large area of brine on the ocean basin

These craters mark the formation of brine pools, from which salt has seeped through the sea floor and encrusted the nearby substrate.

Chimaeridae fish and seep mussels at edge of brine pool.

A brine pool is a volume of brine collected in a seafloor depression. These pools are dense bodies of water that have a salinity three to eight times greater than the surrounding ocean. Brine pools are commonly found below polar sea ice and in the deep ocean. Brine pools below sea ice form through a process called brine rejection[1]. For deep-sea brine pools, the salt can come from one of two processes: the dissolution of large salt deposits through salt tectonics[2] or geothermally heated brine issued from tectonic spreading centers[3]. The brine often contains high concentrations of hydrogen sulfide and methane, providing energy to chemosynthetic animals that live near the pool. These creatures are often extremophiles and symbionts.[4][5] Deep-sea and polar brine pools are toxic to marine animals due to their high salinity and anoxic properties.

Characteristics [ edit ]

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 size from less than 1 m2 to as large as the 120 km2 and 200 m deep orca basin[2]. 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 shores of the pool where bacteria and their symbionts grow near the highest concentrations of nutrient release[6].

Formation [ edit ]

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.

Brine Rejection 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. On a localized scale, that rejected brine collects in seafloor depressions. In the absence of mixing, the brine will become anoxic in a matter of weeks.[1] Salt Tectonics During the middle Jurassic period, the Gulf of Mexico was a shallow sea that dried out, producing a thick layer of salts 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.[2] Geothermal Heating At earth's oceanic tectonic spreading centers, plates are moving apart allowing new magma to rise and cool 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.[3]

Support of life [ edit ]

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 "breath" the environment and experience cerebral hypoxia due to the lack of oxygen and toxic shock due to the hyper-salinity. When observed by submarines or Remotely Operated Vehicles (ROV), brine pools are found to be littered with dead fish, crab, amphipods, and other organisms that ventured too far into the brine. Dead organisms are then preserved in the brine for years without decay due again to the anoxic nature of the pool preventing decay and creating a fish "grave yard".[6]

Despite their inhospitable nature, 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 seep mussels living at the edge of the pool. This ecosystem is dependent on chemical energy, and relative to almost all other life on Earth, has no dependence on energy from the Sun.[7]

Like the chemosynthetic communities surrounding hydrothermal vents, brine pool communities offer some of the most compelling evidence yet to suggest extraterrestrial life exists. Chemosynthetic communities offer excellent evidence for extraterrestrial life because they prove that life can exist without light or oxygen: characteristics thought necessary to support life prior to the 1977 discovery of chemosynthetic communities in the deep ocean.[8]

Examples [ edit ]