Below the halocline, the salinity of ocean water

One of the most interesting features of DHABs is the zone where they meet the normal seawater above them. Because of its high salt content, the brine in a DHAB is so dense that it mixes very little with the seawater above. Instead, there is a narrow layer of water where, as you move toward the basin, the salt concentration goes from normal seawater salinity to hypersaline (very salty). Along that gradient, the density of the water goes from that of normal seawater to very high, and the oxygen concentration drops from normal seawater concentrations to zero. That transition zone is called the halocline. At some basins it is only a meter (39 inches) thick.

Índice Show

A Density Gradient
Like in the Halocline

A Density Gradient

Many tiny particles in the sea, including bits of fecal matter and dead organisms, get trapped in the halocline because they are not dense enough to fall through it. Some get caught at the top of the halocline, some stop falling within the halocline, and others make it to the bottom of the halocline but cannot fall all the way into the DHAB. Where they stop depends on their density (recall that density = mass ÷ volume).

All that organic matter is a rich source of nutrients for organisms that are able to live in the high salt and low oxygen conditions of the halocline. Scientists have discovered a vigorous community of many kinds of bacteria, archaea, and protists (microbial eukaryotes) that either eat the organic particles or prey on the organisms that eat the particles.

Like in the Halocline

So far, research shows that the microbial community in the halocline is unique—few of the organisms that live in the halocline are also found in the normal seawater just above it or in the DHAB water just below it. The microbial community varies across the halocline, too. Researchers have found that the microbial community living at the top of the halocline is distinct from the community living toward the bottom of the halocline, separated only by a couple of meters (6 feet) or even less! Microbiologists think that is because the changes in salt and oxygen levels through the halocline provide a range of conditions over a very short distance, and each set of conditions can support different species of microbes.

Other unique life forms may live in the sediment where the halocline meets the seafloor. Remember that a DHAB is a basin, with walls that come up like the sides of a bowl. The halocline sits on top of the very salty water in the basin and touches the sides of the basin. Researchers call that area the “bathtub ring” because it is like the ring of soap scum and dirt that forms on a bathtub when the water is drained out. The sediment in that narrow zone has a little bit of oxygen and less salt than sediments inside the DHAB. These conditions provide a good habitat for different forms of life than those that live in DHAB sediments.

Researchers call the area that touches the sides of the basin the “bathtub ring” because it is like the ring of soap scum and dirt that forms on a bathtub when the water is drained out.

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In oceanography, a halocline (from Greek hals, halos 'salt' and klinein 'to slope') is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water.[1] Because salinity (in concert with temperature) affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m3 results in an increase of seawater density of around 0.7 kg/m3.

Halocline visible at the cenote Chac Mool, Mexico. The freshwater lies above the denser saltwater. In this photo, the visible water distortion from the halocline can be seen below the diver.

In the midlatitudes, an excess of evaporation over precipitation leads to surface waters being saltier than deep waters. In such regions, the vertical stratification is due to surface waters being warmer than deep waters and the halocline is destabilizing. Such regions may be prone to salt fingering, a process which results in the preferential mixing of salinity.

In certain high latitude regions (such as the Arctic Ocean, Bering Sea, and the Southern Ocean) the surface waters are actually colder than the deep waters and the halocline is responsible for maintaining water column stability, isolating the surface waters from the deep waters. In these regions, the halocline is important in allowing for the formation of sea ice, and limiting the escape of carbon dioxide to the atmosphere.

Haloclines are also found in fjords, and poorly mixed estuaries where fresh water is deposited at the ocean surface.[2]

A halocline can be easily created and observed in a drinking glass or other clear vessel. If fresh water is slowly poured over a quantity of salt water, using a spoon held horizontally at water-level to prevent mixing, a hazy interface layer, the halocline, will soon be visible due to the varying index of refraction across the boundary.

A halocline is most commonly confused with a thermocline – a thermocline is an area within a body of water that marks a drastic change in temperature. A halocline can coincide with a thermocline and form a pycnocline.[3]

Haloclines are common in water-filled limestone caves near the ocean. Less dense fresh water from the land forms a layer over salt water from the ocean.[1] For underwater cave explorers, this can cause the optical illusion of air space in caverns. Passing through the halocline tends to stir up the layers.

Graph

Plot of temperature and salinity in the Arctic Ocean at 85,18 north and 117,28 east dated Jan. 1st 2010.[4]

In the graphical representation, three layers can be discerned:

About 50 m (160 ft) of low salinity water "swimming" on top of the ocean. The temperature is −1.8 °C (28.8 °F), which is very near to the freezing point. This layer blocks heat transfer from the warmer, deeper levels into the sea ice, which has considerable effect on its thickness.
About 150 m (490 ft) of steeply rising salinity and increasing temperature. This is the actual halocline.
The deep layer with nearly constant salinity and slowly decreasing temperature.[4]

Thermocline – A cline based on difference in water temperature,
Chemocline – A cline based on difference in water chemistry,
Pycnocline – A cline based on difference in water density.

Underwater diving portal

Hypersaline lake – Landlocked body of water that contains concentrations of salts greater than the sea
Isopycnal – Line connecting points of a specific density or potential density
Osmotic power – Energy available from the difference in the salt concentration between seawater and river water
Thermohaline circulation – Part of large-scale ocean circulation
Thin layers (oceanography)

^ a b White, William B; Culver, David C (2012). Encyclopedia of Caves. Academic Press. p. 157. ISBN 978-0-12-383832-2.
^ Svensson, Torbjörn (6 February 1981). "Water Exchange and Mixing in Fjords" (PDF). www.chalmers.se. Chalmers University of Technology. p. 159. Retrieved 13 July 2020.
^ Garrison, Tom (2006). Enhanced Essentials of Oceanography. Cengage Learning. p. 115. ISBN 0-495-11372-7.
^ a b "U.S. National Oceanographic Data Center: Global Temperature–Salinity Profile Programme. June 2006. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Oceanographic Data Center, Silver Spring, Maryland, 20910".