Sea Ice Safety

There are two types of ice in the Ross Sea: ice shelves, and sea ice. Ice shelves are essentially the tips of continental ice sheets and glaciers that are floating directly on the ocean as they flow outward from the continents (they are no longer sitting on ground). The Ross Ice Shelf is essentially an outlet glacier tongue for the East and West Antarctic Ice Sheets that has extended so far out that it is now floating over the Ross Sea. Sea Ice, in contrast, forms directly from freezing of the freshwater component of the ocean (the salt from the ocean will sink toward the deeper part of the ocean as the sea ice freezes at the surface). Here is a neat little diagram that illustrates the difference between the two, just imagine that Antarctica is on the left side of the diagram and the Ross Sea is on the right side; the green lines indicate the direction of flow on the glacier; and the map of the Southern Ocean below the diagram may help you get oriented on the location of the Ross Sea and McMurdo Station:
The Ross Ice Shelf is essentially an outlet glacier tongue for the East and West Antarctic Ice Sheets that has extended so far out that it is now floating on the Ross Sea. Sea Ice, in contrast, forms directly from freezing of the freshwater component of the ocean (the salt from the ocean sinks toward the deeper part of the ocean as the sea ice freezes at the surface).
Source: The Alfred Wegener Institute, via Sustainable Guernsey (click on Image for link and larger image)
Map of Antarctica and the Southern Ocean: Note, of relevance to the image above, the location of the Ross Ice Shelf, within the Ross Sea, and our location on the map (McMurdo Station).
Source: LIMA (Landsat Image Mosaic of Antarctica) Project via (click the image for the larger version)

One of the many cool and unique things about the Southern Ocean, and the Ross Sea in particular, is that the temperature of the ocean water is about -1.9ºC, just below freezing!! You’re probably sitting there thinking “Below freezing?? How can water be colder than ice?? You guys are crazy!” The answer has a lot to do with salinity. The waters of Antarctica are essentially isolated from the rest of the ocean because of what we call the Antarctic Circumpolar Current, which  flows (as the name suggests) around and around Antarctica, isolating it from the rest of the ocean currents. Because of this, warmer water from the equator cannot mix with the the colder waters of the Antarctic ocean. This is important, you see, because recall that when the sea freezes in the winter it is only the freshwater component that freezes at the surface, and the salt essentially exsolves (comes out of solution) from that freezing seawater and goes back into the deeper part of the Ross Sea. This saltier seawater in the Ross Sea partly contributes to lowering the freezing temperature of the ocean around Antarctica. An easy way to test this idea is to stick two cups of water in your freezer:  one with clean drinking water (freshwater), and one of water mixed with a spoon-full of table salt (saltwater). The water in your “clean” cup will freeze faster than the water in your “salty” cup.

Why is all of this information about ice shelves and sea ice important to us? Simply put, it is important because we live on an island (Ross Island), and in order to go anywhere outside of McMurdo Station to do work and research we have to travel across the frozen ocean. We use a range of vehicles that are adapted to travel on snow and ice, which includes snowmobiles and pick-up trucks with tracks, but I will leave the vehicle descriptions for a later post. I will only talk about one of those vehicles: the hägglund!
A Hägglund!! Jenn Erxleben, our driver and instructor for the US Antarctic Program’s Field Safety Training Program, is happy to be out on the ice with Hägglund 007 for a beautiful day of training.
© A. Padilla

The Hägglund is an articulated all-terrain tracked vehicle (as you can see above) specially designed for harsh climates and the types of conditions with a lot of snow and ice that we find in Antarctica (it was first developed in Sweden in 1974 for military use). They are amphibious, so they can travel through water as well. We use these types of vehicles (with tracks) particularly to travel around on the Sea Ice. This is how science groups studying marine organisms get out to their fishing and diving holes, and it is how McMurdo’s FSTP (Field Safety Training Program) gets around to set up safe travel routes to different locations around Ross Island and on the Sea Ice.

A few weeks ago (I am behind on posts) I got to do Sea Ice Safety training with FSTP, and it was a really awesome experience. The Sea Ice training is basically a course designed to teach you how to safely assess whether the sea ice (the frozen ocean) is safe enough for you to travel on it with a variety of vehicles, ranging from tractors and pick-up trucks, to  snowmobiles and hägglunds. First we learn how to identify features on the ice (like breathing holes that seals make, and cracks, and pressure ridges) that may be problematic for safe travel on the ice. Linear features typically indicate that there is a discontinuity below the snow & ice cover, like a crack in the ice. A pressure ridge is simply a ridge (see picture below) in the ice that forms because of compression of the ice in much the same way that mountains are formed. As an example, lets assume part of the ice fractures and a crack opens up separating the two sides of the ice. If that crack remains open long enough the water in between may freeze into a thinner layer of ice, creating a stair-stepping pattern in the ice that can then get covered by snow-pack (that’s the snow that gets blown around and compacted by the strong winds). That crack with thinner ice is now a weak feature of the sea ice, and when strong winds come racing down the shelf and sea ice they can open that crack further, or they can create enough pressure to begin pushing the two sides of the ice toward each other and forming a pressure ridge along the crack.

We also learn how to dig a trench properly in order to expose those features (our main concern are cracks in the ice), how and where to drill holes through the ice, and how to measure the thickness of the ice in order to assess whether it is safe to cross it and with which vehicles. We found two cracks on the ice that we profiled (measured their width, thickness of the ice on both sides of the crack) and determined that one of them was safe for us to cross on our hägglund, but the second (and bigger) crack was too wide and as a result the ice was too thin for us to cross safely. Below, I attempt to depict this process with the pictures that I took throughout the day:

A view of the Ross Sea Ice in the McMurdo Sound. Can you spot any continuous linear features that may indicate a discontinuity (like an ice crack)?
© A. Padilla
See that linear feature in the center of the picture? It kind of looks like a wide snow walkway within the sea ice. That is an indication of a discontinuity within the ice below the snowpack; it is the first crack we spotted during our training.
© A. Padilla
In order to analyze the ice cracks, we need to expose the top of the ice itself, which as you can see in this picture can be covered by a significant amount of snow-pack. In this case, it the snow cover is only a few inches thick, but well packed (which makes it more resistant). I dug up this ice crack myself! After that, we can measure the thickness of the ice on both sides of the crack as well as within the crack in order to determine if it is safe to travel across it with our vehicles.
© A. Padilla
An impossible to miss linear feature on the sea ice: pressure ridges. These features are a result of compression of the sea ice along big cracks (similar to the way that mountains are formed). This particular pressure ridge extends southward from Hut Point, and was a major obstacle for northbound vehicle crossings early in the season. We picked this feature to practice our newly-acquired skills on sea ice safety. Our group was the first group of the season to measure the ice thickness of this pressure ridge and assess it for vehicle crossings! Woot woot!
© A. Padilla
Once we identified the feature that we want to assess (in this case the Hut Point Pressure Ridge), we can begin with step 1, which is to dig the trench and remove the snow-pack across the feature to expose the cracks within the ice. Rory Welsch (in this picture) did a great job digging (and I helped too).
© A. Padilla
Step 2 is to drill holes across the ice so that we can drop a special measuring tape into them and measure the sea ice thickness. Harry House (my Sea Ice Safety training partner — also, casually, McMurdo Station’s Winter Site Manager), begins the drilling process after we’ve dug a trench across the ice crack.
© A. Padilla
I take a turn drilling a hole through the ice to measure its thickness on one side of the crack.
© A. Padilla
Harry House measures the thickness of the ice by dropping a measuring tape through the hole we drilled. The measuring tape has a thin and flat metal bar attached to the end of it. We drop the tape with the bar vertically, and because the measuring tape is attached in the middel, once it reaches the bottom of the hole it will turn horizontally and get stuck at the base of the ice when we pull on it. That’s how we get the ice thickness.
© A. Padilla
I am kneeling inside the trench that we dug across the Hut Point Pressure Ridge. During our transect, we drilled about 15 holes across the exposed cracks (it had multiple events, as in it cracked, then healed (froze), then cracked and healed again, leaving a stair-like geometry across the crack). The thickness of the ice across this transect varied from ~35 to ~75cm.
© A. Padilla
Another view into the trench across the Hut Point Pressure Ridge. It is approximately 3 m long, and about ½ m wide.
© A. Padilla
A good view of Mt. Discovery from the Sea Ice. Most of the features that you can see on the ice are from drift (strong winds blowing ice and snow around over the Sea Ice).
© A. Padilla
After our long day of training, we got to see the transition between the Ross Sea Ice and the Ross Ice Shelf. Here I am standing over the crack (pointing at it) that marks the edge of the Ross Ice Shelf. The part of the ice that I’m standing on is multiple meters thick, whereas the Sea Ice on the other side of that is on the order of 1m thick (though it varies).
© A. Padilla
I got to see Mt. Erebus for the first time this day! Here it looms high above the Hut Point Peninsula, with McMurdo Station tucked into the Bay. Hut Point (and Scott’s Discovery Hut) is at the left end of the image, and roughly in the center is Ob Hill.
© A. Padilla
At day’s end. The lighting was just phenomenal! You can see Mt. Discovery in the background.
© A. Padilla

It was a great learning experience, and we had fantastic weather (hardly any wind, which is one of the most dangerous weather-related features of Antarctica), and I learned a lot of great stuff that I need to know for my safety. I am now all trained and ready to explore the Sea Ice!!


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