The cold temperatures of the poles lead the seawater there to be the highest density in the world. These pictures
show an interesting phenomenon that takes place under the Antarctic ice sheet. As ice forms at the surface the
ions that are forced out saturate the surrounding water creating pockets of super-salty brines. Their freezing
temperature drops substantially below that of normal seawater – which allows them to get as cold as -6 or 7
degrees Celsius. The highly dense cold and salty brines drill down through the ice and when they meet the
normal seawater underneath, with a normal melting/freezing point of -1.5 degrees Celsius, the much colder
brines cause the first water they touch to freeze. The result is the formation of a hollow tube of ice as the brines
descend. The tube is made of frozen normal seawater. Within it descends the super salty brines from above. You
can see what happens to these fluids as they hit the seafloor – they don’t mix easily because their density is so
much higher than the surrounding water. Instead they stay on the bottom and spread out across the seafloor.
This same thing happens on a larger scale in the oceans as a whole. This bathtub cross-section of the oceans shows
the equator in the middle, the Arctic on the left, and the Antarctic on the right. Notice that ALL the deep water in
the world’s oceans is formed by cold dense waters in the poles sinking and spreading out laterally. What’s left is a
thinner layer of warmer surface water sitting over cold dense polar waters. Even at the equator, where the surface
waters can be quite warm, the water at depth is cold polar water. The boundary between these two water layers is
called the a pycnocline – pycno means density – cline means changing. It is the zone where the density changes
substantially from the surface layer to the deep layer beneath.