STRONTIUM ISOTOPIC, CHEMICAL, AND SEDIMENTOLOGICAL EVIDENCE FOR THE EVOLUTION OF LAKE-LISAN AND THE DEAD-SEA

Precise strontium isotope ratios, combined with chemical analyses and sedimentological information, are used to monitor the water sources an d the evolution of the Dead Sea and its late Pleistocene precursor, La ke Lisan (70-18 kyr B.P.). The materials analyzed include bulk aragoni te, water-leached soluble salts, and residual aragonite and gypsum fro m the Lisan Formation in the Perazim Valley (near the SW shore of the Dead Sea). The residual aragonite and the associated soluble salts dis play systematic fluctuations in Sr-87/Sr-86 ratios between 0.70803 and 0.70806 and from 0.70805 to 0.70807, respectively. In individual solu ble salt-residual aragonite pairs, the soluble salt displays a higher Sr-87/Sr-86 ratio. Gypsum samples yield Sr-87/Sr-86 ratios similar to the soluble salts from adjacent layers in the section. This shows that , in individual samples, the source of Sr in aragonite was distinct fr om that in soluble salts and the gypsum. The sterility of the Lisan se diments, their strictly nonbioturbated fine lamination, and their high content of chloride salts indicate that Lake Lisan was a saline, or e ven hypersaline water body. In the absence of alternative sources of H CO3- and SO42- the abundance of primary aragonite and gypsum in the Li san column reflects an import of very large volumes of freshwater into the otherwise saline lake, resulting in a density stratification of t his water body. The history of the upper water layer and that of the l ower brine is reflected in the chemical and strontium isotope composit ion of the aragonite and in that of the associated soluble salts and i n the gypsum samples, respectively. Whereas the bicarbonate and much o f the Ca2+ required for aragonite crystallization were supplied by the freshwater, the complementary Ca2+ (and Sr2+) were added by the lower brine. The upper water layer of Lake Lisan acted as a SO42- capacitor during the lake's rise periods. It was removed therefrom, as prominen t gypsum beds, upon climatic-induced (drier period) mixing or even com plete overturn of the lake. The evolution of Lake Lisan took place bet ween two distinct modes. The first was characterized by an extensive s upply of freshwater and resulted in a rise of the lake's level, a (den sity) layered structure, and precipitation of aragonite. The second mo de was marked by a diminishing freshwater input, resulting in mixing o r complete overturn of its water, and precipitation of gypsum. These t wo modes reflect the climatic evolution of the region in the late Plei stocene which fluctuated between drier and wetter periods. The transit ion to the Holocene is accompanied by the dry up of Lake Lisan and its contraction to the present Dead Sea. Copyright (C) 1997 Elsevier Scie nce Ltd.