干旱区生态环境知识资源中心

A main process affecting interstitial water and solutes at shallow depths on the newly formed shores of the Dead Sea (DS) is evaporation. The evaporation rate from the unsaturated zone, as calculated based on stable isotope profiles, ranges from 0.5 to 36 mm yr-1. The high tritium concentrations (20-40 TU as compared with < 10 TU in recent precipitation) at a depth of 0-3 m are best explained by the presence of a tritium-rich immobile water phase. It is suggested that this surface tritium enrichment in the unsaturated zone is a general phenomenon which occurs in semi-arid to arid areas where flushing downward by vertical recharge is negligible. In the deeper parts of the unsaturated zone flushing of the old saline water by recent relatively fresh meteoric water is the main process. Two mechanisms for flushing are suggested: (1) penetration of relatively fresh groundwater and flood water into the clastic unit and horizontal movement of water in perched horizons; and (2) capillary rise from the water table of regional groundwater or from perched horizons. The flushing process, even in this arid area, is rapid, diluting in some places the original DS solution by as much as ten-fold in < 30 yr.

The main implication of the present study is that the evaporitic sediments can undergo rapid chemical and isotopic changes which, later, can be found preserved in the sequence. Thus the isotopic and chemical record of some of the phases found in saline ancient evaporitic sequences may have a freshwater signature.