PS100 - Weekly Report No. 6 | 22 - 28 August 2016
At the 79° Glacier
Yesterday we finished our investigations in the close vicinity of the 79°N glacier. Now R/V Polarstern is exploring the southern end of Westwind Trough on the inner shelf. We had already paid a visit to the outer part of this depression earlier in the expedition.
Throughout the last week many different research activities were performed close to the calving front of the 79°N glacier. This lucky opportunity arose, because westerly winds had driven both the sea ice and the ice bergs - that had only days before occupied the bay in front of the glacier - toward the east. Thus, over several days we were able to survey the sea floor, collect sea floor sediment samples, observe the distribution and the circulation of Atlantic waters and collect water samples in this area. In addition the friendly weather allowed us to run a large number of helicopter-based operations. Therefore all geodetic and seismological stations could be recovered from the mainland of Greenland und two ice radar systems were installed. Over the course of one year the latter will continuously record the amount of melting at the base of the glacier that is in contact with sea water. Next summer the radar system will be recovered again.
In this weekly report we will highlight the geodetic work. During this expedition the geodetic program comprises ten stations, which were installed on bedrock with a special marker and measured for the first time in 2008. A first reoccupation of some stations was already carried out in 2009. The main goal of the measurements is to obtain information about the horizontal and vertical land movement relying on the global navigation satellite systems (GNSS). The vertical movement is strongly related to the deformation of the Earth crust associated with historical and recent ice mass losses in the region (glacier melt & recession). The long-term component of these movements can be regarded as the glacial isostatic adjustment (GIA). In the area of investigation, the estimated magnitude of this effect amounts up to five millimeters per year. The horizontal displacement is predominantly associated with the movement of continental plates.
In the last few weeks, eight of ten planed stations were successfully set up. In addition, one more station was set up on the 79°N glacier in order to survey the response of the glacier’s floating ice tongue to ocean tides. Therefore, a location in the center of the ice stream was chosen, where the glacier is already afloat. At this station, the antenna was mounted on top of an iron rod fixed to the glacier by three additional anchors. To reach the stations on shore, 16 helicopter operations were done. On each field site the antenna was directly mounted on the reference point to avoid a vertical offsets. An aluminum box contained a receiver to store the observed signals, a charging controller and a battery. Two additional solar panels are used to charge the battery. To facilitate an evaluation of vertical land movements with sufficient accuracy, every station has to record measurements for at least three complete 24 hour cycles (from 0:00 to 24:00 UTC). In this configuration and by performing a complex analysis relying highly accurate reprocessed satellite orbits, an accuracy of about 2-3 millimeters in horizontal and 4-5 millimeters in vertical component will be possible. This allows us to determine the above-mentioned GIA effect with a sufficient accuracy. GNSS observations on the bedrock are crucial, since they provide the only ground truth to constrain the GIA modeling. Our results will allow us to improve both models of both the glacial history and the recent mass balance in North-East Greenland.
Many greeting from R/V Polarstern,
Torsten Kanzow, Benjamin Ebermann and Andrei Krämer