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:
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!
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:
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!!