Chimaeridae fish and seep mussels at edge of brine pool.
A brine pool, sometimes called an underwater, a deepwater lake, or a brine lake, is a volume of brine collected in a seafloor depression. The pools are dense bodies of water that have a salinity that is three to eight times greater than the surrounding ocean. Brine pools are commonly found below polar sea ice and in the deep ocean. Those below sea ice form through a process called brine rejection. For deep-sea brine pools, salt is necessary to increase the salinity gradient. The salt can come from one of two processes: the dissolution of large salt deposits through salt tectonics or geothermally heated brine issued from tectonic spreading centers. The brine often contains high concentrations of hydrogen sulfide and methane, which provide energy to chemosynthetic organisms that live near the pool. These creatures are often extremophiles and symbionts. Deep-sea and polar brine pools are toxic to marine animals due to their high salinity and anoxic properties, which can ultimately lead to toxic shock and possibly death. The frequency of brine pool formation coupled with their uniquely high salinity has made them a candidate for research regarding ways to harness their properties to improve human science.
Brine pools are sometimes called sea floor "lakes" because the dense brine does not easily mix with overlying seawater creating a distinct interface between water masses. The pools range in area from less than 1 square metre (11 sq ft) to as large as the 120 square kilometres (46 sq mi) Orca Basin. The high salinity raises the density of the brine, which creates a surface and shoreline for the pool. Because of the brine's high density and lack of mixing currents in the deep ocean, brine pools often become anoxic and deadly to respiring organisms. Brine pools supporting chemosynthetic activity, however, form life on the pool's shores where bacteria and their symbionts grow near the highest concentrations of nutrient release. Patchy, reddish layers can be observed floating above the dense brine interface due high densities of halophilic archaea that are supported by these environments. These shores are complex environments with significant shifts in salinity, oxygen concentration, pH, and temperature over a relatively small vertical scale. These transitions provide a variety of environmental niches.
Brine pools are created through three primary methods: brine rejection below sea ice, dissolution of salts into bottom water through salt-tectonics, and geothermal heating of brine at tectonic boundaries and hot spots.
When sea water freezes, salts do not fit into the crystalline structure of ice so the salts are expelled. The expelled salts form a cold, dense, brine that sinks below the sea ice to the sea floor. Brine rejection on a oceanic scale is associated with the formation of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AAW) that play a large role in global thermohaline circulation (THC). On a localized scale, that rejected brine collects in seafloor depressions forming a brine pool. In the absence of mixing, the brine will become anoxic in a matter of weeks.
During the Middle Jurassic period, the Gulf of Mexico was a shallow sea that dried out, producing a thick layer of salt and seawater derived minerals up to 8 km thick. When the Gulf refilled with water, the salt layer was preserved from dissolution by sediments accumulating over the salt. Subsequent sedimentation layers became so heavy that they began to deform and move the more malleable salt layer below. In some places, the salt layer now protrudes at or near the seafloor where it can interact with seawater. Where seawater comes in contact with the salt, the deposits dissolve and form brines. The location of these surfacing Jurassic era salt deposits is also associated with methane releases giving deep ocean brine pools their chemical characteristics.
At earth's oceanic tectonic spreading centers, plates are moving apart, allowing new magma to rise and cool. This process is involved in creating new sea floor. These mid-ocean ridges allow seawater to seep downward into fractures where they come in contact with and dissolve minerals. In the Red Sea for example, Red Sea Deep Water (RSDW) seeps into the fissures created at the tectonic boundary. The water dissolves salts from deposits created in the Miocene epoch much like the Jurassic period deposits in the Gulf of Mexico. The resulting brine is then superheated in the hydrothermal active zone over the magma chamber. The heated brine rises to the seafloor where it cools and settles in depressions as brine pools. The location of these pools is also associated with methane, hydrogen sulfide, and other chemical releases setting the stage for chemosynthetic activity.
Support of life
Due to the methods of their formation and lack of mixing, brine pools are anoxic and deadly to most organisms. When an organism enters a brine pool, they attempt to "breathe" the environment and experience cerebral hypoxia due to the lack of oxygen and toxic shock from the hyper-salinity. If organisms cannot surface long enough to retreat to the rim, they quickly die. When observed by submarines or Remotely Operated Vehicles (ROV), brine pools are found to be eerily littered with dead fish, crabs, amphipods, and various organisms that ventured too far into the brine. Dead organisms are then preserved in the brine for years without decay due to the anoxic nature of the pool preventing decay and creating a fish "graveyard."
Despite the harsh conditions, life in the form of macrofauna such as bivalves can be found in a thin area along the rim of a brine pool. A novel genus and species of bivalves, known as Apachecorbula muriatica, have been found along the edge of the "Valdiva Deep" brine pool in the Red Sea. There have also been recorded instances of macrofauna brine pools at the seawater interface. Inactive sulfur chimneys have been found with affiliated epifauna such as polychaetes and hydroids. In fauna like gastropods, capitellid polychaetes, and top snails have also been found to be associated with brine pools in the Red Sea. Such species typically feed on microbial symbionts or bacterial and detritus films.
While organisms can typically flourish on the outskirts of a brine pool, they are not always safe from harm here. One possible reason for this is that underwater landslides can impact brine pools and cause waves of hypersaline brine to spill out into surrounding basins, thus negatively affecting the biological communities which live there.
Despite their inhospitable nature, brine pools can also provide a home, allowing organisms to flourish. Deep-sea brine pools often coincide with cold seep activity allowing for chemosynthetic life to thrive. Methane and hydrogen sulfide released by the seep is processed by bacteria, which have a symbiotic relationship with organisms such as seep mussels. The seep mussels create two distinct zones. The inner zone, which is at the edge of the pool, provides the best physiological conditions and allows for maximum growth. The outer zone is near the transition between the mussel bed and the surrounding seafloor, and this area provides the worst conditions causing these mussels to have lower maximum sizes and densities. This ecosystem is dependent on chemical energy and, relative to almost all other life on Earth, has no dependence on energy from the Sun.
An important part of the study of extreme environments such as brine pools is the function and survival of microbes. Microbes help support the larger biological community around environments like brine pools and are key to understanding the survival of other extremophiles. Biofilms contribute to the creation of microbes and are considered the foundation by which other micro-organisms can survive in extreme environments. The research into the growth and function of artificial extremophile biofilms has been slow due to the difficulty in recreating the extreme deep-sea environments they are found in.
One major idea involves harnessing the salinity of brine pools to use as a power source. This would be done using an osmotic engine which draws the high salinity top water through the engine and pushes it down due to osmotic pressure. This would cause the brackish stream (which is less dense and has a lighter salinity) to be propelled away from the engine via buoyancy. The energy created by this exchange can be harnessed using a turbine to create a power output.
It is possible to study liquid brine in order to harness its electrical conductivity to study if liquid water is present on Mars. A HABIT (Habitability: Brines, Irradiation, and Temperature) instrument will be part of a 2020 campaign to monitor changing conditions on Mars. This device will include a BOTTLE (Brine Observation Transition to Liquid Experiment) experiment to quantify the formation of transient liquid brine as well as observe its stability over time under non-equilibrium conditions.
A third idea involves using microorganisms in deep-sea brine pools to form natural product drugs. These microorganisms are important sources of bioactive molecules against various diseases due to the extreme environment they inhabit, giving potential to an increasing number of drugs in clinical trials. In particular, a novel finding in a study used microorganisms from the Red Sea brine pools as potential anticancer drugs.
Deep sea brine pools have also been a large interest in bioprospecting in the hope that unlikely environments might serve as sources of biomedical breakthroughs due to unexplored biodiversity. Some areas have been found to host antibacterial and anticancer activities in biosynthetic clusters. Other novel antibiotic resistance enzymes have been found that are useful in various biomedical and industrial applications.
^Antunes, André; Olsson-Francis, Karen; McGenity, Terry J. (2020), "Exploring Deep-Sea Brines as Potential Terrestrial Analogues of Oceans in the Icy Moons of the Outer Solar System", Astrobiology: Current, Evolving, and Emerging Perspectives, Caister Academic Press, 38, pp. 123-162, doi:10.21775/9781912530304.06, ISBN978-1-912530-30-4, PMID31967579
Hartmann, M., Scholten, J., Stoffers, P., & Wehner, F. (1998). Hydrographic structure of brine-filled deeps in the Red Sea--new results from the Shaban, Kebrit, Atlantis II, and Discovery Deep. Marine Geology,144(4), 311-330. doi:10.1016/s0025-3227(97)00055-8