Paired 10Be sampling of polished bedrock and erratic boulders to improve dating of glacial landforms: an example from the Western Alps

Cosmogenic nuclide dating of glacial landforms may lead to ambiguous results for ice retreat histories. The persistence of significant cosmogenic concentrations inherited from previous exposure may increase the apparent exposure ages for polished bedrocks affected by limited erosion under ice and for erratic boulders transported by glaciers and previously exposed in high-altitude rock walls. In contrast, transient burying by moraines, sediments and snow decreases the apparent exposure age. We propose a new sampling strategy, applied to four sites distributed in the Arc and Arve valleys in the Western Alps, to better constrain the factors that can bias exposure ages associated with glacial processes. We used the terrestrial cosmogenic nuclide ¹⁰Be (TCN) to estimate the exposure time from paired sampling of depth profiles in polished bedrock and on overlying erratic boulders. For a given sampling site, the exposure ages for both the polished bedrock and boulder are expected to be the same. However, in six cases out of seven, boulders had significantly higher ¹⁰Be surface concentrations than those of the associated polished surfaces. In present and past glacial processes, the ¹⁰Be distribution with depth for boulders and bedrocks implies the presence of an inheritance concentration of ¹⁰Be. Our study suggests that ¹⁰Be concentrations in erratic boulders and in polished bedrocks provide maximum and minimum exposure ages of the glacial retreat, respectively. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Moraine pebbles and boulders yield indistinguishable Be ages: A case study from Colorado, USA

Cosmogenic exposure dating of moraines during the last two decades has vastly improved knowledge on the timing of glaciation worldwide. Due to a variety of geologic complications, such as moraine degradation, snow cover, bedrock erosion and isotopic inheritance, samples from multiple large boulders (>1–2 m) often lead to the most accurate moraine age assignments. However, in many cases, large boulders are not available on moraines of interest. Here, I test the suitability of pebble collections from moraine crest surfaces as a sample type for exposure dating. Twenty-two ¹⁰Be ages from two Pleistocene lateral moraine crests in Pine Creek valley in the upper Arkansas River basin, Colorado, were calculated from both pebble and boulder samples. Ten ¹⁰Be ages from a single-crested Bull Lake lateral moraine range between 3 and 72 ka, with no statistical difference between pebble (n = 5) and boulder (n = 5) ages. The lack of a cluster of ¹⁰Be ages suggests that moraine degradation has led to anomalously young exposure ages. Twelve ¹⁰Be ages from a single-crested Pinedale lateral moraine have a bimodal age distribution; one mode is 22.0 ± 1.4 ka (three boulders, two pebble collections), the other is 15.2 ± 0.9 ka (two boulders, five pebble collections). The interpretation of the two age modes is that two glacier maxima of similar extent were attained during the late Pleistocene. Regardless of moraine age interpretations, that ¹⁰Be ages from pebble collections and boulders are indistinguishable on moraines of two different ages, and in two different age modes of the Pinedale moraine, suggests that pebble collections from moraine crests may serve as a suitable sample type in some settings.     

Cosmogenic surface exposure dating the last deglaciation in Denmark: Discrepancies with independent age constraints suggest delayed periglacial landform stabilisation

Cosmogenic nuclide surface exposure ages are determined from in situ ¹⁰Be and ³⁶Cl analysis of 38 rock surfaces found in different glacial landforms in Denmark. Dating of erratic boulders and adjacent ice-sculpted bedrock on the island of Bornholm in the western Baltic Sea reveals almost identical values. This suggests that little if any inherited nuclides are present in the sampled boulders. West of the Last Glacial Maximum (LGM) ice margin in Denmark ages reflect exposure from the Middle Weichselian. East of the LGM margin exposure ages from 35 samples show Late Weichselian ages in a range between 20.6–11.9 ka. To test to what extent these dates reflect the onset of deglaciation immediately after cessation of active glacier flow, surface exposure ages are evaluated against independent chronologies of Late Weichselian ice-sheet fluctuations in southwestern Scandinavia. The Bornholm dates agree with the independent age model, however, in the data set for eastern Denmark only less than half the surface exposure ages lie within the expected age envelope. This apparent mismatch is most likely due to post-glaciation shielding and delayed surface stabilisation compared to the timing of ice-margin retreat. Thus ages from boulders resting in dead-ice moraines and mass wasting landscapes underestimate deglaciation by 3–6 thousand years. The results quantify the impact of exhumation and landform stabilisation on cosmogenic surface exposure ages on millennial scales. We conclude, that interpretation of cosmogenic exposure ages should include careful evaluation of possible post-depositional landform transformation in attempts to fine tune ages of e.g. end moraine features. With reference to independent age models we critically evaluate glacier advance – retreat scenarios from regions around the southern Baltic that alone are based on weighted average ages of cosmogenic exposure dating.     

A locally calibrated,late glacial 10Be production rate from a low-latitude,high-altitude site in the Peruvian Andes

Well-dated records of tropical glacier fluctuations are essential for developing hypotheses and testing proposed mechanisms for past climate changes. Since organic material for radiocarbon dating is typically scarce in low-latitude, high-altitude environments, surface exposure-age dating, based on the measurement of in situ produced cosmogenic nuclides, provides much of the chronologic information on tropical glacier moraines. Here, we present a locally calibrated ¹⁰Be production rate for a low-latitude, high-altitude site near Quelccaya Ice Cap (∼13.95°S, 70.89°W, 4857 m asl) in the southeastern Peruvian Andes. Using an independent age (12.35 +0.2, −0.02 ka) of the late glacial Huancané IIa moraines based on thirty-four bracketing radiocarbon ages and twelve ¹⁰Be concentrations of boulders on the moraines, we determine a local production rate of 43.28 ± 2.69 atoms gram⁻¹ year⁻¹ (at g⁻¹ yr⁻¹). Reference ¹⁰Be production rates (i.e., production rates by neutron spallation appropriate for sea-level, high-latitude sites) range from 3.97 ± 0.09 to 3.78 ± 0.09 at g⁻¹ yr⁻¹, determined using scaling after Lal (1991) and Stone (2000) and depending on our assumed boulder surface erosion rate. Since our boulder surface erosion rate estimate is a minimum value, these reference production rates are also minimum values. A secondary control site on the Huancané IIIb moraines suggests that the ¹⁰Be production rates are at least as low as, or possibly lower than, those derived from the Huancané IIa moraines. These sea-level, high-latitude production rates are at least 11–15% lower than values derived using the traditional global calibration dataset, and they are also lower than those derived from the late glacial Breque moraine in the Cordillera Blanca of Peru. However, our sea-level, high-latitude production rates agree well with recently published, locally calibrated production rates from the Arctic, New Zealand, and Patagonia. The production rates presented here should be used to calculate ¹⁰Be exposure ages in low-latitude, high-altitude locations, particularly in the tropical Andes, and should improve the ability to compare the results of studies using ¹⁰Be exposure-age dating with other chronological data.     

10Be surface-exposure age dating of the Last Glacial Maximum in the northern Pamir (Tajikistan)

Knowledge of the spatial and temporal variations in Alpine glaciations is essential for reconstructing the regional and global timing of ice ages. This study investigates glacial deposits at the mouth of the Muksu catchment in the northern Pamir using ¹⁰Be surface-exposure age dating. We sampled boulders from the furthest downstream recessional moraine (20 samples) and five lateral moraines (41 samples) near the former terminus of the Fedchenko Glacier, the longest (∼72 km) present-day Alpine glacier of the Pamir. After the identification of outliers, the boulder population of the recessional moraine yielded a mean exposure age of 17.5 ± 1.9 ka. The maximum exposure age of the lateral moraines, collected ∼5 km up-valley of the recessional moraine, is 18.2 ± 1.7 ka. The boulder ages reflect glacial deposition during the Last Glacial Maximum (Marine Isotope Stage 2) in the region; they are in accordance with published glacial deposition ages in the western Tian Shan.     

Cosmogenic 21Ne exposure dating of young basaltic lava flows from the Newer Volcanic Province,western Victoria,Australia

Cosmogenic ²¹Ne was utilised to determine exposure ages of young subaerial basaltic lava flows from the Newer Volcanic Province, western Victoria, Australia. The ages (36–53 ka) determined from co-existing cosmogenic ²¹Ne and ³He in olivines separated from basalts are consistent within analytical uncertainties with ages previously determined by cosmogenic ³⁶Cl exposure dating. This paper illustrates the potential of cosmogenic neon exposure ages in studying the eruption, surface morphology, and erosion history of young volcanic rocks, which are difficult to date using other conventional methods, such as K-Ar or ⁴⁰Ar/³⁹Ar dating. The present study demonstrates that combined cosmogenic ³He and ²¹Ne dating, specifically measured cosmogenic ³He/²¹Ne ratios, on the same samples, is powerful for evaluating the validity of calculated cosmogenic ³He and ²¹Ne surface exposure ages.     

A quantitative assessment of snow shielding effects on surface exposure dating from a western North American 10Be data compilation

To better assess the spatiotemporal variations of the snow shielding effect on surface exposure dating, we compiled a dataset of 1341 ¹⁰Be ages from alpine moraines and glacially eroded valleys across western North America, and conducted a sensitivity test with both modern and time-integrated snow data covering the same region. Our analyses reveal significant differences in snow shielding both across our geographic domain and through time. In our time-integrated experiments we find snow-based exposure age corrections as low as 3.5% in the Great Basin region and high as 28.4% in the Pacific Northwest for samples dating to the Last Glacial Maximum (LGM) when no wind-sweeping is assumed. As demonstrated with our time-varying snow conditions with a global climate model and a positive degree day model, modern snow conditions across western North America cannot account for the varying snow patterns under large scale climate shifts since the LGM. The snow-based exposure age corrections from the modern data differ from those calculated by our time-varying model by up to 17% across our model domain. In addition, we find that the ¹⁰Be ages calculated under two end-member scenarios regarding wind-sweeping effects, specifically whether boulders were shielded only when the total snow accumulation exceeded boulder heights or were always shielded when the snow was present, can differ by ∼7.6% on average for LGM aged samples. Our analyses provide a model-based estimates of the spatiotemporal variability and complexity of snow shielding effects on surface exposure dates across western North America and highlight the need to consider snow depth variations both spatially and temporally when conducting surface exposure dating in terrains where snowfall accumulation is significant.     

Defining rates of landscape evolution in a south Tibetan graben with in situ‐produced cosmogenic 10Be

The Tangra Yum Co graben is one of the active structures that accommodate the east‐west extension of the southern Tibetan Plateau and hosts one of the largest Tibetan lakes, which experienced lake‐level changes of ~200 m during the Holocene. In this study, cosmogenic ¹⁰Be is employed to: (1) quantify catchment‐wide denudation rates in fault‐bounded mountain ranges adjacent to the Tangra Yum Co graben; (2) date palaeo‐shorelines related to the Holocene lake‐level decline; and (3) determine the age of glacial advances in this region. The fault‐bounded, non‐glaciated mountain range north of Tangra Yum Co – and presumably most other areas around the lake – erode at low rates of 10–70 mm/ka. Owing to the slow erosion of the landscape, the sediments delivered to Tangra Yum Co have high ¹⁰Be concentrations. As a consequence, accurate exposure dating of sediment‐covered terraces and beach ridges is difficult, because the pre‐depositional ¹⁰Be concentration may exceed the post‐depositional ¹⁰Be concentration from which exposure ages are calculated. This difficulty is illustrated by a rather inaccurate ¹⁰Be exposure age of 2.3 ± 1.4 ka (i.e. an error of 60%) for a terrace that is located 67 m above the lake. Nevertheless, the age is consistent with luminescence ages for a series of beach ridges and provides further evidence for the decline of the lake level in the late Holocene. At Tangra Yum Co exposure dating of beach ridges via ¹⁰Be depth profiles is not feasible, because the pre‐depositional ¹⁰Be component in these landforms varies with depth, which violates a basic assumption of this approach. ¹⁰Be ages for boulders from two moraines are much older than the early Holocene lake‐level highstand, indicating that melting of glaciers in the mountain ranges adjacent to Tangra Yum Co has not contributed significantly to the lake‐level highstand in the early Holocene. Copyright © 2015 John Wiley & Sons, Ltd.     

Postglacial to Holocene landscape evolution and process rates in steep alpine catchments

Climate change and high magnitude mass wasting events pose adverse societal effects and hazards, especially in alpine regions. Quantification of such geomorphic processes and their rates is therefore critical but is often hampered by the lack of appropriate techniques and the various spatiotemporal scales involved in these studies. Here we exploit both in situ cosmogenic beryllium-10 (¹⁰Be) and carbon-14 (¹⁴C) nuclide concentrations for deducing exposure ages and tracing of sediment through small alpine debris flow catchments in central Switzerland. The sediment cascade and modern processes we track from the source areas, through debris flow torrents to their final export out into sink regions with cosmogenic nuclides over an unprecedented five-year time series with seasonal resolution. Data from a seismic survey and a 90 m core revealed a glacially overdeepened basin, filled with glacial and paraglacial sediments. Surface exposure dating of fan boulders and radiocarbon ages constrain the valley fill from the last deglaciation until the Holocene and show that most of the fan existed in early Holocene times already. Current fan processes are controlled by episodic debris flow activity, snow (firn) and rock avalanches. Field investigations, digital elevation models (DEMs) of difference and geomorphic analysis agree with sediment fingerprinting with cosmogenic nuclides, highlighting that the bulk of material exported today at the outlet of the subcatchments derives from the lower fans. Cosmogenic nuclide concentrations steadily decrease from headwater sources to distal fan channels due to the incorporation of material with lower nuclide concentrations. Further downstream the admixture of sediment from catchments with less frequent debris flow activity can dilute the cosmogenic nuclide signals from debris flow dominated catchments but may also reach thresholds where buffering is limited. Consequently, careful assessment of boundary conditions and driving forces is required when apparent denudation rates derived from cosmogenic nuclide analysis are upscaled to larger regions. © 2018 John Wiley & Sons, Ltd.     

Effects of long soil surface residence times on apparent cosmogenic nuclide denudation rates and burial ages in the Cradle of Humankind,South Africa

In situ cosmogenic nuclides are an important tool for quantifying landscape evolution and dating fossil-bearing deposits in the Cradle of Humankind (CoH), South Africa. This technique mainly employs cosmogenic 10-Beryllium (¹⁰Be) in river sediments to estimate denudation rates and the ratio of 26-Aluminium (²⁶Al) to ¹⁰Be (²⁶Al/¹⁰Be), to constrain ages of sediment burial. Here, we use ¹⁰Be and ²⁶Al concentrations in bedrock and soil above the Rising Star Cave (the discovery site of Homo naledi) to constrain the denudation rate and the exposure history of soil on the surface. Apparent ¹⁰Be-derived denudation rates obtained from pebble- to cobble-sized clasts and coarse-sand in soil (on average 3.59 ± 0.27 m/Ma and 3.05 ± 0.25 m/Ma, respectively) are 2-3 times lower than the bedrock denudation rates (on average 9.46 ± 0.68 m/Ma). In addition, soil samples yield an average ²⁶Al/¹⁰Be ratio (5.12 ± 0.27) that is significantly lower than the surface production ratio of 6.75, which suggests complex exposure histories. These results are consistent with prolonged surface residence of up to 1.5 Ma in vertically mixed soils that are up to 3 m thick. We conclude that the ¹⁰Be concentrations accumulated in soils during the long near-surface residence times can potentially cause underestimation of single-nuclide (¹⁰Be) catchment-wide denudation rates in the CoH. Further, burial ages of cave sediment samples that consist of an amalgamation of sand-size quartz grains could be overestimated if a pre-burial ²⁶Al/¹⁰Be ratio calculated from the surface production is assumed. © 2019 John Wiley & Sons, Ltd.     

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

Schmidt‐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 (SHD_mean) and the maximum age (SHD_max) of surface boulders were estimated for whole ramparts, crests and distal fringes. Interpretation was further assisted by reference to R‐value distributions. SHD_mean ages (with 95% confidence intervals) ranged from 520 ± 270 years to 5375 ± 965 years, whereas SHD_max 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 SHD_mean 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. SHD_max 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.     

Chronology of Lateglacial ice flow reorganization and deglaciation in the Gotthard Pass area,Central Swiss Alps,based on cosmogenic 10Be and in situ 14C

We reconstruct the timing of ice flow reconfiguration and deglaciation of the Central Alpine Gotthard Pass, Switzerland, using cosmogenic ¹⁰Be and in situ ¹⁴C surface exposure dating. Combined with mapping of glacial erosional markers, exposure ages of bedrock surfaces reveal progressive glacier downwasting from the maximum LGM ice volume and a gradual reorganization of the paleoflow pattern with a southward migration of the ice divide. Exposure ages of ∼16–14 ka (snow corrected) give evidence for continuous early Lateglacial ice cover and indicate that the first deglaciation was contemporaneous with the decay of the large Gschnitz glacier system. In agreement with published ages from other Alpine passes, these data support the concept of large transection glaciers that persisted in the high Alps after the breakdown of the LGM ice masses in the foreland and possibly decayed as late as the onset of the Bølling warming. A younger group of ages around ∼12–13 ka records the timing of deglaciation following local glacier readvance during the Egesen stadial. Glacial erosional features and the distribution of exposure ages consistently imply that Egesen glaciers were of comparatively small volume and were following a topographically controlled paleoflow pattern. Dating of a boulder close to the pass elevation gives a minimum age of 11.1 ± 0.4 ka for final deglaciation by the end of the Younger Dryas. In situ ¹⁴C data are overall in good agreement with the ¹⁰Be ages and confirm continuous exposure throughout the Holocene. However, in situ ¹⁴C demonstrates that partial surface shielding, e.g. by snow, has to be incorporated in the exposure age calculations and the model of deglaciation.     

Dating alluvial fan surfaces in Owens Valley,California, using weathering fractures in boulders

A wide range of sedimentological and geomorphological field research depends on the availability of accurate and detailed depositional age models. Although exposure dating techniques such as cosmogenic nuclide and luminescence dating are now widely available, they remain expensive and time‐consuming, and this frequently limits the density of age constraints and the resolutions of age models for many study areas. We present a simple and effective, field‐based approach for extending and correlating existing age models to un‐dated surfaces. In Owens Valley, California, we make use of detailed beryllium‐10 (¹⁰Be) chronologies reported for four different alluvial fan systems, to precisely calibrate the rate at which weathering fractures are enlarged in granitic surface boulders. We show that these fractures have widened at a time‐integrated rate of 1.05 ± 0.03 mm ka^?1 for at least 140 ka at this location, and this relationship can be represented by a linear regression that makes them ideal chronometers for surface dating. Our analysis offers a new approach to refining the uncertainties of both surface erosion rate and cosmogenic age estimates at this location. Ultimately, we integrate our observations to devise a robust age calibration for clast fracture widths in Owens Valley, and we demonstrate its application by estimating the ages of 27 additional local fan surfaces. We present an updated and extended stratigraphy for eight Sierra Nevada fan systems in total, with exceptional age control. This novel approach to dating sedimentary surfaces is inexpensive and easily applied in the field, and has the potential to significantly increase the temporal and spatial density of age constraints available for a particular study area. Copyright © 2014 John Wiley & Sons, Ltd.     

Dating carbonate rocks with in-situ produced cosmogenic Be: Why it often fails

Surface exposure dating of carbonate rocks using cosmogenic ¹⁰Be is problematic. We have performed step-wise leaching of calcite-rich samples in order to investigate the reasons for this. Results on different grain size fractions clearly indicate the source of atmospheric ¹⁰Be is clay. We demonstrate that partial-leaching procedures, which result in moderate pH levels will not release ¹⁰Be (in-situ produced or atmospheric) due to the instant re-absorption on grain surfaces. By contrast, under strongly acidic conditions, all absorbed ¹⁰Be is leached from aluminosilicates giving abnormally high ¹⁰Be concentrations and consequently exposure ages that are too old. Dating is only possible if samples do not contain any clay minerals or if they can be removed prior to carbonate dissolution.     

Can in-situ cosmogenic 14C be used to assess the influence of clast recycling on exposure dating of ice retreat in Antarctica?

Cosmogenic nuclide exposure dating of glacial clasts is becoming a common and robust method for reconstructing the history of glaciers and ice sheets. In Antarctica, however, many samples exhibit cosmogenic nuclide ‘inheritance’ as a result of sediment recycling and exposure to cosmic radiation during previous ice free periods. In-situ cosmogenic ¹⁴C, in combination with longer lived nuclides such as ¹⁰Be, can be used to detect inheritance because the relatively short half-life of ¹⁴C means that in-situ ¹⁴C acquired in exposure during previous interglacials decays away while the sample locality is covered by ice during the subsequent glacial. Measurements of in-situ ¹⁴C in clasts from the last deglaciation of the Framnes Mountains in East Antarctica provide deglaciation ages that are concordant with existing ²⁶Al and ¹⁰Be ages, suggesting that in this area, the younger population of erratics contain limited inheritance.     

Glacier response to the change in atmospheric circulation in the eastern Mediterranean during the Last Glacial Maximum

In this study, we document glacial deposits and reconstruct the glacial history in the Karagöl valley system in the eastern Uludağ in northwestern Turkey based on 42 cosmogenic ¹⁰Be exposure ages from boulders and bedrock. Our results suggest the Last Glacial Maximum (LGM) advance prior to 20.4 ± 1.2 ka and at least three re-advances until 18.6 ± 1.2 ka during the global LGM within Marine Isotope Stage-2. In addition, two older advances of unknown age are geomorphologically well constrained, but not dated due to the absence of suitable boulders. Glaciers advanced again two times during the Lateglacial. The older is exposure dated to not later than 15.9 ± 1.1 ka and the younger is attributed to the Younger Dryas (YD) based on field evidence. The timing of the glaciations in the Karagöl valley correlates well with documented archives in the Anatolian and Mediterranean mountains and the Alps. These glacier fluctuations may be explained by the change in the atmospheric circulation pattern during the different phases of North Atlantic Oscillation (NAO) winter indices.     

“Difference Dating”: A novel approach towards dating alpine glacial moraines

The East Antarctic Ice Sheet responds sluggishly to shifts in climate. To capture subtle changes in Antarctic climate, researchers have focused instead on smaller alpine and cirque glaciers that fringe the ice sheet throughout the McMurdo Dry Valleys. The exposure ages of glacial moraine boulders scatter widely and often incorporate large amounts of inheritance, prohibiting the construction of a regional deglaciation chronology. We present a new sampling technique that takes advantage of ubiquitous desert pavements and allows for the detection of inheritance in overlying glacial moraine boulders. Our approach requires a large sample set, but offers increased confidence in modeling moraine age, an acceptable trade-off considering the need for more refined Antarctic paleoclimate reconstructions. Using the beryllium-10 system in sandstone quartz, we show that single exposure ages collected from moraine boulder tops are frequently inaccurate, and consistently over- and underestimate moraine age. Difference Dating offers a new approach to dating alpine glacial moraines independent from traditional boulder exposure age dating.     

Regional Be production rate calibration for the past 12 ka deduced from the radiocarbon-dated Grøtlandsura and Russenes rock avalanches at 69° N, Norway

Two rock avalanches in Troms County – the Grøtlandsura and Russenes – were selected as CRONUS-EU natural cosmogenic ¹⁰Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring ⁹Be (typically < 1.5 ppb); and (c) have reliable minimum radiocarbon ages of 11,424 ± 108 cal yr BP and 10,942 ± 77 cal yr BP (1σ), respectively. Quartz samples (n = 6) from these two sites contained between 4.28 × 10⁴ and 5.06 × 10⁴ at ¹⁰Be/g using the 1.387 Myr ¹⁰Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced ¹⁰Be concentrations by 10%. Using the ¹⁰Be half-life of 1.387 Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total ¹⁰Be production rate of 4.12 ± 0.19 at/g/yr (1σ). A corresponding error-weighted mean spallogenic ¹⁰Be production rate is 3.96 ± 0.16 at/g/yr (1σ), respectively. These are in agreement within uncertainty with other ¹⁰Be production rates in the literature, but are significantly, statistically lower than the global average ¹⁰Be production rate. This research indicates, like other recent studies, that the production of cosmogenic ¹⁰Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total ¹⁰Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Hølen avalanche) to be 7.5 ± 0.3 kyr (1σ). This age suggests that the rock avalanche occurred shortly after the 8.2 kyr cooling event, just as the radiocarbon ages of the Grøtlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation.     

Single grain optically stimulated luminescence dating of glacial sediments from the Baiyu Valley,southeastern Tibet

The Qinghai-Tibetan Plateau is an important area for the study of Quaternary glaciation. Optically stimulated luminescence (OSL) dating has the potential to contribute to the chronology of glaciation in this region, but it is important to assess the accuracy of OSL dating of these glacial sediments. In this study, single grain quartz OSL signals are examined for five glacial samples collected from the moraines outside the Baiyu Valley, southeastern Tibet. The quartz grains exhibit poor luminescence characteristics, with a small proportion of grains passing the screening criteria. Grains which pass the screening criteria have relatively low signal intensity, leading to D_e values with large uncertainties. MAM and CAM were used to determine D_e values for these samples. The OSL ages are consistent with the sequence of events derived from the geomorphological relationship of the samples, and also with previous published radiocarbon ages. However, it is more difficult to reconcile the OSL ages and the terrestrial cosmogenic nuclide (TCN) ¹⁰Be ages. Analysis of both single grain quartz OSL data and TCN ¹⁰Be data is complex in this area. Further work is required to increase confidence in the OSL ages generated for the glacial sediments from this region.