Composition of anoxic hypersaline brines in the Tyro and Bannock Basins, eastern Mediterranean

Abstract

At present two brine-filled depressions have been identified in the eastern Mediterranean - The Tyro Basin (33°53′N, 26°02′E) and the Bannock Basin (34°20′N, 20°02′E). In both areas, the main basin is surrounded by satellite basins that used to have or still have hypersaline and anoxic conditions. The brines are thought to originate from dissolution of outcropping or underlying (Messinian) evaporitic salt layers. The transition from normal seawater to brine water occurs at a slightly different water depth in the two basins; at 3383 dbar in the Tyro Basin and at 3330 dbar in the Bannock Basin. At this transition, salinity increases sharply, whereas the concentration of dissolved oxygen drops rapidly to zero across a depth interval of only a few metres. The brines in the anoxic Tyro and Bannock Basins are of almost identical salinity (10 times that of seawater). However, between the Tyro and Bannock brines differences occur in the major (and trace) element composition. The Tyro brine is characterized by a relatively high Na content, whereas the Bannock brine is more enriched in K, Mg and SO4. These differences are related to the composition of the (Messinian) evaporitic salt that serves as a source for each of these brines. The differences in evaporite composition result from differences in the depositional ‘stage’ of the two salt deposits. The Bannock brine results from dissolution of a late-stage evaporitic salt deposit, whereas the Tyro brine originates from an earlier-stage evaporite. In contrast to the homogeneous brine in the Tyro Basin, a double-layered brine is observed in the Bannock Basin. The lower brine (II) has a slightly higher salinity than the upper brine (I) and the brines also differ in composition. Brine I has a higher concentration of Ca and Sr, and a lower concentration of Mg, S and SO4. The concentrations of these ions in the Scirocco brine deviate even more from those in brine II, which is thought to indicate a higher degree of dolomitization in the Scirocco Basin.