Subaqueous perennial (holomixis and meromixis)

Surface waters within freestanding perennial brine lakes or seaways routinely fluctuate between stratified and unstratified conditions. In the stratified system, an upper fresher (less dense) water mass sits atop a more saline (denser) lower water mass. The narrow zone of transition between the two masses in the brine column is called the halocline. Often a stratified saline lacustrine system is also thermally stratified (heliothermal), with warmer waters in the lower water mass and a zone of transition is called the thermocline. Its position is typically identical in the column to that of the halocline. Thermal stratification with a cool top and a hot base is opposite to that found seasonally in many temperate freshwater lakes and so is sometimes called reverse stratification and also described as heliothermal layering.

Water mass zonation in a perennial brine lake or seaway - meromictic versus holomictic

The term meromictic is used to describe a permanently stratified water mass where surface layers may mix, but the bottom tier does not disappear. The upper water mass that periodically mixes is the mixolimnion, and the lower permanent mass is the monimolimnion. Oligomictic is used to describe stratified water masses that mix or homogenise for short irregular periods every few years.

Mixing in a saline water mass is controlled by the evaporation and concentration of the upper water mass until it reaches a density equivalent to the lower water mass. Then mixing or overturn occurs. Substantial density differences between the upper and lower masses in most density stratified systems means diffusive mixing across the halocline is insignificant. A stratified system is a stable system, so that when a hypersaline water mass is density-stratified, there is little bottom precipitation of salts.

Sedimentation in the central basin in a density-stratified setting is mainly by a pelagic rain of crystallites, which first formed either in the uppermost part of the upper water mass or by brine mixing at the halocline. Whenever the upper and lower water masses equilibrate and homogenise, bottom nucleation of salts is possible even at the base of deep brine columns, as is occurring in the Dead Sea today. Whenever a homogenised water mass restratifies, the rate of brine reflux into sediments below the sediment brine interface slows and ultimately stops, for there is no ongoing mechanism to resupply brines denser than pore waters in the substrate.

Hydrological layering, and it s long term stability, controls the type of sedimentary textures of the evaporites

Holomixis permits the deposition of a coherent salt layer across the whole basin floor, beneath both shallow and deep brine columns. Density stratification allows evaporitic salts to crystallise only in the upper water mass or at the brine-brine interface, so bottom nucleation tends to occur on the shallower lake floor, where it lies above the halocline. That is, long-term (ectogenic) column stratification means bottom nucleation of salts can only occur where the upper salt-saturated brine-mass intersects the sediment bottom, with a pelagic settling of salts occurring deeper out in the depositional basin, as in the Dead Sea before February 1979 (Warren, 2016). The bottom growth of crystals cannot happen on a deep bottom located beneath a density-stratified system as there is no mechanism to drive ongoing supersaturation in the lower water mass. For the same reason, constant brine reflux driving sinking of a dense brine into sediments beneath the floor of the evaporite basin can only proceed if significant regions of the overlying brine mass are holomictic.

When salts are accumulating beneath a holomictic brine mass, textures in bottom nucleates is controlled by the stability of the overlying brine column. When the overlying column is deep (>30-100m) then, other than areas on the deep bottom of local phreatic spring-fed outflows, there is no general hydrochemical mechanism to drive fluctuations in bottom-brine chemistry. The resulting deep bottom precipitates tend to be monomineralogic crystal clusters, possibly encased by re-transported material washed in from the shallower surrounds. In contrast, when the overlying brine column is shallow (<30m and typically <5-10m) then the chemistry and stability of the brine vary on a shorter-term (daily-weekly) basis, so more layered bi-mineralogic bottom-nucleates can accumulate as layered to laminated salt beds. In addition, all evaporite sediments can be reworked by bottom currents, with similar textures to those that characterise siliciclastic and mechanically-modified carbonate sediments. Deposition of capillary salts (sabkha deposits) occurs in subaerial settings, wherever the saline capillary zone intersects the land surface.

Salinity (temperature-heliothermal) layering in a brine lake water column shows metabolic stacking of the halobiota tied to the presence of anoxic waters below the halocline and oxic waters above. This oxic/anoxic salinity/density related layering in the brine column of the lake controls the distribution of the microbial communities, and so stacks their metabolic requirements, pathways and products, including biomarkers (Warren 2016, Chapter 9).

Density stratification also layers the biota living in the various stratified brine layers. Primary prokaryote producers that are light-dependent and, along with halotolerant photosynthetic eukaryotes, make up the base of the food chain in most modern hypersaline ecosystems. But there are other classes of green and purple sulphur microbes that are lithotrophs and chemotrophs; they too can metabolise sulphur independent of a light source (they are chemoautotrophs, not photoautotrophs). In many saline microbial communities, these non-sulphur bacteria regularly inhabit unlit layers beneath the lit-community layers above, where the aerobic cyanobacteria and anaerobic purple and green sulphur bacteria (photosynthesizers) thrive, while a community of sulphate reducers and fermenters live below.

This split between photic and nonphotic communities ofter ties to the position of the halocline. Such metabolic layering is seen in both microbial mats and stratified brine columns. In addition, these lithotrophic microbes can be a significant biomass component in deep seafloor seep communities and in cavern waters formed by the action of sulphuric acid where gypsum is a byproduct. It is why when stratified meromictic saline water becomes holomictic, there is an accompanying release of H2S and ammonia gas.