Dead Sea: Mystery of the "salt fingers" solved

Salt crystal growth at the bottom of the Dead Sea is only apparently contradictory to physics

In the Dead Sea, thick layers of salt crystals form not only on the edge, but, strangely, in the middle of the sea as well - how and why, researchers have only now found out. © David Leshem / iStock
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Physical mystery: Actually, the growing salt-crystal formations at the bottom of the Dead Sea are unlikely to exist - because their formation is physically only partially explainable. But now researchers have solved the puzzle. Accordingly, these crystals are formed because small turbulences on the water surface repeatedly foul fingers of salty, warm water in the depth. These "salt fingers" sink, become cold and then rain salt crystals down to the bottom.

The Dead Sea is one of the saltiest waters in the world - and its salinity continues to increase. Because the ever-increasing water withdrawal from the Jordan, its main inflow, hardly allows fresh water to flow. At the same time, the heat evaporates so much water that the level of the Dead Sea drops by about a meter per year. As a result, the lake shrinks, its edges are salt-encrusted and its banks are already thousands of collapse holes torn open.

Also cables and measuring instruments at the bottom of the lake are covered by a thick salt crust. © Nadav Lensky / Geological Survey of Israel

Enigmatic rain of salt crystals

But there is one more phenomenon that has recently appeared on the Dead Sea: since the late 1970s, researchers have observed that a true "snowstorm" of salt crystals rains down from the middle layers of the lake. There, the crystallized salt forms a layer that grows by about ten centimeters per year.

The remarkable thing, however, is that there really should not be this "snow" of salt crystals in the Dead Sea. For at first glance this contradicts the rules of physics. The reason: The warm, especially salty surface water due to evaporation lies on a ten degrees colder, less salty water layer. Because of its higher density, this cold layer acts as a barrier and should actually prevent the sinking of the saltier surface water and thus also the crystallization of the salt. But that's obviously not the case.

Salty water fingers

A solution to this riddle has now been found by Raphael Ouillon of the University of California at Santa Barbara and his colleagues. They modeled the conditions in the Dead Sea in a physical model and looked for mechanisms that could explain the formation of salt crystals in the deeper water. They paid special attention to the processes at the boundary layer between the warmer and cooler water layers. display

It turned out to be surprising. Because even the tiniest turbulence could cause tiny "fingers" of warm surface water to protrude into the deeper water. "At first these fingers are too small to see. But they interact with each other as they sink, thus forming ever larger structures, "explains Ouillon.

Similar to the warm, downwardly growing water fingers, similarly fine "fingers" rise from the cooler, less salty water towards the surface. "As a result, warm, salty water fingers form beneath the boundary layer and become colder, saltier fingers over them, " the researchers report.

"Unique system"

The trick: These millimeter-small "fingers" made of warm surface water are strongly enriched with salt. When they sink into the deep, they get into colder water layers and gradually cool off. However, because cold water can dissolve less salt than warm water, the salt in these fingers begins to crystallize out kleine small white salt crystals are formed, which sink to the bottom of the Dead Sea.

This is behind the mysterious salt deposits at the bottom of the Dead Sea. AGU

"These processes take place on a very small scale, with salt fingers that are only millimeters to centimeters thick, " explain Ouillon and his team. But the sheer volume of such salt fingers is enough to transport large quantities of salt into the cooler middle layer of water over time. This is saturated with salt and the salt crystallizes out. This is the reason why thick crusts of salt crystals form not only along the shallow lakeshore, but also in its deep center.

"This makes the Dead Sea a unique system, " says co-author Nadav Lensky of the Israeli Geological Service. Because it is the only deep, layered salt lake in the world in which halite crystals are deposited in this way.

Explanation also for primordial salt deposits?

The new findings explain not only why the salt layer at the bottom of the Dead Sea is getting thicker over time. They also provide clues as to how the hundreds of meters of salt layers of many rock salt occurrences once formed. For example, the gradual dehydration of the Mediterranean almost six million years ago led to the deposition of kilometer-thick layers of salt.

According to the researchers could have run in the extremely salty residual water of the Mediterranean, a similar process as today in the Dead Sea. "One of the basic characteristics of such evaporite basins is that the salt rock layers get thicker towards the middle, " they explain. But this is typical for the crystallization by the "salt fingers". Because there is no colder deep layer at the edges of such waters, there is also no concentrated crystallization of the salty water fingers there.

Thus, the Dead Sea is today the only salt water that has these "salt fingers". In the course of geological history, however, there could have been bays, lagoons and lakes with similar conditions. The Dead Sea also gives us an insight into the processes that shaped our planet millions of years ago. (Water Resources Research, 2019; doi: 10.1029 / 2019WR024818)

Source: American Geophysical Union

- Nadja Podbregar