Schmidt-hammer Exposure-age Dating Research Articles

Quantifying the effects of carborundum rock surface smoothing for Schmidt hammer R‐value metrics measured on glacial erratics in the North American Great Lakes region

AbstractSchmidt hammer R‐values, which reflect the degree of rock surface weathering, can be paired with independent rock surface exposure ages to produce Schmidt hammer exposure‐age dating (SHD) calibration curves that are used to provide rapid estimations of exposure ages for undated rock surfaces. In this Letter, we lay the groundwork for later establishing a SHD calibration curve based on glacial erratics deposited by the Laurentide Ice Sheet in the North American Great Lakes region. First, we establish a reference R‐value for a calibration boulder, which may be used to facilitate direct comparison of R‐values collected by different individuals using mechanical N‐type Schmidt hammers, specifically. We then assess how no, light or heavy use of a carborundum puck to smooth rock surfaces affects resulting R‐value data. Lastly, we compare two different R‐value metrics on the same rock surface: Rmean (n = 30) versus ΔR (Rmax − Rmin of five consecutive R‐values on the same spot). In all assessments, Rmean is similar for erratic surfaces with no and light surface treatment, and both are significantly lower than Rmean of surfaces with heavy treatment. Thus, we advise against the use of heavy surface treatment in SHD applications. We observe no relationship between inferred erratic age and either Rmean or ΔR, which could arise from inferred ages of erratics being inaccurate, from inferred ages of erratics being too similar or from measuring too few erratics from each landform.

Implications of an Early Holocene climax in solifluction and related periglacial activity on Juvflye, Jotunheimen (southern Norway), as revealed by Schmidt-hammer exposure-age dating

Schmidt-hammer exposure-age dating (SHD) was applied to ~180 medium- to large-scale solifluction features on the northern edge of Juyflye, Jotunheimen (southern Norway) using an electronic Schmidt-hammer (RockSchmidt) and an improved local SHD age-calibration equation. Age estimates from four different types of solifluction landforms were analysed and compared with those from recalibrated estimates from patterned ground previously investigated on Juvflye. Average SHD-age estimates are c. 9.8 and 9.3 ka for the two dominant morphological types of solifluction features (‘type A’ boulder tongues and ‘type B’ stone-banked solifluction lobes) and c. 8.6 ka for sorted stripes and circles. Our results indicate that active formation of all investigated types of solifluction features, sorted stripes, and sorted circles ceased in the Early Holocene, prior to the onset of the regional Holocene Thermal Maximum (HTM) at c. 7.7 ka. Formation of all of these periglacial landforms appears to have commenced shortly after local deglaciation (c. 11.4 ka) in water-saturated till. Alternative origins are rejected, including the possibility of development before the last glaciation, survival beneath cold-based glaciers, and exhumation in the Early Holocene. Cessation of activity is attributed to changing ground conditions affecting active layer processes, particularly reduced soil moisture and pore water pressure. Temporal variations of the altitudinal permafrost limits had little or no impact on the timing of either the Early Holocene climax in activity or subsequent stabilisation. Caution is therefore urged in the utilisation of large-scale solifluction and patterned ground landforms as palaeoclimatic indicators.

Schmidt-hammer exposure-age dating of seven paraglacial rock-slope failures in the Bolungarvík-Suðureyri area (Westfjords, Iceland)

Many rock-slope failures (RSF) impact Iceland's Westfjords morphology. Five RSF areas, totalling seven RSFs, were studied in the western fjords around Bolungarvík-Suðureyri: Óshólar, Minni-Hlið, Meira-Hraun, the Vatnadalur valley and part of the Sunddalur valley.The aim of this article is to provide chronological data of the RSFs and to subsequently obtain a better chronosequence of post-glacial events, none of which have yet been dated in the Westfjords. More particularly, we discuss a paraglacial hypothesis of their occurrence.For this purpose, the Schmidt-hammer exposure-age dating (SHD) method is used on the RSF deposits, and the rebound values obtained from the impacted blocks are converted to age estimates using radiocarbon ages obtained from the different sites in order to calculate a SHD age calibration. Then, we establish a chronology of the setting up of the events. In this way, seven RSFs have been dated and different chronological generations of deposit emplacement have been exhibited for some of them. Thus, various periods of instability affected mountain slopes. The Óshólar RSF is thought to have been deposited firstly at around 12,500 cal BP, the Minni-Hlið RSF at ∼9800 cal BP, the Meira-Hraun RSF at ∼8700 cal BP, the Vatnadalur RSFs at ∼9000 cal BP, ∼ 6900 cal BP, and ∼ 4300 cal BP, and the Sunddalur RSF at ∼9600 cal BP. These RSFs therefore correspond to so-called paraglacial events, which took place at the beginning of deglaciation of the area during the first half of the Holocene.

Schmidt-hammer R-values from glacially-scoured bedrock surfaces across glacier-foreland boundaries: Insights into Holocene weathering rates with implications for exposure-age dating

Schmidt-hammer R-values were measured on glacially-scoured bedrock outcrops located inside and outside of 11 ‘Little Ice Age’ glacier-foreland boundaries in the Jotunheimen and Jostedalsbreen regions of southern Norway. Analysing paired samples differing in exposure age by ~10,000 years constitutes a field experiment on chemical weathering rates within and between regions. Mean R-values (± 95 % confidence intervals) from inside sites were 65.9 ± 0.6 and 66.9 ± 0.6 for rock surfaces composed of pyroxene-granulite gneiss in Jotunheimen at altitudes of 990–1360 m above sea level and granite and granitic gneiss in the Jostedalsbreen region at 270–620 m a.s.l. The corresponding values from outside sites of 39.9 ± 0.9 and 39.0 ± 0.9 were significantly lower, indicating a higher degree of chemical weathering. In contrast, regional differences in mean R-values were insignificant. A similar pattern is reflected in indices of rock weathering (39.0 % for Jotunheimen and 41.6 % for Jostedalsbreen), and weathering rate (2.8 R-value units and 3.0 units per 1000 years, respectively). These results imply an estimated minimum age resolution of Schmidt-hammer exposure-age dating of ~265–315 years and a maximum age range of ~20,000 years. They suggest potential application of the Schmidt hammer to both studies of weathering rates and exposure-age dating at the regional scale, despite lithological variation associated with different rock types and climatic variation associated with altitudinal differences of up to 1000 m between the sites.

Deglaciation of the highest mountains in Scandinavia at the Younger Dryas–Holocene transition: evidence from surface exposure‐age dating of ice‐marginal moraines

Surface exposure–age dating was applied to rock surfaces associated with ice‐marginal moraines at elevations of ~1520–1780 m a.s.l. on the slopes of Galdhøpiggen and Glittertinden, the two highest mountains in Scandinavia located in the Jotunheimen mountains of central southern Norway. This is important for understanding the pattern and timing of wastage of the Scandinavian Ice Sheet at the Younger Dryas–Holocene transition. Cosmogenic exposure dating (here 10Be dating) of boulders from the moraine ridges yielded overall mean ages (corrected for glacio‐isostatic uplift, surface erosion and snow shielding) of ~11.6 ka from Galdhøpiggen and ~11.2 ka from Glittertinden. Similar 10Be ages were also obtained from additionally collected proximal and distal erratic boulders and bedrock samples. These enabled age calibration of Schmidt‐hammer R‐values and independent Schmidt‐hammer exposure‐age dating (SHD) of the moraine ridges, which yielded comparable mean SHD ages of ~10.8 and ~10.6 ka from the Galdhøpiggen and Glittertinden sites, respectively. Taking account of the age resolution and other limitations of both dating techniques, the results suggest that the two sets of moraines have approximately the same age but that neither technique can distinguish unambiguously between moraine formation in the late Younger Dryas or Early Holocene. Together with features of moraine‐ridge morphology and estimates of equilibrium‐line altitude depression of ~360–575 m (corrected for land uplift), the results imply moraine formation during short‐lived re‐advances of active glaciers, at least the lower reaches of which were warm‐based. It is concluded that the local glaciers remained active and advanced during deglaciation either very late in the Younger Dryas or very early in the Holocene, possibly in response to the Preboreal Oscillation at ~11.4 ka. The study supports the concept of a thin Younger Dryas ice sheet and places time constraints on the timing of final deglaciation in southern Norway.

Schmidt Hammer exposure-age dating of glacial landforms in the Cairngorm Mountains, Scotland

Schmidt Hammer exposure-age dating of glacial landforms in the Cairngorm Mountains, Scotland

Age, origin and palaeoclimatic implications of peri- and paraglacial boulder-dominated landforms in Rondane, South Norway

Boulder-dominated landforms of periglacial, paraglacial and related origin constitute a valuable, but often unexplored source of palaeoclimatic and morphodynamic information. The timing of landform development initiation and its subsequent stabilization can be linked to past climatic conditions offering the potential to reconstruct cold climatic periods. In this study, Schmidt-hammer exposure-age dating (SHD) was applied to a variety of boulder-dominated landforms (sorted stripes, blockfield, rock-slope failure, paraglacial alluvial fan) in Rondane, eastern South Norway for the first time. On the basis of old and young control points a regional SHD calibration curve was established and successively utilized for the calculation of surface exposure ages for individual landforms. The chronological investigation of development and stabilization of the respective landforms permitted an assessment of Holocene climate variability in Rondane and its impact on overall landform evolution. Our obtained SHD age estimates ranged from 11.44 ± 1.22 ka (ST-D2) to 4.09 ± 1.51 ka (AF1) showing their inactive and relict character. Most surface exposure ages for sorted stripes clustered between 9.88 ± 1.35 ka and 9.25 ± 1.21 ka, hence indicating stabilization during the late stage of the Erdalen Event or shortly thereafter. It is inferred that the blockfield formed prior to the Last Glacial Maximum, was protected by cold-based ice throughout glaciation and shortly reactivated during the Erdalen Event only to subsequently becoming inactive. The surface exposure age of a rock-slope failure (7.58 ± 0.73 ka) falls into the early phase of the Holocene Thermal Maximum (HTM, ~8.0–5.0 ka). This indicates permafrost degradation and/or increasing hydrological pressure negatively influencing slope stability. The paraglacial alluvial fan with its four subsites yielded ages between 8.73 ± 1.63 ka and 4.09 ± 1.51 ka. The old exposure ages point to fan aggradation following regional deglaciation due to paraglacial processes, whereas the younger ages can be explained by increasing precipitation during the onset neoglaciation at ~4.0 ka. Our results underline the importance of meltwater for the activation of periglacial landforms in a continental climate and indicate that the Erdalen Event and immediately following onset of the HTM had major impact on landscape evolution in Rondane. Our obtained surface exposure ages from boulder-dominated landforms in Rondane give important insights into the local palaeoclimatic variability during the Holocene.

Schmidt hammer exposure-age dating of periglacial and glacial landforms in the Southern Swiss Alps based on <i>R</i>-value calibration using historical data

Abstract. As a contribution to the palaeoenvironmental history reconstruction of the Alpine periglacial domain, this study focuses on the Schmidt hammer exposure-age dating (SHD) of (peri-)glacial landforms using rebound-value (R-value) calibrations for 10 stations in the Scaradra glacier forefield (north-eastern part of the Ticino Canton, Lepontine Alps) and for 13 stations in the Splügenpass region (located between Switzerland and Italy, Rhaetian Alps). Linear calibration based on the known age of several moraines of the Scaradra glacier assessed by historical cartography allowed the reconstruction of the glacier fluctuations around the end of the Little Ice Age. Timing of deglaciation and of rock glacier development was defined in the Splügenpass region using the calibration of exposure ages based on two mule tracks built in 300 CE and 1250 CE, respectively. Discussion on R-value analysis and calibration improves the knowledge on the potential use of SHD for numerical-age dating in Alpine geomorphological studies.

New evidence for active talus-foot rock glaciers at Øyberget, southern Norway, and their development during the Holocene

Synthetic aperture radar interferometry (InSAR) measurements demonstrate that lobate, blocky depositional landforms, located in southern Norway at an altitude of ~530 m above sea level, with an estimated mean annual air temperature of ~1.6°C, currently exhibit deformation attributed to viscous creep. Five years of InSAR measurements for six lobes demonstrate average surface velocities of 1.2–22.0 mm/year with maximum rates of 17.5–55.6 mm/year. New Schmidt-hammer exposure-age dating (SHD) of two proximal lobes reveals mid-Holocene ages (7.6 ± 1.3 ka and 6.0 ± 1.2 ka), which contrast with the early-Holocene SHD and 10Be ages obtained previously from distal lobes, and late-Holocene SHD ages presented here from two adjacent talus slopes (2.3 ± 1.0 ka and 2.4 ± 1.0 ka). Although passive transport of boulders on the surfaces of these small, slow-moving rock glaciers affected by compressive flow means that the exposure ages are close to minimum estimates of the time elapsed since lobe inception, disturbance of boulders on rock glaciers is a source of potentially serious underestimates of rock-glacier age. Rock-glacier development at Øyberget began shortly after local deglaciation around 10 ka before present and continued throughout the Holocene in response to microclimatic undercooling within the coarse blocky surface layer of the talus and rock-glacier lobes. We suggest this enhanced cooling lowers mean annual surface-layer temperature by at least ~3.6°C, which is needed at such a low altitude to sustain sporadic permafrost and avoid fast thawing as atmospheric temperatures rise. Our results point to circumstances where inferences about rock glaciers as indicators of regional climate should be interpreted with caution, and where they may be less useful in palaeoclimatic reconstruction than previously thought.

Interpretation, age and significance of a relict paraglacial and periglacial boulder-dominated landform assemblage in Alnesdalen, Romsdalsalpane, southern Norway

A boulder-dominated landform assemblage in an alpine setting in Norway is described, interpreted and dated using a combination of Schmidt-hammer exposure-age dating (SHD) and cosmogenic 10Be surface-exposure dating. The origin and formation of these features is determined from their morphology, topographic setting and age. Three main components of the assemblage are recognized. First a ~300 m long and ~160 m wide, tongue-shaped rock-slope failure with a SHD age of 11.49 ± 0.82–12.96 ± 0.86 ka is identified on the basis of its planform, thickness, ‘runout distance’ and association with a fractured backing scarp. This is interpreted as a relict paraglacial landform triggered by deglaciation. Second, a 400-m long unequivocal pronival (protalus) rampart with an SHD age of 6.95 ± 0.97–8.35 ± 1.21 ka is identified on the basis of its narrow ridge form aligned close to the foot of an extensive talus slope. Although formation of the rampart probably commenced with deglaciation, the younger SHD ages indicate formation at a diminishing rate through the Holocene. Third, a ~550 m by ~150 m lobate component with transverse ridges, longitudinal and transverse depressions and enclosed pits, previously identified as a rock glacier, is dated to 11.33 ± 0.83–13.33 ± 0.90 ka with the Schmidt hammer and between 13.3 ± 2.0–14.7 ± 2.0 ka with 10Be. This third component may also be a product of rock-slope failure but attribution to rock-glacier creep associated with either interstitial ice (permafrost) or buried glacier ice during the Allerød Interstadial – Younger Dryas Stadial, and to which one or more rock-slope failures contributed debris, cannot be rejected. The advantages and limitations of compiling such evidence for identifying, differentiating and classifying apparently similar boulder-dominated colluvial landforms are exemplified.

Holocene alluvial fan evolution, Schmidt‐hammer exposure‐age dating and paraglacial debris floods in the SE Jostedalsbreen region, southern Norway

The evolution of several sub‐alpine alluvial fans SE of the Jostedalsbreen ice cap was investigated based on their geomorphology and Schmidt‐hammer exposure‐age dating (SHD) applied to 47 boulder deposits on the fan surfaces. A debris‐flood rather than debrisflow or water‐flow origin for the deposits was inferred from their morphology, consisting of low ridges with terminal splays up to 100 m wide without lateral levees. This was supported by fan, catchment, and boulder characteristics. SHD ages ranged from 9480±765 to 1955±810 years. The greatest number of boulder deposits, peak debris‐flood activity and maximum fan aggradation occurred between ̃9.0 and 8.0 ka, following regional deglaciation at ̃9.7 ka. The high debris concentrations necessary for debris floods were attributed to paraglacial processes enhanced by unstable till deposits on steep slopes within the catchments. From ̃8.0 ka, fan aggradation became progressively less as the catchment sediment sources tended towards exhaustion, precipitation decreased during the Holocene Thermal Maximum, and tree cover increased. After ̃4.0 ka, some areas of fan surfaces stabilized, while Late‐Holocene climatic deterioration led to renewed fan aggradation in response to the neoglacial growth of glaciers, culminating in the Little Ice Age. These changes are generalized within a conceptual model of alluvial fan evolution in this recently‐deglaciated mountain region and in glacierized catchments. This study highlights the potential importance of debris floods, of which relatively little is known, especially in the context of alluvial fan evolution.

Development of a Holocene glacier-fed composite alluvial fan based on surface exposure-age dating techniques: The Illåe fan, Jotunheimen, Norway

Holocene development of a composite alpine alluvial fan is reconstructed and dated based on its geomorphology and Schmidt-hammer exposure-age dating (SHD), supplemented by terrestrial cosmogenic-nuclide dating (TCND), lichenometry, radiocarbon dating, soil development, and palaeohydrological analysis. SHD ages of 7080 ± 450 to 8220 ± 440 years and a TCND age of 6075 ± 1220 years from the fan surface indicate that fan aggradation occurred in the early Holocene through episodic debris floods (phase 1) in response to paraglacial processes following deglaciation of the catchment (~9.7 ka). Stabilisation during the Holocene Thermal Maximum (phase 2) was followed by re-activation of the fan, involving entrenchment and terrace formation (phase 3), which occurred in response to neoglaciation after ~5.5 ka and culminated in the Little Ice Age. The main control on fan development was the effect of the changing extent of glaciers in the catchment on sediment supply, initially evinced in high-magnitude debris floods (hyperconcentrated flows) and later in water floods (typical fluvial processes). Aggradation switched to stabilisation as paraglacial activity waned, while later entrenchment was associated with relatively low bedload. Results provide the basis of a conceptual model of alluvial fan evolution for glacierized catchments related to Holocene environmental change.

Snow-avalanche boulder fans in Jotunheimen, southern Norway: Schmidt-hammer exposure-age dating, geomorphometrics, dynamics and evolution

ABSTRACTEleven snow-avalanche boulder fans were dated from two high-alpine sites in Jotunheimen using Schmidt-hammer exposure-age dating (SHD) and lichenometry. Average exposure ages of the surface boulders ranged from 2285 ± 725 to 7445 ± 1020 years and demonstrate the potential of SHD for dating active landforms and diachronous surfaces. Application of GIS-based morphometric analyses showed that the volume of rock material within 10 of the fans is accounted for by 16–68% of the combined volume of their respective bedrock chutes and transport zones. It is inferred that the fans were deposited entirely within the Holocene, mainly within the early- to mid Holocene, by frequent avalanches carrying very small debris loads. Relatively small transport-zone volumes are consistent with avalanches of low erosivity. Excess chute volumes appear to represent subaerial erosion in the Younger Dryas and possibly earlier. Debris supply to the fans was likely enhanced by early-Holocene paraglacial processes following deglaciation, and by later permafrost degradation associated with the mid-Holocene Thermal Maximum. The latter, together with the youngest SHD age from one of the fans, may presage a similar increase in geomorphic activity in response to current warming trends.

Schmidt-hammer exposure-age dating (SHD) of sorted stripes on Juvflye, Jotunheimen (central South Norway): Morphodynamic and palaeoclimatic implications

Measurements with an electronic Schmidt-hammer (RockSchmidt) were conducted on 23 sites of sorted stripes (periglacial patterned ground) on Juvflye, Jotunheimen (central South Norway). All were located above the current lower limit of alpine permafrost. Performing Schmidt-hammer exposure-age dating (SHD) based on application of a new local age-calibration equation for RRock-values yielded SHD-ages between 7975 ± 370 and 6660 ± 355 years ago, which are closely comparable to results obtained previously from sorted circles at the same location. The age estimates are interpreted as ‘composite’ ages indicative of upfreezing of boulders, lateral sorting, and subsequent stabilisation. Formation of patterned ground essentially ceased with the onset of the regional Holocene Thermal Maximum (HTM). Neither sorted stripe sites at higher altitude, continuously underlain by permafrost during the entire Holocene, nor those at lower altitudes affected by re-aggradation of permafrost in the late Holocene show signs of significant recent morphodynamic activity. Likely explanations for early- to mid-Holocene stabilisation include (1) substantial changes of soil moisture conditions and related thermodynamics within the active layer affecting frost action, (2) loss of fine-grained substrate matrix from the coarse stripes and hence reduced frost susceptibility, and (3) exhaustion of supply of boulders from the fines-dominated areas. Whereas the sorted stripe data set as a whole did not reproduce the altitudinal gradient characteristic of sorted circles on Juvflye, the strength of the relationship between sorted stripe mean RRock-values and altitude increased with declining slope gradient. Although interpretation of SHD-ages for patterned ground remains challenging, this successful application of the electronic Schmidt-hammer, with its increased efficiency and technical improvements over the mechanical Schmidt-hammer, offers considerable potential for future SHD-studies in both morphodynamic and palaeoclimatic contexts.

Age determination of glacially-transported boulders in Ireland and Scotland using Schmidt-hammer exposure-age dating (SHD) and terrestrial cosmogenic nuclide (TCN) exposure-age dating

AbstractSchmidt-hammer exposure-age dating (SHD) was applied at 15 sites with glacially-transported granite boulders in parts of northern and western Ireland and southwest Scotland that had been exposed by retreat of the last British-Irish Ice Sheet (BIIS) or Younger Dryas (YD) ice masses. Seven of these surfaces had previously been dated using terrestrial cosmogenic nuclide (TCN) exposure-age dating. Application of the granite calibration equation of Tomkins et al. (2018c) indicated a close correspondence between the SHD ages and the TCN ages (within 1σ or 2σ uncertainties). These findings demonstrate that SHD ages can be of comparable accuracy, precision, and reliability to TCN ages and are a strong argument for the more extensive use of SHD in some Quaternary dating projects. However, surface ages obtained by both SHD and TCN dating should not be accepted uncritically; they must be assessed in relation to the wider geological, geomorphological, and geochronological evidence. Evaluation of eight SHD ages, for which corresponding TCN ages are not available, indicate that most are consistent with current theory and field evidence, but some anomalous age estimates occur.

Early-Holocene glacier fluctuations of northern Grovabreen, western Norway

Marginal moraines on the northern side of Grovabreen, a plateau glacier in inner Sunnfjord, Western Norway, have been mapped and morphostratigraphically correlated with the Erdalen Event and possibly the Finse Event and the ‘Little Ice Age’. Schmidt-hammer exposure-age dating was used to evaluate the age of the most distinct marginal moraines by measuring the degree of surface weathering on boulders. The lithostratigraphy of lake sediments was analysed in a core from Dalevatnet in order to correlate this stratigraphy with the marginal moraines in the catchment area of the lake and reconstruct the early-Holocene history of Grovabreen. The lake catchment was deglaciated at 10,750 cal. yr BP. Two readvances of an outlet glacier in Fagredalen were dated to between 10,340 and 9960 cal. yr BP, correlated with the two-peaked Erdalen Event. A readvance shortly after 8450 cal. yr BP can most probably be correlated with the 8200 cal. yr BP Finse Event.

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

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.

Small rock‐slope failures conditioned by Holocene permafrost degradation: a new approach and conceptual model based on Schmidt‐hammer exposure‐age dating, Jotunheimen, southern Norway

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 (≤103 m3), rather than on the corresponding features of massive RSFs (~108 m3), 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.

Investigations on blockfields and related landforms at Blåhø (Southern Norway) using Schmidt-hammer exposure-age dating: palaeoclimatic and morphodynamic implications

ABSTRACTSchmidt-hammer exposure-age dating (SHD) was performed on blockfields and related landforms on Blåhø, Southern Norway. By developing a linear high-precision age-calibration curve through young and old control points of known age from terrestrial cosmogenic nuclide dating, it was possible to gain landform age estimates based on Schmidt hammer R-values. The aim of this study is to relate formation and subsequent stabilization of the landforms investigated to climate fluctuations since the Last Glacial Maximum (LGM) and to explore the palaeoclimatic implication of such periglacial landforms. The SHD ages range from 19.14 ± 0.91 ka for the Rundhø blockfield to 5.32 ± 0.73 ka for the lowest elevation rock-slope failure. The R-value frequency distributions obtained on the landforms studied indicate complex, long-term formation histories. Landforms above 1450 m a.s.l. share comparable SHD ages and seem to have stabilized during the Karmøy/Bremanger readvance (∼18.5–16.5 ka). The lower elevation rock-slope failures most likely occurred during the Bølling-Allerød interstadial (∼14.7–12.9 ka) and the Holocene Thermal Maximum (∼8.0–5.0 ka). The results contrast with the established model that rock-slope failures occur within the first millennia following deglaciation. Instead of the inferred ice coverage above 1450 m a.s.l. until 15.0 ± 1.0 10Be ka, our results suggest severe periglacial and ice-free conditions occurred earlier. Landforms above 1450 m a.s.l. do not show any form of reactivation during cold periods within the Late Glacial and Holocene. Our SHD results suggest that the landforms investigated were (at least partly) generated prior the LGM and survived beneath cold-based ice or were located on nunataks.

Age constraints of rock glaciers in the Southern Alps/New Zealand – Exploring their palaeoclimatic potential

Two rock glaciers in the valley head of Irishman Stream in the central Ben Ohau Range, Southern Alps/New Zealand, have been investigated using the electronic Schmidt-hammer (SilverSchmidt). Longitudinal profiles on both features reveal a consistent trend of decreasing R(Rebound)-values and, hence, increasing weathering intensity and surface-exposure age on their numerous transverse surface ridges from rooting zone towards the front. Previously published numerical ages obtained by terrestrial cosmogenic nuclide dating (TCND) allowed the calculation of a local Schmidt-hammer exposure-age dating (SHD) age-calibration curve by serving as the required fixed points. Age estimates for the lowermost rock glacier surface ridges fall within the early Holocene between 12 and 10.5 ka and indicate a fast disappearance of the Late Glacial glacier formerly occupying the valley head, followed by the initiation of rock glacier formation around or shortly after the onset of the Holocene. Although it cannot be judged whether the rock glaciers investigated were active within the entire Holocene or only repeatedly during multiple episodes within, their location and intact morphology exclude any substantial glacial activity at Irishman Stream during the Holocene. This has considerable regional palaeoclimatic implications because it opens for the hypothesis that climatic conditions during early Holocene were possibly comparatively dry and favourable for rock glacier initiation, but less so for glaciers. It would also challenge the view that air temperature is the sole major climate driver of glacier variability in the Southern Alps. More work utilising the palaeoclimatic potential of rock glaciers in the Southern Alps is advised.