Brittle Landscapes: A Case Study of Crevasse Development in Firn

Recently formed crevasses can now be observed more than 100 km upstream from the outlet of Sermeq Kujalleq (Jakobshavn Isbræ, JI), challenging previous assumptions that the ice sheet is crevasse-free at 2000 m elevation. The recent appearance of these open surface crevasses is a strong indicator of change migrating inwards on the ice sheet. Investigating the formation and evolution of these large transverse crevasses is important, as their presence may signal shifting firn mechanics which may amplify the impacts of climate change on ice sheet stability.

We are working on a case study of one particular crevasse field located at site T131, which is ~130 km upstream of the JI grounding zone. Open surface crevasses appeared around 2001 in optical satellite imagery (Landsat-7), but radar imagery (ERS-1) confirm the prior existence under snow and firn cover as early as 1991. We have lowered a tethered LiDAR robot into an open crevasse to map its geometry beyond the line of sight accessible to humans. Direct LiDAR measurements indicate a crevasse depth exceeding 37.6 m, with width-based extrapolations placing the full depth between 44 and 58 m. A firn and ice density profile from nearby Crawford Point (50 km away) reveals the firn-ice transition at ~124 m depth, confirming that these crevasses are forming within the firn. 

Offsets between airborne radar and laser altimetry suggest the presence of ice slabs formed by refreezing meltwater in the near surface firn downstream of our crevasse field. Repeat firn density profiles from nearby site T4 (47 km away) show an increase in firn density from ~600 to ~700 kg/m³ above 20m depth between 1967 and 2019. Repeat firn density profiles from higher elevations above the crevasse field show no such recent increase in near-surface density. A transition toward more brittle firn conditions, associated with the appearance of refrozen ice layers within the near surface, may be responsible for the recent opening of this crevasse field. 

The crevasse field examined here is not unique; similar fields are appearing across the region, suggesting a regional transition toward more fracture-prone firn.