Ionic proportions change as seawater concentrates and salts are precipitate. Magnesium sulphates are typical precipitates at the bittern stage.
Today, the chemical make-up and the proportions of the major ions in seawater are near-constant across all the world’s oceans. Seawater is dominated by Na and Cl, with lesser amounts of SO4, Mg, Ca, K, CO3 and HCO3. Modern seawater is a Na–(Mg)–Cl–(SO4) water, with a density of 1.03 gm/cc and salinity around 35 ± 2‰. When seawater evaporates, a predictable suite of primary evaporite salts crystallise from increasingly concentrated hypersaline waters.
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. Carnallite and epsomite are the dominant bittern precipitates from modern marine brines. Brine density by this stage of concentration is more than 1.30 gm/cc and brine viscosity and feel of the brine when rubbed between the fingers approaches that of olive oil.
Mineral proportions in modern seawater and their salt precipitates
Much of the field and laboratory work done on evaporating water has been done in closed conditions. Usiglio (1849) just took a container of seawater and monitored its progress as he evaporated it to complete dryness. McCaffrey et al. (1987) studied the evolution of seawater in a series of man-made salt ponds in the Bahamas and supplemented this with laboratory work at higher salinities and densities not found in the ponds, as did Laborde (1983). Ideally, after depositing gypsum and halite, supersaline seawater evaporated to complete dryness deposits a predictable suite of Mg and K sulphate minerals, with the majority of the salt volume dominated by halite. The most problematic endgame in this evolving marine brine system is predicting what will be the bittern salt sequence. This is especially obvious when one tries to compare the suite of bittern salts seen in ancient marine evaporites to modern bittern; there are apparent discrepancies related to brine leakage, backreaction and evolving Phanerozoic seawater chemistries. These problems can be addressed at varying levels of chemical, hydrological and chronological complexity. In the end, many of the discrepancies can be related to the simple statement, "In nature, there is no such thing as a totally closed, static system" (Warren, 2016; Chapter 2).