RELICS OF EVAPORATED SEA-WATER IN DEEP BASINS OF THE EASTERN MEDITERRANEAN

Reexamination of data on hypersaline bottom brines from the deep anoxi c Tyro and Bannock basins in the Eastern Mediterranean reveals that de spite their similar chlorinity (6.0 mole/kg H2O) their chemical compos ition is significantly different. The brine in the Tyro basin has a Na -chloride composition (Na/Cl approximately 1) with conspicuously low r atios of conservative elements to chloride (Br/Cl = 2.4 x 10(-4), Li/C l = 1.4 x 10(-5), B/Cl 1.6 x 10(-4)), indicating dissolution of halite . In contrast, brines from the Bannock basin are characterized by an M g-chloride signature (Na/Cl = 0.78) with relatively high ratios of con servative elements to chloride, (Br/Cl = 1.7 x 10(-3), Li/Cl = 5.2 x 1 0(-5), B/Cl = 8.4 x 10(-4)). The contents of Na, Mg, K, Li, Cl, Br, an d B in the deep Bannock brines (brine II) are identical to those in ev aporated 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 su ggested that brine II from the Bannock basin is a relic of ancient eva porated 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 forma tion of the deep basins in the Bannock area.