Along with surface melting, ice-stream activity is one of the primary ways in which mass is lost from the Greenland ice sheet (Figure 1). Ice streams are pathways of fast-moving ice that transport large quantities of mass from the ice-sheet interior to the margins. Reliable knowledge of their dynamics is essential if we are to understand their impact on the ice-sheet mass balance and stability under future climate scenarios.
Finding traces of paleo ice streams
Rapid and long-term ice flow leaves traces, such as geomorphological imprints at former ice-stream beds in regions that are now ice-free. These so-called subglacial landforms allow reconstructions of the long-term ice-stream activity of large former ice sheets, such as the Laurentide Ice Sheet in North America during the Last Glacial Period. Ice streams of today’s ice sheets can be detected by, satellite observations which provide ice surface flow velocities of Antarctica and Greenland. Much more difficult, however, is to reveal how the ice streams of these two contemporary ice sheets have changed over the past thousands of years.
Ice streams do not only leave traces on the bed of the ice sheets by carving their way through the landscape, they also leave an imprint on the internal ice stratigraphy. The originally more or less flat layers of former surfaces of the ice sheet, which are then buried by accumulation, can be used as a marker for the internal deformation of the ice. These so-called isochrones, layers of ice of the same age, can be made visible by sending radar waves into the ice and recording the reflections at these layers (Radio echo sounding; RES). In areas with active streaming, like in the region of the Northeast Greenland ice stream (NEGIS) or the Petermann glacier, RES surveys showed that fast ice flow leaves a characteristic imprint on this ice-internal stratigraphy.
Airborne radar data reveal: Holocene ice-stream shutdown in Northeast Greenland
In our study (Franke et al., 2022 1 ) we found that similar traces were present in regions which show no ice-stream activity today but must have been ice streams before. The type and orientation of the deformation of the isochrones reveal different types of ice streams. A comparison of the present-day and paleo ice stream radar structures even allows to constrain the timing of activity of the paleo ice streams into the Holocene (~ 11.5 ka ago).
Acquiring high-resolution RES data in the centre of the Greenland ice sheet requires long-term planning and large logistical resources. The data in the study was acquired during several airborne RES campaigns with AWI’s (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research) research aircraft Polar6 Basler BT-67 (Figure 2) and during NASA’s Operation Ice Bridge in central Northeast Greenland. The AWI radar system consists of a multi-element antenna array mounted underneath the fuselage of the aircraft (here shown in Figure 2 for the setup with Polar6). Due to a large bandwidth, multiple waveforms and advanced processing routines, the radar system is capable of imaging the internal stratigraphy of the Greenland ice sheet in high resolution. One of the logistic centres for the airborne RES operations was the camp of the East Greenland Ice-Core Project (EastGRIP; https://eastgrip.org/), where an ice core is drilled into the Northeast Greenland Ice Stream (NEGIS). The drill site is located far in the interior of the GrIS, which allowed to operate efficiently in close proximity to the onset region of NEGIS in central Greenland.
By analysing deformed and folded isochrones derived from RES, we have revealed the former activity of two paleo ice streams, both of which extended far into the GrIS (Figure 3). This represents strong observational evidence that the ice flow in the GrIS is temporally and spatially variable, and that ice streams hold the potential to adapt rapidly to changing boundary conditions. Although the detailed cause of the ice-stream shutdown and ice-flow regime reconfiguration is unknown, we see the potential that similar adjustments of the glaciological conditions with ongoing warming may lead to similar ice-stream reconfigurations with implications for sea level rise.
1 Franke, S., Bons, P. D., Westhoff, J., Weikusat, I., Binder T., Streng, K., Steinhage, D., Helm, V., Eisen, O., Paden, J. D., Eagles, G. and Jansen, D. (2022). Holocene ice-stream shutdown and drainage basin reorganization in northeast Greenland. Nature Geoscience, 15, 995–1001, https://doi.org/10.1038/s41561-022-01082-2