Alpine Sediment Research Articles

Debris Cover Limits Subglacial Erosion and Promotes Till Accumulation.

Glaciers are commonly conceptualized as bodies composed of snow and ice. Yet, many glaciers contain a substantial amount of rock, especially those abutting steep mountains. Mountain slopes erode, depositing rocks on glaciers below. This loose rock (or debris) is buried in glaciers and melts out lower down creating a debris cover. Debris cover reduces ice melt, which changes the shape and movement of glaciers. Glacier movement, specifically basal sliding, efficiently sculpts landscapes. To date, we know little about the impacts of surface debris on conditions below glaciers. To help remedy this, we run numerical model simulations which show that debris‐covered glaciers erode slower than glaciers unaffected by debris. Reduced melt under surface debris lowers sliding speeds and causes sediment to accumulate at the bed, potentially establishing conditions for surging. The influence of surface debris cover on the subglacial environment may hold substantial implications for alpine sediment storage and landscape evolution.

Contributions of climate, vegetation and soil to the alpine sediment carbon accumulation rate in central China since the Middle Holocene

Large amounts of carbon in alpine sediments have been expected to be sensitive to climate change, but how carbon accumulation responds to climate change remains unclear. Thus, we explored the impact of different factors on the carbon accumulation rate (CAR) of alpine sediments by combining a variety of climatic variables, vegetation data and erosion indicators based on two alpine sediment successions on Taibai Mountain, the highest peak in central and eastern mainland China. One succession is near the modern treeline (Paomaliang Swamp, PML) and the other is located at the upper forest line (Sanqing Chi, SQC, a small lake). We used our previously published organic carbon content data and for the first time calculated the CAR, and further used pollen and physicochemical indicators to quantify the contributions from climate, vegetation and soil. We found that their contributions varied during different periods and between the two sediment successions. For the PML succession, from 5850 to 4000 calendar years before present (cal. a BP), the CAR was low, which was related to low annual temperatures, low vegetation cover and strong soil erosion. From 4000 to 2400 cal. a BP, a high CAR coincided with high annual temperatures, high vegetation cover and weak soil erosion. From 2400 to 200 cal. a BP, the CAR decreased, mainly attributed to low vegetation cover. Local vegetation cover had major impacts on the CAR in the SQC succession during the Middle–Late Holocene. In general, the local factor interpretation rate in SQC (83%) was higher than that of PML (47%), related to the vegetation stability of continuous forest and the treeline. This study highlights the important role of the local environment in determining carbon accumulation in the alpine region.

Talus slope geomorphology investigated at multiple time scales from high‐resolution topographic surveys and historical aerial photographs (Sanetsch Pass, Switzerland)

AbstractTalus slopes are common places for debris storage in high‐mountain environments and form an important step in the alpine sediment cascade. To understand slope instabilities and sediment transfers, detailed investigations of talus slope geomorphology are needed. Therefore, this study presents a detailed analysis of a talus slope on Col du Sanetsch (Swiss Alps), which is investigated at multiple time scales using high‐resolution topographic (HRT) surveys and historical aerial photographs. HRT surveys were collected during three consecutive summers (2017–2019), using uncrewed aerial vehicle (UAV) and terrestrial laser scanning (TLS) measurements. To date, very few studies exist that use HRT methods on talus slopes, especially to the extent of our study area (2 km2). Data acquisition from ground control and in situ field observations is challenging on a talus slope due to the steep terrain (30–37°) and high surface roughness. This results in a poor spatial distribution of ground control points (GCPs), causing unwanted deformation of up to 2 m in the gathered UAV‐derived HRT data. The co‐alignment of UAV imagery from different survey dates improved this deformation significantly, as validated by the TLS data. Sediment transfer is dominated by small‐scale but widespread snow push processes. Pre‐existing debris flow channels are prone to erosion and redeposition of material within the channel. A debris flow event of high magnitude occurred in the summer of 2019, as a result of several convective thunderstorms. While low‐magnitude (<5,000 m3) debris flow events are frequent throughout the historical record with a return period of 10–20 years, this 2019 event exceeded all historical debris flow events since 1946 in both extent and volume. Future climate predictions show an increase of such intense precipitation events in the region, potentially altering the frequency of debris flows in the study area and changing the dominant geomorphic process which are active on such talus slopes. © 2020 John Wiley & Sons, Ltd.

Reduced sediment supply in a fast eroding landscape? A multi-proxy sediment budget of the upper Rhône basin, Central Alps

Alpine water and sediment supply influence the sediment budget of many important European fluvial systems such as the Rhine, Rhône and Po rivers. In the light of human induced climate change and landscape modification, it becomes increasingly important to understand the mechanisms of sediment production and supply in Alpine sediment systems. This study aims to investigate the modern sediment budget of the upper Rhône basin, one of the largest Alpine intramontane watersheds, located in the Central Alps of southwestern Switzerland. Major areas of sediment generation are fingerprinted by framework petrography, heavy mineral concentrations and bulk geochemistry. The relative contributions of the three major sources to the sediment of the trunk Rhône river are identified by compositional mixing modelling. Concentrations of the terrestrial cosmogenic nuclide 10Be measured in quartz separated from fluvial sediments provide spatially averaged denudation rates for selected tributary basins. Results from sediment fingerprinting and mixing modelling suggest that tributaries located in the North and the East of the catchment are generating most of the sediment transported by the Rhône river to its primary sedimentary sink in Lake Geneva. Despite having some of the highest denudation rates within the basin, tributaries located in the southern area of the Rhône basin are relatively underrepresented in the sediment budget of the Rhône river. These tributaries are severely affected by human activities, for example through sediment mining as well as water and sediment abstraction in large hydropower reservoirs. Together, these processes reduce the basin-wide sediment discharge by about 50%, thereby explaining most of the observed compositional pattern. In addition, there is evidence suggesting that large amounts of glaciogenic sediments are currently supplied by retreating glaciers. Glaciogenic material with its low 10Be concentrations can lead to a significant overestimation of denudation rates and thus limit the applicability of cosmogenic nuclide analysis in such glaciated settings.

Quantifying postglacial sediment storage and denudation rates in a small alpine catchment of the Făgăraș Mountains (Romania)

The study of sediment production, transport, storage and discharge in alpine drainage basins is an essential prerequisite for a better understanding of the postglacial evolution of the alpine landscape. To get an overview on sediment production and alpine landscape evolution in Romania, the current study presents the first alpine sediment storage quantification in the Romanian Carpathians. Postglacial denudation was quantified within the small alpine catchment of the Doamnei Valley (3.62km2), located in the central part of the Făgăraș Mountains. The quantification of sediment volumes was performed through a combined approach consisting of: (i) detailed geomorphological mapping of sediment storage landforms, by means of high accuracy field and remote mapping of sediment storage landforms, (ii) shallow geophysical investigations and (iii) geographic information systems modeling techniques. A total of 64 ground penetrating radar profiles were conducted through the valley for sediment thickness determination of individual landforms. Through parallel profiling, 5 electrical resistivity tomography profiles were also performed for the comparison of bedrock depths in order to determine the overall degree of accuracy of the geophysical investigations applied. In total, 79 sediment storage landforms were identified. Talus sheets were found to be the most dominant landforms within the investigated area, followed by talus cones, moraines and fluvio–torrential deposits. Sediment volume for the Doamnei Valley was calculated to be 7.08±1.42 106m3, corresponding to a mean sediment thickness of 4.20m, with the hanging cirques and valleys subsystem storing 48.58% of the total sediment volume, despite covering just 22% of the investigated area. Sediment volume was used in the determination of mean annual denudation rates for the entire catchment (0.20mm/y±0.04mm/y) as well as for mean annual mass transfer (406.2±31.6t/km2/y), based on a time span of 13ka.

Evidence of paraglacial and paraperiglacial crisis in Alpine sediment transfer since the last glaciation (Ticino, Switzerland)

The “paraglacial” and “paraperiglacial” concepts were introduced in the second half of 20th Century for describe processes, landforms and deposits directly conditioned by deglaciation (paraglacial), respectively by permafrost degradation (paraperiglacial). They represents theoretical models describing the transition from glacial to periglacial, or more generally non glacial conditions (paraglacial model), and from periglacial to temperate conditions (paraperiglacial model). Evidences of sediment transfer conditioned by these processes were described in particular in the Arctic and Subarctic domains. These evidences are less generalised in the Alps and they consider rarely both concepts, integrating periglacial landforms and deposits in source to sink sediment transfer in a single catchment. Here we present evidences of para(peri)glacial sedimentary crises by quantifying sediment transfer from the periglacial zone to the delta in Lake Maggiore for the Ticino River catchment (southern Swiss Alps). Compilation and revision of chronological data, the assessment of sedimentation rates in the Ticino Valley, of progradation rates of the Ticino River delta and of rockwall erosion rates in the periglacial zone, allowed empirical models of sediment transfer to be produced. These models highlights significant high sedimentation rates in the valley floor during the beginning of the deglaciation, and significant rates of rockwall erosion during periods of intense temperature warming and intense permafrost degradation (such as at the beginning of Bølling and during the Preboreal), showing a very good correspondence with paraglacial and paraperiglacial theoretical models. Sediment transfer evolution during the entire Lateglacial and the first half of Holocene in the southern Swiss Alps may then be explained by a combination of a paraglacial erosion phase related to the deglaciation and of two paraperiglacial erosion phases related with significant periods of temperature warming during the Bølling/Allerød and the first part of the Holocene.

Tracing Alpine sediment sources through laser ablation U–Pb dating and Hf‐isotopes of detrital zircons

AbstractAlpine provenance studies based on conventional methods such as sandstone framework grain and heavy mineral analyses are now enhanced by improved techniques in laser ablation inductively coupled plasma mass spectrometry detrital zircon analysis. Although the conventional methods appear to have reached their limits of resolution in palaeogeographic problems, laser ablation inductively coupled plasma mass spectrometry U–Pb dating of detrital zircons adds the time dimension to the provenance analysis. Hafnium‐isotope ratios measured on dated zircons give further information on the origin of the magmas in which the detrital zircons have grown. This study reports detrital zircon U–Pb dating and Hf‐isotope results from sandstone formations related to rifting, drifting and subduction settings at different stages of the Alpine Tethys development. This study is a first evaluation of the correlation between U–Pb age and isotopic features of detrital zircons aimed at describing source terranes in different palaeogeographic domains in the Alpine Tethys area. Pan‐African/Cadomian (Ediacaran–Ordovician), Variscan (Middle Devonian–Carboniferous) and Post‐Variscan (Permian) detrital zircon populations are present in nearly all palaeogeographic settings, but in varying amounts. Single Mesoproterozoic and Palaeoproterozoic detrital zircons are found as minor populations. When comparing the northern and southern margins of the Alpine Tethys, the southern margin detrital sources are characterized mostly by a decreased occurrence or by the absence of Silurian–Devonian zircons. A major distinction between northern (Helvetic, North and Middle Penninic domains) and southern (Austro‐Alpine and South Alpine domains) detrital sources is the occurrence of Triassic zircons at the southern Alpine Tethys margin during rifting and subduction stage sedimentation. Hafnium‐isotope ratios measured on uppermost Permian–Triassic zircons from the South Alpine domain suggest a continental crust derivation of the hosting magmas, as expected in a continental rift environment. In the late stage of Alpine convergence (Late Cretaceous–Palaeogene), the Permian–Triassic zircons are reworked into basins situated on the northern Alpine Tethys margin.

A new modelling approach to delineate the spatial extent of alpine sediment cascades

The delineation of sediment cascades in alpine geosystems requires the identification and localisation of the effective geomorphic processes. In order to classify the terrain on that basis, modelling approaches originally designed for natural hazard zonation were extended. Four sub-models are used to simulate geomorphic processes: (1) potential initiation sites, (2) process pathways, (3) run-out distances and (4) geomorphic activity (erosion, transport and deposition areas). By overlaying the datasets comprising the spatial extent of geomorphic activity, a map of geomorphic process units, i.e. a map with units of homogeneous process composition can be generated. As the modelling of process pathways includes a topology of process links, the map can be used to address several geomorphologic problems, e.g. process interaction, sediment routing, hillslope-channel coupling and the identification of areas of predominant erosion and deposition (sediment storages). This paper presents the design of the modelling approach and selected results of rockfall, slope-type and torrent bed-type debris flow modelling in an alpine basin (Lahnenwiesgraben) in the Northern Limestone Alps, Germany.

Volume changes of Alpine sediment stores in a state of post-event disequilibrium and the implications for downstream hydrology and bed load transport

The cascading system of a catchment in the Bavarian Alps is in a state of disequilibrium after an extreme dambreak flood event. Large parts of talus cones were undercut by fluvial erosion and act as additional sediment sources for fluvial sediment transport, demonstrating the geomorphic coupling between individual subsystems of a sediment cascade. Using terrestrial laser scanning (TLS), high resolution budgeting of sediment input from a talus cone was possible in the post-event field season. Further investigations of changes in the main sediment sources and direct measurements of bed load transport in the river enabled the authors to calculate a preliminary sediment balance. Total bed load flux leaving the catchment in the first post-event field season (year 2006) was c.7900 tons (one-third of the mass exported by the dambreak flood). The input from the main sediment sources was c.5061 t. Hence, the sediment balance has become negative; more sediment was exported from the catchment than stored within.

Sediment generation and delivery from large historic landslides in the Southern Alps, New Zealand

There is much variability of sediment yield in mountain river systems because it is influenced by both periodic (e.g., snowmelt, monsoonal circulation) and episodic (high-intensity rainstorms, earthquakes) controls. In the Southern Alps of New Zealand, landslides are dominant mechanisms of conveying large amounts of debris to river channels. This paper attempts to quantify catastrophic modes of sediment delivery by presenting three recent examples of large landslides on the western slopes of the Southern Alps. A combination of ground surveys, DEM-based volume calculations, and air photo interpretation attributes immediate postfailure sediment yields in excess of 70,000 t km −2 a −1 in the short term. Such landslide-derived sediment pulses pose significant hazards to downstream settlements and infrastructure on alluvial fans, where they cause massive aggradation, increased flooding frequency, and large-scale channel avulsion. In addition to intramontane valley floor obliteration or channel metamorphosis, these geomorphic impacts may thus occur far beyond the initial locations of slope failure. These catastrophic off-site effects involve apparent net sediment delivery rates in the order of 10 5–10 6 m 3 a −1. The accurate quantification of the relative contribution of debris from the disturbing landslide to overall sediment flux is extremely difficult. Nonetheless are quantitative estimates of landslide-derived sediment production and delivery important data to develop a more comprehensive understanding of the variability in alpine sediment flux. Such data are essential not only for use in hydraulic engineering or catchment management, but also in gauging potential future impacts from rainstorm- and earthquake-induced landsliding in the Southern Alps.

Sand Composition and Sedimentary Evolution of a Late Quaternary Depositional Sequence, Northwestern Adriatic Coast, Italy

ABSTRACT In order to detect temporal and spatial variations in detrital supply in the upper part of the last depositional sequence of late Pleistocene-Holocene age, we conducted a combined petrographic and stratigraphic study in the Romagna coastal plain, south of the Po delta, Northern Italy. We analyzed heavy minerals and the bulk composition of sands from cores and surface samples. By using petrographic and stratigraphic data on composition of modern beach and major river deposits, new aspects of detrital dispersal mechanisms and the depositional history of the study area are documented. The heavy-mineral distribution, coupled with the dolostone and volcanic rock fragment contents of bulk sands, allow identification of three different petrofacies: Petrofacies A, Petrofacies B, and Petrofacies C, which have been interpreted as of apenninic, mixed Eastern Alps/Po River basin, and Po River catchment basin provenance, respectively. Distribution of the three petrofacies changed through time in response to landward and seaward shifting of the coastline. During the late Pleistocene lowstand, a pure Apenninic provenance characterized the Romagna coastal plain (Petrofacies A). During the Holocene transgression, when the shoreline migrated tens of kilometers west of its present position, eastern Alpine sediment sources fed littoral facies (Petrofacies B), probably as a result of southward transport by littoral drift. This sediment supply continued during the early regressive phases and was cut off by a change in coastal morphology related to the development of the early Po delta. This led to the establishment of a sediment supply entirely related to the Po River catchment basin (Petrofacies C). Changes in the compositional signature of sand in the youngest beach ridges mark the abandonment of the early Po delta due to an avulsion event and testify to the establishment of a coastal system fed by rivers draining the Apennines (Petrofacies A). We emphasize sand petrography as an important tool in studying the internal architecture of sandy clastic depositional units on detailed spatial scales, and its use in deciphering the depositional history of complex sedimentary successions. FIG. 1. Location of the study area. End_Page 829------------------------

Temporal and Spatial Representativeness of Alpine Sediment Yields: Cascade Mountains, British Columbia

Temporal and Spatial Representativeness of Alpine Sediment Yields: Cascade Mountains, British Columbia

Development and Application of a Method to Estimate Short-Term Alpine Sediment Transfer in the Colorado Front Range, USA

Clastic sediment movement was evaluated in Roaring River Watershed in the Mummy Range of the Colorado Front Range. Photogeologic techniques and fieldwork on identifiable landforms were used to develop a terrain classification of sediment transfer. Terrain units formed the bases for assessing contemporary processes of sediment transport and for testing its usefulness in evaluating sediment flux. Contemporary behavior of terrain units suggests relative stability (short-term, i.e., 50 years) of the present system. The hillslope sediment transfer system appears to act as a closed system and is dominated by internal transfer of sediment moving on slopes with little clastic material entering the fluvial system. The method proposed here can be used by mountain park and wilderness area managers as a framework for determining alpine and subalpine ecosystem disturbance that results from recreational impact.

Metamorphic pressure estimates and sediment volumes for the Alpine orogeny: an independent control on geobarometers?

Theories which regard the deep erosion (20–30 km) of orogenic belts as an important factor in the thermal development of the continents rely on pressure estimates from metamorphic geobarometers to give an estimate of the depth scale for this process. The imprecision of geobarometers, and the apparent lack of sediments that would result from this amount of erosion, are frequently-made arguments for much smaller erosion depths. The Alps are a well-studied young mountain belt and afford the opportunity to compare overburden masses inferred from geobarometry with sediment masses in the surrounding sedimentary basins. This comparison suggests that metamorphic geobarometry has not significantly overestimated burial depths, and that the average erosion to date has been over 15 km, with a maximum of 25 km or more; it is probable that the total erosion in the belt will eventually average 20–25 km, with a maximum of around 40 km. A significant feature of the sediment distribution is that over 50% of the volume is outside the neighbouring along-strike sedimentary basins. This fact, which accounts for previous low estimates of Alpine sediment volumes, follows from the response of the lithosphere to loading by continental thickening and may be a common feature of post-orogenic sedimentation.