Fig. 1. Lake Vostok location map and survey area. The inset Antarctica location map is the surface slope of the ERS-1 ice surface topography [20]. Lake Vostok is circled. The survey area map is the ice surface elevation in meters above sea level [21] contoured at a 10-m interval. The dark solid lines indicate the grid of the aerogeophysical profiles of the 2000–2001 Lake Vostok survey [3]. The white filled star denotes the location of the Vostok Ice Core. Map scale is in kilometers.

Fig. 2. Hydrological potential in the Lake Vostok region. The contour intervals are 0.1 MPa. The potential (Φ [Mpa]) is calculated based on the ice density (ρ_ice=900 kg m⁻³), lake water density (ρ_lake=1000 kg m⁻³), gravitational constant (g), bedrock/lake elevation (Z_lake) and ice surface elevation (Z_s), where Φ=ρ_iceg(Z_s−Z_lake)+ρ_lakegZ_lake. The surface elevations are from Studinger et al. [3] and are in meters above sea level with respect to the World Geodetic System 1984 ellipsoid. This reference system is used for all subsequent maps. The projection is polar stereographic with an origin at 90°S, 80°E, a scale factor of 0.989, a false easting of −515.050 km and a false northing of −1104.530 km. This projection is used in all subsequent maps.

Fig. 3. Deep internal layer elevation maps in the Lake Vostok area. (a) The elevation of internal layer 2 inside the boundaries of the lake, and the bedrock elevation outside of the lake. The contour interval of internal layer 2 is 75 m, and the grid interval is 5 km. The bedrock elevation is from Studinger et al. [3]. The lake boundary is defined by the presence of a distinct subglacial lake reflector. The elevation is in meters above sea level with respect to the World Geodetic System 1984 ellipsoid. (b) The elevation of internal layer 3 with a contour interval of 75 m. The circular white filled areas represent areas where it was not possible to resolve the internal layer.

Fig. 4. Structures in the deep internal layers across northern Lake Vostok. (a) Three panel figure of the identification of structures A to P in internal layers 2–4 for each of the individual Y lines of the Lake Vostok aerogeophysical survey. The lake boundary is the dotted line. The location of the Y line profiles (Y04–Y12) are denoted by thin black lines. The elevation of the individual internal layers along each Y line (grey lines) is plotted along track (about a median), with positive values projected to the false west in the polar stereographic projection. The median values range from 1400 to 2200 m for layer 2, from 1050 to 1500 m for layer 3, and from 800 to 900 m for layer 4. Individual distinct structures (A–P) along each profile are identified by filled circles. (b) Three panel figure of the elevation of internal layers 1–3 in profiles Y07, Y09 and Y11, with the identification of structures G to L. The distance between structures G and K is denoted by a solid line for profile Y07, a dashed line for profile Y09 and a dash-dot line for profile Y11. The varying distance between these two structures from profile to profile illustrates the widening downstream across the lake.

Fig. 5. Ice flow over Lake Vostok. (a) Internal layer 2 elevation (50 m contour interval) and interpreted structure tracking or flowlines. Flowlines from structure tracking between individual Y radar profiles (presented in Fig. 4) are denoted by the red dashed lines and red crosses. Flowlines interpreted from structures tracked on the internal layer 2 elevation map are denoted by purple solid lines. Areas of grounding, identified from the basal reflectors, are within the circled dashed black lines. Areas of enhanced surface ice flow, identified from the surface ice thickness map and presented in (e), are outlined by blue solid lines. (b) Interpreted flow field at a 5 km interval over the lake (blue arrows) with the input flowline constraints (red lines). (c) A comparison of the InSAR-derived velocity field in the vicinity of Lake Vostok [17] with the ice flow trajectories over Lake Vostok derived in this paper and presented in (b). The length of the arrows for the InSAR-derived flow field is scaled with respect to the magnitude of the velocity. (d) Ice surface elevation (10 m contour interval) in the Lake Vostok area (from [3]) with the downhill gradient of the surface elevation (black arrows) calculated at a 10-km interval, and flowlines A–V (white arrows) identified in this paper and presented in (a). The lengths of the arrows for the gradient of the surface elevation are normalized to be able to show the different magnitude vectors over the relatively flat lake surface and the surrounding ice sheet. The areas encircled by red dotted lines identify the areas of noticeable coherent surface gradient. The dominant directions of the surface gradient for these two areas are denoted by solid red arrows. (e) Ice thickness between internal layer 1, a shallow internal layer and the ice surface. The circular areas of thickening, labeled as coves C1–C8, identifiable over the lake at the western shoreline are denoted in pink. The pink dashed arrows indicate the inferred surface ice flow from east to west. (f) Total ice thickness over the lake with the location of the coves identified in (e). Note that only three of the northern coves (C1, C2 and C4) are associated with resolvable thickening of the total ice sheet.

Fig. 6. Distribution of an accreted ice/glacial ice reflector in the ice-penetrating radar and ice flow over Lake Vostok. (a) Distribution of identifiable accretion ice reflector over all of Lake Vostok on a basemap of the elevation of internal layer 2 (contoured at a 50-m interval), with a subset of the flowlines identified in Fig. 5a from tracking of structures in internal layers 2–4. The light blue filled circles denote areas that appear to be the locus of the accreted ice trajectories. (b) Illustration of the origin of the accretion ice reflector at a ridge within Lake Vostok in a radar profile [see (a) for location of profile].