It’s Snowing Salt. The Strange Phenomenon Happening Deep in the Dead Sea

Salt giants and other striking formations in the Dead Sea reveal how evaporation and fluid dynamics shape Earth’s geological past and present.

The Dead Sea represents a unique convergence of conditions: it lies at the lowest point on Earth’s surface and contains one of the planet’s highest salt concentrations. This extreme salinity makes the water unusually dense, and its distinction as the deepest hypersaline lake produces remarkable, often temperature-driven processes beneath the surface that scientists are still working to understand.

Among the most intriguing features are the so-called salt giants — vast accumulations of salt within the Earth’s crust.

“These large deposits in the earth’s crust can be many, many kilometers horizontally, and they can be more than a kilometer thick in the vertical direction,” said UC Santa Barbara mechanical engineering professor Eckart Meiburg, lead author of a paper published in the Annual Review of Fluid Mechanics. “How were they generated? The Dead Sea is really the only place in the world where we can study the mechanism of these things today.”

Although massive salt deposits are also present in places such as the Mediterranean and Red seas, the Dead Sea is the only location where they are actively forming. This makes it an unparalleled site for investigating the physical processes that govern their development, including how their thickness varies across space and time.

Evaporation, precipitation, saturation

In their study, Meiburg and co-author Nadav Lensky of the Geological Survey of Israel describe the fluid dynamics and sediment transport processes currently shaping the Dead Sea. These processes are controlled by several factors, most notably the Dead Sea’s classification as a terminal salt lake — a body of water with no natural outflow. Evaporation is therefore the only means of water loss, a process that has been shrinking the lake for thousands of years while leaving behind extensive salt deposits. In recent decades, the damming of the Jordan River, its primary inflow, has intensified this decline, with the water level now dropping at an estimated rate of about 1 meter (3 feet) per year.

Temperature differences within the water column also play a key role in the formation of salt giants and related features such as salt domes and chimneys. For much of its history, the Dead Sea was “meromictic” (stably stratified), with a warmer, less dense surface layer resting above a cooler, saltier, denser layer at depth.

From meromictic to holomictic conditions

“It used to be such that even in the winter when things cooled off, the top layer was still less dense than the bottom layer,” Meiburg explained. “And so as a result, there was a stratification in the salt.”

This balance shifted in the early 1980s when partial diversion of the Jordan River reduced freshwater inflow, allowing evaporation to dominate. At that point, surface salinity reached levels comparable to the deep waters, enabling the two layers to mix. This change transformed the lake from meromictic to holomictic (a lake in which the water column overturns annually). Today, stratification still occurs, but it persists only for roughly eight months during the warmer part of the year.

In 2019, Meiburg and colleagues observed an unusual process in summer: the precipitation of halite crystals, or “salt snow,” typically associated with colder months. Halite (commonly known as rock salt) forms when salinity exceeds the amount water can dissolve, making the deeper, colder, denser layers the usual site of precipitation in winter. However, during summer, the researchers found that while evaporation raised the salinity of the upper layer, the warmth of the water allowed salts to keep dissolving there. This produced a condition called “double diffusion,” where patches of the warmer, saltier water near the surface cooled and sank, while portions of the deeper, cooler water warmed and rose. As the denser upper layer cooled further, salt began to precipitate, creating the unexpected “salt snow” phenomenon.

Salt snow and giant formations

The combination of evaporation, temperature fluctuations and density changes throughout the water column, in addition to other factors including internal currents and surface waves, conspire to create salt deposits of various shapes and sizes, assert the authors. In contrast to shallower hypersaline bodies in which precipitation and deposition occur during the dry season, in the Dead Sea, these processes were found to be most intense during the winter months. This year-round “snow” season at depth explains the emergence of the salt giants, found in other saline bodies such as the Mediterranean Sea, which once dried up during the Messinian Salinity Crisis, about 5.96 to 5.33 million years ago.

“There was always some inflow from the North Atlantic into the Mediterranean through the Strait of Gibraltar,” Meiburg said. “But when tectonic motion closed off the Strait of Gibraltar, there couldn’t be any water inflow from the North Atlantic.” The sea level dropped 3-5 km (2-3 miles) due to evaporation, creating the same conditions currently found in the Dead Sea and leaving behind the thickest of this salt crust that can still be found buried below the deep sections of the Mediterranean, he explained. “But then a few million years later the Strait of Gibraltar opened up again, and so you had inflow coming in from the North Atlantic and the Mediterranean filled up again.”

Meanwhile, salinity fluxes and the presence of springs on the sea floor contribute to the formation of other interesting salt structures, such as salt domes and salt chimneys, according to the researchers.

In addition to gaining a fundamental understanding of some of the idiosyncratic processes that can occur in evaporating, hypersaline lakes, research into the associated sediment transport processes occurring on the emerging beaches may also yield insight on the stability and erosion of arid coastlines under sea level change, as well as the potential for resource extraction, the authors state.

Reference: “Fluid Mechanics of the Dead Sea” by Eckart Meiburg and Nadav G. Lensky, 11 September 2024, Annual Review of Fluid Mechanics.
DOI: 10.1146/annurev-fluid-031424-101119

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