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Gallala W.,Faculte des science de Gabes | Gaied M.E.,University of Sfax | Essefi E.,Ecole Nationale des Ingenieurs de Sfax ENIS | Montacer M.,University of Sfax
Journal of African Earth Sciences | Year: 2010

This paper is meant to study the stratigraphy, the mineralogy, the microstructure and the geochemistry of Pleistocene calcretes from eastern Tunisia in order to infer the environmental factors intervening in their formation.Samples of eight profiles of Pleistocene calcretes from eastern Tunisia were examined on the basis of a variety of techniques including Optical Microscopy (OM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), chemical analysis and Atomic Absorption Spectrophotometer (AAS) techniques. Then, the obtained data underwent a statistical analysis on the basis of Factor Analysis (FA) and Principal Component Analysis (PCA).On the basis of field missions, five different horizons have been differentiated from bottom to top of all profiles: nodular, powdery, massive Brecciated and laminar horizon. The mineralogical study shows two minerals categories inversely proportional: calcite and (quartz and the clay). It shows also shows that Palygorskite is the dominant clay mineral. The escarpment edge is capped by a limestone containing fibrous palygorskite. Finally, superficial calcrete are described: a brecciated horizon which occurs in pockets on the plateau surface. This study about eastern Tunisia revealed the occurrence of successive cycles of calcretisation. Pedogenesis, water table oscillation, sedimentogenesis and stromatogenesis are the intervening factors in the calcretisation process. During the Pleistocene, they interfered with each other according to the climatic pulsations. From the studied case, it may be noticed that the formation of each calcrete horizon is the result of a dominating process that takes place during a distinguishable stage. In the first stage, the pedogenic process is developed by palygorskite formation including authigenic replacement or formation from a precursor mineral, neoformation from the breakdown products of such minerals or neoformation from suitable solutions. In the second stage, the powdery horizon is formed in the slope of the distal zone which presents a drained environment. In the third stage, several diagenetic processes (cementing, compaction, dissolution...) contribute to the formation of the laminar and massive horizon. Since it is exposed to dryness for a long period, the massive horizon is harder and more compact. In the fourth stage, the banding of light-dark in the laminar horizons reflects a dry-wet season alternation seasons. Dark beds are formed by the stromatolitic cover were developed during the wet season, whereas light beds were developed in an extremely arid climate argued by the presence of the detrital grains. In the fifth stage, the brecciated horizon, which occupies the channels, is formed by well rolled concretions, which present a dismantling material of Early and Middle Pleistocene calcretes after the Post-Villafranchian compressive phase. Thus, calcretisation seems to have been controlled by periods of uplift and stability of the slope, given that calcrete formation might be inhibited by the activation of the sedimentation of colluvial materials as a consequence of the tectonic activity. We also suggest that groundwater and biological activity may play a significant role in the development of pedogenic, sedimentological and polygenetic calcrete cycles within the same sedimentary basin. The alternation of dry and wet climatic periods may be responsible for the calcrete genesis. © 2010 Elsevier Ltd. Source

Mahmoudi S.,Faculte des science de Gabes | Srasra E.,Center des Materiaux | Zargouni F.,Departement de Geologie
Journal of African Earth Sciences | Year: 2010

The Valangenian-Hauterivian clays of Tunisia located at Djebel Oust were characterised by chemical analysis, X-ray diffraction and scanning electron microscopy in order to determine if these clays are appropriate for the manufacturing of ceramic products for construction. The mineralogical study shows that these clays are mainly of illitic type. However, other clay minerals, such as kaolinite, I/Sm mixed-layer, and chlorite, are also present. The chemical analysis indicates that the clays are notably siliceous, carbonated and aluminous. The content of iron is variable. Under firing tests on clay raw material until 1200. °C, quartz, cristobalite, hematite, mullite and sillimanite minerals were also identified. The amount of interstratified I/Sm and chlorite is higher in the samples located at the west zone of Djebel Oust than those located at the east. However, under firing behaviour of both types of raw samples seems to be similar. As conclusion of this study, these clays meet the firing specifications required for the manufacturing of ceramic products for construction. © 2010. Source

Leclaire S.,CNRS Center of Evolutionary and Functional Ecology | Czirjak G.A,Leibniz Institute for Zoo and Wildlife Research | Hammouda A.,Faculte des science de Gabes | Gasparini J.,University Pierre and Marie Curie
BMC evolutionary biology | Year: 2015

BACKGROUND: Complex communities of bacteria inhabit the feathers of all birds. Under normal conditions, individuals maintain a healthy state by defending themselves against these potential invaders by preening. The immune system is only triggered when bacteria gain access into the body. Preening is, however, costly and may trade-off with investment in the immune system. To shed light on how birds balance the trade-off between immunity and preen secretions when facing high or low feather bacterial load, we experimentally manipulated feather bacteria load of feral pigeons (Columba livia), and investigated the effects on immune defenses.RESULTS: Birds facing high feather bacterial load had lower immune response to PHA skin-swelling test (a measure of induced pro-inflammatory capacity) than controls, while birds facing low feather bacterial load had higher blood bacterial killing ability (a measure of the capacity to eliminate bacterial pathogens) than controls. No other components of the immune system (i.e., hemagglutination and hemolysis capacity of plasma, primary and secondary responses to KLH and quantity of blood parasites) were found to be affected by feather bacterial load.CONCLUSION: Pigeons had previously been shown to adjust preening to feather bacterial load. The decrease in the energetically costly inflammatory response of birds experiencing high bacterial load suggests a trade-off between investment in preen secretion and immunity and reinforces the idea that feather microbiota may have a strong impact on the ecology and evolution of the avian host. Source

Urbain X.,Catholic University of Louvain | Lecointre J.,Catholic University of Louvain | Mezdari F.,Faculte des science de Gabes | Miller K.A.,Columbia University | Savin D.W.,Columbia University
Journal of Physics: Conference Series | Year: 2012

Total and partial cross sections have been measured for the mutual neutralization of H- and H+ by means of a merged and inclined beam set-up. The low energy data between 10 meV and 5 eV contradict one previous set of measurements, while above 5 eV the data fall in excellent agreement with previously published results by two other groups. Source

Chokri M.A.,Faculte des science de Gabes | Hammouda A.,Faculte des science de Gabes | Tavecchia G.,CSIC - Mediterranean Institute for Advanced Studies
Bird Study | Year: 2011

Capsule Crèches formed early in the season lasted longer than those formed later, but a longer crèching period did not appear to confer higher chick survival. Aims To investigate the ecological factors influencing the benefit to parents of crèching behaviour by measuring chick survival. Methods Mark-recapture was used to model apparent daily survival of 505 chicks during the crèching period in three different crèches. We contrasted models with different tipping points to assess possible differences across crèches in chick survival during the first week and in the moment at which chick departures began. Results We did not find a clear difference across crèches on daily chick survival during the first stages of the crèche. By modelling chick apparent survival as a linear function of time we showed that the latest formed crèche dispersed more rapidly. Conclusions The two crèches formed early in the season lasted longer than the one formed later but chicks did not appear to have a higher survival over the first week of crèching. We suggest that a longer period at the crèche should result in a higher survival in the period soon after fledging because chicks leave the crèche 4-7 days older than other chicks. Furthermore, early crèches are synchronous with those of other species breeding in the same area, thus perhaps diluting predation. We discuss the limitations of our analysis and the possible implications for the community of waterbirds breeding at our study site. © 2011 British Trust for Ornithology. Source

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