Thermohaline stratification and double diffusion diapycnal fluxes in the hypersaline Dead Sea

Abstract

The Dead Sea is a hypersaline terminal lake, experiencing negative water balance, increasing salinity, and NaCl (halite) crystallization. We observed atypical evolution of the thermohaline stratification in comparison to most lakes due to the role of salt crystallization and diapycnal fluxes across lake layers. We characterized the dynamics of the thermohaline properties of the lake strata through high‐resolution continuous measurements of temperature profiles, novel water sampling methods, and observation of vertical profiles of salt crystallization. The diapycnal fluxes across the metalimnion were explained by Double Diffusion (DD) salt fingering driven by instability between warmer saltier water above cooler less salty water. The DD flux is associated with: (1) sharpening of the metalimnion from a 20 m wide transition in early summer, to staircase, ultimately merging to a single sharp sub‐meter step, (2) salinity decline from the epilimnion starting from mid‐summer synchronous with increasing salinity and temperature of the hypolimnion, and (3) active halite crystallization in the hypolimnion. We hypnotize that the salt fingering mechanism in saturated brines reveals a unique asymmetry; i.e., the descending cooling fingers become supersaturated and crystallize halite, whereas the ascending warming fingers becomes undersaturated. The DD flux in the Dead Sea is shown to be fundamental in the dynamics of stratification, providing a framework for general understanding DD flux in hypersaline environments. The finding that the epilimnion experiences seasonal halite undersaturation whereas the hypolimnion continuously precipitates salt by DD flux, has wide implications on the understanding of the dynamics of deposition of evaporitic rocks.

Seasonal dynamics of internal waves governed by stratification stability and wind: Analysis of high‐resolution observations from the Dead Sea

“…Interestingly, concurrent with the internal waves pycnocline fluctuations, double diffusion salt fingering also acts across the pycnocline interface in the Dead Sea, effectively advecting heat and salt between the strata (Arnon et al ). This phenomenon includes staircases of density at the interface that are likely to be affected by the internal waves fluctuations.…”

“…The samples were kept warm to avoid salt precipitation prior to laboratory analysis. Accurate density measurements were critical for this study and were performed in the laboratory (DMA 5000) at 30°C, with an accuracy of ±0.005 kg m −3 (Gertman et al ; Arnon et al ; Sirota et al ). The density difference between the epilimnion and hypolimnion is based on the difference of samples from 10 and 50 m. This difference is positive under holomictic conditions, as presented in this study.…”

“…Ignoring chemical composition variations is justified for short‐term simulations (days to weeks) that are needed for internal waves simulations. However, in longer simulations (months to years), it would be preferable to account for halite precipitation and dissolution (Arnon et al ; Sirota et al , ) and for double diffusive fluxes (Arnon et al ); both of these are beyond the scope of this article.…”

“…C,D) and diminishes toward the winter due to surface cooling (Gertman and Hecht ;Arnon et al ; Sirota et al ). The temperature, salinity, and density of the upper layer are governed by atmosphere–lake interactions, salt precipitation, and double diffusion diapycnal fluxes; Arnon et al () distinguish between these governing factors, showing that in mid‐summer and late summer, double diffusion plays a major role in reducing salinity from the epilimnion, whereas halite precipitation plays a major role in autumn and winter (Arnon et al ; Sirota et al ). Throughout the seasonal cycle, temperature plays the major role in controlling the density of the upper layer and thus on the density gradient (Fig.…”

Seasonal variations of halite saturation in the Dead Sea

“…Until the 1980s, the lake was meromictic with a stable diluted epilimnion [ Neev and Emery , ]. Since then, as a response to the negative water balance, the level has declined at the rate of ∼1 m/yr [ Lensky et al ., ] and the salinity of the epilimnion increased, leading to holomictic conditions, with summer stratification of warmer and saltier stable epilimnion (depth of thermocline ∼25 m) and a vertically mixed water column during winter [ Steinhorn , ; Anati et al ., ; Anati , ; Gertman and Hecht , ; Arnon et al ., ]. The maximum temperature and salinity differences between the epilimnion and hypolimnion during summer reaches 12°C and 2.5 kg/m 3 , respectively (salinity units are presented here in quasi‐salinity units σ 30 , kg/m 3 , see methods).…”