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1 Terra Antartica 2004, 11(1), Terra Antartica Publication Ice Thickness and Bedrock Map of Matusevich Glacier Drainage System (Oates Coast) V. DAMM Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover, Germany Received 1 October 2003; accepted in revised form 23 August 2004 Abstract - As part of the geophysical project of the joint German-Italian Antarctic expedition, GANOVEX VIII-ITALIANTARTIDE XV helicopter-borne radio echo soundings (RES) were taken to measure the thickness of the ice cover in Oates Land, Antarctica. Based on 2500 km of RES data acquired in the drainage system of the Matusevich Glacier, maps of ice thickness and bedrock elevation were compiled. Maximum ice thickness was 2500 m, with an average ice cover of 2000 m in the south eastern part of the survey area. A remarkably deep graben structure beneath the Matusevich Glacier can be followed to 70 S, where it bends to the west and joins the Wilkes Basin. The subglacial relief between the Lazarev Mountains, at the western flank of the Matusevich Glacier, and the Archangel Nunataks at the eastern side of the Wilkes Basin shows a relatively uniform hilly landscape, separated from the USARP Mountains by the Matusevich drainage basin. INTRODUCTION The major scientific goal of the 1999/2000 joint German-Italian Antarctic expedition was to study the crustal structure of the northern part of the Wilkes Basin in order to understand the nature of the boundary of the East Antarctic shield (Bozzo & Damaske, 2001). Several ground-based and airborne geophysical methods were used to supplement the geological field work for this purpose. A helicopterborne radio-echo sounding (RES) survey was conducted primarily to provide the necessary data for ice correction procedures for the aeromagnetic and gravity surveys in the Oates Coast area (Fig. 1). In addition, ice thickness data in the region are of interest for glaciological investigations to support calculations of the mass balance of the Antarctic ice sheet. Few ice thickness data for the near coastal area of George V Coast and Oates Coast between 144 and 160 E were available before this program. Some analogue data already existed from the Mertz Glacier area and a single profile crossing the Wilkes Basin was acquired during Scott Polar Research Institute/National Science Foundation/Technical University of Denmark airborne missions in the 1970 s (Drewry & Meldrum, 1978). In December 1999, within the framework of the XV Italian Antarctic Expedition, an Italian Twin Otter operation to Lake Vostok measured several long-range RES profiles in the Mertz and Matusevich glacier area and near the coast (Tabacco et al., 2002). The helicopter-borne RES surveys of GANOVEX VIII- ITALIAANTARTIDE XV mainly cover the area of the Matusevich drainage basin with parallel survey lines at an average spacing of 20 km. Fig. 1 - GANOVEX VIII-ITALIANTARTIDE XV survey flights for Radio Echo Soundings solid lines (dotted lines: Italian RES survey flights from December 1999) (WH - Wilson Hills, LM Lazarev Mountains, AN Archangel Nunataks). *Corresponding author (volkmar.damm@bgr.de)

2 86 V. Damm Tab. 1 - RES survey data acquired during 1999/2000 joint German- Italian expedition GANOVEX VIII-ITALIANTARTIDE XV. Target Area Oates Land George V Land Number of survey flights 14 4 Number of data points Length of flight tracks 2500 km 600 km FIELD SURVEYS The GANOVEX VIII-ITALIANTARTIDE XV radar surveys cover two areas: Oates Land in the east and George V Land in the west. The eastern part of the target region in Oates Land comprises mainly the drainage area of the Matusevich Glacier. This region represents the main survey area, which is bordered by the Wilson Hills (159 E) at the eastern side and extends to 154 E longitude in the west. The Matusevich Glacier basin is flanked by the outcrops of the Wilson Hills in the east and the Lazarev Mountains in the west. Another group of outcrops in this area is formed by the Archangel Nunataks further to the west. The polar plateau reaches altitudes up to 1500 m in the south-west of the eastern target region at 70 S. A dense survey grid with a line spacing of 20 km covers an area of about km 2 up to 150 km inland of the front of the Matusevich Glacier (Fig. 1). Poor flying conditions during the end of the expedition prevented coverage of the western target area in Mertz glacier, George V Land being achieved at a similar density. A brief overview of the field data is given in table 1. RADAR SYSTEM AND DATA COLLECTION The helicopter-borne RES system of the Federal Institute of Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe - BGR) is a 150 MHz pulse radar. The system has a high versatility for aircraft installation and can be operated both from helicopter and fixed wing airplanes using different antenna sets. For helicopter surveys an antenna is suspended 15 m below the aircraft. The maximum theoretical depth penetration is 4000 m of ice according to the maximum possible record length of reflection signals predetermined by the data acquisition. The system was successfully used during several expeditions to Antarctica (Damm et al., 2001). The technical specifications of the system configuration used for GANOVEX VIII- ITALIANTARTIDE XV is given in table 2. The 150 MHz transmitter pulse length can be set to 12 ns, 60 ns and 600 ns allowing a broad range of applications for either near surface investigations or for detection of subglacial morphology. Longer pulses increase the maximum depth penetration, while shorter pulses improve the vertical resolution. Tab. 2 - General characteristics of the RES system. Transmitter frequency Pulse length Peak pulse power Pulse repetition frequency Antenna Receiver Sampling interval Data acquisition rate Record length Depth penetration in ice Vertical resolution in ice GPS-Positioning 150 MHz 12 ns / 60 ns / 600 ns 1.6 kw 20 khz Corner reflector with 2 dipoles, 15 db antenna gain 2 channels (120 db) Minimum 1 ns 0.1 Hz Maximum 48 µs (optional) 4000 m 1 m / 5 m / 50 m (depending on selected pulse length) Trimble 4000S, DGPS Two different radar receivers were used during the GANOVEX VIIII-ITALIANTARTIDE XV mission. The receiver for pulse lengths of 60 ns and 600 ns used logarithmic amplifiers, thus providing the envelope of the reflection signals. The dynamic range of this receiver is about 120 db allowing the detection of weak signal reflections. In the case of shorter pulse lengths of 12 ns or 60 ns, a linear receiver was used. The linear receiver output is directly sampled, which results in a lower dynamic range compared to that of the logarithmic signal detection, but optimizes the detection of internal structures. One of these two receiver units had to be selected prior to the survey data acquisition. The data acquisition unit was a VXI Bus (Tektronix) based system with a highest sampling rate of 10 9 samples per second (minimum sampling interval of 1 ns). The maximum trace length is 15,000 samples. The sampling interval and trace length were selected depending on the estimated ice thickness and could be changed during the survey. Table 3 gives an overview of data acquisition parameters, signal stacking rates and the resulting spacing of single data points after stacking at an average flight speed of 50 kts (26 m/s). The clearance height over ground ranged from 300 to 500 m. Most data during the GANOVEX VIIII- ITALIANTARTIDE XV mission was acquired using the logarithmic receiver to allow larger sampling intervals and reduce data volume and increase data acquisition rate. Spacing between subsequent reflection points was between 3 and 10 m. Two GPS (Trimble Geodetic Surveyor 4000S) receivers (rover and base station) were used for acquisition of navigation data during GANOVEX VIIII-ITALIANTARTIDE XV operations. The rover unit was installed on board, with the antenna mounted on the aircraft roof. The base station was installed near the coast within a 200 km radius to all flight positions. The data recording was 2 Hz. The GPS rover station was used to

3 Ice Thickness and Bedrock Map of Matusevich Glacier Drainage System (Oates Coast) 87 Tab. 3 - Data acquisition parameters. Receiver Sampling interval Signal stacking Data acquisition rate Surface point spacing Linear 2 ns Hz 21 m Hz 78 m Linear 4 ns Hz 10.5 m Hz 65 m Logarithmic 10 ns 4 7 Hz 3.7 m 16 3 Hz 7.8 m synchronize the system time of the VXI data acquisition unit. After differential GPS (DGPS) processing a horizontal accuracy better than 1 m and a vertical accuracy better than 3 m were obtained for base lines shorter than 200 km. The positions of the radar reflections were calculated during the postprocessing based on the corrected navigation data and time synchronization. CALCULATION OF ICE THICKNESS AND BEDROCK ELEVATION The software package ReflexW (Sandmeier Scientific Software) was used for data processing and picking reflections. The high quality of signal reflections from the surface and subsurface of the glacier allowed extensive use of the integrated automatic picking algorithm. The surface elevation of the glacier was deduced by the two-way travel time of the prominent surface reflections, which represent the terrain clearance of the aircraft, subtracted from the DGPS altimeter data. The accuracy in the detection of the surface reflection is within 10 ns (equivalent to 1.5 m). Taking into account the accuracy of the DGPS data the error in the surface elevation detection is less than 5 m expressed in meters above the WGS84 ellipsoid. The ice thickness was calculated from the two-way travel time difference between the surface and subsurface reflection using an in-ice velocity of 168 m/µs for all lines. The varying and unknown thickness of firn, with higher propagation velocity than in solid glacier ice, was not considered and the error in estimating ice thickness can result in an positive bias of 5 to 15 m depending on firn thickness. Since the antenna footprint of the highly directive corner reflector antenna covers an area of 500 to 5,000 m 2 depending on the terrain clearance, each data point represents the mean ice thickness within a 30 to 40 m radius circle. The results show glacier thicknesses ranging from a few hundred meters to more than 2500 m. MAPS OF ICE THICKNESS AND BEDROCK ELEVATION The data points provide a reasonable database for constructing maps of ice thickness and subice morphology. All surface reflection data were used to calculate a digital elevation model of the survey area. The dataset was extended with topographic data of the Antarctic Digital Database (BAS et al., 1993) available for this area. All data points representing outcrops and nunataks were set to zero ice thickness and provide supplementary data that can be used for constructing the map of ice thickness as fixed points for grid interpolations. The gridding algorithm used for the map compilation (Figs. 2 to 5) had to be able to take into consideration irregular data distribution with high data density along the flight lines and partly non-parallel survey profiles. The bi-directional gridding method (BIGRID), implemented within the program Oasis Montaj (Geosoft Inc.) was used for this purpose. BIGRID is ideal in these situations, especially if there is a high sample density down the lines relative to the line separation. The optimum distance between grid nodes was determined to be 1 km. Areas with an insufficient data density were not included into the interpolation and are left blank on the map. The analogue data, acquired during the 1970 s (Drewry & Meldrum, 1978) were not included into the gridding process because of their low navigation precision. The subice topography within the survey area shown in figure 4 is the difference grid between the topography given in the Antarctic Digital Database supplemented by the DGPS survey elevations, and the grid of ice thickness values based on the survey data. MORPHOLOGICAL AND GLACIOLOGICAL INTERPRETATION OF THE RESULTS The map of ice thickness (Fig. 3) is characterized by thick ice in the range of 1800 to 2500 m in the south-western area towards the Wilkes Basin and high variability in the north east. The Matusevich Glacier basin deepens to the south. The deepest part of the basin is the uppermost section of the Matusevich glacier, and here a major ice stream draining the polar plateau feeds into the glacier from the southwest. The width of this ice stream is 30 km and it is bordered to the north by the Archangel Nunataks. The maps of subice topography (Figs. 4 & 5) show a shallow elevation of the bedrock in the area connecting the Archangel Nunataks with the Wilson Hills to the south-east. The main structural feature in the survey area is the graben beneath Matusevich

4 88 V. Damm Fig. 2 - Surface topography of the survey area based on ERS-1 data. glacier with its deepest part in the south, where it is of more than 850 m below sea level. The bedrock elevation increases towards the coastline and is at 320 m b.s.l. around the grounding line. The distance between grounding line and calving front was 35 km in 1999/2000. The thickness of the Matusevich glacier decreases from 2200 m at S, E to 500 m at the grounding line and is 160 m at the edge at S, E. MASS BALANCE CALCULATIONS An estimate of the mass flux across the grounding line can be made from the ice thickness data obtained for Matusevich Glacier, together with spot measurements of surface velocity. A high density of ice thickness data is available for the area close to the grounding line, which allows an increase of the original grid resolution from 1 km to 200 m along a traverse profile with a total width of 19.5 km as shown in figure 2. The maximum ice thickness across the profile was 508 m at a point located 7.5 km from the left (eastern) margin, decreasing near-parabolically to both margins of the ice tongue (see example of survey data in Fig. 6). The cross section area (A) is calculated to be 4.35 ± 0.07 x 10 6 m 2, assuming an ice thickness error of 5 m and a horizontal position error of 1 m based on differential GPS coordinates. Two differential GPS ice velocity measurements were made near the centre line of the floating tongue of Matusevich Glacier at S; E and S; 157,12 E. These are labelled M1 and M2 in figure 3. M1 is located 7 km from the grounding line, and M2 is located 16.5 km further down glacier, 11 km from the calving front. The observation period was 2.5 days and the velocity magnitudes were 887 ma -1 and 896 ma -1 with uncertainty estimates of 1% at M1 and M2, respectively, with vectors parallel to the ice tongue orientation (Korth, 2000; Korth, personal communication). A constant velocity profile with depth was assumed on the floating ice tongue since basal shear stress is negligible at the ocean interface. On the left (west) margin the ice tongue is well anchored on rock, so that ice velocity at that region Fig. 3 - Map of Ice Thickness (M1, M2 locations of GPS stations for measurements of ice flow velocities, solid red line cross section used for mass balance calculations, arrow limited solid red line position of line section shown in figure 6).

5 Ice Thickness and Bedrock Map of Matusevich Glacier Drainage System (Oates Coast) 89 Fig. 4 - Map of Subice Topography (arrow limited solid red line position of line section shown in figure 6). was assumed to be zero. The right margin floats freely and a constant velocity profile was assumed. Assuming a standard power flow law for ice, with n=3 (Paterson, 1999), the mean velocity (v m ) across the grounding line (x-direction) can be estimated as: v m = width v(x) dx = 0.9 v c = 802 m a -1, where v c is the centre line velocity. Based on the precision obtained by differential GPS, the velocity precision is estimated to be ± 10 m a -1. Considering the value of v m = 802 ± 10 ma -1 and A = 4.35 ± 0.07 x 10 6 m 2, a total ice flux of 3.5 ± 0.1 km 3 is estimated. This ice flux at the grounding line can be compared with the total mass input due to surface snow accumulation in the glacier catchment. The drainage area of Matusevich glacier has been determined to be 19,500 km 2 by Frezzotti et al. (2002) based on the Digital Elevation Model of Antarctica provided by Remy et al. (1999). According to the accumulation map compiled by Giovinetto and Bentley (1985), a mean surface accumulation rate of 260 kg m -2 a -1 is estimated for the drainage basin. The accumulation rate distribution shows even higher levels of accumulation near the coast. Using the mean accumulation rate of 260 kg m -2 a -1 the total estimated mass input results in 5.1 Gt a -1 for Fig. 5-3D-Image of Subice Topography.

6 90 V. Damm Fig. 6 - Line section of RES survey data crossing the Matusevich glacier (for position see Figs. 3 & 4). Matusevich glacier, which indicates an excess mass of = 1.6 Gt a -1. However the flux is measured 7 km downstream from the grounding line and basal melt may be very high where the glacier starts to float. To explain the flux imbalance of 1.6 Gta -1 would require a basal melt rate of 12 m a -1 over the 7 x 19.5 km 2 floating section. A lowerer melt rate would mean that the accumulation rate by Giovinetto and Bentley (1985) is overestimated, as pointed out by Frezzotti (2002). If the system is in balance, then the mass flux estimated near the grounding line of the Matusevich Glacier indicates that the accumulation rate for the Matusevich drainage area should not exceed 180 kg m -2 a -1. A major calving event occurred in February 2002, where almost the whole ice tongue broke up, forming a 21.5 km x 8 km iceberg. The estimated flux data in combination with a remeasurement campaign could be used to test the influence of ice tongues on retarding glacier flow, and any relationship between their break up and an increase of glacier out flux. CONCLUSIONS The RES data acquired during the GANOVEX VIII-ITALIANTARTIDE XV campaign supply new information to improve the data base of Antarctic ice cover and bedrock topography provided by the international project BEDMAP. The subice topography map of the Matusevich drainage system shows icecovered structural features which point to a morphological connection between the Archangel Nunataks and the Wilson Hills. The ice flux calculations based on the ice thickness and velocity data lead to the conclusion, that either high basal melt rates occur close to the grounding line or accumulation rates are remarkable lower than previously suggested. Acknowledgements - The field data was acquired during the 1999/2000 joint German-Italian expedition, GANOVEX VIII-ITALIANTARTIDE XV, funded by the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Hannover, Germany and the Italian Programma Nazionale di Ricerche in Antartide (PNRA). Special thanks to Gernot Reitmayr and Gino Casassa for their assistance to improve this paper. REFERENCES BAS, SPRI and WCMC Antarctic Digital Database and user s guide and reference manual, J.W. Thsomson (ed.) Cambridge, Scientific Committee on Antarctic Research, 156 pp. CD-ROM and manual. [Multinational collaborative project.] Bozzo E. & Damaske D., GANOVEX VIII ITALIANTARTIDE XV Antarctic Expedition , Terra Antartica Reports, 5, 116 p. Damm V. & Eisenburger D., Bericht über hubschraubergestützte Radar-Eisdickenmessungen im zentralen Queen Maud Land während der Antarktisexpedition GeoMaud 1995/96. Field Report, Federal Institute for Geosciences and Natural Resources, Hannover, 23 pp. Damm V. & Eisenburger D., Helicopter-borne radio echo sounding survey in the Oates Coast area and around the Mertz Glacier. Terra Antartica Reports, 5, Damm V., Eisenburger D., Jenett M., Ice thickness data acquired using a helicopter-borne pulse radar system. Proc. Symp. Remote Sensing by Low-Frequency Radars, Naples, Sept Drewry D.J. & Meldrum D.T., SPRI Folio Series, Pol. Rec., 19(120), Frezzotti M., Raffi R., Guarracino M., Mancini M., Glaciological Map of Matusevich Glacier Area (Oates Coast, East Antarctica). Terra Antartica Reports, 7, 1-8. Giovinetto M.B. & Bentley C.R., Surface Balance in Ice Drainage Systems of Antarctica. Antarctic Journal of the United States, 20(4), Korth W., Realization of the Geodetic and Gravimetric Project. Terra Antartica Reports, 5, Paterson W.S.B., The Physics of Glaciers. Pergamon Press, (3 rd Edition), 496 p. Remy F., Shaeffer P., Legresy B., Ice flow processes derived from the ERS-1 high resolution map of the Antarctica and Greenland ice sheets. Geophys. J. Int., 139, Rignot E. & Thomas R.H., Mass Balance of Polar Ice Sheets. Science, 297, Tabacco I., Bianchi C., Zirizzotti A., Zuccheretti E., Forieri A. & Della Vedova A., Airborne radar survey above Vostok region, east-central Antarctica: ice thickness and Lake Vostok geometry. Journal of Glaciology, 48(160),