What are brine pools and how are they formed?

The environments that exist on Earth can be something of a mystery, especially within the ocean, of which only 5% has been explored.

Brine pools are one of the most hostile and bizarre habitats in the marine environment. Underwater lakes are mystical sights, since the interface of seawater and brine can be seen to be flowing, or when a submersible lands on it, not only does it balance on the briny water but ripples can be seen lapping against its shores.

Diagram of a vertical cross-section of an average brine pool formation, with descriptions of the differences in densities. (Source).

These alien phenomena hold many new discoveries that alter the ways in which organisms can exist.

Found mainly in the Gulf of Mexico, but are also known to occur in Antarctica and the Red Sea. The salt lakes within the Gulf of Mexico are formed via the movement of large deposits of salt that have been buried since the Jurassic era, this movement is defined as salt tectonics. One such brine pool is named the ‘Jacuzzi of Despair‘, which is a vast brine pool in the Gulf of Mexico, named as such because of its warm temperature (19˚C) and the waterfall spilling out of from one of its edges.

Sediment has settled on top of the salt layer over the course of millennia, trapping ancient deposits beneath.

The salt layers buried beneath the sediments of the Gulf of Mexico. These salt deposits are responsible for forming brine pools, when they are uncovered and exposed to the seawater. (Sourced).

Geological events cause cracks in the surface of the sediment exposing the salt deposits to seawater. More specifically, in the Gulf of Mexico, mud volcanoes expose the salt, which mixes with the seawater forming very dense brine. The hypersaline water is between 3 and 8 times denser (depending on location) than the surrounding ocean which causes it to sink and accumulate on the ocean basin to form bodies of water that can range in size; from small puddles to vast lakes.

Source of the salt can develop other ways. In Antarctic brine pools, the salt content is a result of sea ice formation. As sea ice develops, salt is excluded, mixes with surrounding seawater which becomes denser, and thus, sinks to the bottom of the ocean forming a pool of brine.

The extreme conditions and the life surrounding

A high concentration of methane is released from the seabed within and surrounding the brine pools. A symbiotic relationship between chemosynthetic bacteria and cold-seep mussels enables both organisms to benefit each other and live successfully in the methane-rich areas surrounding the brine pools. The bacteria chemically synthesise the methane, converting it into chemical energy and nutrients for both organisms to not just survive the toxicity, but to thrive. The mussels live along the edges of brine lakes in such abundance that entire mussel beds form a ‘beach’ surrounding the lake edges.

Diagram depicting brine pools and the mussel beds surrounding the edges of the underwater shoreline. (Sourced).

The hypersaline waters of the lakes are highly toxic to organisms. The deadly brine will claim any victim (animal) entering its toxic layer and the extreme saline will embalm it. Cut-throat eels (a very fitting name) swim around the briny pools of death searching for any sustenance available and even take to diving through the deadly surface to forage for something to eat. Remaining  within the toxic brine too long left it writhing into knots, as a result of toxic shock as viewed on Blue Planet II.

Eel diving into the misty brine (left) and writhing as a result of toxic shock from the severe salinity (right). (Sourced)

Deep-sea eels can force themselves into open mussels shells to feed on the contents inside. Crabs and Squat Lobsters scavenge through the husks of mussels and other dead organisms, with all mobile animals  accidentally or even knowingly venturing into the hypersaline depths where they will surely die.

Why do organisms inhabit such hostile environments?

The extreme conditions are unforgiving, killing animals with no second chances. Yet, there are reasons for life to hang around; one of which are the warmer temperatures (18.3˚C) surrounding this environment. Another reason is the effect of the distinct contrasts between the mussel ‘reefs’, teeming with animal life and the toxic ‘deadpools‘, littered with dead and dying animals; the extreme salinity embalming its victims, creating a graveyard of animals that can be preserved for years within the brine.

The contrast provides a highly abundant feeding ground for scavengers such as eels, fish, crabs and squat lobsters to extract dead or dying organisms across the shores and within the deadly brine. There are no predators present for the scavenging animals, therefore the only true ‘killer’ is the Jacuzzi of Despair. I suppose that could be quite comforting – one less thing to worry about. But the preserving brine constantly maintains an efficient food chain and ecosystem.

 

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