During Pleistocene-Holocene times a sequence of lakes occupied at different times the depression in the Jordan Arava valley. The largest of these lakes was Lake Lisan, which was the latest precursor of the present Dead Sea. The sediments of the Lake Lisan consist of alternating layers of evaporitic aragonite and detritus composed of calcite, dolomite and clay, both a few mm thick. It is believed that the laminae represent seasonal precipitates. Because of
the dry conditions of the Dead Sea region the sediments
have been preserved and show no signal of diagenesis or alteration.
We have analyzed U and Th isotope compositions of the aragonites with TIMS. The aragonites yield 230Th-234U ages between 26 to 72 ky. The 232Th concentration is partly controlled by detrital material and has been corrected. Trace element concentrations (e.g. Sr, Li, Pb) were measured by ICP-MS.
The detailed sampling of the Prazim Valley section allow us to monitor the behaviour of U isotopes and trace element ratios through time. Fig. 1 displays 234U/238U activity ratios U/Ca and Sr/Ca concentrations versus height of the section. 234U/238U, U/Ca (molar) and Sr/Ca (molar) ratios range between 1.46 and 1.52, 0.89 and 1.81*10-6 and 5.64 to 8.36*10-3 respectively. Sr/Ca ratios show a secular decrease along the section, with change in the slope at about 23 m. Similar trend was observed in the Masada section north of the Prazim section (Katz et al., 1977).
In finer details it appears that 234U/238U and U/Ca ratios fluctuate along the Prazim section. They increase until a maximum at about 20 m (43 ky) and decrease towards the top of the section.
The water supplies for the present-day Dead Sea are
fresh water sources with low Sr/Ca (2-4*10-3), high U/Ca
(3-6*10-4) and 234U/238U (1.5-1.8) ratios and saline brine sources with high Sr/Ca (~1*10-2) but low U/Ca (~5*10-9) and 234U/238U (1.1-1.3) ratios. We suggest that the slight fluctuations in 234U/238U activity and U/Ca ratios reflect changing contributions of fresh and salty water to the surface layer of the lake. While the secular decrease in Sr/Ca suggests a continuous input of fresh water to Lake Lisan (Katz et al., 1977), the fluctuation in 234U/238U and Sr/Ca ratios probably monitor changes in the freshwater input/evaporation rate ratio.
Katz, A., Kolodny, Y., Nissenbaum, A., Geochim. Cosmochim. Acta 41, 1609-1626 (1977).
Fig. 1: U isotopes and trace element ratios versus height, Prazim Valley section.