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Institute
Obwohl die pleistozänen Vereisungen bereits lange Gegenstand geowissenschaftlicher Forschung sind, wird das jüngste Glazial bezüglich Ausmaß und Chronologie kontrovers diskutiert. Für die spanischen Zentralpyrenäen wird derzeit, wie auch für andere Vergletscherungsgebiete während des Spätpleistozäns, eine zum globalen letzten glazialen Maximum (gLGM) frühere und damit asynchrone Vereisung angenommen. Aufgrund der zeitlichen Differenz von 10 ka bis 30 ka vor dem gLGM ist in den spanischen Zentralpyrenäen eine im Vergleich zu den gLGM-zeitlich geprägten Vergletscherungsgebieten differenzierte Geomorphodynamik und Bodenbildung zu erwarten. Ziel der vorliegenden Arbeit ist die Darstellung der spätquartären Bodenentwicklung und Geomorphodynamik in zwei benachbarten Tälern der spanischen Zentralpyrenäen, die beide von gLGM-asynchronen Gletschervorstößen geprägt wurden. Die Untersuchungen wurden mittels einer Kombination sedimentologischer und bodenkundlicher Methoden durchgeführt, um neben der Identifikation der Sedimentfazies die pedogene Entwicklung darzustellen. Die chronologische Einordnung der Befunde erfolgte mittels 15 optisch stimulierter Lumineszenzdatierungen (OSL-Datierungen) und einer 14C-Datierung.
Die Ergebnisse belegen eine prozessual sehr ähnliche Landschaftsentwicklung wie in den mitteleuropäischen Glazial- und Periglazialgebieten. Zwar konnten in der vorliegenden Arbeit keine rißzeitlichen Sedimente datiert werden, die intensive Lessivierung auf den als rißzeitlich beschriebenen lössbedeckten Terrassen wird aber auf die Entwicklung einer Parabraunerde während des letzten Interglazials zurückgeführt. Mit dem Würm beginnt sowohl im Glazialraum als auch im Extraglazialraum eine Phase aktiver Geomorphodynamik. Für den Periglazialbereich können Lössanwehung um 61 ± 4 ka und Solifluktion um 55 ± 5 ka nachgewiesen werden. Die über glazialen Sedimenten hangenden und von Solifluktion und Lösseintrag geprägten Hauptlagen wurden zwischen 14,4 ± 1,2 ka bis 10,6 ± 0,8 ka gebildet. In den rezent von Lessivierung gekennzeichneten Hauptlagen wurden bei den mikromorphologischen Untersuchungen keine Pedorelikte einer früheren Bodenbildung gefunden. Dies weist unter Berücksichtigung der Altersdifferenz von etwa 20 ka zwischen den hier datierten Hauptlagen und dem bisher angenommenen Alter des letzten Gletschervorstoßes um 35 ka entweder auf eine markante Stabilität der Landschaft ohne jegliche Bodenentwicklung über einen Zeitraum von 20 ka oder auf eine massive Erosion vor 14,4 ka. Aufgrund der methodisch problematischen OSL-Datierungen, auf denen die Asynchronitätstheorie basiert, muss die gLGM-asynchrone Chronologie der spanischen Zentralpyrenäen stark angezweifelt werden. Diese Schlussfolgerung wird untermauert durch die in der jüngsten Literatur mehrfach beschriebenen maximalen Gletschervorstöße in den östlichen Pyrenäen, die zum gLGM-synchrone Alter aufweisen, und in Anbetracht der Befunde aus der vorliegenden Arbeit.
This thesis deals with geomorphic processes associated with Small-Scale Opencast Mining (SSOM) in the Gatumba sector of the Western Highlands of Rwanda. In this area tin and tantalum mining was carried out for decades, and it has brought about increase in the wealth and standard of living of the people. Though a lot has been done and achieved in soil erosion research, and despite the enormous wealth coming from mining in Rwanda and Gatumba sector in particular, the negative environmental impacts resulting from mining activities were overlooked by miners and stakeholders and are scarcely addressed in the research yet. This trend was bound to last since there were no valid guidelines for assessing impacts and reclamation of mine sites for the operators in the sector. It was recently, in 2007, that restoration of mining areas has received great attention from the Rwanda Geology and Mine Authority.
The objective of the research was to assess and to increase the understanding of geomorphic impact produced by SSOM. In this regards, the following aspects which indicate the types and spatial distribution of geomorphic processes were measured: (i) investigate the landforms typical of SSOM, (ii) map the watershed morphometry of the study area, (iii) analyse the most relevant properties of soils in term of geomorphology, and (iv) to assess the soil loss potential in the study area.
Primary and secondary data were collected from Government agencies and personal observations. Desk study to review papers and relevant literatures, field observation and experiments, laboratory analyses, mapping and modelling using RUSLE were combined together to develop a practical and integrated methodological approach to effect and realize the objectives. The argument guiding this analytical approach is that physical processes produced by mining cannot be assessed through a single method. The first step mainly concentrated on defining mining sites suitable for assessing geomorphic processes. Within the two studied mines, namely Ruhanga and Gatare, different plots were identified based on the post-mining land uses, to investigate the level of soil and landscape degradation by comparing them with that of control sites located outside of mining influence. Modelling using RUSLE in GIS interface enabled to quantify soil loss potential within the mines and the watershed.
Results indicate that the direct processes associated with opencast mining commonly involve pitting and trenching. Indirect and less conspicuous processes emerge as a long-term consequence of mining. They include depletion of organic matter, compaction or loosening of soil particles in mine sites mostly reflected by low content in organic matter, low rating in soil stable aggregates, and often high soil bulk densities which are variably distributed within the mine sites. This could explain the restriction or lowering, or the rapid infiltration of water into the soil during field experiments, as a consequence probably of the sealing of soil pores or the formation of fissures around mine shafts, from which slides or slumps occur. In average, soil organic matter comprises between 1 and 2.5 % on control sites and ex-mine cultivated sites. organic matter content of soils on reclamation sites was in the same ranges with that of control and ex-mine cultivated sites, but could reach 4,8 % in topsoil of some sample locations. The ex-mine self-recovering sites present much lower organic matter content which doesn’t exceed 2 % in general. The soil aggregation rating of the area is from very low (8%) to low (≤13.5 %). In general, the bulk density ranges between 1.29 to 1.56 g cm-3, and locally can attain 1.76 g cm-3. As a consequence, the total porosity changes locally. Application of correlation and multiple regression models showed a strong influence of soil organic matter on the bulk density. Infiltration tests performed on different experimental sites showed differences of rates in water intake rates as in infiltration curves as well. Infiltration rates are variably distributed over the mines. They range from very slow (3.8 mmhr-1) to rapid (111.18 mm hr-1). Atterberg Limits analyses showed that soils of Gatumba Mining District do not possess extreme properties and they are suitable for engineering purposes. The liquid limit ranges between 51 % and 26 %, whereas the plastic limit is comprised between 22 % and 18 %. The highest plastic index (PI) determined was 29 % and the later soil had a plastic index of 6 %.
The average soil loss in the Gatumba watershed is 27.45 t ha-1 yr-1 with a standard deviation of 0.891. More than 65% of land are prone to high rates of soil loss (exceeding 10 t ha-1 yr-1), and an increasing soil erosion follows increasing slope and land use patterns. The maxima of soil erosion rates are found in Upper Kibilira, Kirombozi and Gisuma catchments respectively. Based on different scenarios applied to quantify soil erosion rates, we found that more the organic matter content from 0. 5%, 2% and >2%) is increased with better support practice (from contour, strip and terrace), more the soil erosion potentials are decreased linearly, in the order of 18.8 to 17.8 t ha-1 yr-1 from 2% to >2 % of organic matter content respectively.
To make SSOM more environmentally sustainable, there is a need of developing integrated practices and cross-cutting approaches that reduce the environmental impact of mining operations, and leave mine sites in an acceptable state for reuse by people or systems. Practices such as isolation of soil and earth material, stabilization of slopes and amendments of sites to be restored should be highly considered in the process of rehabilitation of affected sites.