Collection and processing

All the radar data were collected using the British Antarctic Survey’s Deep Look Radio Echo Sounder (DELORES). This is a monopulse radar operating in the 1–4MHz range that is based on designs developed by the University of Washington,WA, USA, (Reference FujitaGades, 1998) and St Olaf College, MN, USA (Reference Vaughan, Cor, Smit, Pritchar and ShepherWelch and Jacobel, 2003). The transmitter uses a ±2000V pulser that has a variable firing rate between 1 and 5 kHz. The antennae are resistively loaded wire dipoles, and the receiver uses a chassis computer with a 100 MHz digitizing card and is capable of stacking 3000 traces. The system is towed behind a snowmobile at 10–20 km h^–1, so the distance between traces was 5–10 m. I collected approximately 2500 km of radar profiles during two Antarctic field seasons. The data were processed using the ReflexW software in the following sequence: edit; background removal to suppress the effects of the direct (airwave) arrival; bandpass filter to increase the signal-to-noise ratio; amplitude correction to compensate for spherical spreading; and Kirchhoff migration to focus diffracted energy to its source point.

Data description

Four radar profiles are presented (Fig. 2), each positioned orthogonal to the flow direction, distributed along the length of the ice stream from 40 km upstream of the grounding line to 240 km upstream (Fig. 1). Average flow speed is 87 ma^–1 for the most upstream profile (R067), 129 ma^–1 (R125), 276ma^–1 (J300) and 375ma^–1 at the downstream profile (R200). The profiles will now be described from upstream to downstream.

Fig. 2.

Radar profiles. Boxes in (a–d) show location of detailed images in Figure 3a–d respectively. Ice flow is out of the page in each case. (a) Profile R067 (south–north). Data collected with a 1.2 MHz monopulse radar and processed using a bandpass filter, spherical spreading correction and migration. The migration aperture for this display was kept narrow to suppress migration noise from shallow bedrock reflections at the south end of the profile. Flow-speed profile taken from InSAR data (solid line) and from ground-based global positioning system (GPS) measurement (red star). (b–d) Profiles R125 (south–north) (b), J300 (west–east) (c) and R200 (west-southwest–east-northeast) (d) with selected isochrones picked out for clarity.

Determination of margin locations

The margins of sector B flow were determined from the radar data by recognizing distinctive patterns of reflections that mark the margins, which are common to each profile. On profile R067, the southern margin is marked by a short-wavelength syncline at 3.8 km (Fig. 2a) which is spatially correlated with a 500 m step in the bed. Above the syncline there is a complex pattern of reflections with varying dips. There is also a reduction in the amplitude of the isochrones at this point. On the other three profiles, the margin closest to the Ellsworth Mountains is marked by a sharp termination of the isochrones over the full depth, and the termination lies immediately beneath the complex pattern.

The northern or eastern margin of the sector B flow is marked on each profile by two distinctive reflector groups that are closely associated (Fig. 3). On each profile there is a 1 km wide zone of tightly folded reflections at depths below 1000 m (marked by polygons in Fig. 3). Above this zone are a group of faint curvilinear reflections (marked by arrows in Fig. 3) that cross-cut the isochrones. These reflections dip towards north or east with dips that increase with depth, forming isolated curves on the profile. Profile R125 is the upstream member of a grid of six lines spaced 5 km apart (not shown here), and profile R200 is one of a series of 30 lines spaced 500m apart. All of the lines show the same curving, cross-cutting reflections, which indicates that the reflecting surfaces are curved in section and linear in the flow direction and that they do not arise from out-of-plane reflection points.

Fig. 3.

Detailed view of reflector groups marking the boundary between sectors B and C. Arrows mark the ends of faint curvilinear reflections that cross-cut isochronal reflections. Polygons bound a zone of short-wavelength folds. Yellow bar is a 1 km scale; note that the horizontal scales for profiles J300 and R200 have been stretched to allow comparison of the detail. Ice flow is out of the page.

Taken together, these two reflector groups, the zone of tight folds and the curvilinear cross-cutting reflections, constitute a flow-margin signature that marks the boundary between sectors B and C. On profile R067, the pattern of reflectors lies beneath a linear feature in the surface imagery (Fig. 1a) that also corresponds to the edge of enhanced flow measured by InSAR (Fig. 1d). I suggest that the pattern develops in the upstream region where there is flow differential between sectors B and C, and is preserved as the ice advects downstream into the area where little or no flow differential between the sectors remains, i.e. about the location of profile J300.

The inclination of the fold axes in the zone of tight folds (Fig. 3) changes between profiles R125 and J300. On profile R125 the dip is towards the centre of sector B, and on J300 the dip is away from the centre. The flow direction changes through almost 90^˚ over the same distance. This suggests that some lateral vertical strain takes place as the ice flows around the bend.

The cross-cutting, curvilinear reflections that form part of the flow-margin signature are unusual. Reflections on radar profiles over ice sheets are normally considered to arise from one of three mechanisms: (1) density variations in the firn layer and upper ice (Reference RignotRobin and others, 1969), (2) conductivity variations in compacted ice (Reference Ng and ConwaParen and Robin, 1975) or (3) crystal orientation fabrics (COFs) generated in high-strain zones (Reference Frolich, Vaugha and DoakFujita and Mae, 1994; Reference Fujita and MaFujita and others, 1999; Reference Dowdeswell and EvanEisen and others, 2007). Reflectors caused by these mechanisms are usually surface-parallel, and the reflections are considered isochrones (Reference Bogorodsky, Bentle and GudmandseBogorodsky and others, 1985; Dowdes-well and Evans, 2004), or, in the case of COF reflections, the fabric develops parallel to the depositional layering due to a strain-enhancement factor resulting from mechanical anisotropy (Reference Fujita and MaFujita and others, 1999). Reflections that cross-cut isochronal reflections must arise from post-depositional effects. In the present case, erosion or non-deposition can be ruled out (cf. Reference Welch and JacobeWelch and Jacobel, 2005). One possible explanation of the reflections is that they arise from changes in COF due to localized strain within the margin of the ice stream in a location where both sides of the margin are moving but at different rates. There is a narrow but deep depression in the basal topography about 30 km upstream of profile R067 (Fig. 1c): possibly a strain fabric developed due to vertical shear as ice flowed into and out of this depression. Further data are required to determine the origin of these features.

The convergence of flow of sector B ice from the onset area into the main trunk of the ice stream is clearly demonstrated on these profiles. The width of sector B reduces from 35 km at profile R067 to 8 km at R200. From considerations of mass continuity, the reduction in cross-sectional area from profile to profile should be proportional to the increase in flow speed between profiles (Reference Liu, Jeze and LiNg and Conway, 2004).

Calculation of flux

Previous estimates of the flux of ice in the Rutford catchment determined the total flux through the full width of the main trunk of the ice stream, and the average flux determined by these studies was approximately 20 km³ a^–1 (Reference Crabtree and DoakCrabtree and Doake, 1982; Reference Jacobel, Gade, Gottschlin, Hodg and WrighJoughin and Bamber, 2005). Here I consider the fraction of the total that comes only from sector B in order to begin to refine the ice-flow model for Rutford. At this stage, I use only the most simplistic of flux calculations, which assumes that all of the flow is accommodated by basal sliding or subglacial till deformation, i.e. the vertical velocity gradient is zero. The accurate definition of a flux gate for this flow sector would also need to take into account the inclination of the flow margin at depth. Using the flow-margin information from the radar profiles described previously, the cross-sectional area of sector B ice was measured on each profile. This was done in a series of 1 km wide sub-gates to correspond with the resolution of the InSAR flow-speed data (Fig. 1d). The flux for each of the sub-gates was then calculated and the sum taken. The total flux across each profile is shown in Table 1. The flux at the most downstream profile, R200, is estimated as 7.27 km³ a^–1, or about 38% of the total flux through Rutford Ice Stream.

Table 1.

Ice flux through radar profile gates for sector B

The flux increases in the downstream direction. This result is as expected because of the additional mass due to surface accumulation between the gates. Table 1 shows that the surface accumulation required to maintain balance lies in the range (0.54±0.04)–(0.61±0.05)ma^–1w.e. This accumulation value is not significantly different from the measurement of 0.39 ~0.10 ma^–1w.e. for the period 1780–2001 made at ITASE (International Trans-Antarctic Scientific Expedition) core site 01-5, which lies within sector B (Reference Joughin, Rigno, Rosanov, Lucchitt and BohlandeKaspari and others, 2004). The calculated flux values may be biased towards higher values since they were based on the assumption that the vertical velocity gradient through the ice column is zero because basal sliding or till deformation predominates over internal deformation. This assumption is likely valid in the main trunk of the ice stream, where the flow is fastest. There, the bed is smooth and there are extensive areas of soft sediment beneath the ice (Reference Siegert, Payn and JoughiSmith, 1997b, Reference Smith2006; Reference A.MSmith and others, 2007). However, this assumption may not be valid in the onset region, where the bed is only intermittently smooth and there are extensive areas of rough bed with high topographic relief (see especially profile R125 (Fig. 2b)). In these regions, it is likely that the vertical velocity profile varies across the ice stream as the ratios of internal deformation and basal sliding change with varying basal conditions. To quantify such variation in the vertical velocity profile requires a full force-balance analysis (Reference SmithVan der Veen and Whillans, 1989) of the catchment along the lines of those performed in the Lambert Glacier region by Reference Eisen, Haman, Kipfstuh, Steinhag and WilhelmFricker and others (2000) or for Heimefrontfjella by Reference Paren and RobiPohjola and others (2004). While such modelling is beyond the scope of this paper, my observations suggest that a force-balance model for the Rutford system should be subdivided into four sectors and each considered as a semi-independent drainage basin.