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In the rock record, laminated gypsum commonly is interpreted as formed by precipitation from a body of brine. A study of a hypersaline pool at Ras Muhamad, the southernmost tip of the Sinai Peninsula, suggests an alternative interpretation of origin. Algal mats carpet a sea-marginal hypersaline pool, which has a salinity of 127^pmil in the spring when the sulfate concentration has reached 9,700 mg/l; by mid-summer salinity has risen to 314^pmil and, as a result of the precipitation of gypsum, the sulfate level has dropped to 4,570 mg/l. In the spring the SO4/Cl ratio is almost identical with that of seawater (14.0 × 10-2 for seawater; 13.9 × 10-2 for the pool), but has dropped sharply by mid-summer as gypsum precipitates (2.4
Algal mats not only survive, but are remarkably active at this high salinity. Gypsum precipitates as a meshwork between the mats and, depending on the algal growth, concentrates as crystals in laminae ranging in thickness from 1 to 2 cm. Compaction of such laminae as they become part of the rock record would result in much thinner laminae. Dark organic mats alternate with white layers of gypsum. Such a deposit in the rock record has, in the past, been mistaken for a deep-sea deposit.
Laminites of aragonite precipitated by algae alternate with laminae of gypsum. Oncolites, pellets, ooids, and spherulites are particles precipitated within the algal mats.
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