Characterizing ground ice content and origin to better understand the seasonal surface dynamics of the Gruben rock glacier (western Swiss Alps)

Over the recent years, there has been focused international efforts to coordinate the development and compilation of rock glacier inventories. Nevertheless, in some contexts, identifying and characterizing rock glaciers can be challenging as complex conditions and interactions, such as glacier-rock glacier interactions, can yield landforms or landform assemblages that are beyond a straightforward interpretation and classification.

To gain a better understanding of the spatial and temporal complexity of the ongoing processes where glacier-permafrost interactions have occurred, the characterization of the subsurface is quantitively assessed using a petrophysical joint inversion (PJI) scheme (Mollaret et al., 2020), based on electrical resistivity (ERT) and refraction seismic (RST) data. Surface dynamics are assessed using both in-situ and close-range remote sensing techniques. These techniques include stationary GNSS stations to monitor daily and seasonal displacements, and biannual GNSS and UAV surveys to monitor landform-wide surface changes at high spatial resolution.

Both the geophysical data and geodetic data allowed to delineate two zones of the rock glacier: the intact permafrost zone and the complex contact zone where both permafrost and embedded surface ice are present. In the complex contact zone, resistivity values ranging up to MΩm are discernible, indicating very high ice contents (estimated up to 85%). However, in the uppermost zone, the liquid water-to-ice content ratio is greater, which probably indicates an ongoing thermal degradation (melt) of the embedded surface ice. This ongoing thermal degradation is reflected by important ice-melt induced subsidence, which ranges between -0.5 m to -0.7 m over the summer season (03.07.2023 – 07.10.2023). Yet, in winter when ground surface temperatures are below 0°C, the ice melt stops. In the intact permafrost zone of the Gruben rock glacier, the uppermost part of the section shows a distinct 5 m thick layer with low resistivity values and low velocity, which corresponds to the active layer. Right below this layer, a 30 m thick layer with high kΩm resistivity values dominates the lower section of the profile, suggesting widespread ice-saturated sediments. Surface displacement rates in this zone are typical of permafrost creep behaviour, with a gradual acceleration in late spring and a gradual deceleration in winter. Moreover, the coherent nature of the intact zone surface deformation contrasts with the back-creeping and slightly chaotic surface deformation of the complex contact zone.

Favouring a multi-method approach allowed a detailed representation of the spatial distribution of ground ice content and origin, which enabled to discriminate glacial from periglacial processes as their spatio-temporal patterns of surface change and geophysical signatures are (mostly) different.  

 

References

Mollaret, C., Wagner, F., Hilbich, C., Scapozza, C. and Hauck, C. 2020. Petrophysical Joint Inversion Applied to Alpine Permafrost Field Sites to Image Subsurface Ice, Water, Air, and Rock Contents, Frontiers in Earth Sciences, 8(85): 1-23. doi: 10.3389/feart.2020.00085