The Dead Sea negative water budget together with low average annual regional relative humidity (<50%) may push the brine toward carnallite (KMgCl₃·6H₂O) precipitation. While geochemical models focused on the Dead Sea evaporation, there has been little investigation to how brine composition will respond to change in climate once near-equilibrium is reached. To address this, we carried out preliminary natural evaporation field experiments on a series of brines from 02/2025 to 11/2025 at the hyperarid Nahal Og beach (north-west Dead Sea), and monitored the local climate, and brine geochemical and physical parameters.
From late winter to late spring/early summer (May/June) the brines’ most conservative element, Li⁺, concentrated monotonically, while densities rose from 1.25 to 1.40 kg L⁻¹. Evaporation of the brines was enhanced due to the daytime heating of the evaporating brines in experimental beakers that were exposed to direct solar radiation. The water activity (α(H₂O)), a ratio of the partial pressure of vapor above the brine at equilibrium relative to partial pressure of vapor above pure water at equilibrium at the same temperature, showed a continuous decrease from ~0.6 to 0.3, approaching the late season mean atmospheric relative humidity (RH) calculated for the surface water temperatures (st) in the experimental beaker i.e., α(H₂O)≈ RHₛₜ/100. Various X/Li ratios reveal the chloride-evaporite sequence of halite (NaCl), carnallite and bischofite (MgCl₂·6H₂O) precipitation. As α(H₂O) approached mean RHₛₜ (ca. June) and water loss changes became insignificant, relatively significant brine density decreases and dilution of dissolved Li⁺ concentrations were observed. We interpret this reversal as a combination of condensation of atmospheric water onto the brine surface and dissolution of previously precipitated bischofite that releases hydration water back into the brine H₂O reservoir. This study shows that major geochemical and physical changes in brines that precipitate hydrated chloride minerals can occur at near unchanging water level.