Relics of evaporated sea water in deep basins of the Eastern Mediterranean

Reexamination of data on hypersaline bottom brines from the deep anoxic Tyro and Bannock basins in the Eastern Mediterranean reveals that despite their similar chlorinity (6.0 mole/kg H₂O) their chemical composition is significantly different. The brine in the Tyro basin has a Na-chloride composition ( Na Cl ∼ 1) with conspicuously low ratios of conservative elements to chloride ( Br Cl = 2.4 × 10^-4, Li Cl = 1.4 × 10^-5, B Cl = 1.6 × 10^-4), indicating dissolution of halite. In contrast, brines from the Bannock basin are characterized by an Mg-chloride signature ( Na Cl = 0.78) with relatively high ratios of conservative elements to chloride, ( Br Cl = 1.7 × 10^-3, Li Cl = 5.2 × 10^-5, B Cl = 8.4 × 10^-4). The contents of Na, Mg, K, Li, Cl, Br, and B in the deep Bannock brines (brine II) are identical to those in evaporated sea water of a degree of evaporation of 12-13, whereas Ca is enriched and SO₄ is depleted. In contrast to previous studies it is suggested that brine II from the Bannock basin is a relic of ancient evaporated sea water that was slightly modified by sulphate reduction and gypsum dissolution. The interstitial ancient evaporated sea water was liberated and accumulated on the sea floor, probably during the formation of the deep basins in the Bannock area.