New evidence for the origin of hypersaline pore fluids in the Mediterranean basin

The Br/Cl, Li/Cl and B/Cl ratios and boron isotope compositions of hypersaline pore fluids from DSDP Sites 372 and 374 were measured in an attempt to evaluate the origin of the brines. In Site 374 the relationships between the Cl concentrations (up to 5000 mM) and Br/Cl (∼ 12 × 10-3), Na/Cl (as low as 0.1), B/Cl (2.5 × 10-3), and δ11B values (43–55%o)ofthedeepporewererbetween 300 and 055 mfsf,loceted withinthe Mesniniansediments,feflecrremnantoof ; 65-fold evaporated sea water. The original evaporated sea water was modified by: (1) dilution with overlying or less saline water by about 30%; and (2) slight dissolution of NaCl evaporites. The variations in δ11B show a continuous increase in δ11B values with depth in Site 374, up to 66.7%oat adepthof 000 mfsf (UpperPliocene marlsecliments). hhe conspicuous 11B enrichment trend is consistent with elemental boron depletion, which was calculated from the expected boron concentrations of evaporated sea water with corresponding Br/Cl and Na/Cl ratios. Li/Cl variations also show a depletion of Li relative to evaporated sea water. The apparent depletions of B and Li, as well as the 11B enrichment, reflect uptake of these elements by clay minerals at low water/sediment ratios.