The disappearance of the long term meromictic stratification of the Dead Sea

Abstract

Following a lowering of its surface level by 3.5–4 m, the water of the Dead Sea turned over in February 1979, ending a long period of stratification. The “classic” structure (a salinity gradient at a depth of 30–40 m separating the surface layers, through a transitional zone, from a “fossil” water spreading from 60 to 80 m to the bottom 320 m) was replaced by a reduction of the salinity difference between surface and deep waters from 25 g kg⁻¹ in 1960 (50 g kg⁻¹ in 1864) to 3 g kg⁻¹ in 1975, and to −1 in 1978, and by a series of deepenings of the interface between the fossil waters and the upper, seasonally variable layers, from about 80‐m depth in 1975 to 180 m in 1978. The “eroded” deep waters maintained their original properties (mean salinity 275.8 g kg⁻¹, mean potential temperature 21.34°C, mean density 1.2334 g cm⁻³, and tritium content 0.1 TU). In February 1979 the entire water column was essentially homogeneous and stable (Brunt‐Väisälä frequency 5.8 × 10⁻⁴ s⁻¹), with temperature, salinity, and density somewhat higher than those of the former deep waters.The successive deepenings and the complete overturn were caused when the surface (10–25 m) layers cooled during autumn and winter from 37° to 18°C, and, following each dry year, reduced their excess salinity (3 g kg⁻¹) by vertical mixing to a greater depth than in the previous winter. The meromixis of the Dead Sea is ectogenic, caused by freshwater inflow over hypersaline deep layers. The imbalance between the effects of salinity and temperature changes on the density of the water column, and variations (both natural and artificial) in amount and timing of the inflow rate, cause the seasonal changes of the water column to be nonrepetitive; the cumulative extreme development of this is an overturn.