Aim of the project

Ever since the research station Wasa was built in Dronning Maud Land (DML) in 1988/89, glaciologists have been able to maintain a continous scientific program in the Vestfjella-Heimfrontfjella area. One important question addressed is whether this area in DML is in balance with the present climate. Indications of climate change can be detected by means of mass balance studies of glacier systems, where incoming mass is compared with outgoing mass. The glacial systems of  Vestfjella-Heimfrontfjella can potentially contribute to sea-level changes and ocean current disturbances, which in turn may induce other regional/global changes. The mass balances of the Vestfjella-Heimfrontfjella glacial systems are also important in relation to studies of nearby areas, as well as for studies of the entire Antarctic ice sheet.

The aim of our project is to establish mass balances for the two ice streams Plogbreen and Kibergbreen in the Vestfjella-Heimfrontfjella area, in a search for signals of global warming (figure 1). Although the 20 times larger Veststraumen dominates the ice flow in the Vestfjella-Heimfrontfjella area, Plogbreen and Kibergbreen are believed to represent the overall balance in the area.  Practical issues and the availability of data from previous studies also favoured the choice of these two smaller ice streams. The incoming mass for the two systems is given by accumulation rates from previous ice core records integrated over their catchment areas. The outgoing mass is the unknown component that motivated our field work during the SWEDARP 2002/03 expedition. By measuring ice surface velocities and ice geometry for a cross-sectional gate in a confined area of the ice streams, we are able to determine the ice dynamical parameters required for calculations of outgoing mass. The following sections describe the field sites and measuring techniques used in the field as well as preliminary results of the calculated flow through one of the gates.

Field sites

Plogbreen (together with Veststraumen) drains the Ritscherflya ice field and covers approximately 20-25 km in width and 60 km in length from the higher ground at Herculessletta to the grounding line at Riiser-Larsen Ice Shelf (figure 1B). The ice surface decreases from 750 m.a.s.l. at the ice divide on Herculessletta (near Ritscherflya) to about 150 m.a.s.l. at the grounding line 10-15 km northwest of Vestfjella mountain range. The most prominent features in this area are the nunataks of Basen and Plogen, which constrain the ice flow through a channel-like section at about 200-250 m.a.s.l. Previous unpublished radar sounding measurements between Basen and Plogen indicate an overdeepening in the south reaching ice depths of 1 000 m with the base below sea level (unpublished data, Näslund). Ice surface velocities have been measured and reach a maximum of ~100 m a^-1 in the same area (Holmlund et al., 1989). Crevasses in the area are mainly associated with the grounding line and restricted areas with relatively steep slope gradients. However crevasses are also found in large flat areas completely lacking topographic undulations. The snow accumulation rates varies from 0.25 mwe a^-1 on Ritscherflya to 0.40 mwe a^-1 on the Riiser-Larsen Ice Shelf, while average values for Plogbreen range between 0.32- 0.34 mwe a^-1 (Isaksson, 1994).

Kibergbreen, 5-7 km wide and 30-35 km long, flows through the Heimfrontfjella escarpment, draining Amundsenisen at 2 200 m.a.s.l. and merging with Ritscherflya at about 1 200 m.a.s.l. (figure 1C). The bed topography is known only at a few profiles where radar soundings give depths of 300 m at shallow spots down to 1 000 m in deep channels (Hedfors, 2002). Velocity measurements from SWEDARP 1987-1995 expeditions combined with remotedly sensed information (SPOT panchromatic imagery) show that ice movements vary between 30-70 m a^-1 along the main flow direction (Hedfors, 2002). In the upper part near Amundsenisen the glacier is well confined by the nunataks of Mathiesenskaget and Sumnerkammen in the northeast and southwest respectively. Further downstream, at about 1 200-1 400 m.a.s.l., the extent of the ice flow is limited eastward by another glacier, Bonnevie-Svendsenbreen, while in the west an adjacent thin, slower moving ice sheet limits the lateral spreading of Kibergbreen. Accumulation rates upstream Kibergbreen are not well known, but based on ground penetrating radar (GPR) and snowpit studies, values range between 0.1-0.5 mwe a^-1 (Richardson et al., 1997; Richardson-Näslund, 2001; Näslund et al., 1991).

Fieldwork

The field work generally consisted of three phases adopted for a gate on both ice streams: first, the planting and positioning of a stake net (photo 3); second, ice depth measurements along transects within the stake net boundaries; and third, re-measurements of the stake net to obtain ice surface movement vectors. The first and second phase in the survey of Plogbreen took place 23 December 2002 to 2 January 2003 and was completed with the third phase 17 January 2003 to 23 January 2003. The shorter campaign on Kibergbreen was carried out 6 January 2003 to 14 January 2003, i.e. between phase 1 and 3 on Plogbreen.

The positioning of the stake net on the surface of Plogbreen (the number given between brackets represents Kibergbreen) was carried out using repeated L1 carrier phase differential global positioning system (DGPS) measurements of 40(36) evenly spaced aluminium stakes. The stake net on Plogbreen (Kibergbreen) was constructed in a section where the ice flows between the two nunataks, Basen and Plogen (Mathiesenskaget and Sumnerkammen) (photo 4). The frame of the grid covered 3(4) km along and 18(5) km transverse to the ice flow direction. The spatial extent was limited by crevassed areas both upstream and downstream of the chosen areas. The maximum distance between the rover antenna and the reference station was 21(12) km. The instrumentation (MK-1 recievers from DataGrid) was pre-tested to provide 0.01 m accuracy in the horizontal plane (x, y) and 0.25 m accuracy in the vertical plane (z) using static recording mode for 15 minutes. This measurement technique was used consistently throughout both surveys. At Plogbreen we used an accurately established fix point at the Swedish station Wasa (Korth et al., 2000) located on the Basen nunatak, while for Kibergbreen we used a local fix point on the lower Mathiesenskaget (photo 5) calculated directly from our measurements. Considering the severe Antarctic ionospheric conditions in combination with single baseline measurements, the error in achieved positions are probably larger than those specified by the GPS manufacturer. By using only the relative movements of the stakes, we apply similar errors to all measurements when calculating velocity vectors. This way, the data turn out to be very consistent even for very short measurement intervals.

The ice geometry is determined by the ice surface slope and the distance from the ice surface to the bedrock, i.e. ice depth. The ice surface slope gradient was obtained directly from the vertical coordinate (z) in the DGPS survey of the 40(36) stakes. The ice depth of the gates on Plogbreen(Kibergbreen) was determined by 174(84) km of GPR profiling using ground-based 8 MHz dipole antennas with a 20 m antenna separation (photo 2). A Tektronix THS 720 oscilloscope docked to a laptop permitted logging of 2 500 samples per shot using a time window of 20 ms. The GPR recording travelled straight in between all stake columns (perpendicular to the ice flow) and rows (parallel to the ice flow). Each GPR transect was surveyed twice and interpreted separately to ensure the bedrock location within an error-range of ±10%.

Preliminary results

The data from Kibergbreen have still to be processed and analyzed. Presented below are the preliminary results from the measurements on Plogbreen. The ice body is assigned a grid system with three dimensions, x, y and z, with a certain cell size. In this case x– and y-directions will represent the horizontal plane and the z-direction represents the vertical plane counted positively downwards. The data and results are presented in a rotated local grid system (for efficient calculation purposes) ranging from 0 to 3 km in the horizontal x-direction (parallel to the glacier flow direction), 0 to 18 km in the horizontal y-direction (perpendicular to the glacier flow direction), and 0 to 1 200 m in the vertical z plane (ice depth).

Ice surface velocities and elevation from the DGPS campaign are presented in figure 2 (a, b and c) together with the GPR derived ice depth (figure 2d). The results from the GPR campaign suggest there is a significant overdeepening down to 850 m below sea level in the southwestern region of the surveyed area, near the Plogen mountain range, whereafter the bedrock gradually rises to near sea level in the central and northeastern parts of the glacier. This corresponds to a maximum ice depth of ~ 1 100 m near Plogen, ~ 500 m in the central parts – gradually decreasing to ~200 m near the Basen nunatak. The pattern of horizontal surface velocities strongly reflects the bedrock topography in showing fast flowing ice in the deep channel near the Plogen massif (y = 12 to 16 km). Lower velocities are observed from the trough towards the Basen nunatak, with the exception of a local maximum just east of the centerline (y = 7 km) where the bedrock tend to overdeepen slightly. Both of the velocity and geometry data agree roughly with previous measurements in this area (unpublished data, Holmlund and Näslund).

In order to find the outgoing mass, the collected field data is fed into an ice flow model that integrates the ice surface velocity over the ice geometry (figure 3). The model is based on ice dynamical calculations or force balances at the gate (van der Veen and Whillans, 1989; van der Veen, 1999; Hedfors, 2002; and Pohjola et al., in press) and provides a total outgoing volume of 0.55 ±0.05 km³ a^-1. The drainage area of Plogbreen was outlined using satellite imagery in combination with AVHRR-based photoclinometry-derived digital elevation models (data from National Snow and Ice Data Center, Boulder, Colorado), and estimated as 1 420 ±300 km². This gives a total inflow of 0.48 ± 0.1 km³ a^-1 from the upstream area, based on the accumulation rate, 0.33 mwe a^-1 (Isaksson, 1994).

By only considering the median values of the flux components, the Plogbreen system can be said to be in a negative state where the mass leaving the system is greater than that of incoming mass (0.55 > 0.48). This is also suggested from ice core records showing a falling trend in accumulation (-25%) over the past 70 years (Isaksson, 1994). More interesting is the glacier’s proximity to the grounding line (~10 km) and the sub sea-level overdeepened trough (~800 m below sea level) observed towards the Plogen massif. This environmental setting may allow for detection of signals of global warming. Thus, as a speculative comment, we argue that the indications of a negative mass balance can be explained by the recent rise in the global sea level, as it is likely to induce glacier acceleration due to a reduction in resistive forces at the site of the gate. The argument is supported by observations of recent ice shelf break-up at the Riiser-Larsen Ice Shelf in front of Plogbreen. However, more studies of similar kind in this area are required to fully understand the processes that affect the glacier response to climate change. In addition, the data from Kibergbreen remains to confirm or disprove our theory on the overall state of the mass balance in the Vestfjella-Heimfrontfjella area.