Ice motion over Lake Vostok, Antarctica: constraints on inferences regarding the accreted ice

Ice motion over Lake Vostok, Antarctica, is measured using repeat-pass synthetic-aperture radar (SAR) interferometry. The coverage of the lake and the components of the vector field are resolved using 10 overlapping data takes from ascending and descending look directions. Seventy-day temporal baselines provide the sensitivity required to observe the range of ice motion (0-6 m a(-1)) over the lake and the adjacent ice sheet. It is remarkable that the scattering field remained coherent over these time separations. This is critical for interferometric analysis and can be attributed to the low surface accumulation and low air temperature at this elevation. The regional flow of the ice sheet around Lake Vostok is from west to east, perpendicular to the surface elevation contours. As the ice flows past the grounding line, a southward component of motion develops that is correlated with the north-south surface slope along the length of the lake. The surface velocity increases slowly from the northern tip of the lake and then more rapidly south of 77 degrees S. At Vostok station, the ice motion is 4.2 m a(-1). Across the lake and away from boundary effects, the down-lake flow pattern takes on a parabolic profile with maximum velocity close to the center line of the lake. The overall influence of the subglacial lake is the addition of a down-lake motion component to the prevailing west-east motion of the ice sheet. As a result, we estimate 10 % of the mass flowing onto the lake is diverted south. Reconstructions based on the Vostok ice core indicate that the ice was grounded up-glacier from the core site approximately 5000 years ago. This suggests a minimum freezing rate of 40 mm a(-1) for the subglacial accretion ice, 10 times greater than that inferred from thermodynamic modeling of the upper 2 km of the ice core.