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A freeze-on ice zone along the Zhongshan–Kunlun ice sheet profile, East Antarctica, by a new ground-based ice-penetrating radar
Sci. Bull. DOI 10.1007/s11434-015-0732-0
Letter
www.scibull.com www.springer.com/scp
Earth Sciences
A freeze-on ice zone along the Zhongshan–Kunlun ice sheet profile, East Antarctica, by a new ground-based ice-penetrating radar Xueyuan Tang • Bo Sun • Jingxue Guo • Xiaojun Liu • Xiangbin Cui • Bo Zhao • Yun Chen
Received: 12 August 2014 / Accepted: 24 November 2014 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract The 2012/2013 Chinese Antarctic Research Expedition (CHINARE) ’s inland traverse from Zhongshan station to Kunlun station on the East Antarctic ice sheet provided an opportunity to reveal englacial freeze-on ice using ice-penetrating radar. A radar dataset along the profile was collected using a new ground-based radar system with a high frequency of 150 MHz. A typical example of a freeze-on ice structure was revealed in the radar images, similar to that found in the Dome A region. The subglacial stratigraphy showed a new freeze-on ice zone with a length of 10 km near the ice-bedrock interface along the traverse, located 1,044–1,056 km from the coast. Keywords Kunlun station Dome A Freeze-on ice Ice-penetrating radar East Antarctic ice sheet Ice-penetrating radar has generally been used to determine the bed topography and internal structure in the large ice body of the East Antarctic ice sheet [1]. Using aircraftbased ice-penetrating radar systems, Bell et al. [2] first revealed two populations of near-bed reflectors beneath
Electronic supplementary material The online version of this article (doi:10.1007/s11434-015-0732-0) contains supplementary material, which is available to authorized users. X. Tang (&) B. Sun J. Guo X. Cui Y. Chen State Oceanic Administration Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai 200136, China e-mail: [email protected] X. Liu B. Zhao Key Laboratory of Electromagnetic Radiation and Detection Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China
Dome A, the highest ice feature of the East Antarctic ice sheet. Their results showed that the basal bright reflectors and the packages of ice bounded by these reflectors are a result of freeze-on ice, and that the widespread presence of freeze-on ice implies that the accretion mechanisms processes of conductive cooling and glaciohydraulic supercooling in the Gamburtsev Mountains beneath Dome A. Freeze-on ice images can be used to reveal the basal processes in the ice sheet, estimating the distribution of accreted ice, and to find the subglacial lakes, melting zones [2, 3], and the older ice [4]. Thus, imaging freeze-on ice is essential for understanding subglacial water systems, basal melting, and ice sheet dynamics. Preliminary investigation has shown that Dome A was a likely center of early ice sheet growth [5], and that it may be an ideal site for deep ice-core drilling to reveal the oldest ice records [6]. The Chinese Kunlun station (80°25.020 S, 77°06.970 E) was constructed at Dome A in 2009. To investigate the glaciological conditions and drill a deep ice-core in the Antarctic interior, CHINARE has carried out several inland traverses from Zhongshan station to Dome A since 2004/2005. During those traverses, radar data on the ice sheet have been collected by ground-based ice-penetrating radar [7]. Altogether, CHINARE has acquired more than 1,200 km of deep radar profiles along the traverse route using both dual-frequency system with center frequencies of 60 and 179 MHz, and single-frequency multi-polarization system, with both systems from the Japanese National Institute of Polar Research (NIPR). Those radar explorations focused on matters of glaciological interest: ice thickness, bed topography, and internal layering. However, the radar systems did not offer direct evidence for the existence or distribution of the freeze-on ice along the profiles. During the austral summer of 2012/2013, a new radar system was applied to explore the
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Fig. 1 Map of the traverse from Zhongshan station to Kunlun station on the East Antarctic ice sheet. a Surface elevation contours derived from RADARSAT. a0 The red circle marks the region of interest (*20 km 9 20 km in area), of which the radar track lines are shown at right, and the red line ‘‘2’’-AB and the black line ‘‘5’’-CD show the locations at which the radar images of the freeze-on ice in d. b Surface elevation data from Bedmap2, smoothed at a grid spacing of roughly 5 km. c Bed elevation, gridded with a spacing of 5 km in the rectangular region at an estimated uncertainty of ±130 m. The exception is the 10 km 9 10 km region enclosed by radar lines 4–7, where the bed elevation is derived for a propagation speed of 168 m ls-1. d 150 MHz radar profiles of the lines ‘‘2’’-AB and ‘‘5’’-CD in a at the traverse, where the line AB extends from 1,044 to 1,056 km from the coast
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freeze-on ice zones by imaging distinct near-bed reflectors in the formerly echo-free zones. The new radar system was developed by the Institute of Electronics of the Chinese Academy of Sciences and the Polar Research Institute of China. The 150 MHz ice radar with a bandwidth of 100 MHz achieved a pulse width of 0.4–10 ls. A transmitter with a peak power of 500 W and a receiver were fixed symmetrically on a snow vehicle. The snow vehicle moved at a speed of *13 km every hour during the traverse. The stacking traces were recorded at an average interval of 3.5 m, with their locations and surface elevations being recorded simultaneously by the global positioning system. The system could penetrate through more than 3,500 m of ice, achieving a depth resolution of 1.0 m, and producing high-resolution images of the subglacial conditions. Radar data processing was done using ReflexW software. Some of the modules of correction that were used included the following: automatic gain control, antenna ringdown removal, band-pass filtering, and Kirchhoff migration. To calculate the ice thickness, a propagation speed of 168 m ls-1 averaged over the ice column was estimated [7]. Using the assumption that the error range of the radio-wave velocity was ?/–3 % (164–173 m ls-1), an associated bed elevation uncertainty of *6 % was determined [8]. The bed elevations were obtained by subtracting the ice thickness from the surface elevation. Using the radar system, more than 1,300 km of radar profiles along the traverse route and around Kunlun station were collected (Fig. 1). Detailed information about the new radar system and data results will be reported elsewhere. A typical example of bright basal reflector structure observed in the ice sheet profile between Zhongshan and Kunlun is shown in Fig. 1d. Its corresponding location, presented by line ‘‘2’’-AB of Fig. 1a, is 1,044–1,056 km from the coast along the traverse route. The surface elevation of the zone is *3,610–3,750 m from Bedmap2 (Fig. 1b) [9], and the ice thickness is approximately 910–2,250 m (Fig. 1c). The ice thickness is rather shallow compared to that of the overdeepened valleys beneath Dome A [5]. Sequential radar images crossing the zone showed the coherence of the basal reflectors at the crossover point of radar tracks of ‘‘line 2’’-AB and ‘‘line 5’’-CD (Fig. 1b). Two numerical experiments were conducted with different inputs to compare the basal temperature and melting point curves (Fig. S1). The results implied that the basal ice is at the pressure melting point and can drive ice from melting to freeze-on in hydrology. This confirms that the reflectors should be an englacial
freeze-on feature rather than a feature of the bed or the offnadir reflective topography. Furthermore, the freeze-on zone is close to the adjacent subglacial water networks in the Gamburtsev Mountains, where there exist overdeepened valleys and a basal hydrologic system of small water bodies [10]. The gradient of the ice sheet surface and the regional subglacial networks may imply the existence of freeze-on ice as a source for the water bodies. The geometry of the basal reflectors seen, in our radar images (Fig. 1d), was similar to that found as a ‘‘freeze-on ice’’ zone in the Dome A region [2]. The subglacial stratigraphy showed that the freeze-on ice zone has a length of *10 km and an average thickness of *300 m. The foregoing interpretations of the zone mean that the basal reflectors represent the features of a refreezing processing. It is expected that the freeze-on ice radar data will provide input for ice models which will interpret the origin and evolution of the subglacial freeze-on ice and assess the balance and redistribution of the ice sheet mass. Acknowledgments The authors thank the 29th Chinese National Antarctic Research Expedition for their help in the field data collection and Richard C Hindmarsh for providing the ice model. This work was supported by the National Natural Science Foundation of China (41376192 and 40906101), the National Basic Research Program of China (2012CB957702 and 2013CBA01804), the Foreign Cooperation Support Program of Chinese Arctic and Antarctic Administration, State Oceanic Administration, China (IC201214) and the Natural Science Foundation of Shanghai, China (13ZR1445300). Conflict of interest of interest.
The authors declare that they have no conflict
References 1. Siegert MJ, Kwok R (2000) Earth Planet Sci Lett 179:227–235 2. Bell RE, Ferraccioli F, Creyts TT et al (2011) Science 331:1592–1595 3. Fischer H, Severinghaus J, Brooke E et al (2013) Clim Past 9:2489–2505 4. Van Liefferinge B, Pattyn F (2013) Clim Past 9:2335–2345 5. Sun B, Siegert MJ, Mudd SM et al (2009) Nature 459:690–693 6. Tang XY, Sun B, Li YS et al (2012) Adv Polar Sci 23:47–54 7. Cui XB, Sun B, Tian G et al (2010) Chin Sci Bull 55:425–431 8. Macheret YY, Oterro J, Navarro FJ et al (2009) Ann Glaciol 50:49–56 9. Fretwell P, Pritchard HD, Vaughan DG et al (2013) Cryosphere 7:375–393 10. Wolovick MJ, Bell RE, Creyts TT et al (2013) J Geophys Res 118:1–15
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Letter
www.scibull.com www.springer.com/scp
Earth Sciences
A freeze-on ice zone along the Zhongshan–Kunlun ice sheet profile, East Antarctica, by a new ground-based ice-penetrating radar Xueyuan Tang • Bo Sun • Jingxue Guo • Xiaojun Liu • Xiangbin Cui • Bo Zhao • Yun Chen
Received: 12 August 2014 / Accepted: 24 November 2014 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract The 2012/2013 Chinese Antarctic Research Expedition (CHINARE) ’s inland traverse from Zhongshan station to Kunlun station on the East Antarctic ice sheet provided an opportunity to reveal englacial freeze-on ice using ice-penetrating radar. A radar dataset along the profile was collected using a new ground-based radar system with a high frequency of 150 MHz. A typical example of a freeze-on ice structure was revealed in the radar images, similar to that found in the Dome A region. The subglacial stratigraphy showed a new freeze-on ice zone with a length of 10 km near the ice-bedrock interface along the traverse, located 1,044–1,056 km from the coast. Keywords Kunlun station Dome A Freeze-on ice Ice-penetrating radar East Antarctic ice sheet Ice-penetrating radar has generally been used to determine the bed topography and internal structure in the large ice body of the East Antarctic ice sheet [1]. Using aircraftbased ice-penetrating radar systems, Bell et al. [2] first revealed two populations of near-bed reflectors beneath
Electronic supplementary material The online version of this article (doi:10.1007/s11434-015-0732-0) contains supplementary material, which is available to authorized users. X. Tang (&) B. Sun J. Guo X. Cui Y. Chen State Oceanic Administration Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai 200136, China e-mail: [email protected] X. Liu B. Zhao Key Laboratory of Electromagnetic Radiation and Detection Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China
Dome A, the highest ice feature of the East Antarctic ice sheet. Their results showed that the basal bright reflectors and the packages of ice bounded by these reflectors are a result of freeze-on ice, and that the widespread presence of freeze-on ice implies that the accretion mechanisms processes of conductive cooling and glaciohydraulic supercooling in the Gamburtsev Mountains beneath Dome A. Freeze-on ice images can be used to reveal the basal processes in the ice sheet, estimating the distribution of accreted ice, and to find the subglacial lakes, melting zones [2, 3], and the older ice [4]. Thus, imaging freeze-on ice is essential for understanding subglacial water systems, basal melting, and ice sheet dynamics. Preliminary investigation has shown that Dome A was a likely center of early ice sheet growth [5], and that it may be an ideal site for deep ice-core drilling to reveal the oldest ice records [6]. The Chinese Kunlun station (80°25.020 S, 77°06.970 E) was constructed at Dome A in 2009. To investigate the glaciological conditions and drill a deep ice-core in the Antarctic interior, CHINARE has carried out several inland traverses from Zhongshan station to Dome A since 2004/2005. During those traverses, radar data on the ice sheet have been collected by ground-based ice-penetrating radar [7]. Altogether, CHINARE has acquired more than 1,200 km of deep radar profiles along the traverse route using both dual-frequency system with center frequencies of 60 and 179 MHz, and single-frequency multi-polarization system, with both systems from the Japanese National Institute of Polar Research (NIPR). Those radar explorations focused on matters of glaciological interest: ice thickness, bed topography, and internal layering. However, the radar systems did not offer direct evidence for the existence or distribution of the freeze-on ice along the profiles. During the austral summer of 2012/2013, a new radar system was applied to explore the
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Sci. Bull.
Fig. 1 Map of the traverse from Zhongshan station to Kunlun station on the East Antarctic ice sheet. a Surface elevation contours derived from RADARSAT. a0 The red circle marks the region of interest (*20 km 9 20 km in area), of which the radar track lines are shown at right, and the red line ‘‘2’’-AB and the black line ‘‘5’’-CD show the locations at which the radar images of the freeze-on ice in d. b Surface elevation data from Bedmap2, smoothed at a grid spacing of roughly 5 km. c Bed elevation, gridded with a spacing of 5 km in the rectangular region at an estimated uncertainty of ±130 m. The exception is the 10 km 9 10 km region enclosed by radar lines 4–7, where the bed elevation is derived for a propagation speed of 168 m ls-1. d 150 MHz radar profiles of the lines ‘‘2’’-AB and ‘‘5’’-CD in a at the traverse, where the line AB extends from 1,044 to 1,056 km from the coast
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freeze-on ice zones by imaging distinct near-bed reflectors in the formerly echo-free zones. The new radar system was developed by the Institute of Electronics of the Chinese Academy of Sciences and the Polar Research Institute of China. The 150 MHz ice radar with a bandwidth of 100 MHz achieved a pulse width of 0.4–10 ls. A transmitter with a peak power of 500 W and a receiver were fixed symmetrically on a snow vehicle. The snow vehicle moved at a speed of *13 km every hour during the traverse. The stacking traces were recorded at an average interval of 3.5 m, with their locations and surface elevations being recorded simultaneously by the global positioning system. The system could penetrate through more than 3,500 m of ice, achieving a depth resolution of 1.0 m, and producing high-resolution images of the subglacial conditions. Radar data processing was done using ReflexW software. Some of the modules of correction that were used included the following: automatic gain control, antenna ringdown removal, band-pass filtering, and Kirchhoff migration. To calculate the ice thickness, a propagation speed of 168 m ls-1 averaged over the ice column was estimated [7]. Using the assumption that the error range of the radio-wave velocity was ?/–3 % (164–173 m ls-1), an associated bed elevation uncertainty of *6 % was determined [8]. The bed elevations were obtained by subtracting the ice thickness from the surface elevation. Using the radar system, more than 1,300 km of radar profiles along the traverse route and around Kunlun station were collected (Fig. 1). Detailed information about the new radar system and data results will be reported elsewhere. A typical example of bright basal reflector structure observed in the ice sheet profile between Zhongshan and Kunlun is shown in Fig. 1d. Its corresponding location, presented by line ‘‘2’’-AB of Fig. 1a, is 1,044–1,056 km from the coast along the traverse route. The surface elevation of the zone is *3,610–3,750 m from Bedmap2 (Fig. 1b) [9], and the ice thickness is approximately 910–2,250 m (Fig. 1c). The ice thickness is rather shallow compared to that of the overdeepened valleys beneath Dome A [5]. Sequential radar images crossing the zone showed the coherence of the basal reflectors at the crossover point of radar tracks of ‘‘line 2’’-AB and ‘‘line 5’’-CD (Fig. 1b). Two numerical experiments were conducted with different inputs to compare the basal temperature and melting point curves (Fig. S1). The results implied that the basal ice is at the pressure melting point and can drive ice from melting to freeze-on in hydrology. This confirms that the reflectors should be an englacial
freeze-on feature rather than a feature of the bed or the offnadir reflective topography. Furthermore, the freeze-on zone is close to the adjacent subglacial water networks in the Gamburtsev Mountains, where there exist overdeepened valleys and a basal hydrologic system of small water bodies [10]. The gradient of the ice sheet surface and the regional subglacial networks may imply the existence of freeze-on ice as a source for the water bodies. The geometry of the basal reflectors seen, in our radar images (Fig. 1d), was similar to that found as a ‘‘freeze-on ice’’ zone in the Dome A region [2]. The subglacial stratigraphy showed that the freeze-on ice zone has a length of *10 km and an average thickness of *300 m. The foregoing interpretations of the zone mean that the basal reflectors represent the features of a refreezing processing. It is expected that the freeze-on ice radar data will provide input for ice models which will interpret the origin and evolution of the subglacial freeze-on ice and assess the balance and redistribution of the ice sheet mass. Acknowledgments The authors thank the 29th Chinese National Antarctic Research Expedition for their help in the field data collection and Richard C Hindmarsh for providing the ice model. This work was supported by the National Natural Science Foundation of China (41376192 and 40906101), the National Basic Research Program of China (2012CB957702 and 2013CBA01804), the Foreign Cooperation Support Program of Chinese Arctic and Antarctic Administration, State Oceanic Administration, China (IC201214) and the Natural Science Foundation of Shanghai, China (13ZR1445300). Conflict of interest of interest.
The authors declare that they have no conflict
References 1. Siegert MJ, Kwok R (2000) Earth Planet Sci Lett 179:227–235 2. Bell RE, Ferraccioli F, Creyts TT et al (2011) Science 331:1592–1595 3. Fischer H, Severinghaus J, Brooke E et al (2013) Clim Past 9:2489–2505 4. Van Liefferinge B, Pattyn F (2013) Clim Past 9:2335–2345 5. Sun B, Siegert MJ, Mudd SM et al (2009) Nature 459:690–693 6. Tang XY, Sun B, Li YS et al (2012) Adv Polar Sci 23:47–54 7. Cui XB, Sun B, Tian G et al (2010) Chin Sci Bull 55:425–431 8. Macheret YY, Oterro J, Navarro FJ et al (2009) Ann Glaciol 50:49–56 9. Fretwell P, Pritchard HD, Vaughan DG et al (2013) Cryosphere 7:375–393 10. Wolovick MJ, Bell RE, Creyts TT et al (2013) J Geophys Res 118:1–15
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