The geochemical evolution of the Pleistocene Lake Lisan-Dead Sea system

Abstract

The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods. Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10 −2−1 × 10 −2, Na/Ca ratios from 3.6 × 10 −3 to 9.2 × 10 −3, δ 18O PDB values between 1.5 and 7%. and δ 13C PDB from −7.7 to 3.4%.. The distribution coefficient of Na + between aragonite and aqueous solutions, λ A Na , is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator. Sr/Ca ratios and δ 18O values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%). Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m. The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation. Na/Ca ratios in aragonite, which correlate well with δ 13C values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan.