Schmidt‐hammer exposure ages from periglacial patterned ground (sorted circles) in jotunheimen, norway, and their interpretative problems

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.

Key words:

• sorted circles
• periglacial patterned ground
• alpine permafrost
• Schmidt‐hammer exposure‐age dating (SHD)
• RockSchmidt
• Holocene climatic variations
• Jotunheimen

Acknowledgments

The fieldwork was carried out on the Swansea University Jotunheimen Research Expedition 2015, and this paper constitutes Jotunheimen Research Expeditions, Contribution No.199. We thank Andreas Kellerer‐Pirklbauer for valuable comments on the manuscript.

Additional information

Notes on contributors

Stefan Winkler

Stefan Winkler, Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand Email: [email protected]

John A. Matthews

John A. Matthews, Department of Geography, Swansea University, Singleton Park, Swansea SA2 8PP, Wales, UK Email: [email protected]

Richard W. Mourne

Richard W. Mourne, Department of Geography and Environmental Management, University of the West of England, Frenchay Campus, Coldharbour Lane, Bristol BS16 1QY, UK Email: [email protected]

Peter Wilson

Peter Wilson, Environmental Sciences Research Institute, Ulster University, Coleraine BT52 1SA, Northern Ireland, UK Email: [email protected]