Abstract
Periglacial patterned ground (sorted circles and polygons) along an altitudinal profile at Juvflya in central Jotunheimen, southern Norway, is investigated using Schmidt‐hammer exposure‐age dating (SHD). The patterned ground surfaces exhibit R‐value distributions with platycurtic modes, broad plateaus, narrow tails, and a negative skew. Sample sites located between 1500 and 1925 m a.s.l. indicate a distinct altitudinal gradient of increasing mean R‐values towards higher altitudes interpreted as a chronological function. An established regional SHD calibration curve for Jotunheimen yielded mean boulder exposure ages in the range 6910 ± 510 to 8240 ± 495 years ago. These SHD ages are indicative of the timing of patterned ground formation, representing minimum ages for active boulder upfreezing and maximum ages for the stabilization of boulders in the encircling gutters. Despite uncertainties associated with the calibration curve and the age distribution of the boulders, the early‐Holocene age of the patterned ground surfaces, the apparent cessation of major activity during the Holocene Thermal Maximum (HTM) and continuing lack of late‐Holocene activity clarify existing understanding of the process dynamics and palaeoclimatic significance of large‐scale sorted patterned ground as an indicator of a permafrost environment. The interpretation of SHD ages from patterned ground surfaces remains challenging, however, owing to their diachronous nature, the potential for a complex history of formation, and the influence of local, non‐climatic factors.
Highlights
Despite uncertainties associated with the calibration curve and the age distribution of the boulders, the early-Holocene age of the patterned ground surfaces, the apparent cessation of major activity during the Holocene Thermal Maximum (HTM) and continuing lack of late-Holocene activity, clarify existing understanding of the process dynamics and palaeoclimatic significance of large-scale sorted patterned ground as an indicator of a permafrost environment
Subsequent improvement during the last 10 years has seen the combination of Schmidt-hammer relative-age dating with absolute dating techniques, in particular terrestrial cosmogenic nuclide dating (TCND), and the development of Schmidt-hammer exposure-age dating (SHD), which enables the calculation of local or regional calibration curves and provides absolute age estimates for the landforms investigated (Matthews and Owen 2010; Matthews and Winkler 2011; Shakesby et al 2011; Matthews and McEwen 2013; Stahl et al 2013; Winkler 2014)
R-values from the mechanical Schmidt hammer and RRock-values obtained by the RockSchmidt are highly comparable in terms of relative differences between sites, the overall trend, and most other parameters, but the 95
Summary
2015), snow-avalanche impact ramparts (Matthews et al 2015), moraines (Matthews and Shakesby 1984; Evans et al 1999; Aa and Sjåstad 2000; Winkler 2005, Ffoulkes and Harrison, 2014), rock fall/avalanches (Nesje et al 1994; Aa et al 2007), fluvial terraces (Stahl et al 2013) and boulder streams (Wilson et al, submitted) It was used only as a relative-age dating technique based on the principle of relating compressional strength of a bedrock or boulder surface to its degree of surface weathering and, its exposure age (McCarroll 1994; Goudie 2006; Shakesby et al 2006). Calibrated-age estimates of exposure ages of clasts within patterned ground surfaces potentially provide insights into the processes and dynamics of sorted patterned ground formation as well as their chronology by revealing, for example, for how long the majority of clasts have been exposed at the surface or when the clast-rich margins were established or became inactive (see, for example, Washburn 1956, 1979; Goldthwait 1976; French 1988, 2007; Hallet 1990, 2015; Kessler et al 2001; Matsuoka et al 2003; Peterson and Krantz 2008; Ballantyne 2013; Warburton 2013, for more details of patterned ground formation and its classification)
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