Basal ice sequences in Antarctic ice stream: Exposure of past hydrologic conditions and a principal mode of sediment transfer

The brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in Kamb Ice Stream. The upper sequence (7.3 m) comprises clear ice and layers with dispersed and stratified debris. The lower sequence (8.2 m) consists of accretion ice with alternating layers of stratified and solid debris. The two sequences have volumetric debris contents of about 5% and 20%, respectively. We infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the Siple Coast plain where basal freezing currently dominates the basal thermal regime. The basal ice layer contains the equivalent of 2.1 ± 0.4 m of frozen sediment. The high volume of sediment is a result of debris-rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on/off cycles. Fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the Ross Sea. Our results show that sediment wedges near grounding lines may form by melt out of basal ice debris. This is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till. Cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze-on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override Coulomb-plastic substrates.