The Geology of DHABs

DHABs form when large, ancient deposits of salt under the seafloor are exposed to the water in a deep basin that currents do not flow through.

Three separate events must occur to produce a DHAB: massive amounts of salt must be laid down in an area that later becomes the bottom of a deep sea; tectonic activity must create a deep basin in that area; and the salt, exposed to the ocean by the tectonic activity, must dissolve into the seawater and stay in the basin.

To illustrate the formation of a DHAB, let’s look at how it occurred in the eastern Mediterranean Sea.

Event 1 - Depositing salt  

1st Step: In the area we know as the eastern Mediterranean Sea, salt was deposited between 5 and 6 million years ago, at the end of the Miocene geologic period. At that time the sea in this region was so shallow that it eventually dried up.

2nd Step: As the water evaporated, the salts that had been dissolved in the water were left behind. That was a huge amount of salt—each cubic meter of seawater contains about a kilogram (more than 2 pounds) of salt.

3rd Step: The salt deposits, called evaporites, hardened like rock. (Try our evaporite activity.) Then the area flooded with seawater from the Atlantic Ocean to the west. But again, over thousands of years, the sea shrank and dried up, depositing another layer of salt.

4th Step: This flooding and drying took place as many as 70 times before the final flooding, about 5 million years ago, which created the Mediterranean Sea. By then the evaporites were up to 10 kilometers (6.2 miles) thick! Over time, the evaporites became covered by oceanic sediments of clay, sand, and organic matter. Currently this “coat” of sediment is 100 to 200 meters (109 to 218 yards) thick.

 

messinian salts

Click to enlarge

Mediterranean evaporites

The Mediterranean evaporites have been named “Messinian salts” after Messina, a region of Italy. At Messina and other places around the Mediterranean, tectonic activity raised portions of the seafloor and evaporites above sea level. Today, some of these evaporites are visible above ground, and are mined for the salts they contain, such as table salt (sodium chloride, NaCl) and gypsum (calcium sulfate dihydrate, CaSO4·2H2O), which is used to make drywall, plaster of Paris, and fertilizer.

 

Event 2 - Creating a basin 

Geologists have identified at least two ways a deep basin can form in the seafloor and expose the underlying layers of salt. In the eastern Mediterranean these events occurred very recently, in geologic terms. The DHABs that have been found there are only between 3,000 and 35,000 years old.

Fault Line Basin:

messinian salts

One way is that tectonic movements of the seafloor along a strike-slip fault can create a depression that dips deep enough into the sediments to reach the salt deposits underneath. The deposits may be at the floor of the basin or along its sides, or water may reach the salts through a fault or large crack in the seafloor.

Seafloor Folding Basin

Step 1: Another way a basin can form occurs when tectonic movements squeeze the seafloor horizontally, like pushing two edges of a piece of cloth toward each other.

Step 2: When that happens, the seafloor starts to fold into ridges and dips, like an accordion. This puts the seafloor under great stress. As a result, cracks form in the sediment.

Step 3: Seawater can go through the cracks, reach the salt deposits below, and begin to dissolve them. This is similar to the formation of caves in limestone deposits on land (known as karst topography).

Step 4: If enough salt dissolves, the evaporite becomes unable to support the heavy layers of sediment above it. The seafloor collapses, forming a deep basin. The process is very much like the formation of a sinkhole on land. If the seafloor collapses far enough, the resulting basin will reach the salt deposits, which can then dissolve directly into the water in the basin.

 

Event 3 - Getting the salt into the basin 

Once the evaporite is exposed to seawater, it can begin to dissolve into the water. If the basin is shallow or if the currents that flow along the seafloor are able to pass through the basin, the salt will get mixed into the ocean. But if the basin is much deeper than the surrounding seafloor, and if currents do not go through it, the water inside the basin stagnates. As it accumulates more salt, it becomes very dense, which makes it even less able to mix with the less dense seawater above it. As a result, the salt concentration in the basin remains very high.

DHAB

Click to enlarge

The high density in the basin has another big effect as well. When organic matter such as dead organisms and fecal pellets fall into the basin, the microbes that degrade them consume the oxygen in the water. Because basin water is not mixing with the oxygenated water above, the oxygen supply is not replaced. The basin becomes anoxic, which means it has no dissolved oxygen. Eventually the basin becomes a DHAB—deep, hypersaline, and anoxic.