The ice pack: frozen saltwater
A layer of ice on the surface of the sea
Once the Arctic summer is over, the freezing Polar winter (-40°C) sets in, sometime quite suddenly. The surface of the ocean gets colder, and when the water reaches -1.8°C the first ice crystals form. Once there is a thin layer of ice, it insulates the seawater from the colder air and the process slows. From then on, the pack ice slowly gets thicker, with the new ice forming on its underside, until it is about 2 metres thick.
Freshwater or saltwater ?
As it freezes, seawater forms a matrix of crystals of freshwater ice and small drops of saltwater. During the winter, these droplets migrate together to form wider networks and finally whole pockets. As they do so, they also migrate downwards until they “re-enter” the seawater. This means that as the sea ice gets older, it becomes less salty.
The sea ice life cycle
In mid-winter, the pack ice extends over nearly 15 million km2. During the summer, about half of this melts. The remaining pack ice can last 2, 3 or 4 years, and sometimes more, and can grow to be 4 or 5 metres thick. During its lifetime, this surface crust of ice will travel right across the Arctic Ocean, driven along by ocean currents. This is called the Arctic or Transpolar Drift.
A chaotically jumbled raft
Compared to the 4,000 metres of water it floats on, the layer of sea ice is like a thin eggshell. As it is pulled along by the currents and blown by the winds, this fragile shell cracks and opens up to form ice-free leads, while the broken plates of ice ride up on one another forming pressure ridges up to 10 metres high.
The first freeze
The nature of the first thin layer of ice to appear depends on the state of the sea. When the sea is calm, large ice crystals form parallel “needles”, building into a surface layer 1 or 2 cm thick (rather like iced soup). If the sea is rough, the first freeze produces crystals facing in all directions that build up into a thick viscous layer about a metre thick called “grease”. Only then will the surface later truly freeze hard, but this can take between 9 and 36 days.
The ice becomes thicker
While the layer of ice is still thin, the swell and the wind can break it into “pancake ice” but in the end the pancakes will fuse together again to form a layer of young sea ice on which the snow can settle. Within a few weeks, the ice can be up to 60 cm thick. As the bubbles of air and pockets of saltwater are eliminated, the ice gradually becomes more compact, blue and translucent.
The composition of the sea ice
In the Arctic, the salinity of the surface layer of seawater varies between 32 and 33 g/l (compared to an average of 35 g/l for the overall oceans). The first crystals of ice form little slats 1 mm thick that trap tiny (about 10 micrometres) droplets of saltwater as the crystals form a network. Newly formed sea ice can contain up to 22 g/l of various salts. As seawater also contains impurities and air bubbles as well, they too will be trapped in the ice.
Blown by the wind and driven by the currents and tidal streams, the sea ice constantly breaks up and reforms. Channels called leads can open up, even becoming quite large lakes (or polynyas) in the middle of the ice pack in zones where there are upwellings of “warmer” water. Then the plates of ice can move together again, closing the leads, and even colliding so strongly that they form pressure ridges that can rise upwards as high as 10 metres (these are “sails”) or downwards as deep as 60 metres (these are “keels”).
Close to the coast the ice pack can be reshaped by the tide, by input of freshwater (rivers), by coastal currents and by exposure to the sun and wind. Sometimes a large plate of ice can ground on the coast and remain attached to the land. This coastal ice, once hard aground, can become thicker than the rest of the ice pack and can be accompanied by a strip of “fast ice” (occasionally with ice bergs trapped within it) that can sometimes last as long as 10 years.
When the thaw comes
By late May, the snow and the top layer of ice (a few centimetres) begin to melt, forming puddles and ponds on the surface of the pack ice. This water reflects back less of the solar energy than bare ice does and warms faster, thus speeding up the thaw locally. When the sea ice starts to break up, large sections of ice called floes start to drift across the ocean. But in the central part of the Arctic Ocean and along the more sheltered stretches of coastline, the ice stays frozen.
An ice ballet: the Arctic Drift Current
During the warmer months, the enormous jigsaw puzzle of ice floes is borne along by the main Arctic Ocean currents or drifts. There are two main currents in the region: one is a circular current called the Beaufort Gyre, centred on 80°N, 155°W and the other is a transversal current called the Arctic or Transpolar Drift that runs from the Bering Strait to the coast of Greenland. On average, ice floes swirl in the Beaufort Gyre for 5 years and a block of ice takes about 3 years to complete a transpolar journey.
The Arctic currents were discovered thanks to the debris of the vessel “La Jeannette”, shipwrecked in 1881, which was found 3 years later thousands of kilometres from where the ship broke up. Both Nansen in the “Fram” (1893-1896) and Papanine in 1937 used the drift currents.
Studying the ice pack
All aspects of the Arctic ice – ice-free leads, pressure ridges, ice of varying salinity or age, seasonal pulsation, drift rates – are now being studied by scientists on drift stations set up on the pack ice and also using tracking buoys and satellite imagery.
By ship through the pack ice
The amount of ice present is usually expressed subjectively as tenths of the ocean surface, so pack ice with a density of 9/10 or 10/10 is considered impenetrable, except aboard a powerful icebreaker. Jean-Louis Etienne’s sailing vessel “Antarctica” could pursue its journey in pack ice with a density of 4/10 or 5/10.
The upper layer of water of the Arctic Ocean freezes quite easily because its salinity is reduced by the influx of relatively low-salinity water from the North Pacific and by freshwater coming down the major boreal rivers. In winter, this water becomes colder and thus more dense/heavy, so it should plunge towards the ocean depths. However, in the Arctic there is a layer of saltier (and thus denser) water that prevents the surface layer from sinking and keeps the surface water in contact with the cold air for longer.
“One phenomenon that is specific to the higher latitudes is ‘smoking sea’ that occurs in winter. It is usually due to the arrival of colder air over a relatively warm stretch of ocean; this triggers very high evaporation that looks very like smoke rising off the sea”.
(Geostrategy of the Arctic, Economica, 1992).
Thicker ice conducts heat less well and so has a higher freeze rate. “In open leads, the freeze rate can be several centimetres per day, but thick ice adds extra ice much more slowly, growing by as little as a millimetre per day. This is an offsetting effect of thermodynamics which, on the one hand tends to fill in open leads and, on the other hand tends to limit ice growth on the underside of pressure ridges.
(The planet’s ocean – Science et Vie, 1994).
The fact of freezing immobilises the molecules in liquid water that were, until then, constantly moving (thermal agitation). So when pack ice freezes it frees an amount of energy equivalent to that involved in the thermal agitation, and this energy warms up the surrounding air. This energy must come into play again if the ice is to thaw: the energy sets the water molecules in motion once more.
For 1 gram of ice to melt at 0°C it requires nearly 70 calories of energy (latent melting heat), which is 8.5 times less energy that it takes to make 1 gram of water evaporate at about 20°C (latent evaporation heat).
A calorie is defined as the amount of energy required to raise the temperature of 1 gram of water by 1°C. This energy is provided by 4.186 joules of work (1 joule per second of work is equivalent to 1 watt.
The density of saltwater ice can vary widely (between 0.857 and 0.920), depending on its salt and air content. Seawater in the Arctic (density between 1.024 and 1.026) gives good buoyancy.
The expansion coefficient of saltwater ice plays an important role in the changes that the ice pack undergoes. Unlike the expansion coefficient of freshwater ice, that of saltwater ice varies considerably with the temperature. In addition, each chemical salt present in the ice reacts differently to freezing. This means that there will be tensions – and sometimes very strong – within blocks that contain ice with different salinity.
Some useful vocabulary
Jean-Louis Etienne on the ice pack
“From time to time, the pack ice rose up before my very eyes. The different plates of ice can collide and grind against each other with a terrible noise. Sometimes the two plates will rise up and then fall back with a thunderous roar you can hear for kilometres around. Or there will be a loud explosion as the pack cracks and splits, leaving a sinister zig-zag crack of dark sea water. At first the water will evaporate generating a curtain of fog, then when the crack has stabilised, the water will freeze over again”.
The Poles, Arthaud, 1992.
During the Erebus Mission to Antarctica in 1994, the sailing vessel “Antarctica” was trapped in the pack ice for days at a time. The ice in the Ross Sea was unusually compact that year. The crew tried to find a passage through the ice by observing the colour of the sky, which is often darker when it reflects a lead in the ice. On one occasion, the vessel was unexpectedly able to follow in the wake of an American icebreaker.
Did you know ?
> One of the processes used to desalinate seawater is freeze it. If the water is frozen very slowly, the droplets of salty water do not freeze into the ice, giving pure freshwater ice.
> Regions covered by glaciers and/or sea ice tend to have dry climates because the ice cover prevents water vapour evaporating into a low-pressure atmosphere.
> hen the sea freezes, the pressure resulting from ice formation (ice takes up more volume than water) is so great that it can crush the hull of a ship (the “Jeannette” in the Arctic, the “Endurance” in the Antarctic. Even the hull of an oil tanker.
> Saltwater ice becomes less salty with time. The British say that “one-year ice is good for nothing, two-year ice is OK to cook with and only three-year ice will do to make tea”.
The pulsation of the ice pack
To find out more …
- L’Arctique et l’environnement boréal (P. Avérous – CNDP, 1995)
- Les Pôles (J.-L. Etienne – Arthaud – La Nouvelle Odyssée,1992)
- Géostratégie de l’Arctique (Amiral Besnault-Economica-1992)
- La géographie de l’océan (J.-R. Vanney-Oceanis-1991)
- L’océan planétaire (Sciences et avenir, Hors série n° 98-1994)
- L’homme et le climat (J. Labeyrie-Denoël-1993)
- Oceanus (Woods Hole Oceanographic Institution-1986)
- The Nordic Seas (Burton G. Hurdle Ed., Springer-Verlag-1986)
- Encyclopedia Universalis