Friday, December 26, 2014
Ice core drilling is challenging and therefore, very exciting. This season, we had set the objectives very high for the ice core drilling team, Jean-Louis Tison, PhD student Morgane Philippe, and mechanic Kristof Soete, who were all very happy to take that challenge! If you remember well, in 2012, we drilled 120 meters in 11 days, and we encountered some problems on which IPF and Eclipse Ice Drill have been working since then. This year, we planned to drill 3 boreholes: two shallow-firn cores of 30 meters and one ice core of about 160 meters, in 17 days.
After setting up and being stuck for 2 days by the first storm, we could finally start drilling the first 30 meters at Derwael Ice Rise on the 3rd of December. This drilling and the next one went really smooth. Almost every run yielded a 1.40m beautiful piece of firn core, with very few thin ice layers (less than 1mm). The two 30m boreholes were drilled within 3 days, just before the second storm hit us. These boreholes were also “televiewed” which means that we recorded a digital image of the borehole with a special camera, at very high resolution.
On the Roi Baudouin Ice Shelf, drilling the third hole was not such an easy task. There are several reasons for that: one of them is the number of ice layers, sometimes 20 cm thick that the drill encounters regularly during its progression. Another reason is that we reached ice much faster on the shelf, (~30 meters) and the firn – ice transition is always a difficult part to go through, since we have to adjust many parameters. For example, we had to change to more powerful core dogs, designed by IPF, otherwise the drill head could not break the cores anymore.
But the real problems started at 54 m, when the drill got stuck at the bottom… after trying all we could do without damaging the drill (blowing the fuses, using the hand crank, leaving the cable under tension for one night), we decided that we would need to poor diluted antifreeze down the hole, as it has been done in other drilling programs. It was done within 24 hours thanks to a very efficient logistical help from IPF and a tube transport system. To everyone’s relief, the drill was loose the next morning. Surprisingly, the core was in very good state in the barrel, so we decided to try restart in the same hole, and it worked! From that moment, we worked non-stop during two days and two nights, with the whole team taking turns to drill as deep as possible, slowly removing the leftover antifreeze at the same time. Unfortunately, time was running out and we could not miss our plane, unless we wanted to stay in Antarctica until February. We managed to go down to 107m, instead of the 155 meters needed to reach the ocean, but this already represents a lot of valuable material, as well as a very informative televiewer image of the borehole, showing precisely every single ice layer, a signature of about 200 years of the surface melting history in this typical place on the ice shelf. We will soon compare it with all the other data: those from the different radars used in the IceCon project, but also the data from the Benemelt project.
Thursday, December 18, 2014
Back at the station!! And here a very short overview of the accomplishments of the IceCon project. Over the next couple of days, more in-depth accounts will be given here.
We left PEA on 27 November in the evening for a night and a day travel to Derwael ice rise. Camp installment went smoothly, but after one day we got caught in a storm, and not a nice one. Atmospheric pressure dropped very quickly, winds picked up and snow drift was amazing. Furthermore, it was quite warm, meaning that snow melted in contact with persons and goods. We had to leave are tents and sleep in the containers for 2 days. The storm lasted for more than two days and it took another day to clean up camp. For science, this meant a delay, but it is amazing how quickly we picked up after that.
|Getting the contents out of the tents during the storm at Derwael ice rise|
The drill hole of 2012 was retrieved (under three meters of snow) and was again televiewed: this is an instrument that images the interior of the borehole. It was done in 2012 and we hope to see changes in the internal structure when comparing with the 2014 data. The cGPS station was again covered under a lot of snow (as usual), but also here the system worked perfectly through the Antarctic winter and all data were retrieved. In a couple of days Nico managed with the help of Kristof, Raphie, Cristophe and Jan to get also this station installed again for another (third) winter. Another task was to drill two 30 meter firn cores at either side of the ice divide of Derwael. While it would normally take 2 days for each of them, it was remarkably done in less time, including the installation of the drill equipment at either site (4 kilometers apart). We noticed from the drilling in 2012 and subsequent radar surveys that the accumulation variability across the ice divide is very large, with one side having an accumulation rate that is twice as high as the other, due to a so-called foehn effect, leading to more precipitation at one side and erosion due to sublimation and wind scouring at the lee side of the ice rise. If you are in the field, you have the impression that the ice rise is a gently sloping, hardly noticeable rise in the surface elevation, but its effect on mass balance is quite impressive. All boreholes were subsequently linked through radar profiles with our snow radar. Finally, we installed also a whole network of pRES measurements. This new type of radar enables us to measure directly change in the internal structure of the ice, which is necessary to identify the effects of snow compaction and strain to interpret the cGPS data. We already identified that the surface of Derwael ice rise is thinning, but we wish to reveal by how much exactly and what the processes are behind. The pRES data will complement the results from our coffee-cans we installed two years ago and have been functioning for the last two years. Unfortunately, only two of them were still operational (again under three meters of snow!) and we had to abandon them this time. However, the pRES will hopefully give more detailed insight in the dynamics of the ice. Our work at Derwael ended again with a storm, this time less forceful (we could stay in the tents), but it was again a two-day delay (storm + getting the camp cleaned up).
|pRES measurements at Derwael ice rise|
|Digging out the cGPS station|
In the morning of 8 December we moved the camp to the Roi Baudouin ice shelf (RBIS) for another series of exciting measurements. One of them was the third drilling through the ice shelf. Although such a floating ice shelf looks relatively flat with a constant ice thickness between 250 and 300 m, some areas are significantly thinner. These are elongated depressions hardly a couple of hundred meters in width, but stretching from the grounding line (contact of the Antarctic ice sheet with the ocean) to the edge of the ice shelf (more than hundred kilometer). We already found out that these depressions are formed by deeply incised channels at the bottom of the ice shelf so that ice thickness is around 150 m. These features are presumably formed at the grounding line and transported (advected) with the ice flow, but they may also indicate melting at the bottom of the shelf. Therefore we carried out deep drilling through the ice shelf and complemented this with shallow radar surveys as well as pRES measurements that directly can measure bottom melting (if present). The shallow radar was also used to link the borehole in the shelf with shallow cores as part of the BENEMELT project of Jan Lenaerts. The drilling went not so smoothly, but after some interventions, we were capable to drill to 107m. Several pRES measurements were carried out across the channels and the instrument was then finally buried in the snow and left for the winter. It will record every hour the vertical change in the ice shelf. Together with 2 other GPS instruments at the side of the channel to witness the flow, they will send their position every day. The latter is part of the @TweetinIceShelf project (more later).
|Drilling through the ice shelf with the Eclipse drill|
In short, and despite the bad weather at Derwael, the scientific programme was carried out completely. The drill didn't make it all the way to the bottom of the shelf, but what has been recovered is already a wealth of information that will be extremely useful in understanding ice-shelf dynamics. We wish to express our gratitude to the support at PEA, because it was quite something to make the different projects (IceCon, BENEMELT, SISMO) work together and change the plans according to the unpredictability of the weather.