Victoria University Antarctic Research Expedition Science and Logistics Reports 2007-08: VUWAE 52

c. Objectives

c. Objectives

Site survey at Skinner Saddle and Gawn Ice Piedmont

Ground penetrating radar (GPR) measurements provide an image of the internal layering of a glacier and the topography of the ice-rock interface beneath. We applied low and high frequency radar pulses (8 MHz, 35 MHz, 200MHz, and 500MHz) to map the bedrock interface and internal flow structures in the glacier. Those features are identified through reflectors that result from changes in physical and chemical properties, such as dust layers or aerosol and density variations and are thought to represent isochrones (Morse et al., 1998; Vaughan et al., 1999). The choice of antenna frequency involves a trade-off between penetration depth and mapping resolution. The control units were mounted on a Nansen Sledge, pulling transmitter and transceiver antennae. The sledge also carried high precision GPS antenna, which is tied to the temporary GPS base station deployed at the SKS and GIP camps.

Traverses totaling 150km at Skinner Saddle and 35km at Gawn Ice Piedmont have been surveyed with GPR. Excellent isochrone reflections are visible from both the bedrock/glacier interface and in the top part of the profile, which will also be used to investigate geographical and chronological accumulation changes. Further post-processing will enhance the reflectors and will correct for surface topography.

Fig. 2 A) ASTER satellite image of Skinner Saddle and vicinity. See Figure 1 for overview. Image from 31 October 2005. Yellow flag indicates location of camp, red flag indicates proposed drilling location. B) Digital elevation model. X/Y/Z grid in UTM 58 map units. Red lines indicate location of ground penetrating radar survey lines

Fig. 2 A) ASTER satellite image of Skinner Saddle and vicinity. See Figure 1 for overview. Image from 31 October 2005. Yellow flag indicates location of camp, red flag indicates proposed drilling location. B) Digital elevation model. X/Y/Z grid in UTM 58 map units. Red lines indicate location of ground penetrating radar survey lines

Fig. 3 A) ASTER satellite image of Gawn Ice Piedmont and vicinity. See Figure 1 for overview. Image from 08 January 2004. Yellow flag indicates location of camp, red flag indicates proposed drilling location. B) Digital elevation model. X/Y/Z grid in UTM 58 map units. Red lines indicate location of ground penetrating radar survey lines

Fig. 3 A) ASTER satellite image of Gawn Ice Piedmont and vicinity. See Figure 1 for overview. Image from 08 January 2004. Yellow flag indicates location of camp, red flag indicates proposed drilling location. B) Digital elevation model. X/Y/Z grid in UTM 58 map units. Red lines indicate location of ground penetrating radar survey lines

Drilling of shallow firn cores at Skinner Saddle and Gawn Ice Piedmont

As part of the site reconnaissance we drilled a 17m and 13m deep firn core at SKS and GIP, respectively. The drilling system was kindly provided by the Alfred Wegener Institute. The initial data set from these cores allow us to calculate annual accumulation and establish transfer functions with meteorological data to establish the quality and sensitivity of the ice.

Fig. 4: Firn core drilling at GIP

Fig. 4: Firn core drilling at GIP

Submergence Velocity Measurements at Victoria Lower and Evans Piedmont Glacier

The response time of a glacier to changes in accumulation or ablation is dependent on the size and thickness of the ice mass. In general, the response time of cold-based glaciers is positively correlated with the size of its ice mass, leading to long response times in Antarctica. For glaciers in the McMurdo Dry Valleys, with lengths on average of 5-10km and flow rates of 1 to 3 m/a, the response times are thought to range from 1,500a to 15,000a (Chinn, 1987; Chinn, 1998). Consequently, annual variations in surface elevation may only reflect changes in loss rates. As a result surface measurements of mass balance are difficult to interpret in terms of long-term mass balance (Hamilton & Whillans, 2000). This is especially the case in places like the McMurdo Dry Valleys where mass loss is thought to be predominately due to sublimation at ice cliffs and glacier surface caused by wind and solar radiation (Chinn, 1987; Chinn, 1998). For Victoria Lower Glacier (VLG), two mass balance measurements are available in the literature for 1983 and 1991 based on ice cliff characteristics and the motion of the glacier snout (Chinn, 1998). The measurements indicate that VLG was advancing 1.24m/a into Victoria Valley during this time period. However, the small number of observations (2) and the cliff's sensitivity to sublimation (contemporary surface ablation) result in a high uncertainty of longer term mass balance. To determine the longer-term mass balance of the glaciers, unaffected by annual surface variations, three 'coffee-can' or 'submergence velocity' devices (Hamilton et al., 1998; Hamilton & Whillans, 2000) were deployed at Victoria Lower Glacier in 1999/2000 and two at Evans Piedmont Glacier in 2004/05. These are annually re-measured to monitor mass balance changes.

Fig. 6: Submergence Velocity Measurements at VLG

Fig. 6: Submergence Velocity Measurements at VLG