Early Holocene peak of periglacial activity on Juvflye in Jotunheimen/South Norway revealed by Schmidt-hammer exposure-age dating and its morphodynamic implications

Abstract

The potential of periglacial landforms in the context of palaeoclimatic interpretation bases on their connection to climate-driven permafrost conditions with both initial formation and continuing activity. The challenge of obtaining reliable numerical age constraints significantly complicates, however, their utilisation for this purpose. One reason is that many periglacial landforms such as patterned ground, rock glaciers, or various solifluction features represent transitional processes of certain duration rather than clearly defined single events. A related high risks of postdepositional disturbance by frost-related processes has also to be taken into account.Although per se suited for boulder-dominated periglacial landforms, cosmogenic radionuclide dating (CRN) faces the problem that large sample sizes would be required to achieve reliable ages. To overcome this disadvantage, the calibrated-age dating technique of Schmidt-hammer exposure-age dating (SHD) has recently been successfully utilised for obtaining age constraints of such landforms. If robust local or regional SHD age-calibration curves can be established, SHD offers the fundamental advantage of obtaining large sample sizes (hundreds or even thousands of boulders) to overcome the abovementioned limitations of CRN.Recent studies applying SHD on patterned ground and related features in Jotunheimen (South Norway) revealed that the results obtained not only provide a solid basis for palaeoclimatic interpretation but additionally point towards interesting morphodynamic implications. On Juvflye, a high-altitude plateau typical for Jotunheimen, and its transitional upper slopes to Bøver- and Visdalen around 150 periglacial features has been dated applying a local SHD age-calibration curve. These features included sorted circles, sorted stripes, and boulder-banked solifluction lobes in various morphologies and sizes. They covered an altitudinal range between roughly 1,450 and 1,950 m a.s.l. and several different aspects. SHD result show that periglacial activity likely commenced instantly following local deglaciation after the Preboreal Oscillation (PBO, c. 11.45 ka ago). Most important is, however, that all features without exception became definitely inactive prior or latest around the onset of the Holocene Thermal Maximum (HTM, c. 8.0 ka ago). The timing of this stabilisation is surprising because at least high and middle altitudes on Juvflye have been underlain by permafrost during the entire Holocene until today. It seems independent from Holocene fluctuations of the lower limit of permafrost and colder climatic conditions during the Late Holocene and, therefore, challenges also the general application of large patterned ground features as palaeoclimatic indicators for permafrost. Any recent mophodynamic activity on Juvflye is restricted to minor frost-related processes and include micro-scale frost cracking/sorting and solifluction terracettes.The formation of patterned ground and large-sized boulder-banked solifluction lobes restricted to a limited time period during Early Holocene points morphodynamically towards the conclusion that an occurrence of permafrost per se cannot be seen as the sole factor for their efficient formation and continuous activity. Other factors such as soil moisture availability, active layer thickness, or suitable substrate need to be taken into account. A comparison with micro-scale patterned ground features on recently deglaciated glacier forelands in Jotunheimen strongly suggests that a significant influence of soil moisture alongside micro-climatic factors need to be discussed.