Radar scattering in an alpine glacier: Evidence of seasonal development of temperate ice beneath ogives John H. McBride1, Summer B. Rupper1, Scott M. Ritter1, David G. Tingey1, Michelle R. Koutnik2, Annika M. Quick1, Thomas H. Morris1, R. William Keach II1, Landon K. Burgener1, Adam P. McKean1, Jessica Williams1, Joshua M. Maurer1, Durban G. Keeler1, and Robert Windell1 1Department of Geological Sciences, Brigham Young University, P.O. Box 24606, Provo, Utah 84602, USA 2Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100-Copenhagen, Denmark ABSTRACT caused by rock debris or an undulating wave 1958). Together, these processes result in wave expressed on the glacier surface. Wave ogives ogives at the base of the icefall. Posamentier Glacial ogives are transverse topographic, initially develop seasonally just below the ice- (1978) proposed what is now a classic “fold-and- wave-like surface features that form below fall as a series of surface undulations with an thrust–style” geological model for ogive devel- icefalls on some alpine glaciers. Ground- amplitude of several meters (one wavelength opment in which deep debris-laden ice is thrust penetrating radar surveys from the Gorner is thought to be equivalent to 1 year of glacial over near-surface ice with less debris, thus form- glacier system in the Swiss Alps reveal an fl ow; Nye, 1958; Waddington, 1986). Band ing periodic band ogives. This model postulates along-fl ow periodicity in scattering inten- ogives may represent the distal extent of eroded that as the ice fl ows down the glacier from the sity that correlates with ogives. The scatter- wave ogives, with only bands of alternating icefall accumulation zone, shear zones deform ing appears in the ablation zone and occurs debris remaining (Cuffey and Paterson, 2010). the ice into tight, asymmetric folds. These folds at 5–20 m depth. The geometry of the scat- In a seminal study, Goodsell et al. (2002) are truncated by reverse faults verging in the tering mimics that of the ogives, although used surface-deployed radar to reveal a promi- down-glacier direction. Goodsell et al. (2002) exaggerated in amplitude. We interpret the nent subsurface periodic scattering structure invoked this shearing model to explain the ogive- scattering to represent lateral variations in in the alpine glacier Bas Glacier d’Arolla in related radar scattering pattern observed on the water content. We propose that as glacial ice southern Switzerland that correlated with band Bas Glacier d’Arolla. Similarly, Appleby et al. accelerated and stretched through the ice- ogives on the surface. A similar structure was (2010) interpreted band ogives on the Fox gla- fall, seasonal fl uctuations occurred in water observed by McBride et al. (2010) along a small cier to be formed by multiple thrust zones that infi ltration to crevasses during the summer portion of a tributary glacier of the Gorner- brought deeper, banded ice to the surface. and subsequent freezing of that water in the gletscher system, the Zwillingsgletscher, which Over the past decade, surface-deployed radar crevasses in the winter. This seasonally vary- has wave ogives and is located ~25 km south- has been increasingly used to study the internal ing infi lling and freezing locally altered the east of the Bas Glacier d’Arolla. Further, a radar structure of glaciers, taking advantage of the distribution of temperature, creating zones of and deep-drilling study by Eisen et al. (2009) low attenuation of electromagnetic waves in temperate ice with water inclusions that pref- of the Gorner gletscher system drew an associa- solid ice (e.g., Matsuoka et al., 2010). Numer- erentially scatter radar energy. In addition to tion between zones of radar scattering and water ous studies have demonstrated that the interior the scattering pattern, highly refl ective pla- inclusion (or temperature). Questions raised by of a glacier may strongly scatter radar energy nar features associated with these periodic these studies include: why does the radar scat- (e.g., Woodward and Burke, 2007; Bradford regions of temperate ice are interpreted as tering show a periodic pattern and how does the et al., 2009; Eisen et al., 2009). Water-fi lled water-fi lled fractures. A three-dimensional glacial thermal structure interact with fractures voids or rock debris have been hypothesized to rendering of the orientation of these pla- in the ice to possibly infl uence this pattern? In be the cause of such scattering (e.g., Bamber, nar features precludes a “fold-and-thrust” this paper, we further examine this subsurface 1988; Moorman and Michel, 2000; Gusmeroli hypothesis for the formation of the ogives. structure with surface-deployed radar, which et al., 2010; see also Lawson et al., 1998; Arcone leads us to answer these questions. et al., 2000). The dominant point of view is that INTRODUCTION Two mechanisms are widely cited for ogive the onset of water causes the onset of radar scat- formation (mainly focused on band ogives), the tering (e.g., Björnsson et al., 1996). However, Glacial ogives are a classic feature of some shearing hypothesis (Posamentier, 1978) and such studies have usually not attempted to cor- alpine glaciers and have been studied for well the “summer versus winter passage” hypoth- relate radar scattering with surface features of over 100 years, yet they remain one of the most esis (Nye, 1958; summarized by Appleby et al., a glacier (e.g., the topographic variation in the puzzling features of glacial ice (Fisher, 1962; 2010; for a review of the differing mechanisms, ice surface, including ogives). Thus, the rela- Posamentier, 1978; Waddington, 1986; Ham- see Goodsell et al., 2002). The “summer versus tion of scattering to the dynamic processes that brey and Lawson, 2000; Goodsell et al., 2002). winter passage” hypothesis states that as ice affect the overall ice structure and development The term ogive, used in architecture and engi- accelerates across the icefall zone, it is stretched. has not been well established. In this paper, we neering to mean a pointed arch, refers to a peri- In addition, ice passing through in summer loses explore the spatial relation between the ogive odic expression of either banded color changes more ice by ablation than during the winter (Nye, topographic pattern and internal radar scatter- Geosphere; October 2012; v. 8; no. 5; p. 1054–1077; doi:10.1130/GES00804.1; 14 fi gures; 1 animation. Received 14 March 2012 ♦ Revision received 24 June 2012 ♦ Accepted 27 June 2012 ♦ Published online 18 September 2012 1054 For permission to copy, contact [email protected] © 2012 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/8/5/1054/3342005/1054.pdf by guest on 27 September 2021 Wave ogives and radar ing on the Gornergletscher system located in can be readily observed on satellite imagery vicinity of our study area, the Zwillingsgletscher the Alps of south-central Switzerland (Canton (Fig. 1D) as a periodic ribbed pattern that has a north-northwest fl ow direction, which Valais). Our purpose is to fi nd the cause of the decreases in wavelength down the glacier (i.e., changes to a mostly westward fl ow direction fur- unusual periodic scattering structure observed to the northwest). These ogives clearly show ther downslope, well below the icefall. in some alpine glaciers with ogives. We test the that the physical properties of ice originating at The Zwillingsgletscher is part of a hetero- hypothesis that periodic scattering associated and modifi ed through the icefall are maintained geneous glacier system (i.e., it is not isolated with ogives represents seasonal episodes where far down glacier from their source (e.g., Fisher, like the Bas Glacier d’Arolla). For example, the crevasses fi ll with water during the spring and 1962; Waddington, 1986). The particular ogives Zwillingsgletscher is a narrow lobe with well- summer and subsequently freeze during the fall of interest form at the icefall of the Zwillings- defi ned wave ogives, whereas the immediately and winter, causing latent heating of the sur- gletscher and extend more than a kilometer adjacent Grenzgletscher is broad and more com- rounding ice during the colder seasons. If this is down glacier (Fig. 3) from the confl uence of the plex with no easily discernible ogives or other correct, then from summer to fall, the crevasse- Zwillingsgletscher and Grenzgletscher (Fig. 1). periodic topographic pattern. Unlike the Grenz- zone ice is thermally altered, resulting in pock- The Gornergletscher system is one of the more gletscher (Eisen et al., 2009), no subsurface ets of temperate ice during the colder seasons. thoroughly studied alpine valley glacial systems temperature data are available for the Zwillings- Further, maximum subsurface scattering should (Eisen et al., 2009; Walter et al., 2010). Previ- gletscher. Our study area consists of a 2.4-km correlate with the gradient of ogive topogra- ous radar investigations in Canton Valais have length of the Zwillingsgletscher, in addition to phy. In this paper, we interpret the intensity and concentrated on the Grenzgletscher (the eastern separate surveys on the Grenzgletscher (Fig. 1). distribution of scattering to represent different branch of the Gornergletscher system) (Eisen thermal zones in the ice that form due to a com- et al., 2009) and on the Bas Glacier d’Arolla METHODOLOGY bination of fracturing, permeability, and latent (Goodsell et al., 2002), located ~25 km west heat release due to infi ltration and freezing of of our study area. Very different radar patterns We used a Geophysical Survey Systems, Inc. liquid water. and glacial morphologies have been reported (GSSI) 200-MHz antenna unit with two fi lter We acquired radar profi les at multiple fre- for the two glaciers. The Grenzgletscher covers settings, 5–300 MHz (low-frequency version) quency bands, and topographic surveys within a broad area and is several hundred meters thick and 50–600 MHz (high-frequency version), the ablation zones of the Zwillingsgletscher and (Eisen et al., 2009).