The question of whether a true lake can exist beneath the ocean is answered by the deep-sea phenomenon known as the brine pool. These are distinct bodies of hypersaline water collected in seafloor depressions, acting as separate reservoirs from the surrounding ocean. Scientists sometimes refer to these features as deep-sea anoxic basins due to their extreme chemical composition and lack of oxygen.
The Scientific Reality of Brine Pools
A brine pool is defined by its extreme salinity, which is typically three to eight times greater than the surrounding seawater. This hypersaline water is significantly denser than normal ocean water, preventing it from mixing easily with the overlying water column. The density difference is so pronounced that it creates a visible, defined boundary, or interface, on the seafloor.
This sharp division between the two water masses is a form of density stratification known as a halocline. The heavier, salt-saturated water remains trapped in the basin, forming a distinct layer observable by remotely operated vehicles. Brine pools vary greatly in size, with some being less than a square meter, while the largest, like the Orca Basin in the Gulf of Mexico, spans approximately 120 square kilometers.
How Underwater Brine Pools Form
The formation of deep-sea brine pools requires a source of salt that can be dissolved into the water column. One common mechanism involves salt tectonics, the movement and deformation of ancient, buried salt layers. In areas like the Gulf of Mexico, vast salt deposits were left behind when shallow seas evaporated millions of years ago, subsequently buried by heavy sediment.
The pressure from the overlying sediment causes the malleable salt to deform and push upward, sometimes creating cracks in the seafloor. Seawater seeps into these fractures, dissolving ancient salt deposits—such as the Louann salt—into a highly concentrated brine. This dense brine then flows out onto the seabed, pooling in topographic depressions.
A different formation process is driven by hydrothermal activity, commonly seen in areas like the Red Sea. Seawater penetrates the oceanic crust near tectonic spreading centers and contacts high heat sources from magma chambers. The water dissolves minerals and salts from the surrounding rock, becoming superheated and highly saline. This hot, dense brine rises through vents and settles into basins, where its high density prevents dispersal into the surrounding ocean.
Life in the Extreme Brine Pool Ecosystem
The conditions within the brine pool are extremely hostile to most marine life, characterized by anoxic water lacking dissolved oxygen. The brine is also saturated with toxic chemicals, including hydrogen sulfide and methane, dissolved from the underlying salt and sediment layers. For most fish and invertebrates, entry into the pool is fatal due to toxic shock and the extreme difference in salinity, which causes cells to rapidly lose water.
This toxic environment creates a “graveyard effect,” where the bodies of animals that fall into the brine pool are often preserved due to the lack of oxygen and the pickling effect of the salt. However, the harsh chemistry supports a specialized ecosystem of extremophiles, primarily microbes like archaea and bacteria. These organisms are chemosynthetic, using toxic chemicals like methane and hydrogen sulfide as an energy source instead of relying on sunlight.
Life flourishes most abundantly along the boundary where the toxic brine meets the oxygenated seawater. This interface supports dense communities of chemosynthetic bacteria that form the base of a unique food web, attracting larger organisms. Specialized species like mussels, shrimp, and tubeworms thrive near the edges, benefiting from the chemical energy while avoiding the deadly brine.