Why did the Dead Sea once have separate upper and lower layers of water?

The Dead Sea historically maintained separate upper and lower layers of water due to differences in salinity and density. Because the concentration of salt generally increases toward the lake's bottom, the deeper, more saline water became "fossilized"—meaning its high density caused it to remain permanently at the bottom, creating a stratified water column that persisted for approximately three centuries until the late 1970s.1

In this stratified state, the two layers possessed distinct characteristics:

• Upper Layer: Extending to a depth of about 130 feet (40 metres), this water was less saline and rich in sulfates and bicarbonates.1
• Lower Layer: Located beneath a transition zone between 130 and 330 feet (40 and 100 metres), this water was more saline and contained strong concentrations of magnesium, potassium, chlorine, and bromine, along with hydrogen sulfide.1

This stratification was disrupted by a reduction in inflow from the Jordan River, which increased the salinity of the upper layer until it eventually became dense enough to sink, triggering a mixing event known as an overturn.1