Solutions in intertidal and supratidal environments of modern carbonate sedimentation: Their implication on diagenesis

Abstract

When sea water is trapped and evaporated in the intertidal zone of tropical coasts, calcium carbonate precipitates as consequence of increasing concentrations. The precipitation takes place only on surfaces where calcium carbonate is already present. Beachrock formation on Bimini, Bahamas, can be attributed entirely to precipitation of aragonite cement from sea water. Because of the low solubility of calcium salts as compared with the higher solubility of halite and magnesian salts, a chemical separation of calcium and magnesium takes place during evaporation of sea water as well as upon dissolution of sea salt in rain water. Dissolution of sea salt yields magnesium-rich solutions with initial pH values above 9 over a wide range of concentrations prior to equilibration with the atmospheric pCO 2. The counterpart of these magnesium-rich solutions is rich in calcium, exhibits pH values below 9 (around pH 8), and tends to decompose unstable magnesian-calcite and to precipitate calcium carbonate and gypsum during evaporation. The tendency for calcium sulfate to precipitate in an early stage of evaporation, as compared to magnesian salts, but to redissolve readily, as compared with the sluggish dissolution of once-precipitated calcium carbonate, makes gypsum a convenient shortterm depot for calcium ions in intertidal and supratidal environments, where gypsum has to be considered as an ubiquitous reaction partner.