Halite Precipitation From Double‐Diffusive Salt Fingers in the Dead Sea: Numerical Simulations

Abstract

We employ direct numerical simulations in order to analyze the role of double‐diffusive salt fingering in halite precipitation from hypersaline lakes. Guided by field observations from the Dead Sea, which represents the only modern deep stratified lake that precipitates halite under hydrological crisis, we consider a saturated layer of warm, salty brine (epilimnion) overlying a layer of colder, less salty brine (hypolimnion) that is also saturated. The double‐diffusive instability originating in the metalimnion gives rise to an asymmetrical pattern of less salty ascending fingers, accompanied by descending salt fingers that lose heat as they propagate through the metalimnion. The net result is a strong, downward salinity flux that leads to the undersaturation of the epilimnion, while the hypolimnion becomes oversaturated and precipitates halite. These interfacial processes within deep, hypersaline water columns in warm and dry regions suggest a potential route toward the formation of thick halite layers found in the geological record.

Hydroclimatic Controls on Salt Fluxes and Halite Deposition in the Dead Sea and the Shaping of “Salt Giants”

“…As a result of this thermocline sharpening, the maximum vertical temperature gradient increases southward from 1°C/m to 8°C/m (Figures 3f and 3i). The halite crystallization rate below the thermocline, which reflects the preferred downward transport of salt over heat by double-diffusive convection from the epilimnion to the hypolimnion (Ouillon et al, 2019;Sirota et al, 2016), doubles from 0.2 mm/day in the northern part to 0.4 mm/day in the south (Figure 3j and the supporting information). At no location did we observe halite to crystallize above the thermocline, as increasing temperature and downward double-diffusive salt flux leave the epilimnion undersaturated (Arnon et al, 2016;Sirota et al, 2016).…”

“…The coefficients k S and k T represent salt and heat diffusivities, and the diffusivity ratio is . The stratified system is unstable to the double‐diffusive fingering instability when ; such a system, interestingly, generates a faster convective downward flux, from the epilimnion to the hypolimnion, of the slow diffuser (dissolved salt) compared with a slower flux of the fast diffuser (heat flux) (Arnon et al, 2016; Ouillon et al, 2019), that is, , where F T and F S are the downward heat and salt flux, respectively. Hence, the fingering regime reduces halite saturation at the epilimnion leading to undersaturation, while maintaining supersaturation and halite deposition at the hypolimnion (Ouillon et al, 2019; Sirota et al, 2016).…”

“…The stratified system is unstable to the double‐diffusive fingering instability when ; such a system, interestingly, generates a faster convective downward flux, from the epilimnion to the hypolimnion, of the slow diffuser (dissolved salt) compared with a slower flux of the fast diffuser (heat flux) (Arnon et al, 2016; Ouillon et al, 2019), that is, , where F T and F S are the downward heat and salt flux, respectively. Hence, the fingering regime reduces halite saturation at the epilimnion leading to undersaturation, while maintaining supersaturation and halite deposition at the hypolimnion (Ouillon et al, 2019; Sirota et al, 2016). Halite deposition at the hypolimnetic lakefloor (Sirota et al, 2017) slightly reduces the salinity of the residual brine (reduction of ~0.1% from the total salinity, (Sirota et al, 2016)), leading to upward buoyancy flux that mixes the hypolimnion, but does not impact on the thermocline and the overlying epilimnion.…”

“…This configuration triggers double‐diffusive fingering, an instability that evolves in stably stratified environments where two scalars contribute to the density in opposite ways, and diffuse at different rates (Stern, 1960; Turner, 1974). In the Dead Sea, heat is the stabilizing fast diffuser, while salinity is the destabilizing, slow diffuser (Anati & Stiller, 1991; Arnon et al, 2016; Ouillon et al, 2019). Linear stability theory (Radko, 2013) indicates that the emergence of double‐diffusive fingering depends on the stratification stability (density) ratio (Figure 2c), that is, the relative contributions of temperature and salinity in determining the vertical density gradient, where T z and S z denote the vertical temperature and salinity gradients, respectively, with α and β being the thermal and salinity expansion coefficients.…”

The demise of a ‘salt giant’ driven by uplift and thermal dissolution

“…The influence that temperature and heating of the upper water column has on saturation during warmer summer periods overcomes the increasing salinity, resulting in undersaturation of the epilimnetic brine (Sirota et al, 2016). This allows a flux of dissolved salt from the warmer and saltier epilimnion to the less salty and colder hypolimnion, via double diffusion salt fingering across the thermocline (Stern, 1960;Arnon et al, 2016;Ouillon et al, 2019). The result is salt super-saturation in the hypolimnion coeval with salt under-saturation in the epilimnion.…”