DETERMINING WINTER MASS BALANCE AND THE PREVIOUS YEAR SNOWLINE POSITION ON THE JUNEAU ICECFIELD, ALASKA USING GROUND-PENETRATING RADAR

To map spatial patterns of winter mass balance (mass input) in a maritime climate, ~150 km of 400 MHz ground-penetrating radar (GPR) profiles covering an elevation range of ~1000 m were collected across the Juneau Icefield, Alaska in July 2012. Associated measurements of stratigraphy from 11 snowpits were used to help interpret the radar-detected stratigraphy. Results show that the point measurements of stratigraphy can be extrapolated over most of the accumulation zone of the icefield to map variations of mass balance with elevation [b(z)]. Radar-detected stratigraphy in the northwest and southwest branches of Taku Glacier show a b(z) gradient decreasing from the coast towards the mainland; the main trunk of Taku Glacier, which originates from the Mathes-Llewellyn ice divide, shows a similar decrease in b(z) from the divide to the equilibrium line altitude (ELA). The observations are consistent with mass-balance gradients expected in accumulation zones; the notably high mass balance in the upper catchments of the southwest and northwest branches likely result from their close proximity to the ocean. The aspect of each tributary has an influence on b(z) gradient, lowering the overall correlation between elevation and b(z) when all profiles are grouped together. Lastly, a strong reflector also occurs near the ELA and is interpreted as the firn-ice transition or previous year equilibrium line altitude. Sequential mapping of this transition and other visible unconformities between the winter accumulation and deeper firn layers may provide an annual time-series of end-of-season snowlines that can be used to infer the balance state of glaciers.