Gjerde et al. (2025) Seasonal drainage-system evolution beneath the Greenland Ice Sheet inferred from transient speed-up events

Identification

• Journal: ˜The œcryosphere
• Year: 2025
• Date: 2025-11-25
• Authors: Grace P. Gjerde, M. D. Behn, Laura A. Stevens, Sarah B. Das, Ian Joughin
• DOI: 10.5194/tc-19-6149-2025

Research Groups

• Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, USA
• Department of Earth Sciences, University of Oxford, Oxford, UK
• Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, USA
• Polar Science Center, Applied Physics Laboratory, Seattle, USA

Short Summary

This study investigates the seasonal evolution of subglacial drainage beneath the western Greenland Ice Sheet by analyzing transient ice speed-up events using a Global Positioning System (GPS) array. It reveals that late-season melt events produce larger, more uniform velocity responses with less uplift compared to early-season lake drainages, suggesting a shift to a pervasive, cavity-dominated subglacial system with closed channels.

Objective

• To investigate the seasonal evolution of subglacial conditions beneath the western Greenland Ice Sheet by analyzing transient ice speed-up events in response to runoff across the 2011 and 2012 melt seasons.
• To compare the characteristics of early-season lake drainage events with late-season melt events to infer changes in basal hydrology and their impact on ice dynamics.
• To determine if late-season melt or precipitation events contribute to ice-sheet acceleration at a magnitude comparable to that of lake drainages.

Study Configuration

• Spatial Scale: Ablation zone of the western margin of the Greenland Ice Sheet, near North Lake (68.66° N, 49.52° W), approximately 50 km south of the Sermeq Kujalleq catchment and 25 km up-ice-flow from the terminus of Saqqarliup sermia. The North Lake basin is located at approximately 950 m above sea level on ice that is approximately 980 m thick. The study utilized a GPS array of up to 16 receivers and runoff estimates from six local 11 km × 11 km Regional Atmospheric Climate Model (RACMO) grid cells.
• Temporal Scale: Melt seasons of 2011 and 2012. GPS data were collected continuously at 30 s resolution. Transient speed-up events had a median duration of 5 days (432000 s).

Methodology and Data

• Models used:
  • Network Inversion Filter (NIF): Used to invert time series of GPS ice-sheet surface positions for vertical hydro-fracture opening, sub-horizontal slip, and basal-cavity opening, assuming elastic ice behavior.
  • Regional Atmospheric Climate Model (RACMO) (Noël et al., 2015; Noël, 2020): Used to estimate daily mean runoff.
  • TopoToolbox (Schwanghart and Scherler, 2014; Schwanghart and Scherler, 2021): Used to define the ice surface catchment basin.
• Data sources:
  • Global Positioning System (GPS) array: 15 receivers in 2011 and 12 receivers in 2012, deployed at North Lake, western Greenland, collecting data at 30 s resolution. Processed kinematically relative to the Greenland GPS Network (GNET) KAGA base station.
  • ArcticDEM dataset (Porter et al., 2023): 10 m resolution surface topography.
  • Sentinel-2 satellite imagery (European Space Agency): Used for visualization of the study area.
  • Landsat-7 satellite images: Used to identify nearby lake drainage events.
  • BedMachine v3 (Morlighem et al., 2017; Morlighem, 2017): Used for basal topography.

Main Results

• No systematic relationship was found between the magnitude of runoff and the amplitude of transient speed-up events.
• A general trend of increasing velocity responses and decreasing variability in the velocity response was observed across the GPS array as the melt season progressed. The average normalized velocity response (ΔV_N) increased with the day of year (R² = 0.44, p = 0.01 for all events; R² = 0.49, p = 0.02 for regional melt events). The standard deviation of the absolute velocity response (ΔV) decreased with the day of year (R² = 0.36, p = 0.03 for all events).
• Early-season transient speed-ups (often associated with lake drainages) produced highly variable speed-up and pronounced uplift. For example, the 2011/169 lake drainage event showed an average uplift of approximately 0.6 m and an average excess flowline displacement of approximately 0.13 m.
• Late-season melt events produced longer, higher amplitude, and more uniform velocity responses, but did not result in large or coherent uplift patterns. For example, the 2011/238 late-season melt event showed an average excess flowline displacement of approximately 1.2 m and an average uplift of approximately 0.2 m.
• Late-season melt events accommodated a larger fraction of the annual ice motion (2 %–3 % of annual displacement) compared to early-season lake drainages, but their net influence on total annual ice-sheet motion remained small.
• A mid-season speed-up event (2012/180) exhibited the greatest variability in sliding response, attributed to a nearby lake drainage approximately 8 km to the northeast. Meltwater from this event was preferentially transported down the hydraulic potential gradient, pooling in a bedrock basin to the south of North Lake, demonstrating the influence of basal topography on local meltwater flow and ice-bed coupling.

Contributions

• Extends the understanding of subglacial drainage system evolution by providing direct observations of transient speed-up events in the late melt season, a period previously less resolved due to infrequent lake drainages.
• Reveals a distinct seasonal evolution in ice dynamic response: early-season lake drainages cause localized, variable speed-ups with significant uplift, while late-season melt events lead to more uniform, higher-amplitude speed-ups with minimal uplift.
• Infers that by the late melt season, subglacial channels and/or connective flow pathways between cavities likely close, sharply lowering basal transmissivity, while pervasive open moulins allow small, widespread melt inputs to rapidly overwhelm the system and reduce frictional coupling.
• Highlights the persistent influence of basal topography on modulating local patterns of meltwater flow and ice-bed coupling, even for events originating outside the immediate GPS array.
• Quantifies the contribution of late-season melt events to annual ice motion, showing they accommodate a larger fraction than early-season lake drainages, though their overall impact on annual displacement remains minor.

Funding

• National Science Foundation’s Office of Polar Programs (NSF-OPP): ARC-0520077, ARC-1023364, OPP-1838410, ARC-0520382, ARC-1023382, OPP-18-38464
• National Aeronautics and Space Administration’s (NASA’s) Cryospheric Sciences Program: NNX10AI30G, NNX10AI33G
• John Fell Oxford University Press Fund
• UK Natural Environment Research Council: grant no. NE/Y002369/1
• Boston College Undergraduate Research Fellows Program
• UNAVCO
• CH2MHILL Polar Field Services

Citation

@article{Gjerde2025Seasonal,
  author = {Gjerde, Grace P. and Behn, M. D. and Stevens, Laura A. and Das, Sarah B. and Joughin, Ian},
  title = {Seasonal drainage-system evolution beneath the Greenland Ice Sheet inferred from transient speed-up events},
  journal = {˜The œcryosphere},
  year = {2025},
  doi = {10.5194/tc-19-6149-2025},
  url = {https://doi.org/10.5194/tc-19-6149-2025}
}