Extreme aridities in the Middle East during the last interglacial revealed by the halite sections from the Dead Sea cores

Abstract:

Thick sections of halite (tens of meters) from the ICDP Dead Sea Deep Drilling Project core were precipitated during extremely dry periods, when the lake level was low and fresh water availability was scarce. The detailed description of the sedimentological record during periods of halite precipitation shows alternations between halite and mud with gypsum layers that are tens of cm to a few meters thick. The halite alternates between small cumulated crystals that are formed on the water surface and large bottom-growth crystals with low densities of fluid inclusion bands that are formed on the lake floor. The large crystals are associated with mud layers and show more mud between the crystals, while the small cumulate crystals have much less detritus and contain ‘halite rafts’, which are formed under high evaporation conditions.

The thick sections of halite cannot form only due to evaporation of the Dead Sea basin brines, and requires a continuous source of salt from fresh water input and brines discharging from the subsurface in order to facilitate the observed amount of salt over time. The salt is added and accumulated in the brines during glacials and wetter periods, when the lake level is high, and probably stratified, and then precipitates during drier periods, when the fresh water discharge into the lake decreases. Based on the chemical compositions of past Dead Sea lakes, from fluid inclusions in the halite (Na/Cl~0.3-0.6), we estimate that ~3-6 cm of halite precipitates per 1 m of lake level drop. The amount of salt in the core indicates periods of significant lake level drop (up to 100 m), represented by intervals of ~3 m of halite. The mud intervals between the halite segments indicate increases in the fresh water discharge, and possibly the formation of a stratified lake (from freshwater addition), which does not facilitate halite precipitation.

The discharge into the lake can be constrained according to the steady-state discharge with a certain lake level, which suggests a maximal water discharge of between 240 and 900 million m³/y. Assuming a linear relationship with precipitation, this suggests a 25-50% average precipitation relative to the present. Large variations in Br concentration in halite, Na/Cl ratios and Mg concentration in fluid inclusions, may suggest even lower lake levels, and drier conditions.