549 research outputs found
Recherches archéobotaniques du SFB 268 en Afrique de l'ouest
Der Beitrag gibt einen Ăberblick ĂŒber 20 Jahre archĂ€obotanische Studien in West-Afrika. Hauptforschungspunkte waren die holozĂ€ne Vegetationsgeschichte der westafrikanischen Savanne und die Entwicklung des Ackerbaus. FĂŒr das gesamte HolozĂ€n konnte die Existenz klimatisch bedingter Savannen bestĂ€tigt werden. AchĂ€obotanische Daten bezeugen das Auftreten von Ackerbau fĂŒr ungefĂ€hr 2000 v. Chr. und die damit verbundene Entwicklung einer Kulturlandschaft.An overview over 20 years of archaeobotanical studies in West Africa is given. The Holocene vegetation history of the West African savannahs and the development of plant cultivation were major research topics. The existence of climatically induced savannahs throughout the Holocene could be confirmed. Archaeobotanical data indicate the late emergence of agriculture around 2000 BC and the development of a cultural landscape in the course of the last 2000 years.Cet article donne une vue d'ensemble de 20 ans de recherches en Afrique de l'Ouest. L'histoire de la vĂ©gĂ©tation de l'holocĂšne des savanes ouest-africaines et le dĂ©veloppement de la cultivation de plantes Ă©taient des sujets de recherche principaux. L'existence de savanes climatiques dans tout l'HolocĂšne pouvait ĂȘtre confirmĂ©e. Des donnĂ©es archĂ©obotaniques indiquent l'apparition tardive de l'agriculture autour de l'an 2000 av.J.C et le dĂ©veloppement d'un paysage de culture au cours des 2000 ans passĂ©s
A description of the Jacobson topology on the spectrum of transformation group C*-algebras by proper actions
Diese Arbeit liefert eine Beschreibung der Jacobson-Topologie auf dem Spektrum von Transformationsgruppen-C*-Algebren fĂŒr eigentliche G-RĂ€ume X vermittels des Raums Stab(X)^={(x,G_x,sigma) | x in X, sigma in (G_x)^}. Es wird gezeigt, daĂ auf Stab(X)^ eine Topologie und eine G-Wirkung derart definiert werden können, daĂ die aus dem Satz von Mackey-Rieffel-Green bekannte Bijektion zwischen dem Bahnenraum G\Stab(X)^ und dem Spektrum des verschrĂ€nkten Produkts C_0(X)xG ein Homöomorphismus ist. Wir diskutieren verschiedene AnsĂ€tze zur Topologisierung des Raums Stab(X)^ und zeigen, daĂ sie dieselbe Topologie liefern
Early food production in the Sahel of Burkina Faso
This paper is concerned with the transition from hunting and gathering to food production in West Africa, based on evidence from the Sahel Zone of Burkina Faso compiled by field research during the last years. Our study intends to enhance the knowledge about the West African versions of this transition, traditionally seen as one of the most fundamental changes in human prehistory. Embedded in an interregional program the Sahel Zone of Burkina Faso has proved to be one of its most unexpected examples
Paléoenvironnement et préhistoire au Sahel du Burkina Faso
Au Sahel du Burkina Faso, le diagramme pollinique d'Oursi fournit des Ă©vidences sur l'apparition de l'agriculture, il y a environ 3000 ans. A partir de ces faits, le but principal des recherches archĂ©ologiques dans le cadre du SFB 268 Ă©tait de trouver et de fouiller des sites qui seraient Ă mĂȘme d'apporter des informations sur cette pĂ©riode dĂ©cisive autour de 3000 ans avant l'actuel. Pour cela, nous avons concentrĂ© notre travail sur les provinces du SĂ©no et plus particuliĂšrement de l'Oudalan. Le paysage de cette rĂ©gion est marquĂ© par des dunes qui s'organisent grossiĂšrement en cordons d'orientation nord-est/sud-ouest Ă travers notre terrain
Masakwa dry season cropping in the Chad Basin
In the inundation area - the basin of the former larger Lake Chad - a special type of sorghum is grown on the clay soils (firgi). This dry-season guinea corn is also called dwarf sorghum or masakwa. In Kanuri, the dominant language in the region, sorghum is called ngawuli. The dry-season types are called ngawuli firgibe (lit. translated: sorghum of the firgi). During the dry season when the natural vegetation becomes dry and yellow, masakwa fields appear in prominent green covering large areas of the clay plains. The most important natural factor for this specialized dry season cropping is the presence of soils with a high clay content. For a better understanding of masakwa and its related issues, a multidisciplinary sub-project (G1) has been established within the SFB 268 (Joint Research Project: History of Culture and Language in the Natural Environment of the West-African Savannah). This project in which all disciplines participate is entitled: "Natural basis for masakwa cultivation and its meaning for the settlement history of the clay plains (firgi) in the Chad basin"
Einfluss von Mutationen im Bereich der ĂŒberlappenden Leserahmen fĂŒr Polymerase und OberflĂ€chenprotein des Hepatitis-B-Virus auf die FunktionalitĂ€t beider Proteine
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Application of High T/High P Rock Mechanics to the Problem of Extraction of Granitic Magma from its Protolith
The mechanical deformation behaviour of a partially molten granitic protolith is investigated, implications towards a mechanism for extraction of low melt percentages («30 %) are mathematically explored.
For this purpose a series of experiments on Westerly granite (with no added water) are presented in which samples were heated to temperatures between 800 and 1200 °C, at a confining pressure of 250 MPa. Samples were uniaxially deformed to study the effects of constant strain-rate (4 * 10-4 to 2 * 10-7 s-1), creep and stress relaxation tests on the mechanical behaviour. The volumetric percentage of melt (Ί) in samples ranged from 3 % at 800 °C to 77 % a t 1200 °C at heating times between 2.5 and 170 hours. However, none of the samples attained chemical equilibrium, due to the short duration of tests. An equilibrium melt percentage was also not reached. Over this temperature interval the supported strength decreased from 500 MPa to less than 1 MPa (at a constant strain rate of 8 * 10-5 s-1) monotonically. No step-like drop in strength, corresponding to a rheological critical melt percentage (van der Molen & Paterson 1979) was observed. A preliminary flow law for the partially molten rock was obtained to allow extrapolation to lower strain-rates. The viscosity of the melt was estimated at 950 and 1000 °C from distances it could be made to permeate into porous sand under a known pressure gradient. Melt distribution in static tests indicated a very low wetting angle (< 5°) which results in high melt contiguity even at low melt percentages along grain boundaries.
Under all conditions, the solid phases deformed by brittle fracture only. Samples with 3 shear enhanced compaction of melt filled voids by analogy with uniaxial compaction of porous sands. In samples with Ί > 45 % grains were carried about passively in the flowing liquid.
A simple mathematical model is erected to describe a two stage process of extracting low volume percentage, granitic melt from its partially molten protolith with the aid of non-hydrostatic stress. Shear-enhanced compaction drives melt from the protolith into a series of interconnected melt-filled veins, whereupon porous flow through the high-permeability vein network allows rapid drainage of melt to higher crustal levels. Modeling suggests that melt extraction, with Ί11 Pas, is possible within geologically realistic time periods (10 Ma)
Böden und Vegetation in TrockenwĂ€ldern SĂŒdwest-Burkina Fasos
Im SW Burkina Fasos (sechs Monate Regenzeit und durchschnittlich ĂŒber 1000 mm Niederschlag) wurden mehrere TrockenwĂ€lder auf ihr Artenspektrum und die BodenverhĂ€ltnisse hin untersucht. Die Waldformationen fallen durch die Dichte der Gehölzbedeckung, ihren Lianenreichtum und das fast vollstĂ€ndige Fehlen von GrĂ€sern auf. Typische Gehölzarten sind Anogeissus leiocarpus, Diospyros mespiliformis und die Liane Saba senegalensis. Der dichte Strauchunterwuchs und die fehlende Grasschicht verhindern das regelmĂ€Ăige Eindringen von Buschfeuern. Wegen der Dichte und des Alters der BĂ€ume belegen diese Waldformationen, daĂ an den Standorten, zumindest fĂŒr einen sehr langen Zeitraum, kein Feldbau betrieben wurde. Daher konnten sich, auch auf eher als ungĂŒnstig zu bewertenden Böden, TrockenwĂ€lder ausbilden, die zumindest in ihrer Physiognomie der potentiellen natĂŒrlichen Vegetation entsprechen. Jedoch finden sich in den WĂ€ldern oft Spuren menschlicher AktivitĂ€ten aus der Vergangenheit, so z.B. Steinsetzungen, SiedlungshĂŒgel und Gruben. AuĂerdem lassen sich vielfach Anzeichen einer rezenten Nutzung beobachten, so z.B. fĂŒr die Entnahme von Werkholz, das Schneiteln mancher Baumarten zur Viehfuttergewinnung, gelegentliche Beweidung und das Sammeln von Wildpflanzen
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