Although you’ve been following our Alaskan adventures for three whole weeks, you may still be wondering: What even is sea ice? And why are we so interested in it?
Sea ice is frozen ocean water and so unlike a glacier which forms on land from accumulated snowfall. A quick glance might suggest that sea ice is a solid barrier between the water and the air, but it’s actually rather permeable, like skin or soil. Sea ice contains an interconnected network of pores containing brine (salt water) and air. Because of this, sea ice plays a role in the exchange of heat, moisture, and gases between the atmosphere and the ocean.
How does sea ice form?
Microtopography: In the Polar regions, the winter air temperatures are much colder than the water. This leads to the formation of tiny crystals of ice in the sea, which are referred to as frazil. As more and more little crystals of ice form, they begin to gather together and turn into larger pieces and finally a solid sheet. Because the air is so cold, the ice crystals begin to grow downward into the sea. The freezing temperature of salt water is lower than that of fresh water, so the newly formed crystals are almost completely fresh.
The salts are expelled as the crystals freeze, making the water around the new ice saltier. Fingers of ice, columnar crystals, grow downwards as the water and ice lose heat to the atmosphere. The highly concentrated saltwater is expelled into the channels between them, producing what we have been referring to as brine channels.
Macrotopography: When frazil ice joins up and begins to form a larger floating plates of ice, they may be jostled together, broken or even rounded and curled up at the edges so that they look like pancakes (hence the term pancake ice). Landfast ice is attached to the shore, while drift or pack ice farther out moves with currents and winds. As pack ice is constantly moving it often collides with the landfast ice, producing pressure ridges – enormous pile ups of ice blocks, with other blocks pushed under or rafted onto one another. The Inupiat people must chop a path through the pressure ridges in April in order to hunt whales near the ice edge.
The sea ice reaches thicknesses of at least a meter in the first year of growth. If it survives over a melt season (summer), it is called multiyear ice. Multiyear ice can be several meters thick. Even first year sea ice though, forms very large blocks in the pressure ridges. Some of them have a dark layer on the bottom, or even in the middle. This is sediment entrained during formation, sometimes, or an algal layer. If there is a piece of ice that has sediment or algae on the bottom and it get flipped and shoved under another piece of ice, you’ll see a dark layer in the middle of the ice.
Effects of Sea Ice Decline:
First-year sea ice is more porous, and saltier, than multiyear sea ice, which has had summer and winter seasons to drain, flush and refreeze. Overall, we know that in the Arctic the overall extent of sea ice has been declining in recent decades, this can be seen from satellite images of the region. Also, on the whole, a greater proportion is melting each summer (the summer sea ice cover is shrinking). So Arctic sea ice is declining and is increasingly first-year ice.
This has an effect on the marine ecosystem as the ice is a home for the algae; the bottom is a sheltered surface for the algae to grow, rich in nutrients, and translucent to the polar sunrise in the Spring as it is only 1-2 meters thick. Algae is at the bottom of a great marine food chain leading up to whales, a sustainable food source and income source of native peoples in the North. The Inupiat also need the landfast ice to be extensive and thick to support a successful whaling season. So the decline of the sea ice here puts a millennia old ecosystem and 1400 year old culture in jeopardy.
Because of its network of pores, sea ice also plays a role in the transfer of heat and salts from the ocean into the atmosphere. When the structure or volume of the ice is changed, these processes are also affected. This can cause cause changes in the surface heat budget and chemistry of the local ocean, the regional troposphere, and by extension the global atmosphere.
For example, algae release reactive organic halogen gases containing iodine and bromine. When phytoplankton bloom in the polar springtime, the effect is seen in regional tropospheric ozone levels (not the ozone layer). Although bromine and iodine are minor constituents in the composition of seawater, they play a disproportionally large role in tropospheric chemistry – taking part in autocatalytic (self-perpetuating) reactions that reduce tropospheric ozone. If there is less ice to support the algae, it will affect this positive feedback loop.
What is our Research?
So what are we studying? What is our greater scientific question? We are looking at the microstructure of the sea ice to better characterize the pore networks. Sea ice and climate modelers need to make a number of assumptions in order to develop models that explain and predict environmental changes. Among these assumptions are the properties of sea ice, porosity for example. The better the information the modelers have, the better models they can build and the better we will understand our climate – past, present and future.
Back at Dartmouth, we are studying the characteristics of the pore networks in sea ice with a MicroCT scanner (a miniature CAT scan). With this technique, we are able to take X-rays of sea ice, produce two-dimensional cross sectional images, and then construct these images into a three-dimensional model of the sample. We can take this data and quantify the shape and connectivity of the pores and demonstrate how these properties change with depth in the core and with temperature.
One really interesting property that I personally have found through my research with the MicroCT has been the change in anisotropy of the pores along the depth of a sea ice core. Isotropy means sameness (in dimensions for example) in all directions. Our data has shown that the porosity in the top and bottoms of the cores is isotropic while that is the middles is very anisotropic; this means that the pores on the top and bottom of the cores are globular and round, while the pores in the middle of the cores are long and skinny. This characteristic of sea ice microstructure is due to a few reasons:
- The frazil ice at the top is a jumble of small crystals that have been packed together causing the globular pores. Sometimes there is also snow-ice, where you get snow on the top of the ice that is rained on or flooded and then becomes part of the ice. If the snow had any air spaces that were not filled with water, they would tend to be isotropic.
- The anisotropic middle is due to the formation of columnar sea ice explained above. The ice crystals freeze downwards and expel the brine into brine channels.
- At the bottom, there is incomplete columnar growth (a very rough, porous bottom) which can again leave globular pores. The ice is very new and warm.
The novel aspect of our research this time around is that we will be keeping the ice at its in situ temperatures from the time we collect it until the time we analyze it (this is achieved with the ICE-MITTs). Up until now, sea ice cores have been stored at uniform temperatures throughout, or isothermally, such as in a -25˚C freezer. Super freezing the ice, just like warming and refreezing the ice, can cause structural changes internally. We are excited to see how the ICE-MITTs change or improve our data.