Ice sheets behave in similar ways to Earth’s crust—they can form faults, rift, and deform. In particular, lakes that form atop the ice sheet (called “supraglacial lakes”) can contribute to ice sheet dynamics. A new study published in the Journal for Geophysical Research led by Laura Stevens at the University of Oxford utilizes an array of GPS stations to study these lakes across the western margin of the Greenland Ice Sheet, which is especially sensitive to mass loss caused by global warming. 

Increased melting of the ice sheet due to these supraglacial lakes contributes to uncertainty around how much of the ice sheet margin is at risk of increased destabilization. These measurements help us better understand how fracturing of the ice surface in one supraglacial lake basin can instigate—or inhibit—fracturing in neighboring basins, ultimately resulting in drainage of surface water to the ice sheet bed. Understanding how drainage events occur and are linked by shifting ice sheet surface stress loads is paramount to understanding how sensitive the ice sheet’s stability is to surface water intrusion.

Hydrofracturing describes a process of water-induced fracturing of the ice, facilitating the drainage of lake water down to the basal layer of the ice sheet. This drainage lubricates the basal layer, which can destabilize the ice sheet due to loss of traction, and causes further cavity formation. This fracturing process also deforms the surface of the ice sheet and is hypothesized to transfer stress to other supraglacial lake basins in proximity to the initial fracture, which may in turn instigate drainage of those basins via hydro-fracturing. This is known as the “stress transmission” hypothesis. Researchers interested in ice sheet dynamics hope to gain a mechanistic understanding of this process in order to predict where future meltwater pathways could form. 

The study investigates two drainage events of three neighboring supraglacial lake basins over the 2011 to 2012 melt seasons. GPS instruments ringing these lake basins were used to track displacement of the ice surface to measure surface stress that might be related to hydrofracture draining. (The GAGE Facility operated by EarthScope Consortium provided instrumentation and data archiving support for this research.) These surface stress measurements were used to calculate strain rate and stress change estimates across the lake basins over the course of the drainage events. And the observations also served as input conditions to modeling schemes used to reconstruct deformation processes. This array allowed for higher temporal resolution of surface strain rates and velocities than previous studies based on remote sensing methods. Satellite imagery from TerraSAR-X was also used to measure the background velocities of the ice sheet to help account for other contributions of stress. 

The main supraglacial lake assessed in this study is North Lake, which is known to consistently form and drain once a season, presumably by hydrofracturing. Several nearby lakes have also drained at similar times, though not in every melt season. It’s unclear what causes the initial hydrofracturing in North Lake, but similar research conducted in the Larsen B Ice Shelf in Antarctica found that the weight of these lakes causes the ice sheet to bend and flex, leading to fracturing as the ice stretches. Once the ice sheet fractures, the lake water can drain along the fracture to the basal layer underneath the ice sheet. In the case of North Lake, drainage events indicate that the lake can drain in under two hours. Once the weight of the lake has been relieved, the ice sheet rebounds, resulting in stress transfer to the adjacent basins, which could either initiate related drainage or hinder it.

When pairing the GPS station measurements of strain-rate with models used to estimate the most significant contributors to surface stress, the resultant simulations show that the initial hydrofracture at North Lake both encouraged draining of one nearby lake and inhibited draining at another. In the basin to the southwest, draining was observed within hours of North Lake’s initial draining. The surface stresses observed there indicate a similar stretching condition caused by the ice sheet experiencing a rebound, or uplift, due to the draining of the initial lake. However, the opposite stress condition is true in the basin to the north. Here, the basin was compressed, preventing draining through any possible fractures. The study successfully captured hydrofracturing at neighboring lakes, providing strong evidence for the stress transmission hypothesis. 

There are still many unanswered questions about ice sheet hydrofractures, leaving more work for GPS stations in Greenland. The researchers call for more comprehensive consideration of horizontal and vertical ice surface displacement measurements to model the stress coupling between adjacent lake basins. They suggest that accounting for the flexibility of the ice surface is essential for understanding what happens after deformation. It is also still uncertain what the failure threshold of the ice sheet is in order to activate an initial hydrofracture. Improving the temporal sampling resolution of ice surface deformation linked to surface-to-subsurface drainage of water from GPS stations could help to better distinguish one process of fracture formation from another.