New research explaining why salt crystals are piling up on the deepest parts of the Dead Sea’s floor could help scientists understand how large salt deposits formed in Earth’s geologic past.

The Dead Sea, a lake bordered by Israel and Jordan, is nearly 10 times as salty as the ocean. People have visited the Dead Sea for thousands of years to experience its unique healing properties and to float in its dense, buoyant waters.The “Salt Sea” is even mentioned in the Bible.

Much of the freshwater feeding the Dead Sea has been diverted in recent decades, lowering the sea’s water levels and making it saltier than ever.

Scientists first noticed in 1979, after this process had started, that salt crystals were precipitating out of the top layer of water, “snowing” down and piling up on the lake bed. The salt layer on the lake floor has been growing about 10 centimeters (4 inches) thicker every year.

Researchers at the Dead Sea in Israel. Photo courtesy of the Geological Survey of Israel

 

The process driving this salt crystal “snow” and buildup of salt layers on the lake bed has puzzled scientists because it doesn’t make sense according to the laws of physics.

A new study by scientists from the Geological Survey of Israel in Jerusalem and from the department of mechanical engineering at the University of California at Santa Barbara proposes that disturbances caused by waves or other motion create tiny “salt fingers” across the surface of the lake that interact with one another as they funnel down to the lake bed, forming larger structures.

The new finding, published in American Geophysical Union’s journal Water Resources Research, not only sheds light on the physics of the Dead Sea but also helps explain how massive salt deposits formed within Earth’s crust.The Dead Sea is the only hyper saline water body where this salt fingering process is still happening.

When researchers lower cables into the Dead Sea to take measurements, the cables become encased in salt crystals. Photo courtesy of the Geological Survey of Israel

“Altogether this makes the Dead Sea a unique system,” said Nadav Lensky, a geologist with the Geological Survey of Israel and co-author of the study. “Basically, we have here a new finding that we think is very relevant to the understanding of the arrangement of these basins that were so common in Earth’s history.”

A salty mystery

As the Dead Sea has become saltier in recent decades, much of that salt has become concentrated near its surface. During the summer, extra heat from the sun warms the surface of the lake and divides it into a warm top layer sitting atop a colder lower layer. As water evaporates from the top layer in the summer heat, it becomes saltier than the cooler layer below.

Researchers realized the salt snow they observed was originating in this top salty layer, but this warm water doesn’t mix with the cooler water below because it’s so much warmer and less dense. How was salt from the surface entering the cooler layer and plummeting to the bottom?

The new study tested a theory that Lensky and his colleagues first proposed in 2016: When the top layer of the lake is disturbed by waves or other motion, tiny parcels of warm water enter the cooler pool of water below. Heat diffuses more rapidly than salt, so this warm water parcel rapidly cools. As it cools, salt precipitates out and forms crystals that sink to the bottom.

The researchers created a computer simulation of the salt fingers theory. And indeed, the theory correctly predicted the downward flow of salt snow and buildup of salt layers in the middle of the lake’s floor.

The new study also helps explain the formation of massive salt deposits within Earth’s crust.

One notable example is the thick salt layer underneath the Mediterranean Sea. Researchers believe that about six million years ago, the Strait of Gibraltar closed off because of the movements of Earth’s tectonic plates. This cut off the supply of water from the Atlantic Ocean to the Mediterranean, creating a giant shallow inland sea.

After several hundred thousand years, the Mediterranean’s water levels dropped so much that the sea partly or nearly dried out, leaving behind thick deposits of salt.

The new finding suggests these deposits formed during this time in a similar manner to what is happening right now in the Dead Sea. When the Strait of Gibraltar opened again, water flooded the basin and the salt deposits were buried under new layers of sediment, where they remain today.