Patterns Of Glacial Erosion Research Articles

Climatic controls on mountain glacier basal thermal regimes dictate spatial patterns of glacial erosion

Abstract. Climate has been viewed as a primary control on the rates and patterns of glacial erosion, yet our understanding of the mechanisms by which climate influences glacial erosion is limited. We hypothesize that climate controls the patterns of glacial erosion by altering the basal thermal regime of glaciers. The basal thermal regime is a first-order control on the spatial patterns of glacial erosion. Polythermal glaciers contain both cold-based portions that protect bedrock from erosion and warm-based portions that actively erode bedrock. In this study, we model the impact of various climatic conditions on glacier basal thermal regimes and patterns of glacial erosion in mountainous regions. We couple a sliding-dependent glacial erosion model with the Parallel Ice Sheet Model (PISM) to simulate the evolution of the glacier basal thermal regime and glacial erosion in a synthetic landscape. We find that both basal thermal regimes and glacial erosion patterns are sensitive to climatic conditions, and glacial erosion patterns follow the patterns of the basal thermal regime. Cold temperature leads to limited glacial erosion at high elevations due to cold-based conditions. Increasing precipitation can overcome the impact of cold temperature on the basal thermal regime by accumulating thick ice and lowering the melting point of ice at the base of glaciers. High precipitation rates, therefore, tend to cause warm-based conditions at high elevations, resulting in intensive erosion near the peak of the mountain range. Previous studies often assessed the impact of climate on the spatial patterns of glacial erosion by integrating climatic conditions into the equilibrium line altitudes (ELAs) of glaciers, and glacial erosion is suggested to be maximal around the ELA. However, our results show that different climatic conditions produce glaciers with similar ELAs but different patterns of basal thermal regime and glacial erosion, suggesting that there might not be any direct correlation between ELAs and glacial erosion patterns.

The hydrology of glacier‐bed overdeepenings: Sediment transport mechanics, drainage system morphology, and geomorphological implications

AbstractEvacuation of basal sediment by subglacial drainage is an important mediator of rates of glacial erosion and glacier flow. Glacial erosion patterns can produce closed basins (i.e., overdeepenings) in glacier beds, thereby introducing adverse bed gradients that are hypothesized to reduce drainage system efficiency and thus favour basal sediment accumulation. To establish how the presence of a terminal overdeepening might mediate seasonal drainage system evolution and glacial sediment export, we measured suspended sediment transport from Findelengletscher, Switzerland during late August and early September 2016. Analyses of these data demonstrate poor hydraulic efficiency of drainage pathways in the terminus region but high sediment availability. Specifically, the rate of increase of sediment concentration with discharge was found to be significantly lower than that anticipated if channelized flow paths were present. Sediment availability to these flow paths was also higher than would be anticipated for discrete bedrock‐floored subglacial channels. Our findings indicate that subglacial drainage in the terminal region of Findelengletscher is dominated by distributed flow where entrainment capacity increases only marginally with discharge, but flow has extensive access to an abundant sediment store. This high availability maintains sediment connectivity between the glacial and proglacial realm and means daily sediment yield is unusually high relative to yields exhibited by similar Alpine glaciers. We present a conceptual model illustrating the potential influence of ice‐bed morphology on subglacial drainage evolution and sediment evacuation mechanics, patterns and yields, and recommend that bed morphology should be an explicit consideration when monitoring and evaluating glaciated basin sediment export rates.

Glacial erosion by the Trift glacier (Switzerland): Deciphering the development of riegels, rock basins and gorges

A long-lasting question in glacial geology is how and how fast glaciers were able to shape the distinctive landscapes of the Alps. This study contributes to the understanding on the formation of overdeepened basins, especially the processes and the amount of time involved. We examine the remarkably high (150 m) cross-valley bedrock riegel and the associated overdeepening located in front of the Trift glacier in the central Swiss Alps. A combined approach of field survey with measurements of two cosmogenic nuclides, 10Be and in-situ 14C, and a numerical model was used to determine the spatial glacial erosion patterns on the bedrock riegel. Ten samples were taken along two transects; one perpendicular to the glacier flow direction, from outside of the Little Ice Age (LIA) extent down to the centre of the riegel, and the other following the former ice-flow direction across the riegel. Analysis of measured nuclide concentrations shows that the sample outside of the LIA was constantly exposed since the retreat of the Egesen stadial Trift glacier (~11.5 ka). The samples inside the LIA extent indicate a distinct trend of increasing glacial erosion rates from 0 mm/a near the LIA ice margin to high erosion all across the top of the riegel. The resulting minimum glacial erosion rates from samples on the riegel are 0.5–1.1 mm/a (10Be) and 0.6–>1.8 mm/a (in-situ 14C) which correspond to minimum erosion depths of 1.6–>3 m (10Be) and 1–>5 m (14C). The extremely low nuclide concentrations measured at the riegel highlight the substantial erosion (predominantly abrasion) of the bedrock surface during late Holocene glacier coverage. Field observations suggest that the formation of the overdeepening and, as a consequence, the riegel is due to a combination of valley shape, bedrock structures, glacier confluence and hydrology. We further hypothesise that the gorge is a key factor responsible for this impressive overdeepening, by lowering the threshold for the subglacial meltwater, effectively decoupling the height of the riegel from the depth of the overdeepening.

Tectonic controls on rates and spatial patterns of glacial erosion through geothermal heat flux

Glacial erosion has shaped many mountain belts during the cold periods of the Late Cenozoic. The rate of glacial erosion is sensitive to the subglacial environment, including both the subglacial hydrology and the basal thermal regime. Geothermal heat from underlying bedrock is a major contributor to glacier energy budgets, controlling ice dynamics at the ice-bed interface by changing the basal temperature and the supply of meltwater. Despite the known influence of geothermal heat on the subglacial environment, its impact on glacial erosion has received little study. The geothermal heat flux in glaciated mountain ranges varies widely as a function of the tectonic setting. Therefore, if glacial erosion is sensitive to geothermal heat flux, the evolution of glaciated landscapes may depend upon tectonically-controlled geothermal gradients. We explore the impact of geothermal heat flux on the rates and spatial patterns of glacial erosion in mountain ranges using numerical models. We couple a sliding-dependent glacial erosion model with the Parallel Ice Sheet Model (PISM) to simulate the evolution of a synthetic glacial landscape. We find a robust tendency for increasing glacial erosion with increasing geothermal heat flux. The spatial pattern of erosion also varies with the magnitude of geothermal heat flux. At low geothermal heat flux, glacial erosion is consistently focused in major valleys. As geothermal heat flux increases, the area of significant glacial erosion expands into higher elevations and the rate of erosion increases. The location of maximum erosion migrates up-valley as geothermal heat flux increases, suggesting that glacial erosion tends to produce distinct landscapes as a function of geothermal heat flux. Our finding suggests that active mountain belts with high geothermal heat flux will express the glacial buzzsaw effect, in which high elevation topography is preferentially removed by glacial erosion. Glaciers at passive margins with low geothermal heat flux, in contrast, will tend to incise deep valleys at relatively low elevations. Previous work on the interaction between tectonics and landscape evolution has focused on relief generation and fracturing of rocks. Our results introduce a novel potential linkage between tectonics and erosion based on the sensitivity of glacial erosion to geothermal heat.

The changing Patagonian landscape: Erosion and westward sediment transfer paths in northern Patagonia during the Middle and Late Pleistocene

AbstractPleistocene glaciations have promoted important landscape transformations as a result of high rates of erosion and rapid sediment evacuation to adjacent marine basins. In the Patagonian Andes the role of the Patagonian Ice Sheet on landscape evolution, in particular the spatial patterns of glacial erosion and its influence on sediment fluxes, is poorly documented. Here, we investigate the Middle and Late Pleistocene sedimentary record of the continental slope from Ocean Drilling Program (ODP) Site 861, offshore Patagonia (46°S), to evaluate the link between glaciations, mountain range erosion and continental margin strata formation. Petrographic analysis of the sand‐size fraction (0.063–2 mm) and ɛNd and 87Sr/86Sr measurements in the silt‐size fraction (10–63 µm) indicate that glacial erosion over the last 350,000 years has focused within the Patagonian Batholith, with a minor influence of a proximal source to the drilling site, the Chonos Metamorphic Complex. This shows that erosion has focused in the core of the northern Patagonian Andes, coinciding roughly with the location of the Liquiñe‐Ofqui Fault Zone and the zone of concentrated precipitation during glaciations, suggesting a combined climatic and structural control on glacial erosion. Temporal variation in the provenance signal is contemporaneous with a marked change in the stratigraphy of ODP Site 861 that occurred after the glaciation of MIS 8 (~240 kyr ago). Before MIS 8, a restricted provenance signal and coarse lithofacies accumulated on the continental slope indicates spatially restricted erosion and efficient transfer of sediment towards the ocean. In contrast, very high provenance variability and finer continental slope lithofacies accumulation after MIS 8 suggest a disorganized expansion of the areas under erosion and a more distal influence of ice sediment discharge to this site. We argue that this change may have been related to a re‐organization of the drainage patterns of the Patagonian Ice Sheet and flow of outlet glaciers to the continental margin during the last two glaciations.

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

AbstractQuantifying 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.

Cosmogenic 10Be and 26Al exposure ages of glaciations in the Frankland Range, southwest Tasmania reveal a limited MIS-2 ice advance

Abstract New mapping of the glacial geomorphology coupled with 10Be and 26Al exposure age dating of moraines on the flanks of the Frankland Range in south west Tasmania indicate that glacier extent during MIS-2 was far smaller than during earlier glaciations with the ice cover being confined to only the uppermost cirques of the range. Moraines further down the range flanks, ∼50–150 m lower in altitude than the MIS-2 dated advance, indicate that glaciers were only slightly larger during earlier glaciations and, depending on the interpretation of their exposure ages, may range from MIS 7 to MIS 12. These older moraines are nested inside the maximum ice limits of an even more ancient and extensive glaciation, defined by degraded valley floor moraines and coalescing glacio-fluvial fans that remain undated but appear no younger than MIS 12. Patterns of glacial erosion and moraine deposition on the Frankland Range suggest that the more recent glaciations were increasingly influenced by the erosional morphology initiated by earlier glaciers. Microclimatic differences resulting from this earlier glacial topography were particularly influential determinants of glaciation during MIS 2. These results are consistent with emerging evidence from studies of other ranges in southwest Tasmania.

Origin and dynamic significance of longitudinal structures ("flow stripes") in the Antarctic Ice Sheet

Abstract. Longitudinal ice-surface structures in the Antarctic Ice Sheet can be traced continuously down-ice for distances of up to 1200 km. A map of the distribution of ~ 3600 of these features, compiled from satellite images, shows that they mirror the location of fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of metres per annum and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. They can be traced continuously through crevasse fields and through blue-ice areas, indicating that they represent the surface manifestation of a three-dimensional structure, interpreted as foliation. Flow lines are linear and undeformed for all major flow units described here in the Antarctic Ice Sheet except for the Kamb Ice Stream and the Institute and Möller Ice Stream areas, where areas of flow perturbation are evident. Parcels of ice along individual flow paths on the Lambert Glacier, Recovery Glacier, Byrd Glacier and Pine Island Glacier may reside in the glacier system for ~ 2500 to 18 500 years. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow configuration of the ice sheet may have remained largely unchanged for the last few hundred years, and possibly even longer. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

The glacial geomorphology of the Antarctic ice sheet bed

AbstractIn 1976, David Sugden and Brian John developed a classification for Antarctic landscapes of glacial erosion based upon exposed and eroded coastal topography, providing insight into the past glacial dynamics of the Antarctic ice sheets. We extend this classification to cover the continental interior of Antarctica by analysing the hypsometry of the subglacial landscape using a recently released dataset of bed topography (BEDMAP2). We used the existing classification as a basis for first developing a low-resolution description of landscape evolution under the ice sheet before building a more detailed classification of patterns of glacial erosion. Our key finding is that a more widespread distribution of ancient, preserved alpine landscapes may survive beneath the Antarctic ice sheets than has been previously recognized. Furthermore, the findings suggest that landscapes of selective erosion exist further inland than might be expected, and may reflect the presence of thinner, less extensive ice in the past. Much of the selective nature of erosion may be controlled by pre-glacial topography, and especially by the large-scale tectonic structure and fluvial valley network. The hypotheses of landscape evolution presented here can be tested by future surveys of the Antarctic ice sheet bed.

Unequal ice-sheet erosional impacts across low-relief shield terrain in northern Fennoscandia

Much previous work on Late Cenozoic glacial erosion patterns in bedrock has focussed on mountain areas. Here we identify varying impacts of ice sheet erosion on the low-relief bedrock surface of the Fennoscandian shield, and examine the geological, topographical and glaciological controls on these patterns.We combine GIS-mapping of topographical, hydrological and weathering data with field observations. We identify and investigate areas with similar geology and general low relief that show different degrees of ice sheet erosional impact, despite similar ice cover histories. On two transects with a total area of ~84000km2 across the northern Fennoscandian shield, we first establish patterns of glacial erosion and then examine why glacially streamlined areas exist adjacent to areas of negligible glacial erosion. The northern transect includes two areas of exceptional glacial preservation, the Parkajoki area in Sweden and the so-called ice divide zone in Finland, each of which preserve tors and deep saprolite covers. The southern transect, overlapping in the northern part with the first transect, includes areas of well developed glacial streamlining, with bedrock areas stripped of loose material and barely any weathering remnants.For both areas, we firstly present contrasting indicators for ice sheet erosional impact: streamlined and non-streamlined inselbergs; parallel and dendritic/rectangular drainage patterns; and the absence and presence of Neogene weathering remnants. This is followed by an investigation of factors that possibly influence ice sheet erosional impact: ice cover history, ice cover duration and thickness, bedrock type and structure, and topography.We find that the erosional impact of the Fennoscandian ice sheet has varied across the study area. Distinct zones of ice sheet erosion are identified in which indicators of either low or high erosion coexist in the same parts of the transects. No direct impact of rock type on glacial erosion patterns was found, but an indirect control appears clear. Bedrock geology and long-term differential weathering and tectonic evolution determined the topography of the pre-glacial landscape, and these topographic differences subsequently influenced ice sheet dynamics and thereby partly controlled patterns of ice sheet erosion. Ice cover duration and former ice thickness were not significant controls on glacial erosion patterns. Extensive preservation of pre-glacial relief through low glacial erosion is attributed to the maintenance throughout the Pleistocene of divergent flow and frozen-bed conditions in the Fennoscandian ice sheet. In contrast, glacial streamlining and strong glacial erosion were caused mainly by acceleration of flow around major obstacles and flow towards major depressions on the ice sheet bed. The relatively strong ice sheet erosion towards the Gulf of Bothnia is the result of a combination of favourable factors: bedrock structure and river valleys aligned sub-parallel to ice sheet flow and convergent ice flow towards the Baltic.

Controls of initial topography on temporal and spatial patterns of glacial erosion

Here we investigate the influence of initial pre-glacial topography on spatial and temporal patterns of glacial erosion using numerical surface process modelling, including a higher order ice sheet model. First, we consider glacier dynamics when simulating glaciation in two real landscapes, representing plateau-type topography (southeast Australia) and characteristic steady-state fluvial topography (southern Taiwan). We find that the different initial landscape configurations result in distinctly different ice configurations and patterns of basal sliding. The sliding patterns are controlled by ice configuration and the resulting basal shear stresses and by the thermal properties at the base of the ice. We then investigate how these characteristic patterns of basal sliding control glacial erosion and long-term landscape evolution using synthetic representations of the two landscapes. The two landscape configurations result in markedly different spatial and temporal patterns of glacial erosion. However, the resulting landscapes may have similar morphology, irrespective of initial landscapes and glacial erosion patterns being significantly different. The numerical experiments also suggest that, in addition to basal temperature, basal shear stress is important in restricting long-term glacial erosion, which is relevant for the preservation of landforms during glaciations. Specifically, pre-glacial landforms may be eroded although they are initially protected by cold-based ice, when the ice configuration promotes significant basal shear stress (glacial erosion) at the edge of a plateau-like landscape. In contrast, pre-glacial landforms may be preserved irrespective of the ice being warm-based, when low gradients in the ice surface act to limit basal shear stress.

Seasonal-scale abrasion and quarrying patterns from a two-dimensional ice-flow model coupled to distributed and channelized subglacial drainage

Field data and numerical modeling show that glaciations have the potential either to enhance relief or to dampen topography. We aim to model the effect of the subglacial hydraulic system on spatiotemporal patterns of glacial erosion by abrasion and quarrying on time scales commensurate with drainage system fluctuations (e.g., seasonal to annual). We use a numerical model that incorporates a dual-morphology subglacial drainage system coupled to a higher-order ice-flow model and process-specific erosion laws. The subglacial drainage system allows for a dynamic transition between two morphologies: the distributed system, characterized by an increase in basal water pressure with discharge, and the channelized system, which exhibits a decrease in equilibrium water pressure with increasing discharge. We apply the model to a simple synthetic glacier geometry, drive it with prescribed meltwater input variations, and compute sliding and erosion rates over a seasonal cycle. When both distributed and channelized systems are included, abrasion and sliding maxima migrate ~20% up-glacier compared to simulations with distributed drainage only. Power-law sliding generally yields to a broader response of abrasion to water pressure changes along the flowline compared to Coulomb-friction sliding. Multi-day variations in meltwater input elicit a stronger abrasion response than either diurnal- or seasonal variations alone for the same total input volume. An increase in water input volume leads to increased abrasion. We find that ice thickness commensurate with ice sheet outlet glaciers can hinder the up-glacier migration of abrasion. Quarrying patterns computed with a recently published law differ markedly from calculated abrasion patterns, with effective pressure being a stronger determinant than sliding speeds of quarrying rates. These variations in calculated patterns of instantaneous erosion as a function of hydrology-, sliding-, and erosion-model formulation, as well as model forcing, may lead to significant differences in predicted topographic profiles on long time scales.

Glacial hydrology and erosion patterns: A mechanism for carving glacial valleys

Most glacial erosion models assume that erosion rates are proportional to ice-sliding velocity. While recent studies have shown that water plays a major role in modulating sliding velocities, the impact it might have on erosion rates is still unclear. Here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule. Our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion. First, it suggests that erosion might mainly occur during melting seasons, when subglacial water pressure is large and effective pressure is low (i.e., before the channels fully develop and reduce the water pressure at the ice bedrock interface), which promotes sliding and erosion. Second, the distribution of erosion is bimodal with a peak at the equilibrium line altitude and, as we demonstrate, enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds. This has important implications because it explains why glacial erosion can simultaneously set a limit on the mean elevation of mountain ranges and carve large fjords or glacial lakes.

Using a GIS filtering approach to replicate patterns of glacial erosion

AbstractIn order to extend our knowledge of glacial relief production in mountainous areas new methods are required for landscape reconstructions on a temporal resolution of a glacial cycle and a spatial resolution that includes the most important terrain components. A generic data set and a 50 m resolution digital elevation model over a study area in northern Sweden and Norway (the present day landscape data set) were employed to portray spatial patterns of erosion by reconstructing the landscape over successive cycles of glacial erosion. A maximum‐value geographic information system (GIS) filtering technique using variable neighbourhoods was applied such that existing highpoints in the landscape were used as erosional anchor points for the reconstruction of past landscape topography. An inherent assumption, therefore, is that the highest surfaces have experienced insignificant down‐wearing over the Quaternary. Over multiple reconstruction cycles, proceeding backwards in time, the highest summits increase in area, valleys become shallower, and the valley pattern becomes increasingly simplified as large valleys become in‐filled from the sides. The sum of these changes reduces relief. The pattern of glacial erosion, which is to 60% correlated to slope angle and to 70% correlated to relative relief, is characterized by (i) an abrupt erosional boundary below preserved summit areas, (ii) enhanced erosion in narrow valleys, (iii) restricted erosion of smooth areas, independently of elevation, (iv) eradication of small‐scale irregularities, (v) restricted erosion on isolated hills in low‐relief terrain, and (vi) a valley widening independent of valley directions. The method outlined in this paper shows how basic GIS filtering techniques can mimic some of the observed patterns of glacial erosion and thereby help deduce the key controls on the processes that govern large‐scale landscape evolution beneath ice sheets. Copyright © 2010 John Wiley & Sons, Ltd.

Using a GIS to Quantify Patterns of Glacial Erosion on Northwest Iceland: Implications for Independent Ice Sheets

Glacial erosion patterns on northwest Iceland are quantified using a Geographic Information System (GIS) in order to interpret subglacial characteristics of part of northwest Iceland affected by ice sheet glaciation. Ice scour lake density is used as a proxy for glacial erosion. Erosion classes are interpreted from variations in the density of lake basins. Lake density was calculated using two different methods: the first is sensitive to the total number of lakes in a specific area, and the second is sensitive to total lake area in a specific area. Both of these methods result in a value for lake density, and the results for lake density calculated using the two methods are similar. Areas with the highest density of lakes are interpreted as areas with the most intense erosion with the exception of alpine regions. The highest density of lakes in the study area exceeds 8% and is located on upland plateaus where mean elevations range from 400 to 800 m a.s.l. Low lake density (0–2%) is observed in steep alpine areas where steep topography does not favor lake development. The GIS analysis is combined with geomorphic mapping to provide ground truth for the GIS interpretations and to locate paleo-ice flow indicators and landforms. The patterns identified in this study illustrate distinct regions of glacial erosion and flow paths that are best explained by two independent ice sheets covering northwest Iceland during the Last Glacial Maximum (LGM). Areas of alpine glacial landforms and the presence of nunataks within the glaciated region support interpretations that ice-free regions or cold-based ice cover existed on parts of northwest Iceland during the LGM. The methods developed in this study are easily transferable to other formerly glaciated regions and provide tools to evaluate subglacial properties of former ice sheets. The data generated yield important subglacial boundary conditions for ice sheet models of Iceland.

Using a GIS to Quantify Patterns of Glacial Erosion on Northwest Iceland: Implications for Independent Ice Sheets

Glacial erosion patterns on northwest Iceland are quantified using a Geographic Information System (GIS) in order to interpret subglacial characteristics of part of northwest Iceland affected by ice sheet glaciation. Ice scour lake density is used as a proxy for glacial erosion. Erosion classes are interpreted from variations in the density of lake basins. Lake density was calculated using two different methods: the first is sensitive to the total number of lakes in a specific area, and the second is sensitive to total lake area in a specific area. Both of these methods result in a value for lake density, and the results for lake density calculated using the two methods are similar. Areas with the highest density of lakes are interpreted as areas with the most intense erosion with the exception of alpine regions. The highest density of lakes in the study area exceeds 8% and is located on upland plateaus where mean elevations range from 400 to 800 m a.s.l. Low lake density (0–2%) is observed in steep alpine areas where steep topography does not favor lake development. The GIS analysis is combined with geomorphic mapping to provide ground truth for the GIS interpretations and to locate paleo-ice flow indicators and landforms. The patterns identified in this study illustrate distinct regions of glacial erosion and flow paths that are best explained by two independent ice sheets covering northwest Iceland during the Last Glacial Maximum (LGM). Areas of alpine glacial landforms and the presence of nunataks within the glaciated region support interpretations that ice-free regions or cold-based ice cover existed on parts of northwest Iceland during the LGM. The methods developed in this study are easily transferable to other formerly glaciated regions and provide tools to evaluate subglacial properties of former ice sheets. The data generated yield important subglacial boundary conditions for ice sheet models of Iceland.

Glacial erosion at the fjord onset zone and implications for the organization of ice flow on Baffin Island, Arctic Canada

Organized conduits of fast ice flow at the fringe of present and past ice sheets move the majority of ice from ice sheet interior to periphery. These relatively narrow corridors play a critical role in feedbacks between landscape evolution and ice sheet dynamics. Here we combine bedrock 10Be concentrations with distribution of lakes on Baffin Island, Arctic Canada, to constrain the pattern of glacial modification at a fjord onset zone — the region of increasing relief from the continental interior to the fjord head. High lake density and a lack of 10Be inheritance in the valley bottom indicate significant glacial scouring for some duration of the last glacial cycle. In contrast, 10Be inheritance and a lack of lakes at higher elevations indicate a lack of glacial scour and only slight glacial modification despite the entire region being covered by the Laurentide Ice Sheet during the last glaciation. This pattern of glacial erosion at the fjord onset zone is similar to what is found in the outer fjords ∼ 100 km toward the last glacial maximum ice margin, and indicates that ice flow becomes organized well inland from the fjorded coast. The relief-generating process of selective linear erosion occurs at the fjord onset zone and was responsible for evolving Baffin Island fjords even in the late Quaternary.

Combined numerical and geomorphological reconstruction of the Serra da Estrela plateau icefield, Portugal

The paper focuses on reconstructing a plateau icefield surface from field geomorphological data and a physical-based glacier model. The results allow the analysis of the patterns of glacial erosion and the estimation of the palaeo-Equilibrium Line Altitudes (ELA). The study area is the Serra da Estrela, a plateau in Central Portugal rising to ∼ 2000 m ASL. The glaciated area during the Last Maximum of the Serra da Estrela Glaciation (LMGSE) was ∼ 66 km 2. The reconstruction of the topography of the icefield and valley glaciers in the LMGSE is based on the Schilling and Hollin model. It iterates along the valley longitudinal profile and is based on the gradient, on valley-shape indices and on yield basal shear stresses. These variables influence ice thickness and therefore, also the slope of the ice surface allowing for its reconstruction. The key variable is basal yield shear stress and its value was included in the model manually starting from the points of maximum extent of the valley glaciers and following a range known to occur in contemporary conditions. The input values are validated by matching the resulting ice surface to geomorphological features. Where these are absent, a constant value of 100 kPa was used. The icefield was reconstructed from a radiating set of long-sections, along which ice thicknesses were calculated. A DEM of the ice surface was constructed, allowing the estimation of the hypsometric curves of the distinct glacier catchments and the calculation of the palaeo-ELAs. The results show a regional ELA at 1650 m ASL with spatial variations across the icefield reflecting mainly the effect of eastward snow drift. The LMGSE glaciers were very sensitive to minor climatic changes, especially due to the large area of the plateau icefield, and to the positioning of the ELAs, close to, or in the, flat part of the hypsometric curve. The model of the ice surface is of significant value for the analysis of the patterns of glacial erosion at the landscape level. In the Serra da Estrela most of the glacial erosion occurred near the plateau margins and in valley heads, where glacier surface slope was steeper allowing for a faster ice flow and where ice flow concentrated. Strong glacier erosion in the Zêzere valley is linked to the tectonic setting, but also to the confluence of glaciers and to the overfeeding of snow from the plateau.

Glaciation, erosion, and landscape evolution of Iceland

Stratigraphic and sedimentological studies indicate that Iceland has experienced over 20 glaciations during the last 4–5 Myr, in reasonable agreement with the number of glaciations reconstructed from the ∂18O record in deep-sea sediment. The pattern of glacial erosion was to a large part controlled by constructive volcanic processes resulting in increased topographic relief after 2.5 Myr. Between 2.5 and 0.5 Ma valleys up to 400 m deep were excavated into the Tertiary basalts of eastern and south Iceland with an average erosion rate of 10–20 cm ka−1. During the last 0.5 million years rates of erosion increased to 50–175 cm ka−1, with an additional 200 to over 1000 m of valley excavation. Previous estimates of the rate of landscape erosion during the Holocene vary widely, from 5 to 70,000 cm ka−1. We present new studies that define the rates of landscape denudation during the major part of the Holocene (the last 10,200 years): one based on the Iceland shelf sediment record, the other from the sediment record in the glacier-fed lake, Hvítárvatn. Both studies indicate average Holocene erosion rates of about 5 cm ka−1 similar to our erosion rate estimate for 4–5 Ma old strata that has not been subjected to regional glaciation.

Features of glacial valley profiles simply explained

Glacial occupation of alpine valleys results in a distinct signature in the long‐valley profile, including steepening of the profile in the headwaters, flattening at lower elevations, and a step in the profile at the convergence of headwater tributaries. We present analytic results for glacial erosion patterns by making the following assumptions: (1) the initial profile is linear, (2) the width of the valley is uniform, (3) the annual mass balance varies linearly with elevation, (4) the glacier at any time is quasi‐steady, (5) the erosion rate is proportional to ice discharge per unit valley width, and (6) glacial erosion rates far exceed fluvial erosion rates. A steady glacier under these conditions would erode a parabolic divot in the longitudinal valley profile, with its maximum depth coinciding with the down‐valley position of the equilibrium line altitude (ELA). The calculated flattening of the valley floor down valley of the ELA and the steepening of it up valley captures the essence of the glacial signature. When a reasonable probability distribution of ELAs is allowed, the predicted erosion peaks at 30–40% of the down‐valley distance to the glacial limit, and the pattern merges smoothly with the steeper fluvial profile downstream of the glacial limit. Profiles of mass balance that are capped at a maximum value produce shorter glaciers and a slight asymmetry in the expected erosion pattern. Only when we mimic the tributaries of glacial headwaters by specifying a valley width distribution do we obtain the upper step observed in many valley profiles.