Terrestrial Cosmogenic Nuclide Research Articles

Paleoseismic patterns of Quaternary tectonic and magmatic surface deformation in the eastern Basin and Range, USA

AbstractThe competing contributions of tectonic and magmatic processes in accommodating continental extension are commonly obscured by a lack of on-fault paleoseismic information. This is especially true of the Sevier Desert, located at the eastern margin of the Basin and Range in central Utah (USA), where surface-rupturing faults are spatially associated with both regional detachment faults and Quaternary volcanism. Here, we use high-resolution topographic surveys (terrestrial lidar scans and real-time kinematic GPS), terrestrial cosmogenic nuclide (10Be and 3He) exposure dating, 40Ar/39Ar geochronology, and new neotectonic mapping to distinguish between modes of faulting and extension in a transect across the Sevier Desert. In the western Sevier Desert, the House Range and Cricket Mountains faults each have evidence of a single surface-rupturing earthquake in the last 20–30 k.y. and have time-integrated slip and extension rates of <0.1 and ∼0.05 mm yr−1, respectively, since ca. 15–30 ka. These rates are similar to near-negligible modern geodetic extension estimates. Despite relatively low geologic, paleoseismic, and modern extension rates, both faults show evidence of contributing to the long-term growth of topographic relief and the structural development of the region. In the eastern Sevier Desert, the intrabasin Tabernacle, Pavant, and Deseret fault systems show markedly different surface expressions and behavior from the range-bounding normal faults farther west. Pleistocene to Holocene extension rates on faults in the eastern Sevier Desert are >10× higher than those on their western counterparts. Faults here are co-located with Late Pleistocene to Holocene volcanic centers, have events temporally clustered around the timing of Pleistocene volcanism in at least one instance, and have accommodated extension ∼2×–10× above geodetic and long-term geologic rates. We propose a model whereby Pliocene to recent extension in the Sevier Desert is spatially partitioned into an eastern magma-assisted rifting domain, characterized by transient episodes of higher extension rates during volcanism, and a western tectonic-dominated domain, characterized by slower-paced faulting in the Cricket Mountains and House Range and more typical of the “Basin and Range style” that continues westward into Nevada. The Sevier Desert, with near-complete exposure and the opportunity to utilize a range of geophysical instrumentation, provides a globally significant laboratory for understanding the different modes of faulting in regions of continental extension.

Postglacial erosion of bedrock surfaces and deglaciation timing: New insights from the Mont Blanc massif (western Alps)

Abstract Since the Last Glacial Maximum, ∼20 k.y. ago, Alpine glaciers have retreated and thinned. This transition exposed bare bedrock surfaces that could then be eroded by a combination of debuttressing or local frost cracking and weathering. Quantification of the respective contributions of these processes is necessary to understand the links between long-term climate and erosion in mountains. Here, we quantified the erosion histories of postglacial exposed bedrock in glacial valleys. Combining optically stimulated luminescence and terrestrial cosmogenic nuclide (TCN) surface exposure dating, we estimated the erosion rate of bedrock surfaces at time scales from 101 to 104 yr. Bedrock surfaces sampled from the flanks of the Mer de Glace (Mont Blanc massif, European Alps) revealed erosion rates that vary from 3.5 ± 1.2 ⋅ 10−3 mm/yr to 4.3 ± 0.6 mm/yr over ∼500 m of elevation, with a negative correlation between erosion rate and elevation. The observed spatial variation in erosion rates, and their high values, reflect morphometric (elevation and surface slope) and climatic (temperature and snow cover) controls. Furthermore, the derived erosion rates can be used to correct the timing of deglaciation based on TCN data, potentially suggesting very rapid ice thinning during the Gschnitz stadial.

Weathering dynamics of Large Igneous Provinces (LIPs): A case study from the Lesotho Highlands

Basaltic terrains contribute a significant component to the global silicate weathering flux despite their limited areal coverage. A strong correlation between chemical weathering flux and climatic factors such as temperature in volcanically inactive basaltic fields implies that weathering of basalt might have played an important role in maintaining the habitability of the Earth and the balance of the geological carbon cycle. However, the low erosion rate of flood basalt provinces on tectonically quiescent cratons, where the clear climate dependence of weathering rate has been observed, implies a ‘supply-limited’ weathering regime so that weathering fluxes are expected to be controlled by physical erosion rather than climate. This work tests the weathering dynamics of flood basalts by investigating the weathering flux of the Karoo LIP on the Lesotho Highlands where an extremely low rate of denudation has been recorded through terrestrial cosmogenic nuclide dating and sediment flux. Stream chemistry of the rivers from the Lesotho Highlands shows typical characteristics of basalt weathering with Ca and Mg as the dominant cations and HCO3− as the dominant anion. Strong spatial and seasonal variability of solute concentration, possibly linked to a dilution effect, is observed. However, the giant Katse Reservoir of the Lesotho Highlands Water Project (LHWP) effectively averages the spatial and seasonal variability of the solute concentration. The average solution concentration and long-term runoff of the Katse Reservoir give an average atmospheric CO2 consumption rate of 0.26 ± 0.02 × 106 mol/km2/yr (mean ± standard deviation). The chemical weathering flux requires complete alteration of primary silicates and, therefore, a ‘supply-limited’ weathering regime is suggested given the low denudation rate. Nevertheless, the chemical weathering rate of the Lesotho Highlands, as well as other low-eroding basaltic plateaus, is still consistent with the global correlation between temperature and the rate of basalt weathering. A production-limited denudation rate, modulated by temperature, is suggested to reconcile the paradox between the observed temperature dependence of chemical weathering rates and ‘supply-limited’ weathering regimes.

Consistent slow exhumation in a late Cenozoic glaciated landscape: The Presidential and Carter ranges of the White Mountains in New Hampshire, USA

To investigate the ability of the onset of Quaternary glaciations to drive an acceleration in erosion rates we compare short timescale (103–104 yr) erosion rates using the in situ terrestrial cosmogenic nuclide (TCN) 10Be to longer time scale exhumation rates (106–107 yr) using the apatite (UTh)/He method in the Presidential and Carter ranges of the White Mountains in New Hampshire, USA. Our results are generally consistent with but refine previous studies using similar techniques. Resulting (UTh)/He ages and thermal modeling from Mt. Washington in the Presidential Range (n = 4, 91.7 to 117.3 Ma) suggest that exhumation rates from the Late Cretaceous to the present were slow, <0.01–0.02 mm yr−1. Be-10 analyses of high elevation bedrock and talus from the Presidential Range (n = 4; 1.5–1.8 km above mean sea level [amsl]; 18–130 ka) and Carter Range (n = 4; 1.0–1.4 km amsl; 14–22 ka) yield exposure ages that dominantly predate the retreat of the last glacial maximum ice sheet (13.9 ka) and indicate inheritance or inefficient glacial erosion of peaks. A non-linear positive relationship between exposure age and elevation suggests a component of elevation dependent polythermal basal ice conditions. A comparison of 10Be concentrations measured in modern fluvial sand and flood deposits in the region (n = 14; 5–19 × 104 atoms g−1 SiO2) to till (n = 5; ~2–7 × 104 atoms g−1 SiO2), colluvium (n = 2; 2–7 × 104 atoms g−1 SiO2), bedrock, and talus suggests that the modern day sediment budget continues to be dominantly sourced from remobilized glacial sediment rather than postglacial weathering. As such, late Cenozoic glaciation and postglacial processes modified existing topography at lower elevations but did not significantly accelerate exhumation rates in the higher elevation regions of White Mountains of New Hampshire. These results refute the idea of a global acceleration in erosion rates resulting from late Cenozoic climatic cooling and glaciation as a result of frozen-based minimally erosive ice cover in conjunction with low rates of tectonic uplift.

New constraints on the late Quaternary landscape evolution of the eastern Tibetan Plateau from 10Be and 26Al in-situ cosmogenic nuclides

Based on Terrestrial Cosmogenic Nuclide (TCN) constraints from depth profiles of one granitic regolith from Wumingshan and five fluvial terraces from Xianshuihe and Zagunao rivers, we discuss the timing of the last deglaciation, the landscape-scale denudation and fluvial incision rates across the eastern Tibetan Plateau, in relation to previous work. We present a three-dimensional-graph visualization approach and corresponding constraints to better assess the feasibility and applicability of cosmogenic nuclides depth-profile dating. The exposure age (older than 19.4 ka) of the Wumingshan regolith corresponds to the retreat of the palaeo-Daocheng ice cap, which covered the Yidun terrane during the Last Glacial Maximum (LGM). Most basin-wide denudation rate data in the eastern Tibetan Plateau are lower than 130 mm/ka (47%, n = 90), which is consistent with the Wumingshan regolith denudation rate (lower than 52.8 mm/ka), and thus indicate that the landscape-scale denudation has been stabilized after the last deglaciation. Considering the reduction of integrated bulk density due to the accumulation of lower-density loess, we estimate mean exposure ages of Xianshuihe and Zagunao river terraces of 4.0 ± 0.7 ka, 5.9 ± 0.3 ka, 13.4 ± 2.0 ka, and 16.6 ± 1.4 ka. The observed increase in incision rate from 0.39 mm/yr over long timescale (∼600 ka) to 5.88 mm/yr over the last 15 ka at the Xianshuihe river site is probably due to the transition from glacial to interglacial climatic conditions. The fewer abandoned terraces along the Zagunao river after the Heinrich event 1 (H1) indicates that the climate change during the latest glacial-interglacial transition impacted less the landscape evolution in a relatively lower elevation area. Comparison with previously reported fluvial incision rates elsewhere across the eastern margin of the Tibetan Plateau indicates that incision rates are mainly influenced by abrupt climate change or intensified summer monsoon since the early Holocene, but probably controlled by regional tectonic uplift or fluvial headward retreat as the timescale increases. Overall, we propose a synthetic pattern of landscape evolution mainly dominated by long-term tectonic uplift together with fluvial headward erosion, episodically influenced by climatic change throughout the late Quaternary.

Late Quaternary slip rates for faults of the central Walker Lane (Nevada, USA): Spatiotemporal strain release in a strike-slip fault system

Abstract The Walker Lane is a broad shear zone that accommodates a significant portion of North American–Pacific plate relative transform motion through a complex of fault systems and block rotations. Analysis of digital elevation models, constructed from both lidar data and structure-from-motion modeling of unmanned aerial vehicle photography, in conjunction with 10Be and 36Cl cosmogenic and optically stimulated luminescence dating define new Late Pleistocene to Holocene minimum strike-slip rates for the Benton Springs (1.5 ± 0.2 mm/yr), Petrified Springs (0.7 ± 0.1 mm/yr), Gumdrop Hills (0.9 +0.3/−0.2 mm/yr), and Indian Head (0.8 ± 0.1 mm/yr) faults of the central Walker Lane (Nevada, USA). Regional mapping of the fault traces within Quaternary deposits further show that the Indian Head and southern Benton Springs faults have had multiple Holocene ruptures, with inferred coseismic displacements of ∼3 m, while absence of displaced Holocene deposits along the Agai Pah, Gumdrop Hills, northern Benton Springs, and Petrified Springs faults suggest they have not. Combining these observations and comparing them with geodetic estimates of deformation across the central Walker Lane, indicates that at least one-third of the ∼8 mm/yr geodetic deformation budget has been focused across strike-slip faults, accommodated by only two of the five faults discussed here, during the Holocene, and possibly half from all the strike-slip faults during the Late Pleistocene. These results indicate secular variations of slip distribution and irregular recurrence intervals amongst the system of strike-slip faults. This makes the geodetic assessment of fault slip rates and return times of earthquakes on closely spaced strike-slip fault systems challenging. Moreover, it highlights the importance of understanding temporal variations of slip distribution within fault systems when comparing geologic and geodetic rates. Finally, the study provides examples of the importance and value in using observations of soil development in assessing the veracity of surface exposure ages determined with terrestrial cosmogenic nuclide analysis.

The Local Last Glacial Maximum of the southern Scandinavian Ice Sheet front: Cosmogenic nuclide dating of erratics in northern Poland

This paper presents new results of terrestrial cosmogenic nuclide dating of erratics in northern Poland. We report the first exposure ages of erratics located on the pre-Local Last Glacial Maximum and the Local Last Glacial Maximum moraines in Poland. Published radiocarbon ages are calibrated and used as a background indicator of the possible time window for the Local Last Glacial Maximum.The terrestrial cosmogenic nuclide exposure ages presented in this study indicate that: (1) exposure ages of erratics located on the pre-Local Last Glacial Maximum moraines (15.9 ± 1.4 to 101.7 ± 8.5 ka) are not clustered around any specific time interval, (2) the age of the ice sheet retreat from the Local Last Glacial Maximum ice limit in western Poland is 20.7 ± 0.8 ka and the probable duration of the Local Last Glacial Maximum in western and central Poland is between ∼25 ka and ∼21 ka, (3) the age of the ice sheet retreat from the Local Last Glacial Maximum ice limit in eastern Poland is 17.3 ± 0.5 ka and the probable duration of the Local Last Glacial Maximum in eastern Poland is between ∼22 ka and ∼18 ka. Our results show that the Local Last Glacial Maximum ice limit in western and central Poland is probably ∼3 ka older than in eastern Poland.

Evaluating post-glacial bedrock erosion and surface exposure duration by coupling in situ optically stimulated luminescence and &amp;lt;sup&amp;gt;10&amp;lt;/sup&amp;gt;Be dating

Abstract. Assessing the impact of Quaternary glaciation at the Earth's surface implies an understanding of the long-term evolution of alpine landscapes. In particular, it requires simultaneous quantification of the impact of climate variability on past glacier fluctuations and on bedrock erosion. Here we present a new approach for evaluating post-glacial bedrock surface erosion in mountainous environments by combining terrestrial cosmogenic nuclide 10Be (TCN) and optically stimulated luminescence (OSL) surface exposure dating. Using a numerical approach, we show how it is possible to simultaneously invert bedrock OSL signals and 10Be concentrations into quantitative estimates of post-glacial exposure duration and bedrock surface erosion. By exploiting the fact that OSL and TCN data are integrated over different timescales, this approach can be used to estimate how bedrock erosion rates vary spatially and temporally since glacier retreat in an alpine environment.

A homogeneous liquid reference material for monitoring the quality and reproducibility of in situ cosmogenic 10Be and 26Al analyses

Analysis of the in situ produced cosmogenic nuclides 10Be and 26Al has been used for several decades to study Earth surface processes; yet, no reference material has been widely adopted and used by the community to ensure the quality and comparability of sample preparation and isotopic analyses. Such a reference material could facilitate inter-laboratory comparison and serve as a benchmark for quantifying the accuracy, precision, and long-term reproducibility of terrestrial cosmogenic nuclide analyses. Here, we describe a liquid reference material, UVM-A, which is freely available to the community. We created four liters of solution by dissolving CRONUS-A granular reference material [1], and spiking the solution with stable 9Be and 27Al, yielding a solution containing 236.7 ± 0.6 μg g−1 Be and 932.6 ± 1.6 μg g−1 Al (average, 1SD). Initial analyses of UVM-A indicate that the nuclide ratios of the reference material (10Be/9Be 1.43 ± 0.02 × 10−13; 26Al/27Al 4.44 ± 0.05 × 10−13; average, standard error) are similar to those of late-glacial exposure samples, are tightly clustered, and match the nuclide concentrations expected from the CRONUS-A quartz used to prepare the reference material. Use of this reference material will help to quantify uncertainty in cosmogenic exposure ages, burial ages, and erosion rates resulting from sample preparation and isotopic analysis.

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.

Erosive Response of Non-Glaciated Pyrenean Headwater Catchments to the Last Major Climate Transition and Establishing Interglacial Conditions

Non-glaciated mountain headwater catchments feature high-resolution geomorphic archives, which provide important insight into erosive processes and sediment dynamics in mountain ranges. As such the Valle de la Fueva catchments in the southern Pyrenees present high-lying talus remnants, extensive denudation surfaces (pediments), deeply incised tributary ravines, and low-lying fluvial-cut terraces. Based on geomorphic analyses and absolute dating using terrestrial cosmogenic nuclides and optically stimulated luminescence, a (late stage) catchment erosion model for the Valle de la Fueva was elaborated and indicates successive development stages of (i) lasting pedimentation under cold-climate conditions during Marine Isotope Stages 4–2, (ii) rapid fluvial dissection, sediment remobilization and downcutting of ravines in response to the last major climate transition and establishing interglacial conditions, and (iii) late stage fluvial incision after 3–4 ka due to regionally increased flood magnitudes, and/or intensification of agriculture and forest management. Valle de la Fueva headwater catchment analysis indicated that the styles and magnitudes of basin surface processes were directly correlated with the amplitude and nature of paleoclimatic changes, modified by the interplay of environmental parameters. In contrast to large-scale fluvial systems, mountain headwater catchments seemed to be less afflicted with temporal and spatial averaging biases. They are thus useful targets for investigating direct climate change effects, surface process coupling, and non-linear response mechanisms in Quaternary fluvial systems.

Attempts to understand potential deficiencies in chemical procedures for AMS: Cleaning and dissolving quartz for 10Be and 26Al analysis

The purity of the analysed samples (e.g. quartz) with respect to chemical composition and radionuclide contamination is essential for geomorphologic applications using so-called terrestrial cosmogenic nuclides (TCNs). To guarantee this, numerous cleaning and dissolution procedures have been developed. At the DREsden Accelerator Mass Spectrometry (DREAMS) facility, we also work on enhancing the chemical quartz-enrichment methodology from bulk rock and dissolution of quartz. Repeated exposure of the bulk material to acid mixtures (HCl/H2SiF6) at room temperature for cleaning and its monitoring by optical microscopy works for most quartz-rich samples. The quartz dissolution in HF under rather mild conditions (at room temperature on a shaker-table) has the advantage to leave difficult-to-dissolve minerals (e.g., tourmaline, zircon, rutile, sillimanite, kyanite, chromite, corundum), not separated by other physical methods before, as residue. Our comparison with a high-temperature dissolution method (in a microwave) indicates an additional amount of interfering elements, such as in average about 3 mg of Ti, more than 7 mg of Al, and about 22 μg of Be (for 50 g SiO2), is added to the sample, hence showing the superiority of our mild method. This way, we reduce problems for chemistry and AMS, but also ensure better comparability to production rates of cleaner stoichiometric quartz from calibration sites.

Surface Geotechnical Weathering and denudation in the Northeast Margin of Tarim

The research region is located in the northeast margin of Tarim, which is characterized by a typical continental arid desert climate, with a scanty rainfall and large difference in temperature between day and night. The research region is relatively stable in geology and exposed to the long-term weathering and serious erosion of seasonal surface water, which lead to the typical landform of “planation surface” and Yadan. In order to investigate the long-term stability of superficial facilities in this region, a series of methods have been conducted to study the erosion characteristics of soil and rocks in some representative positions, i.e., soil erosion monitoring in different places, modelling analysis of soil erosion, terrestrial cosmogenic nuclides concentration measurement, denudation amount measurement of stone relics, and estimation of regional strata denudation. The results demonstrate that erosion rate is less than 0.3m/104a for sandstone in flat surface, 0.8 m/104a in seasonal gully. The erosion rate of soil is averagely 0.16∼0.19cm/a for palaeo-lacustrine deposition area since late pleistocene epoch, 0.42cm/a for monitoring platform, 3.97cm/a for erosional depression. The denudation rate is estimated to be 0.9 mm/104a for siliceous stone relics, 9.5∼11.6 mm/104a for sandstone and granite relics, and 4.0mm/a for ancient architecture made of soil in research area. By the method of tectonic trend of missing strata, it is estimated that the erosion rate is less than 0.35 m/104a since the tertiary period when a steady stage of uplift and denudation begin in the research region.

Transverse canyon incision and sedimentary basin excavation driven by drainage integration, Aravaipa Creek, AZ, USA

AbstractDrainage reorganization events have the potential to drive incision and erosion at high rates normally attributed to tectonic or climatic forcing. It can be difficult, however, to isolate the signal of transient events driven by drainage integration from longer term tectonic or climatic forcing. We exploit an ideal field setting in Aravaipa Creek Basin of southeastern Arizona, USA, to isolate just such a signal. Aravaipa Creek Basin underwent a period of transient incision that formed Aravaipa Canyon, evacuating a significant volume of sedimentary basin fill and Tertiary bedrock from the previously internally drained basin. We use digital terrain analyses to reconstruct the pre‐incision landscapes of both Aravaipa Creek Basin and the adjacent Lower San Pedro Basin, which we use to quantify the magnitude of incision and erosion since the drainage basins integrated. Terrestrial cosmogenic nuclide burial dates from 10Be and 26Al concentrations in latest stage basin fill in Aravaipa Creek enable us to calculate long‐term incision and erosion rates from 3 Myr to the present. A 10Be concentration–depth profile from the Lower San Pedro Basin confirms that the San Pedro River incised into its high stand deposits prior to 350 000–400 000 years ago. Combining our landscape reconstructions with these age constraints, we determine that the transient rates of incision that created Aravaipa Canyon were 150 m/Myr or more, but that the background rate of erosion since integration is an order of magnitude lower, between 10 and 20 m/Myr. These results support our growing understanding that tectonic and climatic forcings need not apply for all episodes of rapid, transient incision and erosion during landscape evolution. © 2018 John Wiley &amp; Sons, Ltd.

Erratics selection for cosmogenic nuclide exposure dating – an optimization approach

The paper presents a method for the selection of large erratics to be sampled for terrestrial cosmogenic nuclide exposure dating (TCNED) in areas previously covered by Pleistocene ice sheets. Our approach is based on (1) a GIS analysis of an extensive dataset of erratics, (2) field inspection of pre-selected boulders and (3) Schmidt hammer (SH) testing of erratics selected for sampling. An initial database of 491 erratic boulders in NW Poland was filtered using a GIS software, based on their characteristics, digital elevation and surface geology. The secondary data set of pre-selected erratics consisted of 135 boulders – i.e. proper targets for field inspection. Ground-truthing in the field resulted in the final selection of 63 boulders suitable for sampling for TCNED. These erratics are located on moraine plateaux and hills formed during the Saalian glaciation (Marine Isotope Stage 6) as well as Leszno/Brandenburg, Poznań/Frankfurt and Pomeranian Phase ice marginal belts from the Weichselian glaciation (Marine Isotope Stage 2). The GIS desk-based analysis of erratics properties resulted in a 73% reduction of the initial dataset, which demonstrates the added value of this selection technique. The field inspection of pre-selected boulders resulted in a 53% reduction of the number of boulders suitable for TCNED. SH testing of the sampled erratics provided a quantitative proxy of their surface hardness. This allowed the quantification of their weathering degree and identification of erratics potentially affected by postglacial erosion. Our systematic approach to selecting erratics and their SH testing could be a useful tool for other researchers facing the problem of choosing appropriate erratics for TCNED in areas of continental Pleistocene glaciations.

How significant is inheritance when dating rockslide boulders with terrestrial cosmogenic nuclide dating?—a case study of an historic event

Terrestrial cosmogenic nuclide (TCN) exposure dating of boulders is frequently used for rockslide chronology. A well-recognized source of error that cannot be readily quantified is related to inheritance of TCN produced in the rock prior to failure. The effect of inheritance is not constant and will be greatest in the instance of a very recent shallow failure on a high-altitude surface with low event frequencies. We illustrate the effect by measuring 10Be concentrations in six boulders exposed for only 9 years before sampling, on a rock avalanche in Puerto Aysen, Chile. Their apparent exposure ages range from 216 ± 76 to 1755 ± 436 years. The mean apparent exposure age of a statistical cluster of three samples exceeds the real exposure time by 345 ± 36 years (3800%), implying that all sampled rock surfaces experienced pre-failure TCN production. A reconstructed pre-failure topography enables the analysis of possible pre-failure boulder positions and an estimate of the range of possible inherited concentrations along a 2D transect. Despite a maximum failure-mass thickness of 110 m, the boulders seem to have originated from depths shallower than 14 m. Because of the likelihood that large boulders, prioritized for TCN sampling, originate from relatively shallow pre-failure depths owing to surface-near transport with minor turbation, it is necessary to consider potentially inherited TCN concentrations and their effect on the age determination, especially in cases of young rockslides, where the commonly adjusted effects of boulder erosion and snow, ash, or vegetation shielding are negligible in comparison.

10Be-derived sub-Milankovitch chronology of Late Pleistocene alluvial terraces along the piedmont of SW Tian Shan

Using topographic maps and terrestrial cosmogenic nuclide (TCN) 10Be exposure dating methods, we reconstructed a chronology with robust age constraints based on four alluvial terrace abandonment features in two study sites along the SW piedmont of Kepingtage Mountain in SW Tian Shan. Agreement between alluvial terrace abandonment ages and Guliya ice-core δ18O, LR04 stack of benthic δ18O, EPICA Dome C CH4, and solar insolation records suggests that alluvial terraces with 10Be age sequences at 63.7 ± 5.9 ka, 40.9 ± 3.84 ka, 36.2 ± 3.4 ka, and 22.1+5.1–4.7 ka correspond to three wetter climate intervals, possibly triggered by intensified South Asian summer monsoons since the Late Pleistocene. Given that the active thrust Kepingtage Fault experienced sustained uplift during this period, we suggest that the abandonment of alluvial terraces that formed since 22.1+5.1–4.7 ka may be related to sudden tectonic uplift. The occurrence of these four younger well-preserved alluvial terraces may also be related to the fact that the transient climatic incision has been no higher than the temporary tectonic uplift since 22.1+5.1–4.7 ka.

Deglaciation chronology of the Donegal Ice Centre, north‐west Ireland

ABSTRACTDuring the Last Glacial Maximum, Donegal in north‐west Ireland functioned as an independent centre of ice dispersal that separated and fed into the Donegal Bay Ice Lobe (sourced in the Irish Midlands) to the south and the Hebrides/Malin Sea Ice Stream to the north. We report geochronological data that demonstrate marked contrasts in the timing and rate of deglaciation in northern and southern Donegal. In northern Donegal, which occupied an inter‐ice‐stream/lobe location, decoupling from the Hebrides/Malin Sea Ice Stream resulted in formation of a marine embayment along the north coast by ∼22–21 ka, and subsequent slow (∼4 ± 1 m a−1) climatically driven inland retreat of the ice margin to mountain source areas by ∼17 ka. By contrast, in southern Donegal, which lay near the axis of the Donegal Bay Ice Lobe, deglaciation was delayed until ∼18 ka following readvance of ice to a moraine in outer Donegal Bay. The ice margin subsequently underwent net retreat, apparently uninterrupted by readvances, at a net rate of ∼ 18 ± 6 m a−1. A mean terrestrial cosmogenic nuclide age of ∼15.0 ka obtained for samples from the foothills of the Blue Stack Mountains in south‐east Donegal indicates that ice persisted in valley heads and cirques at the beginning of the Lateglacial Interstadial, suggesting that these and nearby mountains supported the last remnants of the Irish Ice Sheet before complete deglaciation of Ireland, and that almost all the shrinkage of the ice sheet in this sector occurred under stadial conditions before the onset of interstadial warming at ∼14.7 ka.

Extent and retreat history of the Barra Fan Ice Stream offshore western Scotland and northern Ireland during the last glaciation

During the Last Glacial Maximum (LGM) the marine-terminating Barra Fan Ice Stream (BFIS), a major conduit of the British Irish Ice Sheet (BIIS), drained much of western Scotland and northwest Ireland with ice streaming onto the continental shelf of the Malin Sea. The extent and retreat history of this ice stream across the shelf, until now, is not well known. In particular, geochronological constraints on the history of this ice stream have thus far been restricted to deep-sea cores or terrestrial cosmogenic nuclide dating onshore, with ages across the shelf absent. To understand the possible external forcing factors acting on this marine terminating ice stream during retreat, improved geochronological constraint on its deglaciation is necessary. Here, we present new geophysical data, marine sediment cores and over forty radiocarbon dates to provide important constraints on maximum extent of the BFIS, as well as the timing and pattern of retreat back across the Malin Shelf. Dated moraines and grounding-zone wedges (GZW) seen in seafloor sub-bottom profiles provide evidence that the BFIS reached the Malin Shelf edge during the LGM and was at its maximum extent around 26.7 ka BP. The presence of two sets of GZWs suggests that the style of retreat was episodic. The new radiocarbon chronology shows that retreat from the shelf edge was underway by 25.9 ka BP, with the majority of the continental shelf ice free by 23.2 ka BP, and that glacimarine conditions were present in the Sea of Hebrides by 20.2 ka BP at the latest. Collectively, these results indicate that the majority of the Malin Shelf was free of grounded ice by ∼21.5–20 ka BP, which is up to 4000 years earlier than previously reconstructed. We attribute this early deglaciation to high relative sea level caused by glacial isostatic depression when the BIIS reached its maximum extent promoting ice shelf and grounding line instability. Two deep troughs, forming reverse bed slopes, aided the continued retreat of the BFIS. This suggests that local ice loading and bed morphology can be significant controls on the destabilisation of a marine-terminating ice stream and can override the influence of ocean and atmospheric temperatures.

Multiple rock-slope failures from Mannen in Romsdal Valley, western Norway, revealed from Quaternary geological mapping and 10Be exposure dating

Oversteepened valley walls in western Norway have high recurrences of Holocene rock-slope failure activity causing significant risk to communities and infrastructure. Deposits from six to nine catastrophic rock-slope failure (CRSF) events are preserved at the base of the Mannen rock-slope instability in the Romsdal Valley, western Norway. The timing of these CRSF events was determined by terrestrial cosmogenic nuclide dating and relative chronology due to mapping Quaternary deposits. The stratigraphical chronology indicates that three of the CRSF events occurred between 12 and 10 ka, during regional deglaciation. Congruent with previous investigations, these events are attributed to the debuttressing effect experienced by steep slopes following deglaciation, during a period of paraglacial relaxation. The remaining three to six CRSF events cluster at 4.9 ± 0.6 ka (based on 10 cosmogenic 10Be samples from boulders). CRSF events during this later period are ascribed to climatic changes at the end of the Holocene thermal optimum, including increased precipitation rates, high air temperatures and the associated degradation of permafrost in rock-slope faces. Geomorphological mapping and sedimentological analyses further permit the contextualisation of these deposits within the overall sequence of post-glacial fjord-valley infilling. In the light of contemporary climate change, the relationship between CRSF frequency, precipitation, air temperature and permafrost degradation may be of interest to others working or operating in comparable settings.