Physics of evaporation

In its broadest definition, evaporation is the process by which molecules in a liquid (water) spontaneously become gaseous (water vapour) and escape the liquid state, while evaporites are the resultant mineral precipitates accumulating in and around an increasingly saline residual brine that has reached a state of supersaturation with respect to a particular mineral salt or salts. Water molecules in the liquid phase are in continuous motion and so will collide. As they collide, they transfer energy to each other in varying degrees, based on how they collide. Evaporation, then, is a simple matter of solution kinetics in this milieu of molecular motion and is a response to varying degrees of heat absorption at the molecular scale.

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The process of evaporation

In its broadest definition, evaporation is the process by which molecules in a liquid (water) spontaneously become gaseous (water vapour) and escape the liquid state, while evaporites are the resultant mineral precipitates accumulating in and around an increasingly saline residual brine that has reached a state of supersaturation with respect to a particular mineral salt or salts. Water molecules in the liquid phase are in continuous motion and so will collide. As they collide, they transfer energy to each other in varying degrees, based on how they collide. Evaporation, then, is a simple matter of solution kinetics in this milieu of molecular motion and is a response to varying degrees of heat absorption at the molecular scale.

Vapour pressure

Vapour pressure controls the rate of evaporation. The rate of evaporation slows as the air space above an air-brine interface becomes more saturated with water molecules. Evaporation continues until an equilibrium is reached when the evaporation of the liquid is the equal to its condensation.  When  vapor pressure exceeds the thermodynamic equilibrium value, condensation occurs in presence of nucleation sites. The equilibrium situation defines vapour pressure. That is, vapour pressure  is defined as the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. Equilibrium vapour pressure is an indication of a liquid's evaporation rate. A substance with a high vapour pressure at normal temperatures is often referred to as volatile. 

Water vapour pressure over a free-standing brine decreases with increasing salinity and increases with increasing temperature. Saturated water vapour pressure over any salt solution is always lower than the vapour pressure of pure water at a given temperature. This is due to the binding of water molecules into hydration sheaths about the various charged ions dissolved in the brine. As the concentration of a surface brine increases, the water vapour pressure decreases in the overlying air mass, reaching its lowest value when the brine is saturated. At very high salinities the pressure difference between the brine and the air immediately above the brine can become so small that evaporation effectively shuts down and the higher salinities necessary to precipitate the bittern salts may not be reached. Hence, wind speed is a significant factor in a brine pan’s chemistry.

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Salinity, brine density versus vapour pressure)above a standing brine, replotted from Bonython, 1966)

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Evaporation factor (freshwater is 1) decreases as salinity/density of the brine increases. Evaporation factor is applied to measured pan evaporation to attain a more realistic estimate. For example, a seawater brine with a salinity around 200‰ has an actual evaporation rate that is only 85% (0.85) of the measured pan evaporation rate.

Aerodynamically, the evaporation rate of a surface brine is the product of wind velocity and the difference in water vapour pressure between the main air mass and the air above the surface of the evaporating brine. Complete humidity-induced shutdowns of evaporite precipitation are rare as the air above an evaporating brine body is usually in continual motion, driven by adiabatic, catabatic and onshore breezes. The salinity of a freestanding brine is a function of wind speed, surface roughness and atmospheric stability in the air above the brine (Figure 2.8a; Myers and Bonython, 1958; Bonython 1966). Elevated above-brine humidity and limited lateral extent (tens of kilometres) create a natural buffer to evaporation above modern coastal lakes or sabkhas. Lower salinity evaporite minerals, gypsum and halite, are the dominant precipitates in modern coastal lakes beneath humid marine air, such as the coastal pans of the Saudi Gulf coast and the coastal salinas of southern and western Australia. Gypsum with minor anhydrite is the dominant salt in both the capillary zone and the phreatic zone of most modern arid coastal hydrologies such as the sabkhas of Abu Dhabi (Kinsman, 1976). Any efflorescences or crusts of bittern salts do not last long, and are recycled or washed away by occasional rainstorms and sheet floods.

Density

As seawater concentrates, the first mineral to precipitate is CaCO3, usually as aragonite. This begins in mesohaline waters where the brine reaches twice the concentration of seawater (40 to 60‰) and achieves a density ≈1.10 gm/cc. As the brine continues to concentrate and approaches four to five times the concentration of seawater, that is 130 to 160‰, gypsum precipitates from penesaline waters with densities around 1.13 gm/cc. At 10 to 12 times the original seawater concentration (340 to 360‰) and densities around 1.22 gm/cc, halite drops out of supersaline marine waters. After halite, the bittern salts (potassium or magnesium sulphates/chlorides) precipitate from supersaline waters at concentrations that are more than 70-90 times that of the original seawater. Density increases drive the process of brine reflux in sediments below an accumulating salt bed.

Specific heat

The specific heat is the amount of heat (joules) per unit mass (1kg) required to raise the temperature by one degree
Increasing the concentration of the salt in a brine decreases the specific heat capacity of the brine. When we dissolve NaCl in water, the ions are held in a rigid cage of water molecules. This cage is rigid enough so that the motions of its molecules are "frozen out". The added heat cannot go into increasing the energy of these frozen motions, so it goes into increasing the energies of the other water molecules in the solution. And so it takes less energy to activate these water molecules per volume, so the specific heat and temperature of the water decreases for a given solar energy input (this creates heliothermy in a stratified brine). The higher the concentration of NaCl, the lower the specific heat capacity of the solution.

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Density increases with concentration (measurements taken in seawater-fed salt pans)

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Specific heat capacity in a brine decreases with increasing concentration. In a brine lake this results in density stratification or heliothermy, where a hotter denser more concentrated brine layer underlies a cooler, less dense brine layer.

Heliothermal brine and density stratification

Increasing density and heliothermy go hand-in-hand during the primary formation of all bedded evaporites. Responses include dense brine sinking into and replacing less dense brine in the underlying sediments. This set of processes come under the heading of brine reflux diagenesis. A combination of thermal and density stratification (either short term or long term - meromixis versus holomixis) creates distinctive textures and mineralogies in most evaporite beds. Brine stratification occurs today in perennial saline lakes in the African Rift Valley, the Dead Sea, Israel and in marine-fed coastal salinas in southen and western Australia.

Lake Hayward, Australia

Lake Hayward, a coastal salina in Western Australia, typifies many such stratified brine system and has ionic proportions in the bottom brines similar to that of seawater. Carbonate muds are accumulating on the microbially bound floor of its small perennial brine pool (2-3 m deep) under a limnology that is density and thermally stratified for much of the year (Figure; Rosen et al., 1995). Meteoric inflow creates a well-defined longterm mixolimnion; this upper, less dense and cooler water mass has salinities ranging from 50,000 to 210,000 ppm. It exists from late autumn to early summer (May to February) as it floats atop a lower denser and warmer water mass (monimolimnion) with salinities in the range 150,000 to 210,000 ppm (). Stratification disappears for a few months each year from midsummer to mid-to-late-autumn (Figure).

During summer the waters of the upper water mass evaporate and concentrate as their bicarbonate content steadily increases. From the onset of stratification in late autumn, across mid winter and on into to early summer the temperature trends of the two water masses are parallel. They form a heliothermic system where the lower water mass is some 15-20°C hotter than the upper water mass. By mid summer (e.g. January 1992) lower water mass begins to cool, while the temperature of the upper water mass continues to rise. Once the temperatures (and densities) of the two water masses equalise they mix as the lake overturns. Waters of the lower water mass (the monimolimnion) now come into contact with the atmosphere once more. While the water masses are stratified, the chemocline and the thermocline are sharply defined across a 10cm interface with a salinity contrast that may be as much as 135,000 -140,000 ppm and a temperature difference of up to 19°C.

The time of mixing is immediately preceded by a sharp fall in the level of bicarbonate in the mixolimnion, suggesting the precipitation of calcium carbonate (mostly aragonite) occurs in the upper water mass from late summer to autumn (FigureA; Rosen et al., 1995). The lower water mass in Lake Hayward was supersaturated with respect to gypsum and anhydrite from late 1990 to February 1992 (FigureB; Rosen et al., 1996). When the lake mixed from January 1991 to May 1991 and from March 1992 until early May 1992, the entire water body was at gypsum saturation, but slightly undersaturated with respect to anhydrite. A ‘whiting’ (a cloudy white appearance to the water body) was observed in the lake in March 92, just at the time of first mixing of the lake. Analysis by scanning electron microscopy (SEM) of the collected filtrate indicated that the ‘whiting’ was composed of gypsum and diatom tests, At that time, there was also a thin (10-20 mm) crust of gypsum on the lake floor. After the ‘whiting’, when the water was unstratified, both the monimolimnion and mixolimnion were near saturation with respect to gypsum.

After mixing, the stratification begins to reform with the next influx of meteoric waters onto the lake surface. In the early stages of setup leading into the longterm stratification it appears stratification is not stable and the water bodies may remix before stable stratification sets in by mid winter (e.g. May-June 1992; Figure).
Lake Hayward illustrates an aspect of pelagic evaporite precipitation that is probably more generalised in many “deepwater” density-stratified settings. Namely, that evaporite crystals grow on the bottom only when the water column is unstratified. This is so even when the lower water mass is at saturation almost year round (see stratification hydrology and meromixis).

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Lake Hayward is one of three seasonally meromictic interdunal lakes in the Yalgorup wetland, (Clifton, Preston, Hayward) located on West Australian coastline, southeast of Perth
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Hydrogeochemistry of Lake Hayward waters, Western Australia (after Rosen et al., 1995, 1996; Warren, 2016; Turner et al., 2019)  A) Plot of temperature regime in upper (mixolimnion) and lower (monimolimnion) water masses and bicarbonate content. As the lake mixes, the thermal and density stratification disappears. Immediately prior to mixing the bicarbonate concentration decreases suggesting precipitation of calcium carbonate (aragonite) (after Rosen et al., 1995). B) Saturation state with respect to gypsum. The monimolimnion was saturated with respect to gypsum from summer 1991 to autumn 1992 and was again near saturation at the start of summer 1992-1993. The mixolimnion was only saturated when the lake water column was homogeneous .