Rock-slope failures (RSFs) constitute significant natural hazards, but the geophysical processes that control their timing are poorly understood. However, robust chronologies can provide valuable information on the environmental controls on RSF occurrence: information that can inform models of RSF activity in response to climatic forcing. This study 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 (≤103m3), rather than on the corresponding features of massive RSFs (~108m3), 92 single-event RSFs are targeted for chronology building. A weighted SHD age–frequency distribution and probability density function analysis indicated 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: (i) the timing of peak SRSF frequency; (ii) the significant lag (~2.2 ka) between the Holocene Thermal Maximum and the SRSF frequency peak; and (iii) 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.
|Number of pages||26|
|State||Published - Oct 2018|
Bibliographical noteFunding Information:
Fieldwork was carried out on the Swansea University Jotunheimen Research Expeditions of 2014–2017. The authors are grateful to Ole Jacob and Tove Grindvold (Leirvassbu) for continuing expedition support; to Atle Nesje and Anne E. Bjune for data and sources of information on uplift rates and climate in southern Norway; and to R.L. Hermanns for a very thorough critical review that led to significant improvement of the manuscript. Anna C. Ratcliffe prepared the figures for publication. This manuscript constitutes Jotunheimen Research Expeditions, Contribution No. 206 (see http://jotunheimenresearch.wixsite.com/home).
© 2018 Collegium Boreas. Published by John Wiley & Sons Ltd
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics