NATURAL PHENOMENA : BRINICLE

A brinicle (brine icicle, also known asice stalactite) forms beneath sea icewhen a flow of extremely cold, saline water is introduced to an area of oceanwater, being the undersea equivalent of a hollow stalactite or icicle.

Known since the 1960s, the generally accepted model of their formation was proposed by the US oceanographerSeelye Martin in 1974.^[1] The formation of a brinicle was first filmed in 2011 by producer Kathryn Jeffs and cameramen Hugh Miller and Doug Anderson for the BBC series Frozen Planet.^[2]

At the time of its creation, a brinicle resembles a pipe of ice reaching down from the underside of a layer of sea ice. Inside the pipe is the supercold, supersaline water being produced by the growth of the sea ice above, accumulated through brine channels. At first, a brinicle is very fragile; its walls are thin and it is largely the constant flow of colder brine that sustains its growth and hinders its melt that would be caused by the contact with the less cold surrounding water. However, as ice accumulates and becomes thicker, the brinicle becomes more stable.

A brinicle can, under the proper conditions, reach down to the seafloor. To do so, the flow of supercold brine from the pack ice overhead must continue, the surrounding water must be significantly less saline than the brine, the water cannot be very deep, the overhead sea ice pack must be still, and currents in the area must be minimal or still. If the surrounding water is too saline, its freezing point will be too low to create a significant amount of ice around the brine plume. If the water is too deep, the brinicle is likely to break free under its own weight before reaching the seafloor. If the icepack is mobile or currents too strong, strain will break the brinicle.

Under the right conditions, including favorable ocean floor topography, abrine pool may be created. However, unlike brine pools created by cold seeps, brinicle brine pools are likely to be very transient as the brine supply will eventually cease.

On reaching the seafloor, it will continue to accumulate ice as surrounding water freezes. The brine will travel along the seafloor in a down-slope direction until it reaches the lowest possible point, where it will pool. Any bottom-dwelling sea creatures, such as starfish or sea urchins can be caught in this expanding web of ice and be trapped, ultimately freezing to death.

The formation of ice from salt waterproduces marked changes in the composition of the unfrozen water. When water freezes, most impurities are forced out of solution; even ice from seawater is relatively fresh compared with the seawater it is formed from. As a result of forcing the impurities out, sea ice is very porous and spongelike, quite different from the solid ice produced when fresh water freezes.

As the seawater freezes and salt is forced out of the pure ice crystal lattice, the surrounding water becomes more saline. This lowers its freezing temperature and increases its density. The lower freezing temperature means that the surrounding water does not freeze to the ice immediately, and the higher density means that it sinks. Thus tiny tunnels called brine channels are created all through the ice as this supersaline, supercooled water sinks away from the frozen pure water. The stage is now set for the creation of a brinicle.

As this supercooled saline water reaches unfrozen seawater below the ice, it will cause the creation of additional ice. If the brine channels are relatively evenly distributed, the ice pack grows downward evenly. However, if brine channels are concentrated in one small area, the downward flow of the cold water, now so saline that it cannot freeze at its normal freezing point, begins to interact with unfrozen seawater as a flow. Just as hot air from a fire rises as a plume, this cold water descends as a plume. Its outer edges begin to accumulate a layer of ice as the surrounding water, cooled by this jet to below its freezing point, ices up. This is a brinicle: an inverted "chimney" of ice enclosing a downward flow of this supercold, supersaline water.

When the brinicle becomes thick enough, it becomes self-sustaining. As ice accumulates around the down-flowing cold jet, it forms an insulatinglayer that prevents the cold, saline water from diffusing and warming. As a result, the ice jacket surrounding the jet grows downward with the flow. It is like an icicle turned inside-out; rather than cold air freezing liquid water into layers, down-rushing cold water is freezing the surrounding water, enabling it to descend even deeper. As it does, it creates more ice, and the brinicle grows longer.

A brinicle is limited in size by the depth of the water, the growth of the overlying sea ice fueling its flow, and the surrounding water itself. In 2011, brinicle formation was filmed for the first time.^[3]