Special Report: Antarctic Ice
I thought it was about time we told you about ice. After all we are surrounded by it.
So what does ice mean to you - those little cube things you get out of the freezer trays and put into a drink? Or maybe the frosting in and around your freezer? For those in Illinois of course it is different - I imagine you know a lot about snow and ice. But let me tell you even more.
There are two types of ice in Antarctica:
The first type of ice is continental ice, which is composed of fresh water. Continental ice is very old because it is formed by snow that has built up and has been compressed over thousands of years. It can reveal a lot about the Earth’s climate in the past.
The second type of ice found in Antarctica is sea ice, which forms around Antarctica each year, and melts in the summer. This ice is made from the direct freezing of salty ocean water and therefore has a different composition to fresh water ice. Scientists are studying sea ice because it holds the key to many questions about changes to the current Earth’s climate.
Apart from sea ice there is also lake ice and river ice but that’s another whole area and we won’t cover them. We have more than enough to cover in sea ice.
Because we are travelling through sea ice, I can look outside and see some of the different types of ice I will talk about. When we have finished our work at Mawson I’ll try and talk a bit more about continental ice and about icebergs - because we will see some of the plateau and ice sheet and there are marvellous icebergs around Mawson. In our talk about icebergs you’ll hear about glacier bergs, tabular bergs, ice islands, bergy bits and growlers - a whole new language.
But let’s get back to sea ice.
Sea ice, which spreads over as much as 19 million square kilometres of the Southern Ocean in late winter, profoundly affects many of the processes which relate to climate. At its greatest extent the area of sea ice exceeds that of the continent, effectively doubling the total area of ‘white’. Because it is white, sea ice reflects the sun’s heat back into space. The resultant Antarctic cooling allows more sea-ice to form and further amplifies changes in climate. But sea ice also acts as a patchy blanket restricting the critical exchange of heat and water between the atmosphere and the ocean. Furthermore, the formation and melting of sea-ice affect ocean circulation in the southern hemisphere. By altering the seasonal habitat of micro-organisms, sea ice has an effect on the food chain. It also has an influence on the absorption of atmospheric carbon dioxide.
The picture the glaciologists have of sea ice is of a complicated and highly variable mixture of different types and thicknesses. Up to 30 km from the land the ice is called fast ice, you can walk on it and it does not move. Further out to sea the ice thickness is variable and patchy and this patchy ice can extend up to 1000 km off the coast of Antarctica. In early spring, the time of maximum ice extent, up to 25% of the pack may consist of thin sea ice less than 30cm thick.
Australian glaciologists try to forecast the future by studying the past. Australia’s scientific program in Antarctica has, as its main goal, the understanding of global climate change. By monitoring continental and sea ice, glaciologists hope to understand the role that Antarctic ice plays in regulating the earth’s climate. Ultimately this should help scientists to predict and allow people to respond to any changes brought about by the greenhouse effect.
OK, but isn’t ice all the same even if it is spread across the ocean and whether it is close to the shore or further out?
No, no, no.
Let’s talk about how sea ice forms and the different stages of its development. The first thing to remember is that sea ice is due to sea water freezing, but sea water freezes at a lower temperature than fresh water because of its salt content. The actual freezing temperature varies depending on the salt content but for discussion purposes assume sea water freezes at about minus 1.8 degrees Celsius.
New ice is the term given to recently formed ice and includes a few varieties - these types of ice are composed of ice crystals which are only weakly frozen together and have a definite form only when they are afloat. The types of new ice are -
- Frazil Ice: Fine spicules or plates of ice suspended in water.
- Grease Ice: A later stage of freezing than Frazil ice - when the crystals have coagulated to form a soupy layer on the surface - grease ice reflects little light giving the sea a matt appearance.
- Slush: Snow which is saturated and mixed with water on ice surfaces or as a viscous floating mass in water after a heavy snowfall.
- Shuga: An accumulation of spongy white ice lumps a few centimetres across - they are formed from grease ice or slush and sometimes from anchor ice rising to the surface.
- Nilas are a thin elastic crust of ice, easily bending on waves and swell under pressure, thrusting in a pattern of interlocking fingers (finger rafting); Nilas have a matt surface up to 10cm in thickness. There are dark nilas under 5cm in thickness and light nilas more than 5cm in thickness.
- Ice Rind: A brittle shiny crust of ice formed on a quiet surface by direct freezing or from grease ice usually in water of low salinity. Thickness to about 5cm and easily broken by wind or swell, commonly breaking into rectangular pieces.
- Young Ice is ice in the transition between nilas and first year ice, 10-30cm in thickness and divided into grey ice and grey-white ice. Grey ice 10-15cm thick, less elastic than nilas and breaks on swell, usually rafts under pressure. Grey-white ice 15-30cm thick - under pressure is more likely to ridge than raft.
- First year ice: This is sea ice of not more than one winter’s growth developing from young ice. Ranges in thickness from 30cm to two metres. Subdivided into thin first year/white ice, medium first year and thick first year depending on the thickness reached.
Old ice: This is sea ice which has survived at least one summer’s melt; has typical thickness of three metres or more. Most topographic features are smoother than on first year ice. May be divided into second year and multi- year ice.
- Second year ice is that which has survived only one summer melt – and can be up to 2.5 metres thick. Because it is thicker than first year ice it stands higher out of the water. In contrast to multi-year ice, summer melting produces a regular pattern of numerous small puddles, with bare patches and puddles usually greenish blue.
- Multi-year ice - up to three metres or more thick and which has survived at least two summers melts. Hummocks are even smoother than in second year ice and the ice is almost salt free. Colour when bare is usually blue, melt pattern consists of large interconnecting irregular puddles and a well developed drainage system.
What’s this about multi-year ice being almost salt free? You remembered me saying sea ice was formed by the freezing of sea water, so what gives? I’m not sure of the chemistry of it but the salts in the sea water gradually leach out as the ice forms, consolidates and ages.
But what happens to the salt? If I kept going here we’d go off into a huge new realm. For the students among you or for anyone else - have a think or do some research as to what this could mean. A couple of clues: as the salts leach out of the ice, the free water around the ice gets more saline and therefore denser; think about ocean currents; look for the words ‘bottom water’.
What other forms can floating ice take?
- Pancake ice - predominantly circular pieces of ice from 30cm to 3m in diameter and up to about 10cm in thickness, with raised rims due to pieces striking against one another. May be formed from grease ice, shuga or slush or as the result of the breaking up of an ice rind, nilas, or even grey ice - under severe conditions of swell or waves. It also sometimes forms at some depth at an interface between water bodies of different physical characteristics, from where it floats to the surface - its appearance may rapidly cover wide areas of water. A few years ago when Gordon was on a helicopter flight over a huge area of fresh pancake ice, he described the effect of a light swell upon the mass of pancakes - he said it looked like poppadoms and hence the term poppadom ice for big concentrations of pancakes with a light swell. No I’m kidding - Gordon did coin the term but it is certainly not in the literature.
- Floe - refers to any relatively flat piece of sea ice 20metres or more across. Floes are divided up into size classes - giant, vast, big, medium, small. The giant ones are over ten kms across, the small ones typically 20-100metres across. Relatively flat pieces of ice less than 20metres across are known as ice cakes or small ice cakes depending on size.
- Floe berg - a massive piece of sea ice composed of a hummock or a group of hummocks frozen together and separated from any ice surroundings - up to five metres above sea level.
- Ice breccia - ice pieces of different ages frozen together.
- Brash ice - accumulations of floating ice made up of fragments not more than two metres across, the wreckage of other forms of ice.
- Pack ice - this is used to include any area of sea ice other than fast ice no matter what form it takes or how it is arranged or how old it is. Pack ice is normally described in terms of its concentration - ie, the fraction of sea surface covered by ice - measured in tenths. From the highest concentration to the lowest the terms are compact pack ice, very close pack, close pack, open pack, very open pack, open water, bergy water, ice free.
Travel through the pack ice can be very easy or it can be very hard, depending on the concentration and the type of ice, for example consolidated pack where there are floes and areas of multi-year ice is very hard to get through. As I write this we are travelling through an area with bands of close pack (seven to eight tenths coverage) interspersed with areas of new ice - a mix of frazil, grease and nilas. So we are travelling quite quickly and only occasionally hit something a bit harder but can generally steer around the harder bits. It was different last night - we came across an area of consolidated pack with a lot of ridging and bands of multi year ice. Given the poor visibility and the darkness, the ship actually stopped and we remained in the one spot for about six hours, till we could get a better feel for what was out there and start to steer toward leads and thinner areas. We were certainly not in any danger - the ship has very powerful motors.
And the last thing to talk about is fast ice.
Fast Ice is sea ice which forms and remains ‘fast’ or fixed firmly, along the coast. It can be attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. The fast ice extends in an unbroken piece to an outer edge in the open sea or the start of the pack ice. Fast ice may be formed in situ from sea water freezing or by the freezing of pack ice of any age against the shore. Fast ice may extend a few metres to several hundred kilometres out from the coast. At Davis the fast ice stretched to about 12 kilometres from the shore. To get in to the point where we could do our work, the ship had to break through this fast ice - that was a slow and difficult process because the fast ice was close to two metres thick with a 25cm layer of snow on top.
Remember that fast ice is simply floating on the sea surface and as the sea is subject to rise and fall with the tides, so is the fast ice. But where the fast ice attaches to the shore line, we have the rise and fall of the ice against the constant of the shore. This point of intersection can be very dangerous - the area is generally referred to as the tide cracks, where the fast ice is broken up, it can be weak in spots but also heavily rafted making the transition, say on foot or by vehicle, between the shore and the ice quite dangerous.
There are some characteristics of the fast ice you might like to search out for yourselves - icefoot, anchor ice and grounded ice.
I think that is more than enough for today.
So when you have your next drink with ice cubes in it, think about sea ice and how varied it is.