Straße am Ende der Welt

Der deutsch-tschechische Architekt und Hochschullehrer Mirko Baum beschreibt seinen Weg zur Architektur und die bestimmenden Einflüsse, die ihn auf diesem Weg prägten: die kultivierte Sachlichkeit von Konstruktionen und Materialien ... und die Perspektive der tschechischen Moderne.

Paleo erosion rates and climate shifts recorded by Quaternary cut-and-fill sequences in the Pisco valley, central Peru

Fluvial cut-and-fill sequences have frequently been reported from various sites on Earth. Nevertheless, the information about the past erosional regime and hydrological conditions have not yet been adequately deciphered from these archives. The Quaternary terrace sequences in the Pisco valley, located at ca. 13°S, offer a manifestation of an orbitally-driven cyclicity in terrace construction where phases of sediment accumulation have been related to the Minchin (48–36 ka) and Tauca (26–15 ka) lake level highstands on the Altiplano. Here, we present a 10Be-based sediment budget for the cut-and-fill terrace sequences in this valley to quantify the orbitally forced changes in precipitation and erosion. We find that the Minchin period was characterized by an erosional pulse along the Pacific coast where denudation rates reached values as high as 600±80 mm/ka600±80 mm/ka for a relatively short time span lasting a few thousands of years. This contrasts to the younger pluvial periods and the modern situation when 10Be-based sediment budgets register nearly zero erosion at the Pacific coast. We relate these contrasts to different erosional conditions between the modern and the Minchin time. First, the sediment budget infers a precipitation pattern that matches with the modern climate ca. 1000 km farther north, where highly erratic and extreme El Niño-related precipitation results in fast erosion and flooding along the coast. Second, the formation of a thick terrace sequence requires sufficient material on catchment hillslopes to be stripped off by erosion. This was most likely the case immediately before the start of the Minchin period, because this erosional epoch was preceded by a >50 ka-long time span with poorly erosive climate conditions, allowing for sufficient regolith to build up on the hillslopes. Finally, this study suggests a strong control of orbitally and ice sheet forced latitudinal shifts of the ITCZ on the erosional gradients and sediment production on the western escarpment of the Peruvian Andes at 13° during the Minchin period.

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.

Hillslope Processes in Temperate Environments

In this chapter, we discuss the factors controlling the mechanisms and rates of hillslope failure in temperate environments with a major focus on the Swiss Alps. We frame this presentation by defining Alpine hillslopes as either strength- or transport-limited hillslopes. We organize this discussion into individual sections that outline how hillslope processes are related to (1) the mechanical strength and bedding orientation of bedrock, (2) the competition between channelized and hillslope processes, (3) hillslope–channel coupling relationships, and (4) fluvial erosion rates. We find that hillslope angles depend on bedrock strength along nonincised channels, but are not related to this parameter in inner gorges. We also find that valley flanks host deep-seated landslides where the bedrock dips parallel to the topographic slope. In the opposite case, the valley sides are dissected by a network of bedrock channels bordered by strength-limited hillslopes. In this chapter, we illustrate that a high ratio between sediment discharge on hillslopes and in channels explains the formation of smooth landscapes with low channel densities and long response times. This chapter considers the formation of strength-limited hillslopes as a consequence of an upslope-directed coupling between channels and hillslopes. The chapter also discusses that soil-mantled hillslopes occur where fluvial incision rates are less than weathering rates of bedrock, which are limited to 0.1–0.3 mm yr−1. We finally present evidence for a decreasing trend of hillslope-derived sediment discharge during the Holocene, but predict an opposite trend in the nearest future as winters are warmer and wetter.

Lateral sediment sources and knickzones as controls on spatio-temporal variations of sediment transport in an Alpine river

Modern mixed alluvial-bedrock channels in mountainous areas provide natural laboratories for understanding the time scales at which coarse-grained material has been entrained and transported from their sources to the adjacent sedimentary sink, where these deposits are preserved as conglomerates. This article assesses the shear stress conditions needed for the entrainment of the coarse-bed particles in the Glogn River that drains the 400 km2 Val Lumnezia basin, eastern Swiss Alps. In addition, quantitative data are presented on sediment transport patterns in this stream. The longitudinal stream profile of this river is characterized by three ca 500 m long knickzones where channel gradients range from 0·02 to 0·2 m m−1, and where the valley bottom confined into a <10 m wide gorge. Downstream of these knickzones, the stream is flat with gradients <0·01 m m−1 and widths ≥30 m. Measurements of the grain-size distribution along the trunk stream yield a mean D84 value of ca 270 mm, whereas the mean D50 is ca 100 mm. The consequences of the channel morphology and the grain-size distribution for the time scales of sediment transport were explored by using a one-dimensional step-backwater hydraulic model (Hydrologic Engineering Centre – River Analysis System). The results reveal that, along the entire trunk stream, a two to 10 year return period flood event is capable of mobilizing both the D50 and D84 fractions where the Shields stress exceeds the critical Shields stress for the initiation of particle motion. These return periods, however, varied substantially depending on the channel geometry and the pebble/boulder size distribution of the supplied material. Accordingly, the stream exhibits a highly dynamic boulder cover behaviour. It is likely that these time scales might also have been at work when coarse-grained conglomerates were constructed in the geological past.

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.

Why Monte Carlo Simulations are Inferences and not Experiments

Monte Carlo simulations arrive at their results by introducing randomness, sometimes derived from a physical randomizing device. Nonetheless, we argue, they open no new epistemic channels beyond that already employed by traditional simulations: the inference by ordinary argumentation of conclusions from assumptions built into the simulations. We show that Monte Carlo simulations cannot produce knowledge other than by inference, and that they resemble other computer simulations in the manner in which they derive their conclusions. Simple examples of Monte Carlo simulations are analysed to identify the underlying inferences.

Episodic Stream Acidification Caused by Atmospheric Deposition of Sea Salts at Acadia National Park, Maine, United States

Major episodic acidifications were observed on several occasions in first-order brooks at Acadia National Park, Mount Desert Island, Maine. Short-term declines of up to 2 pH units and 130-mu-eq L-1 acid-neutralizing capacity were caused by HCl from soil solutions, rather than by H2SO4 or HNO3 from precipitation, because (1) SO4 concentrations were constant or decreased during the pH depression, (2) Cl concentrations were greatest at the time of lowest pH, and (3) Na:Cl ratios decreased from values much greater than those in precipitation (a result of chemical weathering), to values equal to or less than those in precipitation. Dilution, increases in NO3 concentrations, or increased export or organic acidity from soils were insufficient to cause the observed decreases in pH. These data represent surface water acidifications due primarily to an ion exchange "salt effect" of Na+ for H+ in soil solution, and secondarily to dilution, neither of which is a consequence of acidic deposition. The requisite conditions for a major episodic salt effect acidification include acidic soils, and either an especially salt-laden wet precipitation event, or a period of accumulation of marine salts from dry deposition, followed by wet inputs.

The Response of a Small Stream in the Lesni Potok Forested Catchment, Central Czech Republic, to a Short-Term In-Stream Acidification

Lesni Potok stream drains a forested headwater catchment in the central Czech Republic. It was artificially acidified with hydrochloric acid (HCl) for four hours to assess the role of stream substrate in acid-neutralisation and recovery. The pH was lowered from 4.7 to 3.2. Desorption of Ca and MP and desorption or solution of Al dominated acid-neutralisation; Al mobilisation was more important later. The stream substrate released 4.542 meq Ca, 1, 184 meq Mg, and 2,329 meq Al over a 45 in long and I in wide stream segment, smaller amounts of Be. Cd, Fe, and Mn were released. Adsorption of SO42- and desorption of F- occurred during the acidification phase of the experiment. The exchange reactions were rapidly reversible for Ca, Mg and SO42- but not symmetric as the substrate resorbed 1083, 790 and 0 meq Ca, Mg, and Al. respectively, in a 4-hour recovery period. Desorption of SO42- occurred during the resorption of Ca and Mg. These exchange and dissolution reactions delay acidification, diminish the pH depression and retard recovery from episodic acidification. The behaviour of the stream substrate-water interaction resembles that for soil-soil water interactions. A mathematical dynamic mass-balance based model, MASS (Modelling Acidification of Stream Sediments), was developed which simulates the adsorption and desorption of base cations during the experiment and was successfully calibrated to the experimental data.

Long-Term and Seasonal Variations in CO2: Linkages to Catchment Alkalinity Generation

As atmospheric emissions of S have declined in the Northern Hemisphere, there has been an expectation of increased pH and alkalinity in streams believed to have been acidified by excess S and N. Many streams and lakes have not recovered. Evidence from East Bear Brook in Maine, USA and modelling with the groundwater acid-base model MAGIC (Cosby et al. 1985a,b) indicate that seasonal and yearly variations in soil PCO2 are adequate to enhance or even reverse acid-base (alkalinity) changes anticipated from modest decreases of SO4 in surface waters. Alkalinity is generated in the soil by exchange of H+ from dissociation of H2CO3, which in turn is derived from the dissolving of soil CO2. The variation in soil PCO2 produces an alkalinity variation of up to 15 mu eq L-1 in stream water. Detecting and relating increases in alkalinity to decreases in stream SO4 are significantly more difficult in the short term because of this effect. For example, modelled alkalinity recovery at Bear Brook due to a decline of 20 mu eq SO4 L-1 in soil solution is compensated by a decline from 0.4 to 0.2% for soil air PCO2. This compensation ability decays over time as base saturation declines. Variable PCO2 has less effect in more acidic soils. Short-term decreases of PCO2 below the long-term average value produce short-term decreases in alkalinity, whereas short-term increases in PCO2 produce shortterm alkalization. Trend analysis for detecting recovery of streams and lakes from acidification after reduced atmospheric emissions will require a longer monitoring period for statistical significance than previously appreciated.

Increasing Silicon Concentrations in Bohemian Forest Lakes

Long-term trends of dissolved silicon (Si) concentrations in five glacial lakes in the Bohemian Forest, Czech Republic, recovering from acidification show higher mobility of Si from the soil to surface waters despite lower atmospheric deposition of acids. Si increased by 0.95 to 1.95 mu mol yr(-1) (36 to 51%) from 1986-2004 and with increasing pH. A change in soil solution conditions because of a sharp decrease in acidic deposition has led to marked decline in Al mobility and to considerable decreases in dissolved Al, especially Al3+. The increase in Si may be related to: (1) unblocking of the inhibitory effect of dissolved Al on weathering of alummosilicates, (2) biogenic opal (phytoliths) dissolving faster, and/or (3) lower Si precipitation as secondary alummosilicates in soil. The change in Al speciation on the dissolution rate of biogenic silica is critical. A lack of change in Si at sites outside central Europe may be explained by small or no decline in mobility of dissolved Al. The effect of a long-term increase in temperature was probably minor.

Response of a First-Order Stream in Maine to Short-Term In-Stream Acidification

An experimental short-term acidification with HCl at a first-order stream in central Maine, USA was used to study processes controlling the changes in stream chemistry and to assess the ability of stream substrate to buffer pH. The streambed exerted a strong buffering capacity against pH change by ion exchange during the 6-hour acidification. Streambed substrates had substantial cation and anion exchange capacity in the pH range of 4.1 to 6.5. The ion exchange for cations and SO42- were rapid and reversible. The speed of release of cations from stream substrates was Na1+ > Ca2+ > Mg2+ > Aln+ > Be2+, perhaps relating to charge density of these cations. Ca2+ desorption dominated neutralisation of excess H+ for the first 2 hr. As the reservoir of exchangeable Ca diminished, desorption land possibly dissolution) of Al3+ became the dominant neutralising mechanism. The exchangeable land possibly soluble) reservoir of Al was not depleted during the 6-hour acidification. Sulphate adsorption during the acidification reduced the concentration of SO42- in stream water by as much as 20 mu eq L-1 (from 70 mu eq L-1). Desorption of SO42- and adsorption of base cat ions after the artificial acidification resulted in a prolongation of the pH depression. The streambed had the capacity to buffer stream water chemistry significantly during an acidifying event affecting the entire upstream catchment.