Tuesday 22 October 2013

Brinicles - ‘Icicles of Death’

by Danny Stubbs

Stretching down from the surface of the ocean, these Icy spikes give unfortunate bottom dwellers an unpleasant frosty fate. In short, they form when supersaline (very salty) water subsides from the surface of sea ice and sinks towards the seabed, forming an icy sheath during the process. This phenomenon was first filmed by the BBC as part of their series ‘The Frozen Planet’ in 2011.

So how do these sub-zero spears form? One of the main things you need to know is that salt lowers the freezing point of water by acting as an impurity. At 0°C (the freezing point of water and melting point of ice) there is an equal amount of molecules entering and leaving the solid state. Salt interrupts this exchange of water molecules and stops them entering the solid phase, hence the water remains in a liquid form. At a lower temperature the amount of molecules leaving the solid state balances the amount entering, allowing the water to freeze but at a lower temperature than freshwater. As a result saltwater freezes at minus two degrees Celsius.

As the seawater freezes the salt becomes concentrated because it doesn’t fit in with the lattice structure of the ice and it is cooled below zero degrees. This creates an area of water with high concentrations of salt (brine) at temperatures below freezing. Since salty water is denser than pure water and colder water is denser than warmer water, the brine begins to sink and forms a vertical column called a brine plume that stretches towards the seabed. The water around the column is cooled below zero and begins to freeze, forming a ‘Brinicle’.

What makes them important?Brinicles have a biological impact, they freeze any sluggish organisms that are too slow to escape the freezing of the seabed. However they also create areas that never fully freeze in regions such as in the Antarctic, which prove to be a vital refuge for some critters during the bleak winter months.These brine plumes are also important for the climate because they aid the circulation of water throughout the oceans of the world. The heavy supersaline water that sinks and migrates towards the equator helps replenish the cool water that warms and rises in areas such as the tropics. 

Check out these incredible time lapse images of the Brinicles forming.

While these principles are fresh in your mind, think about why we add salt to icy roads in the winter months. Did you figure it out? The salt lowers the melting point of ice so it more readily turns back into liquid form. Ice and salt – much more compelling than you originally thought!