#denisty

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 the bottom 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.

-Amy

References:

http://www.tos.org/oceanography/archive/20-4_fisher.pdf

http://oceanexplorer.noaa.gov/explorations/02mexico/background/brinepool/brinepool.html

http://oceanexplorer.noaa.gov/explorations/02mexico/background/brinepool/brinepool.html

During certain times of the year, lakes separate into different layers according to their density or temperature. Temperature and density directly correspond when it comes to water. Water is its densest at 4 degrees Celsius. (This is because water expands when it freezes as opposed to most other matter. This also explains why ice floats.) Below is a graph which shows the density/temperature relationship for distilled water. Shaded areas show relative difference in density for 5 degrees Celsius temperature changes. 

There are different types of lakes which all hold different types of stratification cycles. The basic cycle for a dimictic lake, which is most common, can be seen in the lowermost picture. This is when two turnovers occur during the course of a year. A turnover is when all of the lake water of different densities and temperatures mix together to get an overall same temperature. The oxygen, which had previously been depleted in the past season, is replenished in the different layers of a lake.  A meromictic lake can be seen below. Meromictic lakes never fully turnover. There is still a bottom layer, monimolimnion, which the absent oxygen can never be replenished.