Cosmogenic 10Be Research Articles

Extensive glaciations between MIS 8 and MIS 5 on the eastern side of the Guliya ice cap, West Kunlun Mountains

Glacial chronology is critical to the reconstruction of former glaciers. The West Kunlun Mountains represent one of the most important areas for understanding the response of glacier to climate change in regions dominated by the Westerlies. However, accurate chronologies for Quaternary glaciations in this area are still lack. In this work, we identified two sets of moraines at the eastern side of the Guliya ice cap in the West Kunlun Mountains and constrained their emplacement times using the cosmogenic 10Be exposure dating method. The outer moraine set, within ~6 km in front of the modern glaciers, was dated to 231.8 ± 14.7 ka ago, and the inner moraine set, within ~3.5 km downstream of the modern glaciers, was dated to 113.7 ± 7.0 ka ago. This suggested that two glacier advances could have occurred during the marine isotope stages (MIS) 8–7 and MIS 6–5, respectively, and the glaciers became progressively less extensive between these two glacial periods at the eastern side of the Guliya ice cap. Comparing our dating results with previous studies in the Gurla Mandhata, the Tashkurgan Valley, the Muztag Ata and Kongur Shan, we concluded that regional aridification was the most likely explanation for the progressively less extent of the two glacier advances in the western Tibetan Plateau regions.

Cosmogenic and Geological Evidence for the Occurrence of a Ma-Long Feedback between Uplift and Denudation, Chur Region, Swiss Alps

We used concentrations of in situ cosmogenic 10Be from riverine sediment to quantify the basin-averaged denudation rates and sediment fluxes in the Plessur Basin, Eastern Swiss Alps, which is a tributary stream to the Alpine Rhine, one of the largest streams in Europe. We complement the cosmogenic dataset with the results of morphometric analyses, geomorphic mapping, and sediment fingerprinting techniques. The results reveal that the Plessur Basin is still adjusting to the landscape perturbation caused by the glacial carving during the Last Glacial Maximum c. 20,000 years ago. This adjustment has been most efficient in the downstream part where the bedrock comprises high erodibility North Penninic flysch and Bündnerschist, whereas glacial landforms are still prominently preserved in the upstream region, comprising low erodibility South Penninic and Austroalpine bedrock. This geomorphic observation is supported by the 10Be based denudation rate and sediment provenance analysis, which indicate a much faster sediment production in the flysch and schist lithologies. Interestingly, the reach of fast denudation has experienced the highest exhumation and rock uplift rates. This suggests that lithologic and glacial conditioning have substantially contributed to the local uplift and denudation as some of the driving forces of a positive feedback system.

Cosmogenic 7Be: Particle size distribution and chemical composition of 7Be-carrying aerosols in the atmosphere in Japan

The particle size distributions of airborne aerosols with 7Be were measured using cascade impactors at Dazaifu, a city in western Japan, in 2018 to observe their seasonal variation. Beryllium-7 was found to be adsorbed to aerosols with a particle size of less than 2.1 μm; in general, particles sized 0.43–0.65 μm had the highest 7Be activity concentrations. The activity median aerodynamic diameter (AMAD) of 7Be fluctuated less over the year within the range of 0.40–0.52 μm, which is the size range of particles that can reach human alveoli, and had an annual mean of 0.43 ± 0.034 μm. The activity concentrations of 7Be were significantly lower in summer, which affected 7Be activity concentration for each particle size fraction. The particle size distribution of 7Be-carrying aerosols was also affected by that of the aerosol particles in the atmosphere. Finally, findings suggest that 7Be was mainly adsorbed to sulfate aerosols (particularly ammonium sulfate aerosols).

Slip Rate and Paleoseismology of the Bolokenu‐Aqikekuduk (Dzhungarian) Right‐Lateral Strike‐Slip Fault in the Northern Tian Shan, NW China

AbstractThe ∼1,000 km‐long Bolokenu‐Aqikekuduk fault (Dzhungarian fault, BAF) is one of the major right‐lateral strike‐slip faults that cut across the Tian Shan Range. Although the BAF plays an important role in accommodating crustal shorting of the Tian Shan and cuts through densely populated cities, little is known about its late Quaternary tectonic activity or earthquake history. Here we quantify the late Quaternary dextral strike‐slip rate of the BAF along the Jinghe segment based on interpretation of high‐resolution remote sensing images and structure‐from‐motion (SfM)‐derived DEMs, field observations and the abandonment ages of displaced alluvial fans, which were constrained by optically stimulated luminescence (OSL) and cosmogenic 10Be depth profile dating methods. Using the offset of fan risers and gullies incised into the fan surfaces, we yielded a right‐lateral strike‐slip rate of 3.2 + 1.4/−1.1 mm/a. Compared with the entire crustal shortening rates across the Tian Shan, the BAF accommodates ∼23% of the N‐S crustal shortening rate. Additionally, a paleoseismic trench reveals four events since the late Pleistocene with an average recurrence time of ∼4–6 ka. The last event occurred after ∼6.6 ka, which is slightly larger than the average recurrence time, suggesting that the BAF represents a potential high seismic risk for the densely populated area nearby.

Tectonically and climatically driven mountain-hopping erosion in central Guatemala from detrital <sup>10</sup>Be and river profile analysis

Abstract. The rise of a mountain range affects moisture circulation in the atmosphere and water runoff across the land surface, modifying the distribution of precipitation and drainage patterns in its vicinity. Water routing in turn affects erosion on hillslopes and incision in river channels on surrounding mountain ranges. In central Guatemala, two parallel, closely spaced mountain ranges formed during two consecutive pulses of uplift, the first between 12 and 7 Ma (Sierra de Chuacús–Sierra de las Minas), and the second after 7 Ma (Altos de Cuchumatanes). We explore the climatic and tectonic processes through which the rise of the most recent range drove the slowing of river incision and hillslope erosion over the previously uplifted range. The 40Ar/39Ar dating of perched volcanic deposits documents the sequential rise and incision of these mountain ranges. Terrestrial cosmogenic 10Be in river sediments indicates that currently hillslopes in the older range erode more slowly than in the younger range (20–150 vs. 300 m Myr−1). These differences mimic the current distribution of precipitation, with the younger range intercepting the atmospheric moisture before it reaches the older range. River channel steepness and deformation of paleovalleys in the new range further indicate that the younger range has been rising faster than the older range up to today. We review how atmospheric moisture interception and river long-profile adjustment to the rise of the new range have contributed to the decline of erosion rates over the old range. We also explore the consequences of this decline and of aridification on the topographic evolution of the older range. The older range undergoes a slow topographic decay, dominated by backwearing, by the stacking of slowly migrating erosion waves along the mountain flanks, and by the formation of pediments around its base. The morphology of the old range is therefore transitioning from that of a front range to that of a dry interior range.

Denudation rates of granitic regolith along climatic gradient in Eastern China

In order to assess the impact of the monsoonal climate on denudation, physical erosion, and chemical weathering processes, we measured cosmogenic nuclides 10Be, 26Al and immobile element Zr in five granitic regolith profiles across Northeast to South China. The 10Be and 26Al concentrations are 2.98 × 104–25.20 × 104 atoms g−1 and 2.18 × 105–16.62 × 105 atoms g−1, respectively. Compiling previous depth profile data from Leymon and Beacon Heights, we find that the 26Al/10Be ratios systematically increase or decrease with the depth. Our numerical model shows that muons contributions can produce higher 26Al/10Be ratios (greater than 6.75) in both steady-state and non-steady state scenarios, which can result in underestimated burial age (initial 26Al/10Be ratio) and exaggerated denudation rates (due to non-negligible production from muons) for the application of dual-nuclide in Earth Science. The denudation rates, chemical weathering rates and physical erosion rates derived from cosmogenic nuclides and Zr are 28–45, 9–31 and 6–22 m Ma−1, respectively. Chemical weathering rates, excluding the JLN profile, comprise a large proportion of the denudation rates (greater than 50%). Despite the wide range of mean annual precipitation (MAP) and mean annual temperature (MAT), the denudation rates vary without an apparent gradient from northeast to south. However, the physical erosion and chemical weathering results show good relationships with climate, suggesting the limitation type changed from kinetic-limited in the north, to supply-limited in the south. Chemical weathering indices (chemical index of alteration, CIA and weathering intensity factor, WIF) also indicate that the extent of chemical weathering strengthens from northeast to south and increases with chemical weathering rates, but decreases with physical erosion rates.

Local summer insolation and greenhouse gas forcing drove warming and glacier retreat in New Zealand during the Holocene

Geological climate archives from the Holocene Epoch provide baseline information concerning natural climate variability. Temperate mountain glacier extent is sensitive to summer air temperature, thus geological records of past glacier length changes are a useful proxy for this climatic variable. Here we present a new cosmogenic 10Be chronology of glacier length changes at Dart Glacier in the Southern Alps, New Zealand. Prominent moraines deposited 321 ± 44 yr ago (n = 11) and 7.8 ± 0.3 ka (n = 5) show glaciers during the Little Ice Age were less extensive than during the early Holocene. This pattern of net Holocene glacier retreat is consistent with emerging data from other catchments in New Zealand and across the southern mid-latitudes. Using the physical framework of a transient global climate model simulation, we suggest that cool summers in the early Holocene were promoted by the local summer insolation minimum, together with low atmospheric greenhouse gas concentrations, causing an early Holocene austral glacial maximum. An insolation-driven reduction in seasonality at southern mid-latitudes may reconcile differences between early Holocene temperature reconstructions where climate proxies have different seasonal sensitivities. We suggest that rising greenhouse gas concentrations after 7 ka caused regional-scale glacier retreat and appear to be the dominant driver of multi-millennial summer temperature trends in the southern mid-latitudes during the present interglacial.

Tracing Water Masses and Assessing Boundary Scavenging Intensity With Beryllium Isotopes in the Northern South China Sea

AbstractThe cosmogenic radionuclide 10Be and its stable isotope 9Be can be used as a tracer for water mass mixing, but such applications are rare in both open oceans and marginal seas. Here we report distributions of dissolved 9Be and 10Be concentrations of the seawater in the surface and water columns along a section from shelf to deep basin in the northern South China Sea (SCS). The concentrations of 9Be and 10Be in surface waters range between 8.8–43.6 pmol/kg and 118–576 atoms/g, respectively. The 9Be in the northern SCS is dominantly sourced from river waters, while 10Be has a prominent input from the western Pacific, resulting in the highest 10Be/9Be ratio of 10.9 × 10−8 closest to the Luzon Strait. Along the depth section, the 9Be concentrations decrease southward while 10Be concentrations increase with the maximum value of 1,237 atoms/g occurring at 2,843 m depth of Station A10. The 10Be/9Be ratios are interpreted with consideration of water mixing and boundary scavenging, which reveal evidence of a branch of the intruding Kuroshio Current (>7.7 × 10−8) in subsurface water and a deep cyclonic current (>8.9 × 10−8). A box model yields a sedimentation flux of 10Be around 1,167 atoms/m2/s, that is, 4.6 times of the atmospheric deposition flux, pointing to active boundary scavenging of 10Be. The residence time of 10Be in the deep water is estimated to be longer than 79 years in the northern SCS. The beryllium isotopes as a water mass tracer will find wide applications in basins where the endmembers are sufficiently differentiated.

Deposition and retention of meteoric 10Be in Holocene Taiwan river terraces

The cosmogenic meteoric 10Be that is produced in the atmosphere and mainly scavenged by rainfall is a valuable tool for determining dates and rates of Earth surface processes. A key prerequisite for its applications is the knowledge of the long-term 10Be depositional flux to Earth's surface. Previous efforts on obtaining 10Be depositional fluxes include general atmospheric circulation modelling (GCM), empirical fitting of rainfall 10Be records, and determining the inventory of soil profiles of known age. Here, we derive 10Be depositional fluxes from Holocene river terrace 10Be inventories across the Taiwan mountain belt. We measured 10Be and 9Be concentrations from three terrace profiles, with 14C ages ranging between 3.83 and 9.05 cal. kyr BP and sampling depths of up to 6.2 m.Based on 10Be inventories of our river terraces, the calculated long-term 10Be depositional fluxes vary from 0.32 to 0.49 × 106 at/cm2/yr in the Taiwan orogen. We identify partial Be retention, surface erosion, multi-phase sediment deposition, and incomplete sampling depth in these terraces as potential causes of flux underestimation. When accounting for each of these factors and their uncertainties, the resulting flux estimates of each terrace show an overlapping range of 0.66–0.88 × 106 at/cm2/yr. This newly constrained flux range representative over millennial timescales is thus recommended for Earth surface applications in the fast-eroding Taiwan orogen, especially in regions with similar climatic conditions as the studied terraces. When comparing to flux estimates from other approaches, the flux from the rainfall-based fitting equation exceeds our new terrace-derived flux by more than a factor of two. We consider such high flux to be unlikely for the Taiwan orogen, and suggest that the control of precipitation on the 10Be flux here may be overestimated in rainfall data fitting. The GCM-derived flux overestimates the 10Be depositional flux in this setting to a smaller degree (at least 23%), which is in accordance with findings from previous global data compilation of 10Be fluxes.

In situ cosmogenic 10Be, 26Al, and 21Ne dating in sediments from the Guizhou Plateau, southwest China

In situ cosmogenic 10Be, 26Al, and 21Ne dating in sediments from the Guizhou Plateau, southwest China

Dissolved and Particulate Beryllium Isotopes in the Pearl River Estuary: Their Geochemical Behavior in Estuarine Water and Potential Contributions From Anthropogenic Sources

The ratio of cosmogenic 10Be and its stable isotope 9Be has been used as a proxy of long-term continental weathering fluxes and denudation rates, but transport processes of these isotopes from river water to estuarine water and seawater, as well as interference of potential anthropogenic source of 9Be on natural 10Be/9Be around populated estuaries are not well constrained. Here, we present results of 10Be and 9Be concentrations of dissolved and reactive particulate phase in the Pearl River Estuary (PRE) and its eight major outlets. The concentrations of Cu, Cd, and Pb are also measured, allowing us to assess their contamination levels and anthropogenic source together with 9Be by the geo-accumulation index (Igeo–reac) and enrichment factor (EF). A wide distribution pattern of dissolved 10Be (137–1,194 at/gwater) and 9Be (0.781–8.31 × 10–12 g/gwater) among these outlets is observed. The distribution coefficients (Kd) of both isotopes between sediment and water are in the order of 105, and on average only 5% of 10Be exists as dissolved form. Compared with total meteoric 10Be deposited on the river basin, 23% of the meteoric 10Be is retained while 38% of 10Be finally escape the estuary and is transported into coastal seawater. Despite the high contamination levels of Cu and Cd, the lower Igeo–reac and EF values of 9Be indicate that 9Be is hardly polluted by anthropogenic source. Thus, the 10Be/9Be in the PRE area is mainly associated with natural processes instead of human activities.

Sorption methods in marine radiochemistry

The review presents the general methodology of using sorption methods to solve problems of marine radiochemistry, including sampling, preconcentration and radiochemical preparation and methods for measuring the activity of radionuclides. The possible methodological errors at various stages of sampling and sample concentration are discussed. The most widely used artificial (90Sr, 134Cs, 137Cs, 239Pu, 240Pu), natural (210Pb, 210Po; radium quartet: 223Ra, 224Ra, 226Ra, 228Ra; thorium isotopes, mainly 234Th) and cosmogenic (7Be, 32P, 33P) radiotracers are considered. The sorption of uranium from seawater is not addressed, since its concentration in seawater is usually calculated from the known dependence of uranium concentration on seawater salinity.The bibliography includes 200 references.

The Late Quaternary slip-rate of the Kichera Fault (North Baikal Rift) from morphotectonic, paleoseismological and cosmogenic 10Be analyses

Late Quaternary slip rates characterize the seismic potential of an active fault in terms of estimating the average expected recurrence period for rupturing episodes. Along the Baikal Rift faults the slip rate distributions are poorly understood. This study provides morphotectonic and paleoseismological analyses of the Kichera Fault that stretches within the North Baikal Rift over a length of 100 km. Exposure ages of key displaced terraces of the Neruchanda River from in-situ produced cosmogenic 10Be depth profiles show that the highest terrace T5 is older than 315 ka, intermediate terrace T3 was abandoned 100.0 ± 30 ka ago and the lowest terrace T1 is as old as 48.3 ± 19.9 ka. These exposure ages, coupled with terrace elevation from the actual Neruchanda River bed, allow us to estimate the incision rates for the corresponding time periods which are 0.1 (maximum), 0.16 ± 0.05 and 0.19 ± 0.08 mm yr−1, respectively. Using the cumulated amplitudes of the terrace displacement, we estimate the vertical slip rates along the Kichera Fault are 0.42 ± 0.13 and 0.19 ± 0.08 mm yr−1 over the last ~100 ka and ~48.3 ka, respectively. Paleoseismological studies including morphomertic analyses of the tectonic scarp and trenching across the fault allowed estimating the age (5.4 ka) and minimum magnitude (6.8) of the last Kichera Fault rupture. Using the estimates of short- and long-term slip rates, we propose mean recurrence intervals of 5.3 and 2.4 ka, respectively. The last interval is shorter than the time since the last earthquake, indicating that the future event could be of greater magnitude to maintain the long-term average slip rate.

The role of glacial dynamics in the development of ice divides and the Horseshoe Intersection Zone of the northeastern Labrador Sector of the Laurentide Ice Sheet

The Labrador Sector of the Laurentide Ice Sheet is characterized by a complex network of ice divides and an extensive landform record outlining two broad and opposing ice flows that are separated by a narrow Horseshoe Intersection Zone (HIZ). This geomorphic system gave rise to contrasting reconstructions, which reflect uncertainties on the temporal evolution of ice divides, the nature and age of the main landform systems and the overall pattern of ice retreat during the last deglaciation. Here, we address these issues through systematic mapping of glacial landforms and ice-flow indicators in a large area of northeastern Quebec and Labrador. The application of cosmogenic (10Be and 26Al) dating to esker-fed glaciomarine deltas and different rock surfaces brings new constraints on the chronological framework and insights on the subglacial thermal regime. Our results outline four main landform assemblages, two of which relating to a major system of opposite ice flows that delineate the position of the eastern Ancestral Labrador ice divide. Our reconstruction shows that the HIZ occurs several tens of km to the west of this divide and that these two features are genetically distinct. The HIZ relates to the late-glacial development of ice streams showing a massive convergent ice flow into Ungava Bay. The upstream extent of this ice flow system shows a strong coupling with the outline of Ungava Bay drainage divide, suggesting that the configuration of the HIZ could be the expression of a topographic control on the late-glacial ice sheet dynamics. Landforms also indicate a significant shift in the deglaciation mode, which evolved from an overall warm-based to an areally-confined cold-based ice retreat near the center of the former ice mass – a change in dynamics that also played a role in the configuration of the HIZ. Overall, areas indicative of cold-based ice conditions are limited to highly elevated terrains, thus ruling out models arguing for an extensive cold-based ice cover over Ungava Bay throughout the last deglaciation.

Cosmogenic in situ 14C-10Be reveals abrupt Late Holocene soil loss in the Andean Altiplano

Soil sustainability is reflected in a long-term balance between soil production and erosion for a given climate and geology. Here we evaluate soil sustainability in the Andean Altiplano where accelerated erosion has been linked to wetter climate from 4.5 ka and the rise of Neolithic agropastoralism in the millennium that followed. We measure in situ cosmogenic 14C directly on cultivated hilltops to quantify late Holocene soil loss, which we compare with background soil production rates determined from cosmogenic 26Al and 10Be. Our Monte Carlo-based inversion method identifies two scenarios to account for our data: an increase in erosion rate by 1–2 orders of magnitude between ~2.6 and 1.1 ka, or a discrete event stripping ~1–2 m of soil between ~1.9 and 1.1 ka. Coupled environmental and cultural factors in the Late Holocene signaled the onset of the pervasive human imprint in the Andean Altiplano seen today.

Hilltop Curvature Increases With the Square Root of Erosion Rate

AbstractThe shape of soil‐mantled hillslopes is typically attributed to erosion rate and the transport efficiency of the various processes that contribute to soil creep. While climate is generally hypothesized to have an important influence on soil creep rates, a lack of uniformity in the measurement of transport efficiency has been an obstacle to evaluating the controls on this important landscape parameter. We addressed this problem by compiling a data set in which the transport efficiency has been calculated using a single method, the analysis of hilltop curvatures using 1‐m LiDAR data, and the erosion rates have also been determined via a single method, in‐situ cosmogenic 10Be concentrations. Moreover, to control for lithology, we chose sites that are only underlain by resistant bedrock. The sites span a range of erosion rates (6–922 mm/kyr), mean annual precipitation (39–320 cm/yr), and aridity index (0.08–1.38). Surprisingly, we find that hilltop curvature varies with the square root of erosion rate, whereas previous studies predict a linear relationship. In addition, we find that the inferred transport coefficient also varies with the square root of erosion rate but is insensitive to climate. We explore various mechanisms that might link the transport coefficient to the erosion rate and conclude that present theory regarding soil‐mantled hillslopes is unable to explain our results and is, therefore, incomplete. Finally, we tentatively suggest that processes occurring in the bedrock (e.g., fracture generation) may play a role in the shape of hillslope profiles at our sites.

Late Quaternary variations in paleoerosion rates in the northern Qilian Shan revealed by 10Be in fluvial terraces

Understanding the pace of erosion through space and time may provide insight into how tectonic and climatic processes conspire to drive landscape evolution. However, determining whether erosion rates in a given locality have varied with past climate change remains challenging, and the mechanisms invoked to explain variations in past erosion rates are not well understood. In this study, we evaluate past rates of erosion during the late Quaternary along the northern Qilian Shan, the range bounding the northeastern margin of the Tibetan Plateau. We measured the concentrations of cosmogenic 10Be from fluvial deposits atop 5 terraces along the Hongshuiba River and average 10Be concentrations at the time of terrace formation. We also determined terrace ages by combining 5 10Be depth profiles and 15 OSL samples of overlying loess. Our results confirm previous dating of this terrace sequence using cosmogenic exposure ages of boulders and depth profiles in terrace gravels and indicate that terrace treads span glacial-interglacial cycles over the past ~200 ka. We exploit this exceptionally dense sampling of 10Be to evaluate differences in cosmogenic 10Be concentrations at the time of terrace formation along the Hongshuiba River. Analysis of 48 individual determinations of “inherited” 10Be concentrations reveal that fluvial sediment in terraces abandoned during the last glacial and penultimate glacial periods are 3–4 times greater than concentrations in sediment atop terraces formed during interglacial times. Paleoerosion rates within the Hongshuiba watershed appear to have been ~200 m/Myr during glacial episodes but increased to ~500–700 m/Myr during interglacial times, similar to recent erosion rates inferred from 10Be in modern sediment. These results point to likely changes in the mean residence time of sediment with the watershed that fluctuate in association with 100-ka glacial-interglacial cycles. We suggest that reductions in erosion rates associated with glacial conditions may have been driven by changes in catchment hydrology during cold and dry periods.

Erosion around a large-scale topographic high in a semi-arid sedimentary basin: Interactions between fluvial erosion, aeolian erosion and aeolian transport

Large-scale aeolian erosion features such as pans usually develop upon a relatively homogeneous erodible sedimentary rock substratum and show simple concave morphologies. However, some erosional depressions in semi-arid to arid settings develop in more complex geological contexts, associating sedimentary basins and outcrops of non-erodible basement. In semi-arid basins, the geomorphic evolution is controlled by interplay between the processes of fluvial erosion and deposition (especially of the finer sediment fraction in ephemeral lakes) and the processes of aeolian erosion and deflation. The occurrence of non-erodible basement outcrops within a region of overall deflation complicates drainage systems and wind-flow patterns. Both are liable to evolve rapidly through time and their interactions remain poorly understood. In this study, we use a detailed geomorphological analysis, including mapping of the aeolian sand-motion direction within the 16,300 km2 Tsagaan Els depression (Gobi Desert, Mongolia) as well as in situ-produced cosmogenic 10Be and 26Al based quantitative constraints, to propose a conceptual model for the morphologic evolution of geologically complex semi-arid basins. Based on theoretical studies of wind-flow patterns around obstacles, we demonstrate that the occurrence of erosion-resistant basement highs inside the basin induces the localization of preferential deflation zones and aeolian and fluvial carving of large-scale relief. The enhanced deflation zones form sub-depocenters controlling local drainage systems that mobilize clastic material and transport it to the deflation zone. These river systems are frequently captured by neighboring streams, which creates unstable conditions of erosion and deposition. Such rapid modifications of the topography through time affect the hydrogeologic structure of the basin and may impact potential sediment-hosted mineral deposits.

Younger Dryas ice margin retreat in Greenland: new evidence from southwestern Greenland

Abstract. To date the final stage in deglaciation of the Greenland shelf, when a contiguous ice sheet margin on the inner shelf transitioned to outlet glaciers in troughs with intervening ice-free areas, we generated cosmogenic 10Be dates from bedrock knobs on six outlying islands along a stretch of 300 km of the southwestern Greenland coast. Despite 10Be inheritance influencing some dates, the ages generally support a Greenland Ice Sheet (GrIS) margin that retreated off the inner shelf during the middle Younger Dryas (YD) period. Published 10Be- and 14C-dated records show that this history of the GrIS margin is seen in other parts of Greenland but with large variations in the extent and speed of retreat, sometimes even between neighbouring areas. Areas with a chronology extending into the Allerød period show no marked ice margin change at the Allerød–YD transition except in northernmost Greenland. In contrast, landforms on the shelf (moraines and grounding zone wedges) have been suggested to indicate YD readvances or long-lasting ice margin stillstands on the middle shelf. However, these features have been dated primarily by correlation with cold periods in the ice core temperature records. Ice margin retreat during the middle and late YD is explained by advection of warm subsurface water at the ice margin and by increased seasonality. Our results therefore point to the complexity of the climate–ice margin relation and to the urgent need for direct dating of the early deglaciation history of Greenland.

Unravelling the Pleistocene glacial history of the Pamir mountains, Central Asia

Several hundred thousand year old moraines preserved in the semi-arid environment of High Mountain Asia attest to Middle Pleistocene glaciations, but the regional correlation of glacial stages and the spatial extent of the glacial advances remain poorly constrained. We examined glacial landforms and Quaternary sediments in the Bartang valley, northwestern Pamir, a region with no previous quantitative glacial chronology. Using cosmogenic 10Be exposure ages, we dated glacially polished bedrock, moraines, and mass wasting deposits. Our data show that the northwestern Pamir was heavily glaciated in the Middle Pleistocene (≥220 ky) with large valley glaciers occupying some of the major valleys in the western Pamir. During the penultimate glacial cycle (191–130 ky) these valleys may have been largely ice free. Catastrophic mega debris flows with volumes ≥0.05 km3 occurred after the ice retreat and reflect paraglacial destabilization of glacial sediments. The age of the best-dated mega debris flow (81 ± 4 ky) is similar to moraine ages ∼70–80 ky documented throughout the Pamir, demonstrating that remobilized sediments may provide valuable age constraints on glacial histories. In order to facilitate regional comparison of glacial chronologies, we developed a Gaussian separation algorithm, which determines a moraine age from a distribution of boulder exposure ages based on the assumption that post-depositional processes prevail over inheritance, and that the oldest boulder ages best represent the timing of moraine formation. We compiled moraine boulder exposure ages from the Pamir and adjacent regions and provide a summary of Middle and early Late Pleistocene glacial cycles of western High-Mountain Asia.