Martin's Antarctic Visit
January 2004
| PROJECT COORDINATORS | ACKNOWLEDGEMENTS |
Martin's Letter: 16 January 2004
This evening, Friday 16 January, we are celebrating an important milestone: we will no longer be using snowmachines to travel to our ice sampling sites. This is a great relief, as the twin-track snowmachines drive like sacks of potatoes: they are heavy, steer poorly and, unlike potatoes, make a lot of noise.
Despite our complaints about the snowmachines, they have been very reliable and made it possible for us to visit all the planned ice core sampling sites that are within a one-day round trip of McMurdo Station. It has been a busy week and time has flown by. Speaking of flying, we are scheduled to use a helicopter on Monday 19 January for a final day of ice coring at more distant sites. More on that in due course.
Tomorrow we have to go to work in the walk-in freezer to begin processing a large number of ice samples. We have not been looking forward to that moment. So far ice core processing has simply involved measuring the temperature of the ice in the field, and then cutting up the cores into 5 cm and 10 cm sections to return to the laboratory to melt before measuring the salinity. That's the easy part. The work in the freezer is more difficult inasmuch as it means working in a small, somewhat poorly lit room at a temperature of -23°C with a windchill created by noisy fans that circulate the cold air.
Sea ice is salty because, when the seawater freezes, a small amount of liquid brine is trapped between the ice crystals. As the ice ages, the brine flows out of the ice so that old ice is much less salty than young ice. We measure the temperature of the ice so that we can calculate the volume of liquid brine that is in the ice. We are interested in comparing how much brine is in Antarctic sea ice compared to Arctic sea ice, particularly in the near surface layers of the ice.
It is summer down here, yet the sea ice still has a thick snow cover that will probably persist until the autumn. If we were in the Arctic in summer the snow would melt away completely and that would lead to a large brine loss and desalination of the near surface ice layers. We are interested in the brine flow and loss process in the near surface layers of Antarctic sea ice which is largely unaffected by meltwater.
We are also interested in how the ice forms in the first place, and what it looks like when brine has drained out of it. That is why we must work in the freezer. We cut up the ice samples to make thick sections that we examine in plain, transmitted light. That reveals large scale structural variability related to the number and size of air bubbles and former brine inclusions. Then we make thin sections that we examine between crossed polarizing filters that reveal the crystal texture and thus the ice growth mechanism.
The primary goal of our study is to obtain better estimates of the thermal conductivity of the sea ice, and identify factors that lead to variability in thermal conductivity and thus the heat flow through the ice. The factors that might affect the thermal conductivity include the brine volume and the crystal texture. Hence the need to obtain ice cores for measurements of salinity and temperature, and to examine the crystal texture in the freezer.
The thermal conductivity of the ice is calculated from temperature measurements that have been made since April 2003 at a site about 3 km from McMurdo Station. Ice temperature is measured using thermistors frozen into the ice. Our New Zealand colleagues installed the thermistors and it was our responsibility to remove them this week. That involved using a steam drill to melt the ice around the thermistors, a process that took only about three hours for two sets of thermistors.
All this time the icebreakers have been busy widening the channel through the landfast ice so that a container ship and fuel tanker can reach McMurdo Station and resupply it and South Pole Station for another year of operation. We went to look at the channel on Thursday and were rewarded with the sight of about 12 Orcas breaching as they followed the chunks and floes of broken ice northward and out of the channel.
Apart from Wednesday, when it was windy and overcast with occasional heavy snow showers, the weather has continued to be glorious: bright sunshine from often clear skies. I did not expect to enjoy such persistently good weather. I don't remember it being this way when I worked at McMurdo Station in January and February 1991 and 1992. Good weather makes life so much easier when you are working in Antarctica (or the Arctic, for that matter). The photographs will give you some sense of the wonderful weather that we have been having.