Schmidt-hammer Exposure-age Dating Ages Research Articles

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.

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.

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.

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.

Time constraints for post-LGM landscape response to deglaciation in Val Viola, Central Italian Alps

Across the northern European Alps, a long tradition of Quaternary studies has constrained post-LGM (Last Glacial Maximum) landscape history. The same picture remains largely unknown for the southern portion of the orogen. In this work, starting from existing 10Be exposure dating of three boulders in Val Viola, Central Italian Alps, we present the first detailed, post-LGM reconstruction of landscape (i.e., glacial, periglacial and paraglacial) response south of the Alpine divide. We pursue this task through Schmidt-hammer exposure-age dating (SHD) at 34 sites including moraines, rock glaciers, protalus ramparts, rock avalanche deposits and talus cones. In addition, based on the mapping of preserved moraines and on the numerical SHD ages, we reconstruct the glacier extent of four different stadials, including Egesen I (13.1 ± 1.1 ka), Egesen II (12.3 ± 0.6 ka), Kartell (11.0 ± 1.4 ka) and Kromer (9.7 ± 1.4 ka), whose chronologies agree with available counterparts from north of the Alpine divide. Results show that Equilibrium Line Altitude depressions (ΔELAs) associated to Younger Dryas and Early Holocene stadials are smaller than documented at most available sites in the northern Alps. These findings not only support the hypothesis of a dominant north westerly atmospheric circulation during the Younger Dryas, but also suggest that this pattern could have lasted until the Early Holocene. SHD ages on rock glaciers and protalus ramparts indicate that favourable conditions to periglacial landform development occurred during the Younger Dryas (12.7 ± 1.1 ka), on the valley slopes above the glacier, as well as in newly de-glaciated areas, during the Early Holocene (10.7 ± 1.3 and 8.8 ± 1.8 ka). The currently active rock glacier started to develop before 3.7 ± 0.8 ka and can be associated to the Löbben oscillation. Four of the five rock avalanches dated in Val Viola cluster within the Early Holocene, in correspondence of an atmospheric warming phase. By contrast, the timing of the main Val Viola rock avalanche, 7.7 ± 0.3 ka during the Holocene Thermal Optimum, suggests a possible causal linkage to permafrost degradation. Overall, Schmidt-hammer proved to be an effective, inexpensive and versatile tool for improving the spatial resolution of Val Viola post-LGM landscape history, starting from existing numerical age constrains.

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

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.

Age assessment and implications of late Quaternary periglacial and paraglacial landforms on Muckish Mountain, northwest Ireland, based on Schmidt-hammer exposure-age dating (SHD)

Schmidt-hammer exposure-age dating (SHD) was applied to a variety of late Quaternary periglacial and paraglacial landforms composed of coarse rock debris on Muckish Mountain, northwest Ireland. Landform ages were determined using a linear high-precision age-calibration curve, derived from young and old control surfaces of known age on the same rock type. The SHD ages represent maximum estimates of the time elapsed since the boulders stabilised and the landforms became inactive. Most ages are also minimum estimates for the start of landform development because older boulders are buried beneath the sampled surface boulders. Ages and 95% confidence intervals obtained for blockfield, boulder lobes and talus indicate these features were likely active during several of the early Holocene cold events evidenced in Greenland ice cores and North Atlantic sediment records. Activity ceased at different times ~9–7ka BP. These landforms are the first indication of a geomorphological response to early Holocene cooling in the oceanic mountains of Ireland. Late Holocene ages, obtained for rock-slope failure run-out debris and debris cone boulders, overlap with shifts to cooler and/or wetter conditions, including the Little Ice Age. Geomorphological impacts associated with these changes in climate have not previously been recorded in the Irish uplands. The SHD results indicate that previously implied timings for the stabilisation of some accumulations of coarse rock debris on mountain slopes are in need of revision.

Improved Schmidt-hammer exposure ages for active and relict pronival ramparts in southern Norway, and their palaeoenvironmental implications

Eight pronival (protalus) ramparts from alpine southern Norway were dated using Schmidt-hammer exposure-age dating (SHD) based on local high-precision linear calibration equations. SHD ages were −1355±1240 to 815±1000years for three active ramparts and 8730±1050 to 14,635±1060years for five relict ramparts. Different parts of the same rampart yielded ages that were in generally good agreement. R-value frequency distributions enabled the identification single-age and mixed-age boulder populations, and the differentiation of active and relict rampart surfaces. The relict ramparts are located outside Younger Dryas glacier limits and probably formed over a period of ~6000years between deglaciation (following the Last Glacial Maximum of the late Weichselian, ~18,000years ago) and the end of the Younger Dryas (~11,700years ago). At that time, glacial debuttressing and permafrost degradation apparently enhanced the release of rock debris from rock faces to the ramparts. The active ramparts, which are located in areas deglaciated after the Younger Dryas, are relatively uncommon in the study area due to diminished paraglacial effects and limited debris supply associated with seasonally frozen ground during the Holocene. Thus, the legacy of glaciers, glacier–permafrost interaction, and the specific type of periglacial environment must all be considered when assessing the palaeoenvironmental and palaeoclimatic significance of pronival ramparts.

Schmidt‐hammer exposure‐age dating (SHD) of snow‐avalanche impact ramparts in southern Norway: approaches, results and implications for landform age, dynamics and development

AbstractSchmidt‐hammer exposure‐age dating (SHD) was applied to the problem of dating the diachronous surfaces of five distal river‐bank boulder ramparts deposited by snow avalanches plunging into the Jostedøla and Sprongdøla rivers in the Jostedalsbreen region of southern Norway. Approaches to local high‐precision linear age calibration, which controlled in different ways for boulder roundness, were developed. The mean age (SHDmean) and the maximum age (SHDmax) of surface boulders were estimated for whole ramparts, crests and distal fringes. Interpretation was further assisted by reference to R‐value distributions. SHDmean ages (with 95% confidence intervals) ranged from 520 ± 270 years to 5375 ± 965 years, whereas SHDmax ages (expected to be exceeded by <5% of surface boulders) ranged from 675 to 9065 years. SHD ages from the Jostedøla ramparts tended to be older than those associated with the Sprongdøla, rampart crests were younger than the respective distal fringes, and use of relatively rounded boulders yielded more consistent SHD ages than angular boulders. The SHDmean ages indicate differences in recent levels of snow‐avalanche activity between ramparts and provide insights into rampart dynamics as boulders are deposited on rampart crests and, in smaller numbers, on the distal fringes. SHDmax ages provide minimum age estimates of rampart age (i.e. the time elapsed since the ramparts began to form) and suggest that at least some of the ramparts have been developing since the early Holocene. Copyright © 2015 John Wiley & Sons, Ltd.

Relict Talus‐Foot Rock Glaciers at Øyberget, Upper Ottadalen, Southern Norway: Schmidt Hammer Exposure Ages and Palaeoenvironmental Implications

ABSTRACTThree relict talus‐foot rock glaciers at Øyberget, upper Ottadalen, were dated using high‐precision Schmidt hammer exposure‐age dating (SHD). A SHD calibration curve was constructed for the local banded gneiss using two control points: (1) fresh bedrock in a road cutting; and (2) glacially scoured bedrock surfaces deglaciated c. 9.7 ka. Two alternative hypotheses are suggested to explain SHD ages (± 95% confidence intervals) of 10 340 ± 1280, 9920 ± 1385 and 8965 ± 1700 years obtained for the three rock glaciers located ~ 1000 m below the present lower limit of mountain permafrost, at a site covered by glacier ice throughout the Late Weichselian. First, the rock glaciers may be paraglacial features, which formed rapidly during deglaciation when enhanced debris supply may have buried residual ice bodies. If this was the case, the SHD ages are a true reflection of rock glacier age. Second, the rock glaciers may have formed during several thousand years of permafrost conditions in the Mid‐Weichselian (possibly during the Ålesund Interstadial, c. 34.5–38.2 ka) and then been preserved beneath a cold‐based ice sheet until renewed exposure during Holocene deglaciation. If the second hypothesis is true, then the true age of the rock glaciers is much older than their SHD ages, the latter failing to record the interval of burial when rock weathering would have been zero. Although evidence is currently insufficient to distinguish between these two interpretations, the paper demonstrates both the potential and limitations of high‐precision SHD in the context of rock glaciers. Copyright © 2013 John Wiley & Sons, Ltd.

Pronival rampart formation in relation to snow-avalanche activity and Schmidt-hammer exposure-age dating (SHD): Three case studies from southern Norway

Pronival (protalus) ramparts at three sites in southern Norway—Smørbotn, Alnesreset and Nystølsnovi—are assessed in relation to evidence of snow-avalanche activity, and dated using Schmidt-hammer exposure-age dating (SHD). The position of each rampart suggests frequent snow avalanches played a major role in rampart formation. The pronival ramparts at Smørbotn and Nystølsnovi are characterized by a high proportion of unweathered, very angular clasts, many clean boulders (without any lichen or moss cover), steep distal slope angles, and a disturbed ground cover of vegetation with characteristic alpine pioneer plant species on crests and proximal slopes. High Schmidt-hammer R-values (mean 56.2–58.5) yielded SHD ages of <2900years and <1550years, respectively. These ramparts are currently active and have probably developed since the early Holocene under environmental conditions similar to those at present. Pronival ramparts at Alnesreset are characterized by highly weathered, mainly angular boulders (virtually none of which is clean), a mature, undisturbed vegetation cover, and lower distal slope angles. Mean R-values of 39.9–40.3 from these ramparts yielded SHD ages within the range 9450 to 14,650years (with 95% certainty), indicating formation during the Younger Dryas Stadial and relict status for most of the Holocene. Our results: (1) show that rockfall is not the only primary process involved in the formation of pronival ramparts; (2) broaden the range of landforms included in snow-avalanche geomorphology; (3) demonstrate the feasibility of dating pronival ramparts; and (4) indicate their potential in palaeoenvironmental reconstruction.