Erosion rates of alpine bedrock summit surfaces deduced from in situ Be and Al

We have measured the concentration of in situ produced cosmogenic ¹⁰Be and ²⁶Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My⁻¹. Individual measurements vary between 2 and 19 m My⁻¹. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates ( 50 m My⁻¹). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing.

We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.     

Erosion in northwest Tibet from in‐situ‐produced cosmogenic 10Be and 26Al in bedrock

Concentrations of in‐situ‐produced cosmogenic nuclides ¹⁰Be and ²⁶Al in quartz were measured by accelerator mass spectrometry for bedrock basalts and sandstones located in northwest Tibet. The effective exposure ages range between 23 and 134 ka (¹⁰Be) and erosion rates between 4·0 and 24 mm ka^?1. The erosion rates are significantly higher than those in similarly arid Antarctica and Australia, ranging between 0·1 and 1 mm ka^?1, suggesting that precipitation is not the major control of erosion of landforms. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role in controlling long‐term erosion rates. The obtained erosion rates are, however, significantly lower than the denudation rate of 3000–6000 mm ka^?1 beginning at c. 5‐3 Ma in the nearby Godwin Austen (K2) determined by apatite fission‐track thermochronology. It appears that the difference in erosion rates within different time intervals is indicative of increased tectonic activity at c. 5–3 Ma in northwest Tibet. We explain the low erosion rates determined in this study as reflecting reduced tectonic activity in the last million years. A model of localized thinning of the mantle beneath northwest Tibet may account for the sudden increased tectonic activity at c. 5–3 Ma and the later decrease. Copyright © 2006 John Wiley & Sons, Ltd.     

Improved discrimination of subglacial and periglacial erosion using 10Be concentration measurements in subglacial and supraglacial sediment load of the Bossons glacier (Mont Blanc massif,France)

Deciphering the complex interplays between climate, uplift and erosion is not straightforward and estimating present‐day erosion rates can provide useful insights. Glaciers are thought to be powerful erosional agents, but most published ‘glacial’ erosion rates combine periglacial, subglacial and proglacial erosion processes. Within a glaciated catchment, sediments found in subglacial streams originate either from glacial erosion of substratum or from the rock walls above the glacier that contribute to the supraglacial load. Terrestrial cosmogenic nuclides (TCN) are produced by interactions between cosmic ray particles and element targets at the surface of the Earth, but their concentration becomes negligible under 15 m of ice. Measuring TCN concentrations in quartz sand sampled in subglacial streams and in supraglacial channels is statistically compliant with stochastic processes (e.g. rockfalls) and may be used to discriminate subglacial and periglacial erosion. Results for two subglacial streams of the Bossons glacier (Mont Blanc massif, France) show that the proportion of sediments originating from glacially eroded bedrock is not constant: it varies from 50% to 90% (n = 6). The difference between the two streams is probably linked to the presence or absence of supraglacial channels and sinkholes, which are common features of alpine glaciers. Therefore, most of the published mean catchment glacial erosion rates should not be directly interpreted as subglacial erosion rates. In the case of catchments with efficient periglacial erosion and particularly rockfalls, the proportion of sediments in the subglacial stream originating from the supraglacial load could be considerable and the subglacial erosion rate overestimated. Here, we estimate warm‐based subglacial and periglacial erosion rates to be of the same order of magnitude: 0.39 ± 0.33 and 0.29 ± 0.17 mm a^?1, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Limited modification of mid-latitude landscapes by ice sheets: The case of northeast Scotland

The paper uses a case study in Scotland to examine the amount and processes of landscape modification by Quaternary ice sheets. There is an inverse correlation between the distribution of landforms of glacial erosion and pre-glacial landscape remnants in northeast Scotland. The implication is that in places ice sheets can preserve a pre-glacial landscape unscathed, while elsewhere they remove the pre-glacial weathered rock. The location of glacial protection or erosion is strongly influenced by the topography and its influence on former ice sheet flow and basal thermal regime. The classic glacially eroded landscape of areal scouring can be produced by the removal of only 10–50 m of weathered rock. Furthermore rock basins, often regarded as the hallmark of glacial erosion, may be directly inherited from the pre-glacial pattern of deep weathering.     

Calculating basal temperatures in ice sheets: an Excel spreadsheet method

The flow of ice sheets and their geomorphological impact is greatly influenced by their basal thermal regime. Calculations of basal temperatures in ice sheets are therefore fundamental in evaluating glacier dynamics and in determining the spatial distribution of zones of erosion and deposition beneath ice masses. Calculations of basal temperatures are not frequently attempted, however, primarily because of the techniques required to solve the heat conduction equation between the ice surface and the base. This paper describes a new Excel spreadsheet method of solving this equation that can readily be applied to both former and contemporary ice sheets. The application of the spreadsheet is illustrated with two examples. The first provides a calculation of basal thermal regime beneath the north eastern part of the Scottish ice sheet during the last glacial maximum; the second shows how basal ice temperatures can be calculated beneath the modern Antarctic ice sheet. Copyright © 2002 John Wiley & Sons, Ltd.     

Geomorphology under ice streams: Moving from form to process

Ice streams are integral components of an ice sheet's mass balance and directly impact on sea level. Their flow is governed by processes at the ice‐bed interface which create landforms that, in turn, modulate ice stream dynamics through their influence on bed topography and basal shear stresses. Thus, ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics. This paper reviews developments in our understanding of ice stream geomorphology from a historical perspective, with a focus on the extent to which studies of modern and palaeo‐ice streams have converged to take us from a position of near‐complete ignorance to a detailed understanding of their bed morphology. During the 1970s and 1980s, our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in Antarctica. Very few palaeo‐ice streams had been identified with any confidence. During the 1990s, however, glacial geomorphologists began to recognise their distinctive geomorphology, which included distinct patterns of highly elongated mega‐scale glacial lineations, ice stream shear margin moraines, and major sedimentary depocentres. However, studying relict features could say little about the time‐scales over which this geomorphology evolved and under what glaciological conditions. This began to be addressed in the early 2000s, through continued efforts to scrutinise modern ice stream beds at higher resolution, but our current understanding of how landforms relate to processes remains subject to large uncertainties, particularly in relation to the mechanisms and time‐scales of sediment erosion, transport and deposition, and how these lead to the growth and decay of subglacial bedforms. This represents the next key challenge and will require even closer cooperation between glaciology, glacial geomorphology, sedimentology, and numerical modelling, together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Subglacial abrasion rates at Goldbergkees,Hohe Tauern,Austria, determined from cosmogenic 10Be and 36Cl concentrations

We report concentrations of cosmogenic ¹⁰Be and ³⁶Cl used to determine erosion depths in the recently deglaciated bedrock at Goldbergkees in the Eastern Alps. The glacier covered the sampling sites during the Little Ice Age (LIA) until c. 1940. The youngest ages calculated from these concentrations match the known exposure time after the post‐LIA exposure of <100 years. The apparent age (no cover, no erosion) of most samples, however, is significantly older. We show that the measured nuclide concentrations represent subglacial erosion depths, rather than exposure times. In particular, erosion depths calculated using ¹⁰Be and ³⁶Cl concentrations of individual samples match well, whereas apparent ³⁶Cl ages are consistently older than ¹⁰Be ages. The bedrock at the ‘youngest’ surfaces was deeply eroded (≥ 297 cm) by the Goldbergkees during the late Holocene. In contrast, bedrock at the margin of the LIA ice extent was eroded ≤35 cm. These values convert to subglacial erosion rates on the order of 0.1 mm/a to >5 mm/a. While modeled erosion rates depend on the duration of glacial cover and erosion intrinsic to the different exposure scenarios used for calculation (700–3300 years), modeled total erosion depths are insensitive (5–20% change). Analysis of erosion depths on the transverse valley profile shows a general trend of greatest erosion part way up the valley side and less erosion under thin ice at the lateral margin. A second profile along the valley axis indicates depth of erosion is greatest where the ice abuts the foot of the investigated bedrock riegel and at its lee side just beyond the crest. Copyright © 2016 John Wiley & Sons, Ltd.     

Ice cap erosion patterns from bedrock 10Be and 26Al,southeastern Tibetan Plateau

Quantifying glacial erosion contributes to our understanding of landscape evolution and topographic relief production in high altitude and high latitude areas. Combining in situ ¹⁰Be and ²⁶Al analysis of bedrock, boulder, and river sand samples, geomorphological mapping, and field investigations, we examine glacial erosion patterns of former ice caps in the Shaluli Shan of the southeastern Tibetan Plateau. The general landform pattern shows a zonal pattern of landscape modification produced by ice caps of up to 4000 km² during pre-LGM (Last Glacial Maximum) glaciations, while the dating results and landforms on the plateau surface imply that the LGM ice cap further modified the scoured terrain into different zones. Modeled glacial erosion depth of 0–0.38 m per 100 ka bedrock sample located close to the western margin of the LGM ice cap, indicates limited erosion prior to LGM and Late Glacial moraine deposition. A strong erosion zone exists proximal to the LGM ice cap marginal zone, indicated by modeled glacial erosion depth >2.23 m per 100 ka from bedrock samples. Modeled glacial erosion depths of 0–1.77 m per 100 ka from samples collected along the edge of a central upland, confirm the presence of a zone of intermediate erosion in-between the central upland and the strong erosion zone. Significant nuclide inheritance in river sand samples from basins on the scoured plateau surface also indicate restricted glacial erosion during the last glaciation. Our study, for the first time, shows clear evidence for preservation of glacial landforms formed during previous glaciations under non-erosive ice on the Tibetan Plateau. As patterns of glacial erosion intensity are largely driven by the basal thermal regime, our results confirm earlier inferences from geomorphology for a concentric basal thermal pattern for the Haizishan ice cap during the LGM. © 2018 John Wiley & Sons, Ltd.     

26Al/10Be ratios reveal the source of river sediments in the Kimberley,NW Australia

We use cosmogenic ¹⁰Be and ²⁶Al in both bedrock and fluvial sediments to investigate controls on erosion rates and sediment supply to river basins at the regional scale in the Kimberley, NW Australia. The area is characterised by lithologically controlled morphologies such as cuestas, isolated mesas and extensive plateaus made of slightly dipping, extensively jointed sandstones. All sampled bedrock surfaces at plateau tops, ridgelines, and in the broader floodplain of major rivers over the region show similar slow lowering rates between 0.17 and 4.88 m.Myr^-1, with a mean value of 1.0 ± 0.6 m.Myr^-1 (n=15), whilst two bedrock samples collected directly within river-beds record rates that are one to two orders of magnitude higher (14.4 ± 1.5 and 20.9 ± 2.5 m.Myr^-1, respectively). Bedrock ²⁶Al/¹⁰Be ratios are all compatible with simple, continuous sub-aerial exposure histories. Modern river sediment yield lower ¹⁰Be and ²⁶Al concentrations, apparent ¹⁰Be basin-wide denudation rates ranging between 1.8 and 7.7 m.Myr^-1, with a median value of 2.6 m.Myr^-1, more than double the magnitude of bedrock erosion rates. ²⁶Al/¹⁰Be ratios of the sediment samples are lower than those obtained for bedrock samples. We propose that these depleted ²⁶Al/¹⁰Be ratios can largely be explained by the supply of sediment to river basins from the slab fragmentation and chemical weathering of channel gorge walls and plateau escarpments that result in diluting the cosmogenic nuclide concentration in river sediments measured at the basin outlets. The results of a mass-balance model suggest that ~60–90% of river sediment in the Kimberley results from the breakdown and chemical weathering of retreating vertical sandstone rock-walls in contrast to sediment generated by bedrock weathering and erosion on the plateau tops. This study emphasises the value of analysing two or more isotopes in basin-scale studies using cosmogenic nuclides, especially in slowly eroding post-orogenic settings. © 2019 John Wiley & Sons, Ltd.     

Estimation of episodic exfoliation rates of rock sheets on a granite dome in Korea from cosmogenic nuclide analysis

We have measured concentrations of cosmogenic ¹⁰Be and ²⁶Al produced in situ at bare bedrock surfaces of successive sheets developing on a granite dome in Korea and calculated the exfoliation rate of sheeting joints. The exfoliation rate was obtained using a simple model in which the sheeting joints experience intermittent denudation, i.e. peeling off along the bedrock face. We find that the average exfoliation (erosion) rate of the episodic peeling‐off process is 5·6 cm/ka^?1. The analysis is useful for understanding the evolution of granite sheeting structures on this dome in Korea. Copyright © 2006 John Wiley & Sons, Ltd.     

In situ cosmogenic ~(10)Be dating of the Quaternary glaciations in the southern Shaluli Mountain on the Southeastern Tibetan Plateau

It is generally considered that four-times ice age happened during the Quaternary epoch on the Tibetan Plateau. However, the research on the chronology of the four-times ice age is far from enough. The Shaluli Mountain on the Southeastern Tibetan Plateau is an ideal place for plaeo-glacier study, because there are abundant Quaternary glacial remains there. This paper discusses the ages of the Quaternary glaciations, based on the exposure dating of roche moutonnée, moraines and gla- cial erosion surfaces using in situ cosmogenic isotopes 10Be. It is found that the exposure age of the roche moutonnée at Tuershan is 15 ka, corresponding to Stage 2 of the deep-sea oxygen isotope, suggesting that the roche moutonnée at Tuershan is formed in the last glacial maximum. The expo- sure age of glacial erosion surface at Laolinkou is 130―160 ka, corresponding to Stage 6 of the deep-sea oxygen isotope. The oldest end moraine at Kuzhaori may form at 421―766 kaBP, corre- sponding to Stages 12―18 of the deep-sea oxygen isotope. In accordance with the climate charac- teristic of stages 12,14,16 and 18 reflected by the deep-sea oxygen isotope, polar ice cores and loess sequence, the oldest end moraine at Kuzhaori may form at stage 12 or stage 16, the latter is more possible.     

Using in situ cosmogenic 10Be to identify the source of sediment leaving Greenland

We use the concentration of in situ ¹⁰Be in quartz isolated from fluvial and morainal sand to trace sediment sources and to determine the relative contribution of glacerized and deglaciated terrain to Greenland's sediment budget. We sampled along the western, eastern, and southern margins of the Greenland Ice Sheet, and collected sediment sourced from glacerized (n = 19) and non‐glacerized terrain (n = 10), from channels where sediment from glacerized and non‐glacerized terrain is mixed (n = 28), from Holocene glacial‐fluvial terraces (n = 4), and from one sand dune. In situ ¹⁰Be concentrations in sediment range from 1600 to 34 000 atoms g^‐1. The concentration of in situ ¹⁰Be in sediment sourced from non‐glacerized terrain is significantly higher than in sediment sourced from glacerized areas, in mixed channel sediment, and in terrace sediment that was deposited during the Holocene. To constrain the timing of landscape exposure for the deglaciated portion of the Narsarsuaq field area in southern Greenland, we measured in situ ¹⁰Be concentration in bedrock (n = 5) and boulder (n = 6) samples. Paired bedrock and boulder ages are indistinguishable at 1σ uncertainty and indicate rapid exposure of the upland slopes at ~10.5 ka. The isotope concentration in sediment sourced from non‐glacerized terrain is higher than in sediment sourced from glacerized terrain because the non‐glacerized landscape has been exposed to cosmic radiation since early Holocene deglaciation. Sediment from glacerized areas contains a low, but measurable concentration of ¹⁰Be that probably accumulated at depth during a prolonged period of exposure, probably before the establishment of the Greenland Ice Sheet. The concentration of ¹⁰Be in mixed fluvial sediment and in terrace sediment is low, and similar to the concentration in sediment from glacerized areas, which indicates that the Greenland Ice Sheet is the dominant source of sediment moving through the landscape outside the glacial margin in the areas we sampled. Copyright © 2014 John Wiley & Sons, Ltd.     

Glacial geomorphology and ice ages in Tibet and the surrounding mountains

Abstract   Since 1973 new data have been obtained on the maximum extent of glaciation in High Asia. Evidence for an ice sheet covering Tibet during the last glacial period means a radical rethinking about glaciation in the northern hemisphere. The ice sheet's subtropical latitude, vast size (2.4 million km²) and high elevation (6000 m a.s.l.) are supposed to have resulted in a substantial, albedo-induced cooling of the Earth's atmosphere and the disruption of summer monsoon circulation. Moraines were found to reach down to 460 m a.s.l. on the southern flank of the Himalayas and to 2300 m a.s.l. on the northern slope of the Tibetan Plateau, in the Qilian Shan region. On the northern slopes of the Karakoram, Aghil and Kuen-Lun Mountains, moraines occur as far down as 1900 m a.s.l. In southern Tibet, radiographic analyses of erratics suggest a former ice thickness of at least 1200 m. Glacial polish and roches moutonnées in the Himalayas and Karakoram suggest former glaciers as thick as 1200–2700 m. On the basis of this evidence, a 1100–1600-m lower equilibrium line has been reconstructed, resulting in an ice sheet of 2.4 million km², covering almost all of Tibet. Radiometric ages, obtained by different methods, classify this glaciation as isotope stage 3–2 in age (Würmian, the last glacial period, ca  60 000–18 000 years ago).     

Study of the erosion rates in the upper Maracujá Basin (Quadrilátero Ferrífero/MG,Brazil) by the in situ produced cosmogenic 10Be method

The present work quantifies the erosive processes in the two main substrates (schists–phyllites and granites–gneisses) of the upper Maracujá Basin in the Quadrilátero Ferrífero/MG, Brazil, a region of semi‐humid tropical climate. Two measuring methods of concentration were used: (i) in situ produced ¹⁰Be in quartz veins (surface erosion rates) and (ii) ¹⁰Be in fluvial sediments (basin erosion rates). The results confirm that (i) erosion tends to be more aggressive close to the headwaters than in the lower parts of the basin and (ii) the region is now affected by dissection. Copyright © 2006 John Wiley & Sons, Ltd.     

Spatial and temporal replicability of meteoric and in situ 10Be concentrations in fluvial sediment

Cosmogenic isotopes, short‐lived radionuclides, elemental concentrations and thermochronometric indicators are measured in river sand to quantify erosion rates and trace sediment sources, and/or infer erosional processes. Interpretations of detrital sediment analyses are often based on the rarely tested assumption of time‐invariant tracer concentration. A better understanding of when and where this assumption breaks down and what sampling strategies minimize temporal and small‐scale spatial variance will improve science done using detrital river sediment. Here, we present new and previously published spatial and temporal replicates measured for in situ and meteoric ¹⁰Be (¹⁰Be_i and ¹⁰Be_m, respectively). Our new data include 113 replicate pairs, taken from agricultural and/or tectonically active watersheds in China months to millennia apart and spatial replicates taken up to 2 km apart on the same day. The mean percentage difference is 10% (?122% to 150%) for both systems considered together; the mode is close to 0% for both systems; and 36% of pairs of samples replicate within our analytical accuracy at 2σ . We find that ¹⁰Be_i replicates better than ¹⁰Be_m (p < 0.01). ¹⁰Be_i replicability is worse in steeper basins, suggesting that stochastic processes (i.e. landslides) affect reproducibility. ¹⁰Be_m replicability is worse in larger basins, suggesting non‐conservative behavior of ¹⁰Be_m as sediment moves downstream. Our results are consistent with the few previously published replicate studies. Considering all replicate data in a wide range of landscapes, in areas with deep erosional processes, replicability is poor; in other areas, replicability is good. This suggests that, in steep, tectonically active, and/or agricultural landscapes, individual detrital sediment measurements do not represent upstream rates as well as larger populations of samples. To ensure that measurements are representative of the upstream watershed, our data suggest that samples be amalgamated either over time or from several places close by in the same channel. Copyright © 2017 John Wiley & Sons, Ltd.     

Sidewall erosion: Insights from in situ-produced 10Be concentrations measured on supraglacial clasts (Mont Blanc massif,France)

Sidewall erosion because of rockfalls is one of the most efficient erosional processes in the highest parts of mountain ranges; it is therefore important to quantify sidewall erosion to understand the long-term evolution of mountainous topography. In this study, we analyse how the ¹⁰Be concentration of supraglacial debris can be used to quantify sidewall erosion in a glaciated catchment. We first analyse, in a glaciated catchment, the cascade of processes that move a rock from a rockwall to a supraglacial location and propose a quantitative estimate of the number of rockfalls statistically mixed in a supraglacial sand sample. This model incorporates the size of the rockwall, a power law distribution of the size of the rockfalls and the mean glacial transport velocity. In the case of the Bossons glacier catchment (Mont Blanc massif), the ¹⁰Be concentrations obtained for supraglacial samples vary from 1.97 ± 0.24 to 23.82 ± 1.68 × 10⁴ atoms g⁻¹. Our analysis suggests that part of the ¹⁰Be concentration dispersion is related to an insufficient number of amalgamated rockfalls that does not erase the stochastic nature of the sidewall erosion. In the latter case, the concentration of several collected samples is averaged to increase the number of statistically amalgamated rockfalls. Variable and robust ¹⁰Be-derived rockwall retreat rates are obtained for three distinct rockfall zones in the Bossons catchment and are 0.19 ± 0.08 mm year⁻¹, 0.54 ± 0.1 mm year⁻¹ and 1.08 ± 0.17 mm year⁻¹. The mean ¹⁰Be retreat rate for the whole catchment (ca. 0.65 mm year⁻¹) is close to the present-day erosion rate derived from other methods. © 2019 John Wiley & Sons, Ltd.     

Hummock corridors in the south‐central sector of the Fennoscandian ice sheet,morphometry and pattern

Subglacial conditions strongly influence the flow of ice‐sheets, in part due to the availability of melt water. Contemporary ice sheets are retreating and are affected by increased melting as climate warms. The south Swedish uplands (SSU) were deglaciated during the relatively warm Bølling‐Allerød interval, and by studying the glacial landforms there it is possible to increase the understanding of the subglacial environment during this period of warming. Across the study area, vast tracts of hummocks have long been recognized. However, recent mapping shows a pattern of elongated zones of hummocks radially oriented, hereafter referred to as ‘hummock corridors’. Morphometric parameters were measured on the hummock corridors using a 2 m horizontal resolution digital elevation model. Corridor width varies between 0.2 and 4.9 km and their length between 1.5 and 11.8 km. A majority of hummock corridors are incised in drumlinised till surfaces. The pattern of hummock corridors shows a clear relation to the overall ice‐flow. Further, hummock corridors do not follow topographic gradients, and in at least one place an esker overlies hummocks on the corridor floor. The lateral spacing of hummock corridors and corridor morphology are similar to tunnel valleys, eskers and glaciofluvial corridors reported elsewhere. Such relationships support a subglacial genesis of the corridors in the SSU by water driven by the subglacial hydraulic gradient and that hummock corridors are forms that can be identified as tunnel valleys and glaciofluvial corridors (GFC). Ages were assigned to hummock‐corridor cross‐sections from a deglacial reconstruction of the Fennoscandian Ice Sheet. By comparing the frequency of corridors per age interval with climate variations from a Greenland ice core, we hypothesize that an increase in the number of corridors is related to the Bølling‐Allerød warming, indicating a higher rate of delivery of surface melt water to the bed at this time. Copyright © 2017 John Wiley & Sons, Ltd.     

Glacial meltwater and sea ice meltwater in the Prydz Bay, Antarctica

It has been well known by oceanographers that the World Ocean Circulation originates inNorth Atlantic near the Greenland, where in wintertime the cooled surface water descends to form North Atlantic Deep Waters (NADW). The NADW, when passing the Antarctic…     

Rates of erosion and landscape change along the Blue Ridge escarpment,southern Appalachian Mountains,estimated from in situ cosmogenic 10Be

The Blue Ridge escarpment, located within the southern Appalachian Mountains of Virginia and North Carolina, forms a distinct, steep boundary between the lower‐elevation Piedmont and higher‐elevation Blue Ridge physiographic provinces. To understand better the rate at which this landform and the adjacent landscape are changing, we measured cosmogenic beryllium‐10 (¹⁰Be) in quartz separated from sediment samples (n = 50) collected in 32 streams and from three exposed bedrock outcrops along four transects normal to the escarpment, allowing us to calculate erosion rates integrated over 10⁴–10⁵ years. These basin‐averaged erosion rates (5.4–49 m Myr^?1) are consistent with those measured elsewhere in the southern Appalachain Mountains and show a positive relationship between erosion rate and average basin slope. Erosion rates show no relationship with basin size or relative position of the Brevard fault zone, a fundamental structural element of the region. The cosmogenic isotopic data, when considered along with the distribution of average basin slopes in each physiographic province, suggest that the escarpment is eroding on average more rapidly than the Blue Ridge uplands, which are eroding more rapidly than the Piedmont lowlands. This difference in erosion rates by geomorphic setting suggests that the elevation difference between the uplands and lowlands adjacent to the escarpment is being reduced but at extremely slow rates. Copyright © 2016 John Wiley & Sons, Ltd.     

Topographic variation in soil erosion and accumulation determined with meteoric 10Be

Understanding natural soil redistribution processes is essential for measuring the anthropogenic impact on landscapes. Although meteoric beryllium-10 (¹⁰Be) has been used to determine erosion processes within the Pleistocene and Holocene, fewer studies have used the isotope to investigate the transport and accumulation of the resulting sediment. Here we use meteoric ¹⁰Be in hilltop and valley site soil profiles to determine sediment erosion and deposition processes in the Christina River Basin (Pennsylvania, USA). The data indicate natural erosion rates of 14 to 21 mm 10⁻³ yr and soil ages of 26 000 to 57 000 years in hilltop sites. Furthermore, valley sites indicate an alteration in sediment supply due to climate change (from the Pleistocene to the Holocene) within the last 60 000 years and sediment deposition of at least 0.5–2 m during the Wisconsinan glaciation. The change in soil erosion rate was most likely induced by changes in geomorphic processes; probably solifluction and slope wash during the cold period, when ice advanced into the mid latitudes of North America. This study shows the value of using meteoric ¹⁰Be to determine sediment accumulation within the Quaternary and quantifies major soil redistribution occurred under natural conditions in this region. © 2018 John Wiley & Sons, Ltd.