The idea of a lake existing within the ocean seems contradictory. While literal freshwater lakes do not form on the deep seafloor, a phenomenon occurs that is visually and physically analogous to a lake, complete with a distinct surface and shoreline. These submerged features are volumes of water vastly different from the surrounding ocean, known scientifically as brine pools. These hypersaline pockets are underwater basins where super-concentrated salt water settles in deep-sea depressions.
Defining Oceanic Brine Pools
Brine pools are defined by their high salinity, typically three to eight times greater than the surrounding ocean water. This extreme salt concentration makes the brine significantly denser, preventing it from mixing with the less dense seawater above. The hypersaline water collects in seafloor depressions, creating a visible interface that resembles a terrestrial lake surface.
The dense brine remains stable, forming distinct “shorelines” where the concentrated water meets the standard ocean floor. Submersible footage often captures visible surface ripples and waves. Pools vary dramatically in size, from small puddles to expansive features like the Orca Basin in the Gulf of Mexico, which covers approximately 120 square kilometers.
The Unique Formation Process
The existence of deep-sea brine pools relies on processes that concentrate salt beyond normal oceanic levels. One common formation mechanism is linked to ancient, massive salt deposits, or evaporites, buried beneath the seabed. These deposits, such as the Jurassic-period Louann salt in the Gulf of Mexico, formed millions of years ago when shallow seas evaporated.
Over time, these salt layers were covered by heavy sediments. Tectonic forces or geological faults caused the malleable salt to deform and push upward, a process called salt tectonics or diapirism. Seawater penetrates seafloor cracks and dissolves the exposed salt, creating a super-saturated fluid. This dense, highly saline fluid then seeps back out onto the ocean floor, pooling in low-lying areas.
A second mechanism involves hydrothermal activity, often found near tectonic spreading centers. Seawater circulates deep into the crust through fractures, dissolving minerals and becoming superheated by magma chambers. This mineral-rich brine then rises back to the seafloor, where it cools and settles into depressions. This process is notable in the Red Sea, where the brine often contains high concentrations of metals in addition to salt.
Extreme Environments and Specialized Ecosystems
The hypersaline nature of brine pools creates an environment toxic to most deep-sea organisms. The high concentration of dissolved salts, coupled with chemical compounds like hydrogen sulfide and methane, makes the water dangerous. Marine life that swims into the pool is often stunned or killed due to chemical shock and lack of oxygen.
The density difference prevents oxygen from the overlying seawater from mixing into the pool, resulting in anoxic (oxygen-free) conditions within the brine. Despite this uninhabitable environment, the pools support specialized life forms known as extremophiles. These organisms, primarily chemosynthetic bacteria and archaea, thrive on chemical energy derived from the hydrogen sulfide and methane present at the brine-seawater interface.
These chemosynthetic microbes form the base of the food web, supporting dense communities of larger organisms at the pool’s “shoreline.” Mussels gather in thick reefs around the edges, hosting symbiotic bacteria that convert the toxic chemicals into usable food energy. Creatures like eels, shrimp, and tubeworms feed on this abundance on the otherwise sparsely populated deep-sea floor.
Notable Examples and Scientific Discovery
Brine pools are primarily found in three major oceanic locations:
- The Gulf of Mexico, where they are often associated with the dissolution of the Louann salt layer.
- The Mediterranean Sea.
- The Red Sea.
These sites provide researchers with a laboratory to study chemosynthetic ecosystems and the limits of life on Earth.
One significant discovery was the NEOM Brine Pools, found in the Gulf of Aqaba in the Red Sea in 2020. Located at a depth of 1,770 meters, the discovery was surprising because it was found relatively close to the coastline, a location not previously expected to host such features.
The anoxic and undisturbed nature of the brine pools makes them exceptional archives of geological and climatic history. Sediment cores extracted from the NEOM Brine Pools have preserved an unbroken record of regional events, documenting tsunamis, earthquakes, and rainfall patterns over the last 1,000 years.
The specialized life forms and anoxic conditions of the pools are studied as potential analogs for the origins of life on Earth. They also guide the search for life on other celestial bodies, such as the icy moons Europa and Enceladus.