Contrasting sediment flux in Val Lumnezia (Graubünden, Eastern Swiss Alps), and implications for landscape development

This paper presents qualitative estimates of sediment discharge from opposite valley flanks in the S–N-oriented Val Lumnezia, eastern Swiss Alps, and relates inferred differences in sediment flux to the litho-tectonic architecture of bedrock. The valley flank on the western side hosts the deep-seated Lumnezia landslide where an area of ca. 30 km² has experienced slip rates of several centimetres per year, potentially resulting in high sediment discharge to the trunk stream (i.e. the Glogn River). High slip rates have resulted in topographic changes that are detectable on aerial photographs and measurable with geodetic tools. In contrast, a network of tributary channels dissects the valley flank on the eastern side. There, an area of approximately 18 km² corresponding to < 30% of the surface has experienced a change in the landscape mainly by rock avalanche and rock fall, and the magnitudes of changes are below the calibration limit of digital photogrammetry. We thus infer lower magnitudes of sediment discharge on the eastern tributaries than on the western valley side, where landsliding has been the predominant erosional process. These differences are interpreted to be controlled by the dip-slope situation of bedrock on the western side that favours down-slope slip of material. Morphometric investigations reveal that the western valley side is characterized by a low topographic roughness because this valley flank has not been dissected by a channel network. It appears that high sediment discharge of the Lumnezia landslide has inhibited the establishment of a stable channel network and has largely controlled the overall evolution of the landscape. This contrasts to the general notion that channelized processes exert the first-order control on landscape evolution and formation of relief and needs to be considered in future studies about landscape architecture, drainage network and sediment discharge.     

Computational validation of static and dynamic plate load testing

In this paper, the static load plate test and the dynamic load plate test by means of the light falling weight device are assessed utilizing numerical simulations. Simplified computational models of the tested subsoil and of the testing devices are developed, which capture the main effects of both the static and the dynamic load plate test. In extensive parametric studies, the impact of various subsoil conditions on the test results and several sources of error are evaluated and discussed. Computational test results of the static load plate test and of the dynamic load plate test are set in contrast to an effort to demonstrate the differences and the common features of the outcomes.     

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s^?1 (equivalent to pure wind velocity of 6 to 8 m.s^?1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s^?1.     

Preliminary investigation results on fabrics and related physical properties of an anisotropic gneiss

In order to evaluate the fabric-dependent anisotropy of a particular gneiss type, we assessed the quartz lattice and shape preferred orientations as well as the microcrack pattern statistics. In the rock mechanics laboratory, several strength and deformability tests on drilled rock samples were run and the seismic wave propagation properties along the rock’s principal strain axes were determined. In the mechanical tests it turned out that despite a distinct stretching lineation with initially extreme grain elongation, the rock performs for the larger part as a transversely isotropic material with the schistosity as plane of isotropy. The anisotropy in seismic wave propagation within the plane of schistosity is attributed primarily to a highly anisotropic microcrack pattern, the distinct gneissose banding and to a lower degree to the LPO of quartz.     

10Be‐derived assessment of accelerated erosion in a glacially conditioned inner gorge,Entlebuch, Central Alps of Switzerland

Inner gorges often result from the propagation of erosional waves related to glacial/interglacial climate shifts. However, only few studies have quantified the modern erosional response to this glacial conditioning. Here, we report in situ ¹⁰Be data from the 64 km² Entlen catchment (Swiss Alps). This basin hosts a 7 km long central inner gorge with two tributaries that are >100 m‐deeply incised into thick glacial till and bedrock. The ¹⁰Be concentrations measured at the downstream end of the gorge yield a catchment‐wide erosion rate of 0.42 ± 0.04 mm yr^‐1, while erosion rates are consistently lower upstream of the inner gorge, ranging from 0.14 ± 0.01 mm yr^‐1 to 0.23 ± 0.02 mm yr^‐1. However, ¹⁰Be‐based sediment budget calculations yield rates of ~1.3 mm yr^‐1 for the inner gorge of the trunk stream. Likewise, in the two incised tributary reaches, erosion rates are ~2.0 mm yr^‐1 and ~1.9 mm yr^‐1. Moreover, at the erosional front of the gorge, we measured bedrock incision rates ranging from ~2.5 mm yr^‐1 to ~3.8 mm yr^‐1. These rates, however, are too low to infer a post‐glacial age (15–20 ka) for the gorge initiation. This would require erosion rates that are between 2 and 6 times higher than present‐day estimates. However, the downcutting into unconsolidated glacial till favored high erosion rates through knickzone propagation immediately after the retreat of the LGM glaciers, and subsequent hillslope relaxation led to a progressive decrease in erosion rates. This hypothesis of a two‐ to sixfold decrease in erosion rates does not conflict with the ¹⁰Be‐based erosion rate budgets, because the modern erosional time scale recorded by ¹⁰Be cover the past 2–3 ka only. These results point to the acceleration of Holocene erosion in response to the glacial overprint of the landscape. Copyright © 2012 John Wiley & Sons, Ltd.     

Paleohydraulic reconstruction of a 40 ka‐old terrace sequence implies that water discharge was larger than today

The evolution of landscapes crucially depends on the climate history. This is particularly evident in South America where landscape responses to orbital climate shifts have been well documented. However, while most studies have focused on inferring temperature variations from paleoclimate proxy data, estimates of water budget changes have been complicated because of a lack of adequate physical information. Here, we present a methodology and related results, which allowed us to extract water discharge values from the sedimentary record of the 40 Ka‐old fluvial terrace deposits in the Pisco valley, western Peru. In particular, this valley hosts a Quaternary cut‐and‐fill succession that we used, in combination with beryllium‐10 (¹⁰Be)‐based sediment flux, gauging records, channel geometries and grain size measurements, to quantitatively assess sediment and water discharge values c. 40 Ka ago in relation to present‐day conditions. We compare these discharge estimates to the discharge regime of the modern Pisco River and find that the water discharge of the paleo‐Pisco River, during the Minchin pluvial period c. 40 Ka ago, was c. 7–8 times greater than the modern Pisco River if considering the mean and the maximum water discharge. In addition, the calculations show that inferred water discharge estimates are mainly dependent on channel gradients and grain size values, and to a lesser extent on channel width measures. Finally, we found that the c. 40 Ka‐old Minchin terrace material was poorer sorted than the modern deposits, which might reflect that sediment transport during the past period was characterized by a larger divergence from equal mobility compared to the modern situation. In summary, the differences in grain size distribution and inferred water discharge estimates between the modern and the paleo‐Pisco River suggests that the 40 Ka‐old Minchin period was characterized by a wetter climate and more powerful flood events. Copyright © 2015 John Wiley & Sons, Ltd.     

Inferring lake depth using diatom assemblages in the shallow,seasonally variable lakes of the Nebraska Sand Hills (USA): calibration,validation, and application of a 69-lake training set

The Nebraska Sand Hills are a distinctive eco-region in the semi-arid Great Plains of the western United States. The water table underlying the Sand Hills is part of the High Plains/Ogallala aquifer, an important water resource for the central Great Plains. Lake levels are affected directly by fluctuations in the water table, which is recharged primarily by local precipitation and responds quickly to climatically induced changes in regional water balance. Instrumental records are available for only 50–100 years, and paleolimnological data provide important insights into the extremes and variability in moisture balance over longer time scales. A set of 69 lakes from across Nebraska was used to establish a statistical relationship between diatom community composition and water depth. This relationship was then used to develop a diatom-based inference model for water depth using weighted averaging regression and calibration techniques. Development of the inference model was complicated by strong intra-seasonal variability in water depth and the linkages between depth and other limnologic characteristics, including alkalinity, water clarity and nutrient concentrations. Analysis of historical diatom communities from eight lakes allowed for the reconstruction of lake-level fluctuations over the past several thousand years. Comparisons of the more recent portion of these reconstructions with the instrumental Palmer Drought Severity Index (PDSI) showed that sediment records may not faithfully reflect short-term fluctuations in water level, except where sedimentation rates are very high. However, large and persistent changes in moisture availability were discernible even in longer, low-resolution records. Thus, diatoms are a useful addition to the tools available for understanding past drought in the central Great Plains, especially when trajectories of change are constrained by data from multiple sites or other proxies.     

Evidence of climatic change from oxygen and carbon isotope variations in sediments of a small arctic lake,Canada

Radiocarbon dating of the organic-rich sediments of Lake Illisarvik in the outer Mackenzie Delta indicates that formation of the lake occurred approximately 9500yr BP, with maximum expansion around 6000 yr BP. Sedimentation rates have remained relatively constant at an average of 0.3mm/yr. ¹³C results on biogenic and inorganic carbonates and organics indicate a change from dominantly terrestrial organics (?27 to ?28%₀) to submerged aquatic vegetation or plankton (?18 to ?23%₀) upon formation of the lake (9500yr BP), and a dramatic return to dominantly terrestrial organics at 5800yr BP (δ¹³C = ?27 to ?30%₀). This latter shift is accompanied by a drastic reduction in the macroflora and fauna populations. ¹⁸O results suggest that a warmer climate than today existed prior to the shift at 5800yr BP.     

The Sukari Gold Mine,Eastern Desert—Egypt: structural setting,mineralogy and fluid inclusion study

The Sukari gold mine (18.8 Mt @ 2.14 g/t Au) is located 15 km west of the Red Sea coast in the southern central Eastern Desert of Egypt. The vein-type deposit is hosted in Late Neoproterozoic granite that intruded island-arc and ophiolite rock assemblages. The vein-forming process is related to overall late Pan-African shear and extension tectonics. At Sukari, bulk NE–SW strike-slip deformation was accommodated by a local flower structure and extensional faults with veins that formed initially at conditions of about 300 °C and 1.5–2 kbar. Gold is associated with sulfides in quartz veins and in alteration zones. Pyrite and arsenopyrite dominate the sulfide ore beside minor sphalerite, chalcopyrite and galena. Gold occurs in three distinct positions: (1) anhedral grains (GI) at the contact between As-rich zones within the arsenian pyrite; (2) randomly distributed anhedral grains (GII) and along cracks in arsenian pyrite and arsenopyrite, and (3) large gold grains (GIII) interstitial to fine-grained pyrite and arsenopyrite. Fluid inclusion studies yield minimum vein-formation temperatures and pressures between 96 and 188 °C, 210 and 1,890 bar, respectively, which is in the range of epi- to mesothermal hydrothermal ore deposits. The structural evolution of the area suggests a long-term, cyclic process of repeated veining and leaching followed by sealing, initiated by the intrusion of granodiorite. This cyclic process explains the mineralogical features and is responsible for the predicted gold reserves of the Sukari deposits. A characteristic feature of the Sukari gold mineralization is the co-precipitation of gold and arsenic in pyrite and arsenopyrite.Editorial handling: H. Frimmel