Schmidt-hammer exposure-age dating (SHD) performed on periglacial and related landforms in Opplendskedalen, Geirangerfjellet, Norway: Implications for mid- and late-Holocene climate variability

Abstract

Schmidt-hammer exposure-age dating (SHD) was applied to a variety of boulder-dominated periglacial landforms in an attempt to establish a local mid-/late-Holocene chronology for the Geirangerfjellet in South Norway. Landform ages were obtained by application of a local calibration curve for Schmidt hammer R-values based on young and old control points comprising fresh road cuts and a bedrock surface in proximity to the study area, respectively. The area was deglaciated ~11.5 ka ago according to independent age information. Investigation of age, formation and stabilization of the periglacial landforms and processes involved allowed assessment of the underlying Holocene climate variability and its relationship to landform evolution. Our SHD ages range from 7.47 ± 0.73 ka for glacially abraded bedrock at the valley bottom to 2.22 ± 0.49 ka for surface boulders of a rock-slope failure. All landforms shared negative skewness and largely have narrow tailed frequency distributions of their R-values. This points to either substantial reworking of the boulders within a landform or continuous debris supply. Our results show that most landforms stabilized during the Holocene Thermal Maximum (~8.0–5.0 ka). The findings do not support the hypothesis that rock-slope failures predominately occur shortly after local deglaciation. Instead, it appears that they cluster during warm periods due to climate-driven factors, for example, decreasing permafrost depth or increasing cleft-water pressure leading to slope instabilities. Periglacial boulder-dominated landforms in the western maritime fjord region seem to react sensitively to Holocene climate variability and may constitute valuable but to date mostly unexplored sources of palaeoclimatic information.

<sup>10</sup>Be-based exploration of the timing of deglaciation in two selected areas of southern Norway

“…The internal consistency of our 10 Be exposure ages from glacially eroded bedrock surfaces with their post-LGM age implies that glacial erosion was sufficient to remove any inherited nuclide concentration, and that the bedrock had been continuously exposed since. This supports the concept that glaciers in fjord landscapes were highly effective erosional agents and consequently warm-based (Aarseth et al, 1997;Matthews et al, 2017), especially in the valleys. This is in agreement with Landvik et al (2005), who claim that frozen-bed conditions throughout the growth and decay of glaciers in coastal environments are unlikely.…”

Paraglacial Rock-Slope Failure Following Deglaciation in Western Norway

“…(2017) considered at least two of the rock glaciers to be primarily paraglacial landforms, at least in terms of debris supply. A protalus rampart in Opplendskedalen that yielded a SHD age of 5.6 ka was regarded by Marr et al (2019) to have initially formed after deglaciation and added to during the Early Holocene by rock debris supplied from a rockwall weakened by permafrost degradation.…”

“…Since then, the incidence of small RSFs in this region appears to have declined significantly Matthews et al (2018). interpret the timing of peak RSF frequency (and its lagging the Holocene Thermal Maximum by ~2 ka) as primarily a response to long-term changes in permafrost depth during episodes of relatively warm climate and transition to a seasonal-freezing climatic regime.Similarly,Marr et al (2019) applied SHD to five small RSFs in Opplendskedalen (Møre og Romsdal), which was deglaciated at ~11.5 ka. Their results show that three features stabilized during the Holocene Thermal Maximum and do not support the hypothesis that RSF activity predominately occurs shortly after local deglaciation.…”