Experience from preparation and application of till/bentonite mixtures

Clay-based layers which serve to minimize infiltration of surface water into waste piles seem to form a promising concept, since their hydraulic conductivity can be kept so low that the net effect of cyclic drying and wetting in Sweden will be that no water penetrates. Problems may arise, however, from physico/chemical processes as well as from swelling, one example being given in this paper.     

325-MHz observations of the ELAIS-N1 field using the Giant Metrewave Radio Telescope

We present observations of the European Large-Area ISO Survey-North 1 (ELAIS-N1) at 325 MHz using the Giant Metrewave Radio Telescope (GMRT), with the ultimate objective of identifying active galactic nuclei and starburst galaxies and examining their evolution with cosmic epoch. After combining the data from two different days we have achieved a median rms noise of  ≈40 μJy  beam⁻¹, which is the lowest that has been achieved at this frequency. We detect 1286 sources with a total flux density above  ≈270 μJy  . In this paper, we use our deep radio image to examine the spectral indices of these sources by comparing our flux density estimates with those of Garn et al. at 610 MHz with the GMRT, and surveys with the Very Large Array at 1400 MHz. We attempt to identify very steep spectrum sources which are likely to be either relic sources or high-redshift objects as well as inverted-spectra objects which could be Giga-Hertz Peaked Spectrum objects. We present the source counts, and report the possibility of a flattening in the normalized differential counts at low flux densities which has so far been reported at higher radio frequencies.     

Urban malaria in Africa: an environmental and socio-economic modelling approach for Yaoundé, Cameroon

Most of the existing predictive malaria risks spatial models use very broad spatial scales, and they are usually built-up for continental or national outlines. These models usually do not account for the complexity of socio-economic variables intervening into the malaria transmission process as well malaria prevention strategies. These spatial and thematic shortcomings are particularly interesting when looking at urban environments. This paper explains why a focus on urban malaria overcomes these shortcomings. A set of environmental variables derived from remotely sensed and ground climate station sources was used to build-up an ecological model. An original data collection process based on GPS measurements and retrospective interviews was introduced for the creation of various geoepidemiological and geosocial variables. Multinomial logistic models were used to predict and evaluate the contribution of various thematically separated groups of variables to malaria risk. Finally, it was possible to statistically and spatially evaluate the contribution of ecologic, socio-ecologic, socio-economic and behavioural characteristics of these groups of variables to malaria risks.     

Quaternary of Chile: The state of research

The aim of this paper is to present a summary of current knowledge about Quaternary climatic changes, sea level fluctuations, tectonic deformation, and volcanic activity in Chile. In the Andean highlands of the hyperarid desert, glaciers and lakes fluctuated repeatedly. Evidence of pluvial periods is not well documented in the marginal desert. On the contrary, pronounced climatic changes are recorded in semiarid Chile. In central Chile two or three major glacial advances have been identified, but they remained confined within the high Cordillera. In the longitudinal valley of the Lake Region geomorphic remnants of four glaciations have been described; ¹⁴C dates are available for the last glaciation. The glacial history of the Fjord Region is still obscure. Whether Pleistocene climatic changes in the northern and southern part of Chile were synchronous or not is a problem which requires further investigation. Sea level fluctuations along the Chilean coast are in part ascribed to glacio-eustatic effects. They left striking sets of step-like marine terraces in northern and central Chile. From a paleontologic point of view the Pleistocene corresponds to a pronounced move toward isolation and endemic development of the marine fauna. The Quaternary tectonic tendency seems to be toward extension and not compression expected as sea-floor spreading compensation. Normal faults limiting uplifted, downwarped, and tilted blocks are common. Folds are rarely found. Northern Chile is characterized by an imposing chain of about 600 stratovolcanoes. They rest on Tertiary ignimbrites which cover the altiplano. Quartz-bearing latite-andesites are predominant. Present volcanic activity is sporadic and weak. South of a conspicuous gap between 27 and 33°S, Quaternary volcanism reappears in the high Cordillera, and many volcanoes have erupted violently within historic times. Rocks are fundamentally andesite or basaltic andesite. Poorly sorted ashes including pumice clasts in the Central Valley south of Santiago are interpreted as volcanic mudflows of late Pleistocene age.     

Tracking Holocene environmental changes in an alpine lake sediment core: application of regional diatom calibration, geochemistry, and pollen

Diatom abundances in the surface sediment samples of 41 mountain lakes in the central Austrian Alps (Niedere Tauern) were related to environmental variables using multi-variate techniques. Canonical correspondence analysis (CCA) revealed that the pH, date of autumn mixing (A _mix), mean August water temperature (T _Aug), dissolved organic carbon (DOC), and relative water depth (Z _rel) made significant contributions to explain the diatom assemblage variation in the lakes of the training set. A weighted averaging partial least square regression and calibration model was used to establish Di-pH (R ² _boot= 0.72, RMSEP_boot= 0.131), and a thermistor measurements-based PLS model for A _mix (R ² _boot= 0.71, RMSEP_boot= 0.006 log₁₀ Julian days). The latter showed a better prediction than T _Aug, and was used in terms of climate change. These transfer functions, together with analyses of loss on ignition (LOI), the total carbon/nitrogen (C/N)-ratios, and selected pollen, were applied to an early to mid-Holocene (11.5–4 cal. ky BP) sediment core section from an Austrian Alpine treeline lake on crystalline bedrock. Additionally, passive sample scores in the CCA of the diatom training set were used to show trends in the variables DOC and Z _rel. During the early Holocene, diatoms indicative of increased pH, extended warm summers, and low water levels dominated. Between 10.2 and 7.6 cal. ky BP it was followed by diatom assemblages that indicated an increase in lake water depth and an earlier A _mix. The multi-proxy data suggest that the A _mix decline is the result of a series of snow-rich summer cool and wet climate fluctuations, which were divided by climate warming at ∼9 cal. ky BP. Increased A _mix, LOI and DOC, and the correspondent decline in the C/N-ratios, show subsequent climate warming between 7.3 and 6 cal. ky BP. The long-term trend in Di-pH indicates the impact of catchment-related processes during the early-Holocene, that were superimposed by climate.     

Monsoonal forcing of Holocene paleoenvironmental change on the central Tibetan Plateau inferred using a sediment record from Lake Nam Co (Xizang,China)

This study focuses on Holocene monsoon dynamics on the central Tibetan Plateau (TP) inferred using a sediment record from Lake Nam Co. A high-resolution (decadal) multi-proxy approach, using geochemical, micropaleontological, and sedimentological methods was applied. Fifteen AMS-¹⁴C ages were used to establish the chronology, assuming a reservoir effect of 1,420 years. Our results point to a first strong monsoonal pulse at Lake Nam Co ~11.3 cal ka BP, followed by colder and drier conditions until ~10.8 cal ka BP. A warm and humid climate from ~10.8 to ~9.5 cal ka BP is related to a strong summer monsoon on the central TP, triggering a lake-level highstand at ~9.5 cal ka BP. Declining minerogenic input after ~9.5 cal ka BP indicates less moisture availability until ~7.7 cal ka BP. Following stable conditions between ~7.7 and ~6.6 cal ka BP, a warm and wet climate is inferred for the time span from ~6.6 to ~4.8 cal ka BP. A change towards drier conditions after ~4.8 cal ka BP points to a weakening of the Indian Ocean Summer Monsoon on the central TP, further diminished after ~2.0 cal ka BP. A short-term wet spell occurred from ~1.5 to ~1.3 cal ka BP. By comparing the results derived from Lake Nam Co with several lacustrine records from the central, northern, and eastern TP, a similar but not synchronous pattern of monsoon-driven paleoenvironmental change is observed. Although the general trend of lake and catchment evolution on the TP during the Holocene is also reproduced in this record, pronounced spatial and temporal offsets with respect to distinct climate events were detected, suggesting periods of non-uniform moisture and temperature evolution.     

Diatoms as indicators of climatic and limnological change in Swedish Lapland: a 100-lake calibration set and its validation for paleoecological reconstructions

This study investigated the distribution of subfossil diatom assemblages in surficial sediments of 100 lakes along steep ecological and climatic gradients in northernmost Sweden (Abisko region, 67.07° N to 68.48° N latitude, 17.67° E to 23.52° E longitude) to develop and cross-validate transfer functions for paleoenvironmental reconstruction. Of 19 environmental variables determined for each site, 15 were included in the statistical analysis. Lake-water pH (8.0%), sedimentary loss-on-ignition (LOI, 5.9% and estimated mean July air temperature (July T, 4.8%) explained the greatest amounts of variation in the distribution of diatom taxa among the 100 lakes. Temperature and pH optima and tolerances were calculated for abundant taxa. Transfer functions, based on WA-PLS (weighted averaging partial least squares), were developed for pH (r² = 0.77, root-mean-square-error of prediction (RMSEP) = 0.19 pH units, maximum bias = 0.31, as assessed by leave-one-out cross-validation) based on 99 lakes and for July T (r² = 0.75, RMSEP = 0.96 °C, max. bias = 1.37 °C) based on the full 100 lake set. We subsequently assessed the ability of the diatom transfer functions to estimate lake-water pH and July T using a form of independent cross-validation. To do this, the 100-lake set was divided in two subsets. An 85-lake training-set (based on single limnological measurements) was used to develop transfer functions with similar performance as those based on the full 100 lakes, and a 15-lake test-set (with 2 years of monthly limnological measurements throughout the ice-free seasons) was used to test the transfer functions developed from the 85-lake training-set. Results from the intra-set cross-validation exercise demonstrated that lake-specific prediction errors (RMSEP) for the 15-lake test-set corresponded closely with the median measured values (pH) and the estimations based on spatial interpolations of data from weather stations (July T). The prediction errors associated with diatom inferences were usually within the range of seasonal and interannual variability. Overall, our results confirm that diatoms can provide reliable and robust estimates of lake-water pH and July T, that WA-PLS is a robust calibration method and that long-term environmental data are needed for further improvement of paleolimnological transfer functions.     

Pilot field experiment with canister-embedding clay under simulated repository conditions

Precompacted annular clay blocks surrounding a casing with a heater were emplaced in a borehole with 20 cm diameter and 2.8 m depth in granite. The heater was in operation for 8 months and raised the temperature of the clay to 176°C close to the casing. At the extraction the clay was completely water-saturated close to the rock and it had experienced an increased water content also close to the hot casing.

Mineralogical changes of the Ca-smectite clay were indicated by a slight increase in CEC and by an XRD-based conclusion that the content of low-charge smectite increased, while the initial content of kaolinite was reduced.     

Zur Entstehung der Trou-Au-Natron-Caldera (Tibesti-Gebirge,Zentral-Sahara) aus photogeologischer Sicht

Aerial photographs of the “Trou au Natron” and “Doôn Kinimi” calderas (Tibesti Mts., Central Sahara) have been interpreted and plotted in order to obtain qualitative and quantitative information. The Trou au Natron sets an example for the possibility to identify the origin of a caldera by means of analysing aerial photographs only. Special attention is paid to the pyroclastics, because they had been used so far as an argument for the explosive origin of the Trou au Natron. It is possible to point out that no immediate relation exists between pyroclastics and the formation of the caldera and that the Trou au Natron — contrary to former opinions — presents a collapse caldera.