Reconstruction of Tweedsmuir Glacier’s surge cycle with historical images

Abstract:

The high velocities associated with glacier surging are caused by enhanced basal slip, which results in expressions of basal topography at the glacier surface. With this in mind, we produced multiple surface DEMs of Tweedsmuir Glacier— a surge-type glacier in the St. Elias Mountains of northern British Columbia, Canada— with an eye to identify changes in basal friction throughout the surge cycle. With these reconstructed DEMs, we aim to: 1) reconstruct the geodetic mass balance of Tweedsmuir Glacier from 1950 to 2019, and 2) make inferences about the surge cycle(s) on various timescales throughout this time frame.

Structure-from-Motion with Multiview Stereo photogrammetry was employed to create DEMs from historic aerial images throughout two surge cycles (1950-present). Differencing these DEMs indicates that thinning rates in the receiving area increased from approximately -5.5m/yr to -7.0m/yr between the first observed quiescent phase (1974-1987) and the on-going quiescent phase (2010-2019). A downward shift in the centerline surface elevation profile is observed between quiescent phases, suggesting that a signal from climate warming may exist on timescales greater than one surge cycle. During the first year of the most recent surge in 2007, the main reservoir transported mass to the receiving area. However, the receiving area continued to thicken at 8.5 m/yr between 2007 and 2010.

Through a wavelet analysis, we identified changes in the spectral characteristics of Tweedsmuir Glacier’s surface topography, which reflect changes in the basal slip-to-deformation ratio throughout the surge cycles. This provides insight on the timescales over which the mass changes observed in goal (1) are driven by changes in glacier dynamics or changes in the surface mass balance. The spectral properties of the surface topography also indicate the wavelengths of basal topography that are expressed on the surface during periods of enhanced basal slip. Overall, this new application of wavelet analysis provides a means of obtaining information about the temporally stationary (topography) and varying (slip ratio) properties of the glacier bed using surface DEMs.