In 2016, researchers proposed that salt fingers could send crystals to the Dead Sea floor. 3 years later, a 2019 study backed the theory, showing why the lakebed gains about 10 centimeters of salt yearly

The Dead Sea has a strange problem. As its water becomes increasingly salty, crystals are falling from the surface and collecting on the bottom, forming a layer that grows by about 10 centimeters every year. Scientists first noticed this salt “snow” in 1979, but the process was difficult to explain. Now, research published in Water Resources Research provides evidence for how tiny movements in the lake can send salt downward.

The report is based on a research paper: Raphael Ouillon et al.,“Halite Precipitation From Double‐Diffusive Salt Fingers in the Dead Sea: Numerical Simulations,”Water Resources Research(2019), DOI:10.1029/2019WR024818.

The answer involves structures called “salt fingers”, which form when warm, salty water from the surface moves into the cooler water below.

Why is salt falling to the Dead Sea floor?

The Dead Sea is nearly 10 times saltier than the ocean. In recent decades, freshwater flowing into the lake has been diverted, causing its water level to fall and its salinity to increase.

During summer, sunlight heats the upper layer of the lake. Evaporation removes water from this surface layer, leaving behind a higher concentration of salt.

This creates two distinct layers. The upper water becomes warmer and saltier, while the water underneath remains cooler.

Normally, the difference in temperature and density means the two layers do not mix easily. That created the mystery. If the salty water stays near the surface, how does so much salt end up on the bottom?

Researchers began considering the role of small disturbances such as waves and other movements in the lake.

In 2016, Nadav Lensky and colleagues proposed that these disturbances could allow tiny parcels of warm surface water to enter the colder water beneath.

When that happens, something unusual follows.

How do the “salt fingers” work?

Heat moves through the water faster than salt does. As a parcel of warm water enters the colder layer, it cools rapidly.

But colder water cannot hold as much salt as the warmer water did. The excess salt therefore comes out of the water and forms crystals.

Those crystals sink toward the bottom.

The process creates what researchers describe as “salt fingers”, narrow structures of salty water that move downward. At first, the fingers can be only a few millimeters or a couple of centimeters thick.

“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,” said Raphael Ouillon, a mechanical engineer at the University of California Santa Barbara and lead author of the study.

Eckart Meiburg, another mechanical engineer at UC Santa Barbara and co-author, explained that the tiny structures occur across the surface and collectively move a substantial amount of salt.

“The initial fingers might only be a few millimeters or a couple of centimeters thick, but they're everywhere across the entire surface of the lake,” said Meiburg. “Together these small fingers generate a tremendous amount of salt flux.”

What did the 2019 study find?

The researchers tested the theory using a computer simulation designed to reproduce how water and salt move inside the Dead Sea.

The results supported the salt-finger explanation. The simulation correctly predicted the downward movement of salt “snow” and the accumulation of salt layers in the middle portion of the lake floor.

The falling water level also plays a role in where those deposits become concentrated. Because freshwater has been diverted from the nearby Jordan River, the Dead Sea is shrinking, leaving the salt layers concentrated toward the central part of the lake.

The finding is important because the Dead Sea provides researchers with a rare natural setting in which to study this process.

According to the researchers, it is the only hypersaline body of water on Earth today where salt fingering of this kind is taking place.

Nadav Lensky, a geologist with the Geological Survey of Israel and co-author of the research, described the Dead Sea as a particularly useful system for understanding how similar basins behaved in Earth's past.

“Altogether this makes the Dead Sea a unique system,” said Lensky. “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.”

Could the same process explain ancient salt deposits?

The research may also help explain salt deposits buried deep inside Earth's crust.

Some of these deposits are enormous, reaching up to a kilometer in thickness. Scientists know that thick salt formations exist in many parts of the world, but the processes responsible for creating them over geological time have not always been clear.

One example lies beneath the Mediterranean Sea.

About six million years ago, movements of Earth's tectonic plates closed the Strait of Gibraltar, cutting off the Mediterranean from the Atlantic Ocean. The Mediterranean effectively became a large, shallow inland sea.

Over hundreds of thousands of years, its water level dropped dramatically, and the sea partly or nearly dried out. Thick deposits of salt were left behind.

When the Strait of Gibraltar eventually reopened, water flooded back into the basin. The existing salt deposits were then covered by newer layers of sediment and remained buried.

The Dead Sea offers researchers a modern example of a process that may have operated in similar environments millions of years ago.

The research therefore does more than explain why salt crystals are piling up beneath the Dead Sea. It provides a way to examine the physics behind the formation of massive salt deposits preserved in Earth's geological record.

FAQs

Why is the Dead Sea getting saltier?
Freshwater diversions have lowered its water level.

What sends the salt downward?
Tiny structures called “salt fingers” carry it toward the lakebed.