In the late 1970s the Dead Sea began to grow more saline as the rivers that fed it were diverted. And since 1979, crystals of salt have precipitated near the top of the lake and “snowed” down to the lakebed. The layer of salt in the deepest part of the lake thickens by about 10 cm each year but until now the way the crystals form wasn’t clear.
With that in mind, Raphael Ouillon of the University of California Santa Barbara, US, and colleagues performed fluid dynamics simulations.
In summer the Sun warms and partly evaporates the top layer of water in the Dead Sea, making it even saltier. The salt snow originates here but the warmer, less dense water around it doesn’t mix with the colder water below. So how the salt was raining down into this colder layer was a puzzle.
In 2016 Nadav Lensky of the Geological Survey of Israel and colleagues took measurements at the Dead Sea and proposed an explanation — “double-diffusive salt fingers”. Now simulations by Lensky, Ouillon and colleagues indicate that this mechanism is correct.
“Initially you form these tiny fingers that are too small to observe… but quickly they interact with each other as they move down, and form larger and larger structures,” says Ouillon.
“The initial fingers might only be a few millimetres or a couple of centimetres thick, but they’re everywhere across the entire surface of the lake,” adds Eckart Meiburg, also of UC Santa Barbara. “Together these small fingers generate a tremendous amount of salt flux.”
The salt fingers form following small disturbances in the water, the researchers believe. Small parcels of warm water enter the colder water below and cool rapidly; heat diffuses faster than salt. Once it’s cooler the parcel can hold less salt and the salt precipitates out, forming crystals that sink to the lakebed.
The team’s simulations correctly predicted the downward flow of salt snow and the build-up of salt layers in the middle of the lake floor.
Salt crust
The finding could also explain other salt deposits. “We know that many places around the world have thick salt deposits in the Earth’s crust, and these deposits can be up to a kilometre thick,” says Meiburg. “But we’re uncertain how these salt deposits were generated throughout geological history.”
About six million years ago, the Strait of Gibraltar closed off, turning the Mediterranean into a shallow inland sea. Over several hundred thousand years the sea partly dried out, leaving salt deposits. The team’s discovery suggests that these deposits formed the same way as those in the Dead Sea. When the Strait of Gibraltar re-opened, water flooded the basin and sediment covered the salt.
The team reported their simulations in Water Resources Research.
