• Abstract and Figures
• Public Full-text

Antarctic Science page 1 of 9 (2009) & Antarctic Science Ltd 2009 doi:10.1017/S0954102009002053 Reassessment of ice mass balance at Horseshoe Valley, Antarctica ANJA WENDT1*, GINO CASASSA1, ANDRES RIVERA1,2,3 and JENS WENDT1- 1Centro de Estudios Cientı´ficos, Av. Arturo Prat 514, Valdivia, Chile 2Centro de Ingenierı´a de la Innovacio´n del CECS, Av. Arturo Prat 514, Valdivia, Chile 3Departamento de Geografı´a, Universidad de Chile, Marcoleta 250, Santiago, Chile *[email protected] Abstract: Horseshoe Valley (80818'S, 81822'W) is a 30 km wide glaciated valley at the south-eastern end of Ellsworth Mountains draining into the Hercules inlet, Ronne Ice Shelf. The ice at Horseshoe Valley has been considered stable; now we use Global Positioning System (GPS) measurements obtained between 1996 and 2006 to investigate ice elevation change and mass balance. Comparison of surface heights on a profile across Horseshoe Valley reveals a slight but significant elevation increase of 0.04 m a-1 ± 0.002 m a-1. The blue ice area of Patriot Hills (,13 km2) at the mount of Horseshoe Valley shows large interannual variability in area, with a maximum extent in 1997, an exceptionally warm summer, but no clear multi-year trend, and an elevation increase of 0.05 m a-1 in eight years, which agrees with the result from Horseshoe Valley. Received 18 July 2008, accepted 24 February 2009 Key words: Ellsworth Mountains, Global Positioning System, ice elevation change Introduction changes in mass balance. So far mass balance changes in the interior of the continent have been attributed to The mass balance of the Antarctic ice sheet is receiving enhanced snowfall in East Antarctica (Davis et al. 2005), increased attention due to its relevance for climate change although this has been disputed by more recent studies (e.g. studies and sea level contribution (Jacka et al. 2004). Mass Monaghan et al. 2006). Ice sheet areas close to rock outcrops balance methods applied in Antarctica include the mass in mountain regions where fixed reference sites can be budget approach where snow accumulation input is deployed are especially suited for monitoring changes in compared to ice flow output (Rignot et al. 2008); repeated elevation and ice velocity using ground-based methods. An altimetry using airborne and satellite data obtained by additional advantage of mountain regions is the frequent radar and laser sensors (Smith et al. 2005, Wingham et al. presence of blue-ice areas (BIAs) due to wind erosion and 2006) to measure ice elevation changes; and determination enhanced sublimation (Bintanja 1999, Sinisalo et al. 2003), of temporal changes in gravity based on satellite data where elevation changes over ice can be measured, avoiding (Velicogna & Wahr 2006). Recent assessments suggest an the effects of possible changes in firn densification. overall negative mass balance for Antarctica, with sea level Glaciological studies at Patriot Hills (80818'S, 81822'W), -1 contribution values that range from -0.14 to 1 0.55 mm a southern Ellsworth mountains, were initiated in 1995 as part (Lemke et al. 2007). A growth in East Antarctica probably of the Chilean Antarctic programme. Patriot Hills are located caused by increased snowfall (Davis et al. 2005) seems to be ,50 km inland from the Ronne Ice Shelf grounding line offset by larger shrinkage from West Antarctica and the (Joughin & Bamber 2005), on the southernmost margin of Antarctic Peninsula due mainly to enhanced flow, associated Horseshoe Valley (Fig. 1). Mass balance results by means of thinning and enhanced basal melting of ice shelves (Rignot & repeat measurements of stakes at Patriot Hills and Horseshoe Thomas 2002, Rignot et al. 2005, Shepherd & Wingham Valley performed by optical survey in 1995, and by GPS 2007). While two satellite radar altimetry studies suggested surveys in 1996 and 1997, have shown that the ice sheet is an overall null or even positive mass balance (Zwally et al. in near steady state (Casassa et al.1998,2004).Herewe 2005, Wingham et al. 2006), other studies conclude a analyse new elevation data at Patriot Hills and Horseshoe substantial mass loss for the continent as a whole but with Valley obtained from GPS surveys in 2004/2005 and January very high uncertainties (Velicogna & Wahr 2006, Rignot of 2006, and compare them with GPS data obtained one et al. 2008). decade earlier. Within the Antarctic, fast flowing ice streams, grounding line areas and ice shelves are particularly sensitive to Network survey y Jens Wendt lost his life in an airplane crash while returning In November/December 1996 an existing network of from an airborne laser height survey on 6 April 2009. ablation stakes near Patriot Hills (Casassa et al. 1998) 1 2 ANJA WENDT et al. Fig. 1. Horseshoe Valley, Antarctica. a. Mosaic of ASTER images. Stake locations are drawn as black dots, grey and white lines indicate ice thickness measurements in 1995–97 and 2007, respectively. Black boxes show the extent of Figs 4, 5, 6 & 8. b. Location of Horseshoe Valley. was extended across the whole width of Horseshoe Valley Due to burial by snow accumulation, most stakes could consisting of three parallel lines with a stake separation not be found during the following resurvey performed in of about 1 km (Fig. 1). Additionally, some stakes were set November/December 2004, as expected. Near the BIA up on the ice cliff between Independence Hills and Redpath where net ablation prevails, about 10 stakes were found and Peaks. measured again. In view of the loss of stakes, a kinematic The position of all stakes was observed by static Global scheme was adapted to resurvey the surface heights at the Positioning System (GPS) measurements of at least ten minutes with geodetic-quality dual-frequency GPS equipment relative to a GPS site in the Chilean base camp ‘‘Teniente Arturo Parodi’’ located on snow, at the southern margin of Horseshoe Valley, about 250 m north of the Patriot Hills BIA. The position of the Parodi base station was determined three times during the campaign, using a reference site on bedrock located 2.6 km to the south-west, on the northern slope of Patriot Hills. One year later (1997), the complete network was resurveyed by GPS with Leica SR9500 receivers and antenna type LEIAT302-GP, and a slightly modified measurement strategy to allow for shorter observation times. Additionally to the base station established at Parodi base camp, two temporary base stations across the valley were deployed to reduce baseline length to 13 km at the most. These temporary base stations operated for one or two days only, with differential observations lasting 20 and 40 minutes, respectively. All stakes were observed in a stop-and-go mode with observation times as short as three minutes, by means of the Leica GPS receiver that does not record data in kinematic mode but keeps track of the phase Fig. 2. Example of survey pattern and plane adjustment to the ambiguities. 2006 kinematic survey around the location of stake H30W. ICE MASS BALANCE AT HORSESHOE VALLEY 3 stake positions. A geodetic site on bedrock in Patriot Hills served as a base station and was operated semi-permanently throughout the whole GPS campaign. For the kinematic survey a Javad Lexon GD receiver and an antenna MarAnt, operating with a sampling interval of five seconds, were mounted on a snowmobile. Around the former stake positions, a regular trefoil pattern of 100 m radius was surveyed to enable the reconstruction of the ice or snow surface height at the exact location. The kinematic survey of the stake locations was repeated in January 2006 with the same GPS equipment as in 2004. The ground pattern around each stake location was slightly modified, now consisting of two concentric circles of about 25 and 50 m radius and two loops of 100 m (Fig. 2). The sampling rate in 2006 was increased to 1 Hz. The Fig. 3. Approximation error of the plane adjustment shown in few stakes on and near the BIA that still existed were Fig. 2. The black line represents the normal probability density additionally surveyed statically. function with the same values for mean and standard deviation. (. 10 km) baselines. Consequently, 10% of the sites Data analysis could not be reliably processed. The results from a few All GPS data (four epochs) were (re-)processed using the repeated measurements of stake sites agree within 1–2 cm commercial software GrafNav 7.60 to provide for a in the vertical in case of fixed ambiguities. However, when consistent analysis procedure. We used dual-frequency carrier- the ambiguity solution failed, bad float solutions can phase measurements. For baselines longer than 10 km cause height differences between repeated sessions of the the ionosphere-free linear combination was employed. same site of up to 1 m. Special care has been taken to The processing of each GPS campaign exhibited some evaluate solutions and reject bad ones, which has resulted peculiarities which are summarized below. in a data gap of 3 km between stakes H17 and H20 within the otherwise evenly spaced profile. Similar to 1996, a mean vertical precision of 0.15 m was considered. 1996 As mentioned above, the Parodi base station of the 1996 2004 survey was established on snow and thus moved due to ice flow by an unknown amount during the campaign. A The kinematic processing of the 2004 measurements baseline to a site on bedrock located at Patriot Hills 2.6 km showed a large number of cycle slips resulting in a to the south-west was observed three times to enable the heterogeneous accuracy of the epochs. About 9% of all correction of this influence. The resurvey of the 1996 base kinematic positions had a vertical standard deviation larger station in 1997 revealed a flow velocity of 1.1 m a-1, than 0.5 m according to the GPS software and were resulting in a displacement of 0.07 m during the three-week excluded from further consideration.

Load more

  9

Copy Link