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 a 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 (Figure). Brine density by this stage of concentration is in excess of 1.30 gm/cc and brine viscosity and feel of the brine when rubbed between the fingers approaches that of olive oil.



Marine brine evolution



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