Science highlights of the 2008-09 season
As we approach Troll Station, we pause to summarize our activities and reflect on our accomplishments this season.
Location: Gygra, 71º 57’ S, 3º 13’ E, 1550 metres a.s.l.
Weather: Clear, -25 C, wind 16 kts
Map of science sites visited en route
We have enjoyed a very successful season, and have met virtually all of our science objectives. Between the radars, ice cores, snow pits, gravity surveys, and thermistor strings, it is sometimes hard to keep in mind the larger picture of what we are trying to accomplish with this project. This was the second field season of a larger four year project to better understand climate change in the Dronning Maud Land sector of East Antarctica. There is significant uncertainty about how this region is changing, in terms of both temperature and accumulation rate. Taken together, our experiments and measurements along the traverse route form an integrated set of observations to better understand the climate here, and how it is changing.
The most detailed climate information will come from analysis of the ice cores we have collected (Diary 14 Feb). Back in our labs in Norway and the United States, we will measure the chemistry and physical characteristics of these cores to determine how much snow falls at the coring sites each year, and how that snow fall rate has changed through time. The hand-drilled cores will span the most recent 50 years; the 30m cores we collected at sites 2, 3, 4, and 6 should record the last 200 years of changes; while the longer cores from sites 5 and 7 should reveal more than 1000 years of climate history. These measurements take time, and won’t be completed before 2010. As with so many other aspect of field work, patience is a virtue.
While the cores will provide very detailed information, they only give information at a very specific place. When comparing the size of the ice core (about 50 square cm) with the size of the ice sheet (about 10 million square km), it is apparent the ice core is a very small sample of the total. The question becomes: how representative is an ice core of a larger area? The radar surveys help us address this issue and extend the reach of the ice cores.
Zoe Courville logging an ice core. Photo: Stein Tronstad.
We used five different radars this season (Diaries Dec 31, Jan 12, Jan 15), which image the internal layers of the ice sheet. The high-frequency radars image the upper 20 – 100 metres of the ice sheet, while Kirsty’s low-frequency radar images all the way to the base of the ice. Ideally, the UAV would have also mapped internal layers using the on-board radar system, but owing to some unexpected complications, this was not possible. The layers revealed by these systems are essentially horizons of constant age, and are used to determine how the accumulation rate varies along the traverse route. In areas with relatively low accumulation rate, these layers are closer to the surface. In areas with relatively high accumulation rate, the layers are buried deeper. The ice core data serves as calibration points for the radar layering, and provides the ages for the near-surface layers detected by the radars. The radar data allows us to extend the ice core data from point measurements into data along the lines covered by the radars.
Anna Sinisalo at work behind her radar screens on board "Sembla". Photo: Stein Tronstad.
Satellite data allows us to further extend the footprint of the ice cores. Since the mid-1970s, satellites have imaged East Antarctica at a variety of wavelengths, but without detailed calibration, it is difficult to understand how to interpret these images. Zoe has been working on measuring the physical properties of the upper 2 metres of the snow in her snow pits (Diaries Jan 16, Feb 9; e.g. grain size, permeability, thermal conductivity, density), and will conduct similar measurements on some of the ice cores. These measurements will make it possible to determine exactly what has been recorded in satellite images. Information on the physical properties, coupled with the ice core data, will make it possible to better understand the changes recorded by satellites over the past 30 years, and will extend the reach of our analyses to cover the entire region.
So while the radar data and a combination of satellite and physical properties data allow us to extend the results of the ice core analyses over a wider area, we also made use of the holes produced by collecting the deep ice cores. Ted has installed two thermistor strings (26 Jan) to directly measure the ice temperature in the deepest core holes. The temperature on the ice sheet surface changes with the weather, but the temperature deeper in the ice sheet only changes very slowly as the climate changes. At 90m depth, the ice temperature is determined by the average temperature over the past 30-50 years. By recording this temperature through time (and these stations should be continuously operational for 3-5 years) we can determine how the surface temperature has changed through time. Though not as detailed as a record from a weather station, since there are no weather stations in this part of the continent, Ted’s thermistor measurements are as close as we can come to measuring surface temperature changes directly.
Taken together, the radar data, snow pit studies, ice core analyses, and satellite data will provide a much better understanding of how climate is changing in this part of Antarctica. Our route this season also provided an opportunity to explore the Recovery Lakes region (Jan 11), an area last studied during a traverse in the 1965-66 field season. As we have read, this area is marked by a series of lakes beneath the ice sheet which lie at the head of the Recovery Ice Stream, one of the largest glaciers draining East Antarctica. The lakes were first discovered via satellite in 2006. There are four well-identified subglacial lakes in this area, and several other potential lakes (Jan 27). The lakes range in extent from 600 sq. km to about 1500 sq. km, making these the largest subglacial lakes in Antarctica, aside from Lake Vostok.
Using our radar systems and high-precision gravity and GPS measurements, we mapped the ice thickness, internal layering, surface topography, and gravity variation along our traverse route and along several shorter side traverses over the lakes. These measurements detail the size and shape of the lakes, and provide information about the water depth, as well as how the ice flows over the lakes. Although the lakes were first discovered by satellite imagery, ground-based surveys are the best way to determine the lake geometry, and the ice flow characteristics. We collected over 800 km of radar data along our side traverses, installed two high precisions GPS stations, and conducted high-precision GPS surveys of the surface topography along our routes. The low-frequency radar has revealed that two of the lakes are actually connected, the average ice thickness over the lakes is between 3400 and 3500m, while the ice thickness over the lake margins and grounded parts of the ice sheet is typically less than 3000m. The high-frequency radar surveys have shown that there are significant differences in accumulation rate across the lake margins, and that there are several areas with virtually no accumulation (so-called glaze areas). The gravity data (Jan 5) will take more time to fully process, but initial results suggest water depth in excess of 100m.
We have had a very productive, very busy season, and as we arrive at Troll Station tomorrow, we can be justifiably proud of our work here. By the time the project ends in 2011, we will have made great progress in understanding how this part of Antarctica fits into the larger climate in the southern hemisphere, how the climate here has changed in the past, and how Dronning Maud Land may change in the future.