The Cryosphere Discussions www.the-cryosphere-discuss.net 1994-0432 1994-0440 1 1 2007 10.5194/tcd-1-1-2007 http://www.the-cryosphere-discuss.net/1/1/2007/ http://www.the-cryosphere-discuss.net/1/1/2007/tcd-1-1-2007.html http://www.the-cryosphere-discuss.net/1/1/2007/tcd-1-1-2007.pdf 1 16 2007-06-15 Direct evidence for radar reflector originating from changes in crystal-orientation fabric O. Eisen olaf.eisen@awi.de I. Hamann S. Kipfstuhl D. Steinhage F. Wilhelms Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven, Germany The origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle- to increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2020–2030 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the ice-core profile and synthetic RES data. Our observations allow to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics. As the conductivity profile of the EPICA shows no distinctive peak at this depths, we exclude changes in conductivity as the reflector origin. This is supported by application of numerical forward modelling of electromagnetic wave propagation, based on the conductivity profile, which is able to reproduce nearby reflections, but fails to reproduce this one. Because of background noise, the permittivity profile based on dielectric does not show prominent signals at these depths. We therefore interpret the observed reflector to originate from this change in crystal fabric. Bogorodsky, V., Bentley, C., and Gudmandsen, P.: Radioglaciology, D. Reidel Publishing Company, Dordrecht, Holland, 1985. Doake, C. S M., Corr, H. J F., and Jenkins, A.: Polarization of radio waves transmitted through Antarctic ice shelves, AG, 34, 165–170, 2002. Dowdeswell, J A. and Evans, S.: Investigations of the form and flow of ice sheets and glaciers using radio-echo sounding, Rep. Prog. Phys., 67, 1821–1861, 2004. Durand, G., Gillet-Chaulet, F., Svensson, A., Gagliardini, O., Kipfstuhl, S., Meyssonnier, J., Parrenin, F., Duval, P., and Dahl-Jensen, D.: Change of the ice rheology with climatic transitions – implications for ice flow modelling and dating of the EPICA Dome C core, Clim. Past, 3, 155–167, 2007. Eisen, O., Wilhelms, F., Steinhage, D., and Schwander, J.: Improved method to determine RES-reflector depths from ice-core profiles of permittivity and conductivity, J. Glaciol., 52, 299–310, 2006. Fujita, S., Maeno, H., Uratsuka, S., Furukawa, T., Mae, S., Fujii, Y., and Watanabe, O.: Nature of radio echo layering in the Antarctic ice sheet detected by a two-frequency experiment, J. Geophys. Res., 104, 13 013–13 024, 1999. Fujita, S., Matsuoka, T., Ishida, T., Matsuoka, K., and Mae, S.: A summary of the complex dielectric permittivity of ice in the megahertz range and its application for radar sounding of polar ice sheets, in: The Physics of Ice Core Records, edited by Hondoh, T., pp. 185–212, Hokkaido University Press, 1 edn., 2000. Fujita, S., Matsuoka, K., Maeno, H., and Furukawa, T.: Scattering of VHF radio waves from within an ice sheet containing the vertical-girdle-type ice fabric and anisotropic reflection boundaries, Ann. Glac., 37, 305–316, 2003. Fujita, S., Maeno, H., and Matsuoka, K.: Radio-wave depolarization and scattering within ice sheets: a matrix-based model to link radar and ice-core measurements and its application, J. Glaciol., 52, 407–424, 2006. Hargreaves, N D.: The radio-frequency birefringence of polar ice, J. Glaciol., 21, 301–313, 1978. Harrison, C H.: Radio echo sounding of horizontal layers in ice, J. Glaciol., 12, 383–397, 1973. ISMASS Committee: Recommendations for the collection and synthetis of Antarctic Ice Sheet mass balance data, Global and Planetary Change, 42, 1–15, 2004. Matsuoka, K., Furukawa, T., Fujita, S., Maeno, H., Uratsuka, S., Naruse, R., and Watanabe, O.: Crystal orientation fabrics within the Antarctic ice sheet revealed by a multipolarization plane and dual-frequency radar survey, J. Geophys. Res., 108, 2499, doi:10.1029/2003JB002425, 2003. Matsuoka, K., Uratsuka, S., Fujita, S., and Nishio, F.: Ice-flow induced scattering zone within the Antarctic ice sheet revealed by high-frequency airborne radar, J. Glaciol., 50, 382–388, 2004. Nixdorf, U., Steinhage, D., Meyer, U., Hempel, L., Jenett, M., Wachs, P., and Miller, H.: The newly developed airborne RES-system of the AWI as a glaciological tool, Ann. Glaciol., 29, 231–238, 1999. Paren, J G.: PRC at a dielektrical interface, J. Glaciol., 27, 203–204, 1981. Paren, J G. and Robin, G. d Q.: Internal reflections in polar ice sheets, J. Glaciol., 14, 251–259, 1975. Robin, G. d Q., Evans, S., and Bailey, J T.: Interpretation of radio echo sounding in polar ice sheets, in: Philosophical Transactions of the Royal Society of London, vol. 146 of \em A\/, pp. 437–505, Royal Society of London, 1969. Wallbrecher, E.: Methoden zum quantitativen Vergleich von Regelungsgraden und Formen strukturgeologischer Datenmengen mit Hilfe von Vektorstatistik und Eigenwertanalyse, N. JB. Geol. Paläontol. Abh., 159, 113–149, 1979. Wilhelms, F.: Explaining the dielectric properties of firn as a density and conductivity mixed permittivity (DECOMP), Geophys. Res. Letters, 32, L16501, \doidoi:10.1029/2005GL022808, 2005. Wilson, J., Russell-Head, D S., and Sim, H M.: The application of an automated fabric analyzer systm to the textural evolution of folded ice layers in shear zones, Ann. Glaciol., 37, 7–17, 2003.