Headward retreat of streams in the Late Oligocene to Early Miocene Swiss Alps

This study uses the widths, the spacing and the grain-size pattern of Oligo/Miocene alluvial fan conglomerates in the central segment of the Swiss Alpine foreland to reconstruct the topographic development of the Alps. These data are analysed with models of longitudinal stream profile development, to propose that the Alpine topography evolved from an early transient state where streams adjusted to rock uplift by headward retreat, to a mature phase where any changes in rock uplift were accommodated by vertical incision. The first stage comprises the time interval between ca 31 Ma and 22 Ma, when the Alpine streams deposited many small fans with a lateral spacing of <30 km in the north Alpine foreland. As the range evolved, the streams joined and the fans coalesced into a few large depositional systems with a lateral spacing of ca 80 to 100 km at 22 Ma. The models used here suggest that the overall elevation of the Alps increased rapidly within <5 Myr. The variability in pebble size increased either due to variations in sediment supply, enhanced orographic effects, or preferentially due to a change towards a stormier palaeoclimate. By 22 Ma, only two large rivers carried material into the foreland fans, suggesting that the major Alpine streams had established themselves. This second phase of stable drainage network was maintained until ca 5 Ma, when the uplift and erosion of the Molasse started and streams were redirected both in the Alps and in the foreland. This study illustrates that sedimentological archives of foreland basins can be used to reconstruct the chronology of the topographic development of mountain belts. It is suggested that the finite elevation of mountainous landscapes is reached early during orogeny and can be maintained for millions of years, provided that erosion is efficient.

Geomorphic coupling between hillslopes and channels in the Swiss Alps

The coupling relationships between hillslope and channel network are fundamental for the understanding of mountainous landscapes' evolution. Here, we applied dendrogeomorphic methods to identify the hillslope–channel relationship and the sediment transfer dynamics within an alpine catchment, at the highest possible resolution. The Schimbrig catchment is located in the central Swiss Alps and can be divided into two distinct geomorphic sectors. To the east, the Schimbrig earth flow is the largest sediment source of the basin, while to the west, the Rossloch channel network is affected by numerous shallow landslides responsible for the supply of sediment from hillslopes to channels. To understand the connectivity between hillslopes and channels and between sources and sink, trees were sampled along the main Rossloch stream, on the Schimbrig earth flow and on the Rossloch depositional area. Geomorphic observations and dendrogeomophic results indicate different mechanisms of sediment production, transfer and deposition between upper and lower segments of the channel network. In the source areas (upper part of the Rossloch channel system), sediment is delivered to the channel network through slow movements of the ground, typical of earth flow, shallow landslides and soil creep. Contrariwise, in the depositional area (lower part of the channel network), the mechanisms of sediment transfer are mainly due to torrential activity, floods and debris flows. Tree analysis allowed the reconstruction of periods of high activity during the last century for the entire catchment. The collected dataset presents a very high temporal resolution but we encountered some limitations in establishing the source-to-sink connectivity at the catchment-wide scale. Despite these uncertainties, for decennial timescales the results suggest a direct coupling between hillslopes and neighbouring channels in the Rossloch channel network, and a de-coupling between sediment sources and sink farther downstream, with connections possible only during extraordinary events.

River loads and modern denudation of the Alps — A review

This paper presents the first comprehensive analysis of sediment and dissolved load across an entire mountain range. We investigate patterns and rates of modern denudation of the European Alps based on a compilation of data about river loads and reservoir sedimentation from 202 drainage basins that are between ca. 1 to 10,000 km2 large. The study basins cover about 50% of the total area of the Alps. Modern glaciated basins have the highest sediment yields of up to 7000 t km− 2 a− 1, which are on average 5 to 10 times higher than in non-glaciated basins. Likewise sediment yield and glacial cover are positively correlated. Instead, relief is a relatively weak predictor of sediment yield. The strong glacial impact in the correlations is due to glacier recession since the 19th century as well as due to glacial conditioning during repeated Quaternary glaciations which have produced the strong transient state of the Alpine landscape. We suggest that this is the major cause for ca. 3 fold enhanced denudation of the western compared to the eastern Alps. Chemical denudation rates are highest in the external Alps dominated by carbonate sedimentary rocks, where they make up about one third of total denudation. The high rates cannot be explained without anhydrite dissolution. We estimated that only 45% of the sediments mobilized in headwaters are exported out off the Alps, most sediments being trapped in artificial reservoirs. The total amount of sediment annually trapped within the Alps equates to 43 Mt. When corrected for sediment storage, we obtain an area-weighted mean total denudation rate for the Alps of about 0.32 mm a− 1. The pre-dam rate might be as high as 0.42 mm a− 1. In total, ca. 35 plus 23 Mt of mass are exported each year out of the Alps as solids and solutes, respectively. These rates are not enough to out pace modern rock uplift. Nevertheless, pattern of sediment yield across the Alps coincides roughly with the intensity of glacial conditioning and modern rock uplift, supporting the hypothesis of an erosion-driven uplift of the Alps.

Water versus ice: The competing roles of modern climate and Pleistocene glacial erosion in the Central Alps of Switzerland

Recent studies have identified relationships between landscape form, erosion and climate in regions of landscape rejuvenation, associated with increased denudation. Most of these landscapes are located in non-glaciated mountain ranges and are characterized by transient geomorphic features. The landscapes of the Swiss Alps are likewise in a transient geomorphic state as seen by multiple knickzones. In this mountain belt, the transient state has been related to erosional effects during the Late Glacial Maximum (LGM). Here, we focus on the catchment scale and categorize hillslopes based on erosional mechanisms, landscape form and landcover. We then explore relationships of these variables to precipitation and extent of LGM glaciers to disentangle modern versus palaeo controls on the modern shape of the Alpine landscape. We find that in grasslands, the downslope flux of material mainly involves unconsolidated material through hillslope creep, testifying a transport-limited erosional regime. Alternatively, strength-limited hillslopes, where erosion is driven by bedrock failure, are covered by forests and/or expose bedrock, and they display oversteepened hillslopes and channels. There, hillslope gradients and relief are more closely correlated with LGM ice occurrence than with precipitation or the erodibility of the underlying bedrock. We relate the spatial occurrence of the transport- and strength-limited process domains to the erosive effects of LGM glaciers. In particular, strength-limited, rock dominated basins are situated above the equilibrium line altitude (ELA) of the LGM, reflecting the ability of glaciers to scour the landscape beyond threshold slope conditions. In contrast, transport-limited, soil-mantled landscapes are common below the ELA. Hillslopes covered by forests occupy the elevations around the ELA and are constrained by the tree line. We conclude that the current erosional forces at work in the Central Alps are still responding to LGM glaciation, and that the modern climate has not yet impacted on the modern landscape.

Identification of erosional mechanisms during past glaciations based on a bedrock surface model of the central European Alps

The bedrock topography beneath the Quaternary cover provides an important archive for the identification of erosional processes during past glaciations. Here, we combined stratigraphic investigations of more than 40,000 boreholes with published data to generate a bedrock topography model for the entire plateau north of the Swiss Alps including the valleys within the mountain belt. We compared the bedrock map with data about the pattern of the erosional resistance of Alpine rocks to identify the controls of the lithologic architecture on the location of overdeepenings. We additionally used the bedrock topography map as a basis to calculate the erosional potential of the Alpine glaciers, which was related to the thickness of the LGM ice. We used these calculations to interpret how glaciers, with support by subglacial meltwater under pressure, might have shaped the bedrock topography of the Alps. We found that the erosional resistance of the bedrock lithology mainly explains where overdeepenings in the Alpine valleys and the plateau occur. In particular, in the Alpine valleys, the locations of overdeepenings largely overlap with areas where the underlying bedrock has a low erosional resistance, or where it was shattered by faults. We also found that the assignment of two end-member scenarios of erosion, related to glacial abrasion/plucking in the Alpine valleys, and dissection by subglacial meltwater in the plateau, may be adequate to explain the pattern of overdeepenings in the Alpine realm. This most likely points to the topographic controls on glacial scouring. In the Alps, the flow of LGM and previous glaciers were constrained by valley flanks, while ice flow was mostly divergent on the plateau where valley borders are absent. We suggest that these differences in landscape conditioning might have contributed to the contrasts in the formation of overdeepenings in the Alpine valleys and the plateau.

Topoclimatological case-study of Alpine pastures near the Albula Pass in the eastern Swiss Alps

Alpine grasslands are an important source of fodder for the cattle of Alpine farmers. Only during the short summer season can these pastures be used for grazing. With the anticipated climate change, it is likely that plant production – and thus the fodder basis for the cattle – will be influenced. Investigating the dependence of biomass production on topoclimatic factors will allow us to better understand how anticipated climate change may influence this traditional Alpine farming system. Because small-scale topoclimatological variations of the main meteorological variables: temperature, humidity, precipitation, shortwave incoming radiation and wind speed are not easily derived from available long-term climate stations in mountainous terrain, it was our goal to investigate the topoclimatic variations over the pastures belonging to the Alp Weissenstein research station north of the Albula Pass in the eastern Swiss Alps. We present a basic assessment of current topoclimatic conditions as a site characterization for ongoing ecological climate change studies. To be able to link short-term studies with long-term climate records, we related agrometeorological measurements with those of surrounding long-term sites run by MeteoSwiss, both on valley bottoms (Davos, Samedan), and on mountain tops (Weissfluhjoch, Piz Corvatsch). We found that the Davos climate station north of the study area is most closely correlated with the local climate of Alp Weissenstein, although a much closer site (Samedan) exists on the other side of the Albula Pass. Mountain top stations, however, did not provide a convincing approximation for the climate at Alp Weissenstein. Direct comparisons of near-surface measurements from a set of 11 small weather stations distributed over the domain where cattle and sheep are grazed indicate that nocturnal minimum air temperature and minimum vapor pressure deficit are mostly governed by the altitudinal gradient, whereas daily maxima – including also wind speed – are more strongly depending on vegetation cover and less on the altitude.

Index-Oriented Methodologies for Landslide Consequence Analysis: An Application to a Mountain Community in the French Alps

Consequence analysis is a key aspect of anchoring assessment of landslide impacts to present and long-term development planning. Although several approaches have been developed over the last decade, some of them are difficult to apply in practice, mainly because of the lack of valuable data on historical damages or on damage functions. In this paper, two possible consequence indicators based on a combination of descriptors of the exposure of the elements at risk are proposed in order to map the potential impacts of landslides and highlight the most vulnerable areas. The first index maps the physical vulnerability due to landslide; the second index maps both direct damage (physical, structural, functional) and indirect damage (socio-economic impacts) of landslide hazards. The indexes have been computed for the 200 km2 area of the Barcelonnette Basin (South French Alps), and their potential applications are discussed.

Holocene climate, fire and vegetation dynamics at the treeline in the Northwestern Swiss Alps

Treelines are expected to rise to higher elevations with climate warming; the rate and extent however are still largely unknown. Here we present the first multi-proxy palaeoecological study from the treeline in the Northwestern Swiss Alps that covers the entire Holocene. We reconstructed climate, fire and vegetation dynamics at Iffigsee, an alpine lake at 2,065 m a.s.l., by using seismic sedimentary surveys, loss on ignition, visible spectrum reflectance spectroscopy, pollen, spore, macrofossil and charcoal analyses. Afforestation with Larix decidua and tree Betula (probably B. pendula) started at ~9,800 cal. b.p., more than 1,000 years later than at similar elevations in the Central and Southern Alps, indicating cooler temperatures and/or a high seasonality. Highest biomass production and forest position of ~2,100–2,300 m a.s.l. are inferred during the Holocene Thermal Maximum from 7,000 to 5,000 cal. b.p. With the onset of pastoralism and transhumance at 6,800–6,500 cal. b.p., human impact became an important factor in the vegetation dynamics at Iffigsee. This early evidence of pastoralism is documented by the presence of grazing indicators (pollen, spores), as well as a wealth of archaeological finds at the nearby mountain pass of Schnidejoch. Human and fire impact during the Neolithic and Bronze Ages led to the establishment of pastures and facilitated the expansion of Picea abies and Alnus viridis. We expect that in mountain areas with land abandonment, the treeline will react quickly to future climate warming by shifting to higher elevations, causing drastic changes in species distribution and composition as well as severe biodiversity losses.

The potential of stomata analysis in conifers to estimate presence of conifer trees: examples from the Alps

To estimate whether or not a plant taxon found in the fossil record was locally present may be difficult if only pollen is analyzed. Plant macrofossils, in contrast, provide a clear indication of a taxon's local presence, although in some lake sediments or peats, macrofossils may be rare or degraded. For conifers, the stomata found on pollen slides are derived from needles and thus provide a valuable proxy for local presence and they can be identified to genus level. From previously published studies, a transect across the Alps based on 13 sites is presented. For basal samples in sandy silt above the till with high pollen values of Pinus, for example, we may distinguish pine pollen from distant sources (samples with no stomata), from reworked pollen (samples with stomata present). The first apparent local presence of most conifer genera based on stomata often but not always occurs together with the phase of rapid pollen increase (rational limit). An exception is Larix, with its annual deposition of needles and heavy poorly dispersed pollen, for it often shows the first stomata earlier, at the empirical pollen limit. The decline and potential local extinction of a conifer can sometimes be shown in the stomata record. The decline may have been caused by climatic change, competition, or human impact. In situations where conifers form the timberline, the stomata record may indicate timberline fluctuations. In the discussion of immigration or migration of taxa we advocate the use of the cautious term ``apparent local presence'' to include some uncertainties. Absence of a taxon is impossible to prove.

Along-Strike Differences in the Southern Alps of New Zealand: Consequences of Inherited Variation in Rheology

Long- and short-term strain variations along the Australian-Pacific plate boundary through the South Island of New Zealand, including a 300% increase in orogen width, coexistence of oblique thrusting on orthogonal structures, and variability in the locus of orogenic gold deposits, coincide with rheologically relevant geological variation. Our model investigates the consequences of thin, strong lower crust in the north and thick, weak lower crust in the south. Solution of the full 3-D mechanical equations reproduces the larger wavelength strain patterns of the orogen. A 3-D perturbation-based analytical solution leads to the identification of the sensitivity of displacement type to minor stress changes. Transition from boundary-normal thrusting to boundary-parallel thrusting occurs at the transition from strong to weak lower crust and is related to an increase in either tau(yz) (shear stress in the yz plane) or the ratio of the coordinate normal stresses, (sigma(yy)/sigma(xx)), where x and y are in the horizontal and z is vertical. Both mechanisms are compatible with the geologically dependent rheological variation employed in our model. Citation: Upton, P., P. O. Koons, D. Craw, C. M. Henderson, and R. Enlow (2009), Along-strike differences in the Southern Alps of New Zealand: Consequences of inherited variation in rheology, Tectonics, 28, TC2007, doi:10.1029/2008TC002353.

A model-data comparison of Holocene timberline changes in the Swiss Alps reveals past and future drivers of mountain forest dynamics

Mountain vegetation is strongly affected by temperature and is expected to shift upwards with climate change. Dynamic vegetation models are often used to assess the impact of climate on vegetation and model output can be compared with paleobotanical data as a reality check. Recent paleoecological studies have revealed regional variation in the upward shift of timberlines in the Northern and Central European Alps in response to rapid warming at the Younger Dryas/Preboreal transition ca. 11700years ago, probably caused by a climatic gradient across the Alps. This contrasts with previous studies that successfully simulated the early Holocene afforestation in the (warmer) Central Alps with a chironomid-inferred temperature reconstruction from the (colder) Northern Alps. We use LandClim, a dynamic landscape vegetation model to simulate mountain forests under different temperature, soil and precipitation scenarios around Iffigsee (2065m a.s.l.) a lake in the Northwestern Swiss Alps, and compare the model output with the paleobotanical records. The model clearly overestimates the upward shift of timberline in a climate scenario that applies chironomid-inferred July-temperature anomalies to all months. However, forest establishment at 9800 cal. BP at Iffigsee is successfully simulated with lower moisture availability and monthly temperatures corrected for stronger seasonality during the early Holocene. The model-data comparison reveals a contraction in the realized niche of Abies alba due to the prominent role of anthropogenic disturbance after ca. 5000 cal. BP, which has important implications for species distribution models (SDMs) that rely on equilibrium with climate and niche stability. Under future climate projections, LandClim indicates a rapid upward shift of mountain vegetation belts by ca. 500m and treeline positions of ca. 2500m a.s.l. by the end of this century. Resulting biodiversity losses in the alpine vegetation belt might be mitigated with low-impact pastoralism to preserve species-rich alpine meadows.