With contributions from Juneau Icefield Research Program students and volunteers: Ellery McQuilkin, Roan Bendana, Tatum Witter, Mary Lamb, Clarissa Hee, Oliver Cole, Amanda Newcombe, Hannah Watt, Francis Neff, and Sam Worman.
Motivation
Low elevation outlet glaciers around the Juneau Icefield, Alaska, are losing mass at a rapid pace (Davies et al., 2024). In large part, this loss is caused by reduced mass transfer from higher elevations to valley glaciers, through icefalls. Many icefalls around the Juneau Icefield have disconnected in the past 50 years, and it is plausible that the Vaughan Lewis (VL) icefall will disconnect in the future. Monitoring changes in icefall behavior informs our understanding of the pace of outlet glacier retreat.
Main Points
Variability in glaciohydraulic tremor and in icequake rates show that ice velocities in the area above the VL are subject to hydrological forcing.
The impact of this forcing is likely to increase with changing environmental conditions. Multiple seasons of data acquisition could allow us to determine whether increasing hydrological forcing on ice-dynamics will have an accelerating or a buffering effect on the demise of the VL.
Ice gradually speeds up towards the icefall
Velocity profile derived from Sentinel-2 optical imagery acquired in the summer of 2023 with CautoRIFT (Liu et al., 2024).
Liu, J., Gendreau, M., Enderlin, E.M. and Aberle, R., 2024. Improved records of glacier flow instabilities using customized NASA autoRIFT (CautoRIFT) applied to PlanetScope imagery. The Cryosphere, 18(8), pp.3571-3590.
Photo shows the top of the icefall and zone of ice speedup above it. The depression in this area sits above the equilibrium altitude but sees considerable melting of seasonable snow.
Environmental Seismology is a useful tool to monitor icefall behavior over time. 5 Nanometrics Pegasus quick- deploy broadband seismic stations were deployed around the Vaughan Lewis Icefall for ~7 days. 3 sensors were installed in the glacier firn layer, and 2 sensors were installed in glacier proximal sediment.
Decrease in rates of high frequency icequakes coincides with step decrease in tremor amplitude recorded next to icefall. Upper panel shows median spectrogram recorded on the vertical channel at station HC. Lower panel shows timeseries of tremor amplitudes for station HC summed between 3 and 10 Hz in each 1 h time-window (with 45 min overlap) in black, and the rate of high amplitude events within the 30 Hz highpass filtered vertical channel. Events were detected with a 2-20 seconds STA/LTA trigger.
We find daily variability in glaciohydraulic tremor amplitude above the Vaughn Lewis icefall. Timeseries of tremor amplitude for stations VL1 and VL2, summed between 3 and 10 Hz, shown in black. Modelled total runoff produced at the glacier surface at location VL1 shown in blue. Estimates are produced with the Energy Balance Firn Model, forced with ERA-5 re-analysis data.
Additional Resources
This is an audio file of the seismic recordings (made with seisound): https://ds.iris.edu/dms/products/seissound/
Acknowledgements
The fieldwork took place on Tlingit Lands (Lingít Aaní). This work was conducted in collaboration with the Juneau Icefield Research Program, its students, and its volunteers. We also received support from the Geological Society of America's Graduate Student Research Grants, the University of Idaho College of Science, and Nanometrics, Inc. References Davies, et al. Nature Communications. (2024); Bartholomaus, et al. Geophysical research letters (2015); Liu, et al. The Cryosphere (2024).