COSMOGENIC AND OSL DATING OF THE LATE PLEISTOCENE SHORELINES OF LAKE LISAN, SOUTHERN ISRAEL: TWO DIFFERENT HISTORIES

 

A series of beach ridges, along the southern end of the Dead Sea, southern Israel, 160-177 meters below sea level were studied in order to establish their relationship to the late Pleistocene Lake Lisan, precursor of the Dead Sea. Optically stimulated luminescence dating of sandy sediments from these ridges yielded ages of 20.0± 1.4 ka and 36.1± 3.3 ka. These ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of Lake Lisan, which suggest that the lake reached its highest stand ~27,000 cal yr B.P.

Five chert clasts from two of these beach ridges were sampled for ²⁶Al and ¹⁰Be measurements. The measured activities indicate low rates of chert bedrock erosion and complex exposure, burial, and by inference, transport histories. The chert clasts were derived from the Senonian Mishash Formation, a chert-bearing chalk, which is widely exposed in the Nahal Zin drainage basin, the drainage system that supplied most of the material to the beach ridges.

By assuming only exposure at the sampling sites, ²⁶Al and ¹⁰Be activities suggest exposure ages that range from 35 to 354 ky. These ages do not correspond with the OSL and soil ages of the beach ridges. Furthermore, using the ratio ²⁶Al/ ¹⁰Be, total clast histories range from 460 to 4300 ky, unrelated to the clasts’ current position and exposure period on the late Pleistocene beach ridges. If the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production, the minimum initial exposure and total burial times before delivery to the beach ridges range from 50 to 1300 ky and 390 to 3130 ky, respectively. Alternatively, the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock. Calculating such rates of rock erosion suggests very low erosion rates between 0.4 and 12 m My^-1.

The relatively long burial periods indicate extended sediment storage as colluvium on slopes and/or as alluvial deposits in Plio-Pleistocene Nahal Zin river terraces and only washed on to the shores of Lake Lisan during the late Pleistocene. The results suggest that using cosmogenic nuclides to date arid-region, terminal lake shorelines is problematic because nuclides are inherited and accumulate from prior periods of exposure.