Small rock-slope failures conditioned by Holocene permafrost degredation: A new approach and conceptual model based on Schmidt-hammer exposure-age dating in Jotunheimen, southern Norway

• Published in: Boreas
• Main Authors: Matthews, J. A., Winkler, S., Wilson, P., Tomkins, M. D., Dortch, J. M., Mourne, R. W., Hill, J., Owen, G., Vater, A. E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2018
• Subjects: Norway; Small Rock; permafrost
• Online Access: http://eprints.uwe.ac.uk/36756/; http://eprints.uwe.ac.uk/36756/1/Rockfalls%20-%20R2%20revised%20text%20%28changes%20ACCEPTED%29.doc; http://eprints.uwe.ac.uk/36756/3/Rockfalls%20-%20R2%20revised%20text%20%28changes%20ACCEPTED%29%20%281%29.pdf; https://doi.org/10.1111/bor.12336

Rock-slope failures (RSFs) constitute significant natural hazards but the geophysical processes which control their timing are poorly understood. However, robust chronologies can provide valuable information on the environmental controls on RSF occurrence: information which can inform models of RSF activity in response to climatic forcing. This paper uses Schmidt-hammer exposure-age dating (SHD) of boulder deposits to construct a detailed regional Holocene chronology of the frequency and magnitude of small rock-slope failures (SRSFs) in Jotunheimen, Norway. By focusing on the depositional fans of SRSFs (≤ 103 m3), rather than on the corresponding features of massive RSFs (~108 m3), 92 single-event RSFs are targeted for chronology building. A weighted SHD age-frequency distribution and probability density function analysis indicate four centennial- to millennial-scale periods of enhanced SRSF frequency, with a dominant mode at ~4.5 ka. Using change detection and discreet Meyer wavelet analysis, in combination with existing permafrost depth models, we propose that enhanced SRSF activity was primarily controlled by permafrost degradation. Long-term relative change in permafrost depth provides a compelling explanation for the high-magnitude departures from the SRSF background rate and accounts for (i1) the timing of peak SRSF frequency, (2ii) the significant lag (~2.2 ka) between the Holocene Thermal Maximum and the SRSF frequency peak, and (3iii) the marked decline in frequency in the late-Holocene. This interpretation is supported by geomorphological evidence, as the spatial distribution of SRSFs is strongly correlated with the aspect-dependent lower altitudinal limit of mountain permafrost in cliff faces. Results are indicative of a causal relationship between episodes of relatively warm climate, permafrost degradation and the transition to a seasonal-freezing climatic regime. This study highlights permafrost degradation as a conditioning factor for cliff collapse, and hence the importance of paraperiglacial processes; a result with implications for slope instability in glacial and periglacial environments under global warming scenarios.