Frozen margin dynamics at Storglaciaren, Sweden

Since the summer of 2006, more than 40 boreholes have been drilled near the frozen margin of Storglaciaren. A variety of instruments have been deployed in these holes to investigate the influence of the temperate-to-cold-bed transition on ice flow dynamics and sediment transport near the margin. Among the instruments are thermistors, slidometers, and water pressure transducers. Four boreholes were used for inclinometry, which was repeated a year later. A stake network in the same region was surveyed with a dGPS and a total station to determine surface velocities. Ground-penetrating radar was used to study the geometry of both the bed and cold-temperate transition surface (CTS). Slidometer and thermistor data indicate that, as expected, basal motion declines toward zero where the freezing isotherm intersects the bed, indicating that the basal thermal transition (BTT) is effectively a slip/no- slip transition. However, in this area the average longitudinal gradients in surface and basal velocity during summer 2007 were -0.025 a-1 and -0.0275 a-1, respectively, indicating that compressive longitudinal stress gradients arising from the BTT are not large. Additionally, radar profiles from the same transects in winter and summer suggest that the CTS (and consequently the BTT) may migrate seasonally. A slidometer that was located within temperate-based sliding ice (about 20 m from the inferred BTT location) in summer, 2007, recorded zero basal velocity during winter but activated during summer, 2008. Comparison of the mean annual and summer surface velocities indicates that such seasonal increases in velocity do not extend far into the cold-based ice at the margin. These observations suggest that the thermally-induced slip/no-slip transition near the margin of Storglaciaren is a gradual and dynamic transition and that the frozen margin does not represent a major impediment to ice flow.