnion’s Na⁺ and Cl^- and a small drop in Br^- and Mg²⁺. The cycle ended with additional drop in lake level, salinity increase reaching precipitation of a halite layer at <11Kya.