2011 Issue

39 Figure 9:Coring Ice We needed a way to reach down to the lower depths. So we took cores of ice. A core is about 9 cm diameter and as much as a meter long (though often shorter, because it breaks up during the coring process). The ice is colder, harder, and more b r i t t l e nea r t he surface. It typically has less brine in it than the lower sec- tions that are closer to the sea water. For each core, we drilled holes and tapped s tainles s steel nails into the holes to measure the resistance between the nails. We used both 2-wire and 4-wire measurements at DC, 100 Hz, 1 k, 10 k, and 100 kHz as well as some experimental work at higher frequencies. One of the biggest challenges was coming up with a method to make good and consistent electrical contact with the ice (using the nails) without cracking and breaking it. The classical model of resistivity shows a square block of ice with plates on either end. Computer simulation of the field lines between the nails have been used to better understand the relationship between the plate-based definition and the method we applied here. We also tested and developed several methods for measuring the horizontal component of resistivity separate from the vertical resistivity, we think a nice step forward in sea ice measurement methodology. Measuring in the polar environment was pretty rough on the equipment (and sometimes on the people). We had several gorgeous, blue-sky days where work was relatively easy. We also had several days with winds up to about 30 knots and temperatures below -10 °C where the blowing snow ice crystals were like frozen sand blasting on both our faces and equip- ment. We struggled constantly with corrosion on everything. It seemed like the tiniest salt snow dust could filter in through the pores of every piece of equipment, and as soon as it thawed to water, immediately went to work corroding the most critical surface it could find. David and I spent an hour or more each day cleaning, resoldering, sanding, and caring for the equipment. We also had to account for or deal with galvanic potential and its ability to impact our measurements. We had to manage all our testing (and writing) with thickly gloved hands. Computers could not be outdoors. We had to be quick in our testing, or the very nature of the ice we were trying to measure would change. Field work is a whole lot more challenging (and interesting) than the same work done in a well-controlled laboratory. What did we learn? Sea ice is very different from the ice cubes in your drink. As salt water freezes, ice crystals form from more of the fresh part of the water, leaving behind the ultra-saline brine liquid. The brine tends to form pockets and then vertical channels through the ice, as gravity pulls it downward. These channels are large enough to see just by holding an ice core up to the sun. They are often 1-2mm in diameter and 10-20 or more cm long. This kind of ice is called ‘columnar’ ice, and the ice crystals tend to form in vertical columns. This type of ice is much more conductive in the vertical direction, following the brine channels, than in the horizontal direction, crossing the brine channels. This is the type of ice we found in the meter or so at our test site. The bottom meter or so of our ice was platelet ice. Platelet ice is formed when super cooled water comes from under the Ross ice shelf and moves up towards the surface. The produces platelets – literally small plates of ice perhaps 2 mm thick and 10 cm in diameter. Platelets tend to rest back to back with several other platelets, and then groups of these platelets gather together in a haphazard orientation. Brine fills the area between the platelets and platelet groups, and eventually the platelets form and freeze together into an ice mass that makes up the bottom meter or so of our cores. They tend to have more brine (higher conductivity) than the columnar ice, and there does not appear to be as much dif- ference between the vertical and horizontal conductivities in this type of ice. There is also a transition zone between the co- lumnar and platelet ice that we saw pretty clearly in our cores. On the Shoulders of Giants Polar science is still very much in its infancy. Barely 100 years ago, Scott, Shackleton, Amundsen and other explorers were risking and sometimes losing their lives to explore the frozen poles by foot, with dogs, ponies, and tractors. We lived and Figure10: Testing the ice, sheltered between the wanagins on a windy afternoon Figure 11: Brine channels show clearly in the columnar ice (left side). The right side is the top part of the core. Figure 12: The anchor and broken chain that originally held the ship Arora. This was a very BAD DAY. continued on page 40