Ice thickness, internal structure and subglacial topography of Bowles Plateau ice cap and the main ice divides of Livingston Island, Antarctica, by ground-based radio-echo sounding Yu.Ya. MACHERET,1 J. OTERO,2 F.J. NAVARRO,2 E.V. VASILENKO,3 M.I. CORCUERA, M.L. CUADRADO,2 A.F. GLAZOVSKY1 11nstitute of Geography, Russian Academy of Sciences, 29 Staromonetny Street, 109017 Moscow, Russia 2Departamento Matematica Aplicada, ETSI de Telecomunicacion, Universidad Politecnica de Madrid, Avenida Complutense 30, ES-28040 Madrid, Spain E-mail: [email protected] 3 Institute of Industrial Research Akadempribor, Academy of Sciences of Uzbekistan, Akademgorodok, St. F. Hujaev28, Tashkent WOT25, Uzbekistan ABSTRACT. We present the results of low-frequency (20 MHz) radio-echo sounding (RES) carried out in December 2000 and December 2006 on the main ice divides of Livingston Island, South Shetland Islands (SSI), Antarctica, and Bowles Plateau, Antarctica, respectively, as well as high-frequency (200 MHz) RES on the latter, aimed at determining the ice thickness, internal structure and subglacial relief. Typical ice thickness along the main ice divides is ~150m, reaching maxima of ~200m. On Bowles Plateau the ice is much thicker, with an average of 265 m and maxima of ~500 m. The bed below the main ice divides is above sea level, while part of the outlet glaciers from Bowles Plateau lies significantly below sea level, down to -120 m. The strong scattering of the radio waves in the areas under study constitutes further evidence that the ice in the accumulation area of the ice masses of the SSI is temperate. Typical thickness of the firn layer in Bowles Plateau is 20-35 m, similar to that found in King George ice cap. A strong internal reflector within the firn layer, interpreted as a tephra layer from the 1970 eruption at Deception Island, has allowed a rough estimate of the specific mass balances for Bowles Plateau within 0.20-0.40 m a"1 w.e., as average values for the period 1970-2006. INTRODUCTION 2002; Travassos and SimSes, 2004) and Livingston Island. At the latter location, until very recently the ice thickness was The South Shetland Islands (SSI) are located at ~115 km to known only at four points in the northeastern part of the the west of the northern tip of the Antarctic Peninsula and island from airborne radio-echo sounding (RES) performed comprise 18 ice-covered islands, King George and Living­ in 1975 by the British Antarctic Survey at a frequency of ston Islands being the two largest (Fig. 1a). Ice-temperature 35 MHz (personal communication from D. Vaughan, 1995), measurements in shallow boreholes have indicated that the showing ice thickness between 200 and 400m. At ice masses in the accumulation areas of these islands and Livingston Island, ice-thickness measurements by seismic neighbouring Nelson Island (Fig. 1a) are temperate (Orheim refraction and reflection techniques were carried out on and Govorukha, 1982; Ren and others, 1995; Hun and Johnsons Glacier, Hurd Peninsula, in the vicinity of the Kang, 2002; Travassos and Simoes, 2004), in contrast with Spanish Antarctic station Juan Carlos I, in 1996/97 and the prevailing cold thermal regime of the ice masses further 1997/98 (Benjumea and Teixido, 2001; Benjumea and south in Antarctica. However, some evidence has been put others, 2003; Navarro and others, 2005). Detailed ice- forward concerning the polythermal structureof the ablation thickness measurements were also carried out at Johnsons area of certain glaciers such as Hurd Glacier, Livingston and Hurd Glaciers by ground-based RES in 1999/2000, Island (Molina and others, 2007). During the last 40years 2000/01, 2001/02, 2003/04 and 2004/05 (Benjumea and the ice caps in King George and Livingston Islands have others, 2003; Navarro and others, 2005; Molina and others, retreated by 6.6% and 4.3% in area, respectively, reflecting 2007). Larger-scale ground-based RES measurements on higher than average climate warming in this region (Simoes Livingston Island were also performed in December 2000, and Bremer, 1995; Macheret and others, 1997a,b; Calvet along the main ice divides of the ice cap (Fig. 1b), and in and others, 1999; Macheret and Moskalevsky, 1999; Simoes December 2006 on the eastern part of the island, at Bowles and others, 1999; Molina and others, 2007). ice plateau and two neighbouring outlet glaciers, Huron According to estimations by Vinogradov and Psareva Glacier and an unnamed glacier north of Mount Bowles, (1990), based on analysis of large-scale (1 :2 500 000) maps both flowing eastwards (Fig. 1b and c). In this paper, we and available ice-thickness measurements, the total area of present and discuss the RES data collected in these two latter the archipelago is 3728 km2, the average ice thickness is areas, as well as related data on the radio-wave velocity in 140 m and the total ice volume is 469.7 km3. the glacier firn-ice sequence. These data are of interest in order to estimate the total ice volume stored in the SSI, and In the SSI, data on ice thickness have only been collected therefore its potential contribution to sea-level rise, as the at Nelson Island (Ren and others, 1995), King George Island Antarctic Peninsula area is known to be very sensitive to (Govorukha and others, 1974; Macheret and others, climate warming. 1997a,b; Macheret and Moskalevsky, 1999; Hun and Kang, 62°W 58°W i-f Vi 62°S 2-King George *°^N«$ £> ^Nelson ?^v*s Livingston $ {7 ® 0 V e 64°S + + _£2L MMtfH CK«i f.^^^mf JwV Mt Bowles .%,V Radar profiles, 200G 0 C2 :.J D.B 1.2 ]A ti!fl« "JAM Fig. 1. (a) Location map for the South Shetland Islands, relative to Trinity Peninsula, Antarctic Peninsula, showing the location of King George, Nelson and Livingston islands, (b) Location of the radar profiles along the western ice divides of Livingston Island and Bowles Plateau, to the east, geo-referenced with respect to a Systeme Probatoire pour ('Observation de la Terre (SPOT) image taken in 2001. (c) Location of the low-frequency (20 MHz) radar profiles done on Bowles Plateau. METHODS Radar profiling Radar equipment and settings In December 2000, RES measurements with VIRL-2 were For radar measurements the following radar equipment was performed along the main ice divides of the ice cap (Fig. 1b). used: in 2000 the monopulse ice-penetrating radar VIRL-2 The total length of radar profiles was 46.5 km. More detailed with a central frequency of 20 MHz (Vasilenko and others, RES measurements with VIRL-6 were carried out at the 2002), and in 2006 its modification, the radar VIRL-6 Bowles ice plateau in December 2006 (Fig. 1b and c). The (Berikashvili and others, 2006), as well as a ground- total length of radar profiling was 101.5 km. In order to penetrating radar Ramac/GPR with 200MHz antennae. calibrate the stand-alone GPS elevation data, measurements VIRL-2 and VIRL-6 were used to determine the ice thickness, with a differential GPS (DGPS) receiver were also made at while Ramac was mainly intended for studying the structure both end-points of every radar profile. In the northwestern of the snow-firn sequence at Bowles ice plateau. In addition and southeastern parts of Bowles Plateau, measurements to radar data, navigation information from a global pos­ with Ramac 200MHz radar were also made in December itioning system (GPS) was recorded for both VIRL and 2006 (Fig. 2). In fact, the latter profiling covered the same Ramac radars, storing the data in a dedicated digital areas as those for the low-frequency radar (Fig. 1 c); however, recording system and a laptop, respectively. Two systems failures in the GPS receiver implied that only a fraction of of transmitter-receiver synchronization were used: a radio the total profiles (those shown in Fig. 2) were properly geo- link for VIRL-2 and an optic fibre link for VIRL-6 and Ramac. referenced. The total length of geo-referenced Ramac radar An odometer was also used to measure the distance along profiles was 33 km. the radar profiles and for transmitter triggering. VIRL-2 and VIRL-6 radars were installed on two Nansen Radio-wave velocity estimates sledges towed by a snowmobile, while Ramac radar was Radar profiles of 2000 and 2006 correspond to accumu­ placed on a plastic pulka. The distance between the lation areas. In order to calculate the thickness of the transmitting and receiving antennae of VIRL-2 and VIRL-6 snow-firn layer and the total snow-firn-ice column was 9 and 11 m, respectively, while for Ramac 200MHz it thickness we need appropriate radio-wave velocity (RWV) was set to 0.6 m. The antennae were arranged collinearly estimates. We rely on several data sources for such and following the profile direction. Radar and GPS data estimates (most of them our own data), all of them obtained were collected at 2-3 m distance between records for VIRL from Johnsons and Hurd Glaciers, not far from Bowles radars and 1 m for Ramac radar. Plateau. These consist of: Fig. 2. (a) Location of the high-frequency (200 MHz) radar profiles done on the northwestern and southeastern parts of Bowles Plateau, (b, c) Layout of the profiles, together with the corresponding firn-layer thickness maps. 1. RWV estimates from VIRL-2 radar diffraction hyperbolae reported for some areas of Hurd Peninsula ice cap during the measured in 1999/2000 and reported in Benjumea and period 1956-2000, though the average thickness change has others (2003), which provide the average RWV for the been of only -5.5 ± 4.4 m; Molina and others, 2007).