Tuesday, June 18, 2013

A Lake in the Ocean?

Imagine you could take a deep-sea submersible down toward the bottom of the Gulf of Mexico. As you approach the bottom, you see a light-colored pool of water rippling below you. You try to descend into it and feel a “bump” as you hit the top of the pool and find your submersible floating on the surface of an undersea lake. It is indeed true, lakes and rivers are found at thebottom of the Gulf of Mexico. These areas are known as cold seeps, or brine lakes. These lakes are only about 10 inches deep, but they have a form a very different habitat than the surrounding ocean. They are much denser than the surrounding ocean water and typically have a salinity level three to five times greater.

The cold seeps are formed by a process known as salt tectonics. During the Jurassic, the Gulf of Mexico was a shallow sea. It was cut off from the rest of the ocean, and eventually the water evaporated, forming huge salt deposits. Rifting caused the ocean connection to reopen and flood the area with sediment and water. The salt deposits were buried and preserved. As the sediment layer grew, it became heavier and heavier, causing the salt below to deform and move. Some of the salt deposits rose near the surface as domes. Other areas of salt seeped out, dissolved, and the denser water sank into lower basins.

The high salinity, high methane environment of the brine lakes creates a hostile environment for most ocean creatures. However, bacteria thrive in these locations. Often, the “shores” of the lakes are covered with methane-using (methantropic) mussels and clams. Giant tube worms with lengths up to three meters have been found in many cold seeps on the Gulf.





Photo of brine lake, East Flower Garden Bank, Gulf of Mexico, credit NOAA

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