Stratigraphie, sédimentologie et magnétostratigraphie du site oldowayen de Fejej FJ-1, Sud-Omo, Ethiopie : premiers résultats

Le gisement oldowayen FJ-1 de Fejej est situé dans le sud-ouest de l’Ethiopie, près de la basse vallée de l’Omo et à proximité de la rive orientale du lac Turkana. La stratigraphie du site FJ-1 est divisée en 5 ensembles qui témoignent de dépôts fluviatiles ou de plaine d’inondation et d’une activité volcanique régionale. Les analyses granulométriques indiquent que le transport avant le dépôt des sédiments a été de courte durée. La minéralogie montre clairement que les principales sources des sédiments sont le socle métamorphique de la chaîne Hamar et le haut plateau basaltique, situés au nord-est de la zone d’étude. La combinaison de ces analyses montre que le transport a été essentiellement assuré par un affluent de la rivière Omo. Les mesures du paléomagnétisme effectuées sur toute la stratigraphie du gisement ont permis de situer le site FJ-1 dans un événement positif. L’apport des autres études telles que la paléontologie, la téphrochronologie et les datations RPE sur quartz blanchis permettent d’attribuer celui-ci à l’évènement d’Olduvai et au niveau archéologique C1 un âge compris entre 1,96 et 1,90 Ma.The FJ-1 site of Fejej is located in the south-western most part of Ethiopia near the lower Omo valley, on the eastern side of Lake Turkana. The FJ-1 site’s stratigraphy is divided into 5 members deposited by river bed and flood plain sediments and a regional volcanic event. Morphoscopic and granulometric studies show that the transport of sediment was brief and the mineralogy has revealed that sediments originated from the metamorphic Hamar Range and from the High Plateau, both located to the northeast of Fejej. Sedimentological studies indicate that transport was essentially assured by a tributary of the Omo River. Palaeomagnetic measurements indicate the positive Olduvai event. The contribution of palaeontology and radiometric dating indicates that the age of the C1 archaeological horizon is between 1,96 and 1,90 Ma

Traditional and Molecular Techniques for the Study of Emerging Bacterial Diseases: One Laboratory’s Perspective

Identification of emerging bacterial pathogens generally results from a chain of events involving microscopy, serology, molecular tools, and culture. Because of the spectacular molecular techniques developed in the last decades, some authors think that these techniques will shortly supplant culture. The key steps that led to the discovery of emerging bacteria have been reviewed to determine the real contribution of each technique. Historically, microscopy has played a major role. Serology provided indirect evidence for causality. Isolation and culture were crucial, as all emerging bacteria have been grown on artificial media or cell lines or at least propagated in animals. With the use of broad-range polymerase chain reaction, some bacteria have been identified or detected in new clinical syndromes. Culture has irreplaceable advantages for studying emerging bacterial diseases, as it allows antigenic studies, antibiotic susceptibility testing, experimental models, and genetic studies to be carried out, and remains the ultimate goal of pathogen identification

Magnetostratigraphy of the Hettangian Langmoos section (Adnet, Austria): evidence for time-delayed phases of magnetization

International audienceStudy of the magnetostratigraphy of the earliest Jurassic (mainly Hettangian) section of Langmoos near Adnet in Austria (Northern calcareous Alps) provides a sequence of 10 magnetic polarity intervals. These intervals are often evident in the natural remanent magnetization. However, rather than a true magnetostratigraphic sequence , palaeomagnetic analyses reveal evidence for successive phases of mag-netization within the section. Evidence includes (1) changes in lithology associated with changes in magnetic polarity, (2) different combinations of components of both polarities, and (3) largely non-antipodal directions. Considering the palaeomagnetic directions, we suggest that the apparent magnetostratigraphy is the result of two or three phases of magnetization which occurred at different stages of the tectonic emplacement of the Northern Calcareous Alps

Reply to the comment by J. E. T. Channell and J. S. Stoner on ‘Magnetostratigraphy of the Hettangian Langmos section (Adnet, Austria): evidence for time-delayed phases of magnetization’

International audienceIn our paper, we have considered that the early origin of haematite in the reddish wackestones from the Langmoos section was questionable for several reasons: (1) no clear reversal test (antipodal directions) is available in this lithology; (2) we have evidence for remagnetization at least in parts of the sequence; and, (3) only one site was studied. The comparison (i.e. a fold test) between the characteristic component obtained from the reddish samples and the ones obtained from the Adnet Formation (Channell et al. 1990, 1992) does not provide a clear answer (the fold test is negative) because the bedding attitude is rather flat in this area. Therefore, no direct comparison can be made between the results from the Langmoos section, which does not belong to the Adnet Formation, and those obtained from the sites near by Adnet. Considering this point, we have only proposed that the magnetization in our samples was acquired before the large rotation of the NCA

Néel temperatures of synthetic substituted goethites and their rapid determination using low-field susceptibility curves

International audienceThe NOel temperatures (TN) of synthetic goethites (c•-FeOOH), substituted with diamagnetic elements (A1, Ga) have been determined from the temperature dependence of their low-field susceptibility using a Kappabrige AC bridge. These temperatures proved to be identical to those derived fi'om more conventional measurements of high-field or remanent magnetization versus temperature, and in good agreement with those inferred fi'om M0ssbauer models. The low-field susceptibility based-technique appears to offer a precise and rapid method of determining T N. While NOel temperature coincides with maximtan unblocking temperature in Ga-goethites, supporting the assumption that antiferromagnetism and parasitic ferromagnetism have a similar origin in goethite, the lack of clear correlation between saturation remanent magnetization and increasing diamagnetic substitution suggests that the weak ferromagnetism of goethite is due to substitution independent defects induced during crystal growth

Volumetric changes in weathered profiles: iso-element mass balance method questioned by magnetic fabric

International audienceA major issue in understanding weathering processes is to determine to what extent fabrics, structures and volumes from the parent rock are preserved through the weathering profile. Isoelement mass balance methods (IMB) are often used to estimate volume changes during progressive weathering, based on the assumption that a given element (either Ti, Zr or Th) is not mobile. The petrofabric of the weathered material is often characterized by a mimetic replacement of primary minerals that fully preserve the primary fabric. This suggests an isovolume weathering as volume change must be associated with shape change due to boundary conditions not allowing horizontal strains. So collapse or dilation should induce vertical compaction or constriction, respectively. The weak petrofabric of weathered materials may be precisely quantified using the anisotropy of low-field magnetic susceptibility technique (AMS). This paper reports, for the first time, a combined IMB and AMS study of two different environments: a complex lateritic sequence on Precambrian metamorphic rocks in Cameroon and a weathering profile on a Pleistocene basaltic flow in Morocco. The lateritic profile, divided into a lower saprolite zone (>12 m) and an upper nodular iron-rich unit (4 m), is characterized by neoformed magnetic minerals (goethite/hematite/spinels) and a weak but rather consistent magnetic fabric. Saprolitization, which induces low susceptibility values (50×10 −9 m 3 /kg), preserves the linear tectonic fabric of the parent gneiss. AMS evidence for isovolumetric weathering agrees with Th based IMB, while Ti and Zr partly indicate apparent collapse and dilation (up to 50%), respectively. Ferralitization in the nodular iron crust, which enhances susceptibilities (500 to 700×10 −9 m 3 /kg, due to spinel phases), induces drastic reductions in anisotropy due to multiple generation of neoformed minerals destroying primary fabric. However, a weak horizontal planar magnetic fabric suggests compaction, in agreement with Th and Ti based IMB while Zr would indicate isovolume weathering. Conversely, in the one meter thick weathering profile on basalt, the strong susceptibility (1.5 to 3×10 −6 m 3 /kg) is dominated by inherited titanomaghemite grains which totally preserve the weak magmatic fabric of the fresh basalt. This again suggests that volume change is minor, while Ti and Zr based IMB indicate 20 to 50% of dilation. In both examples, large volume changes inferred from the IMB in units showing preserved primary AMS fabric appear unrealistic. Our AMS study, together with the large discrepancies between the IMB results produced by various elements, indicate that the assumption of immobility of a given element is probably not fulfilled all along these profiles. Isovolumetric weathering in the saprolitic zone may be more widespread than suggested by IM

Stratigraphie, sédimentologie et magnétostratigraphie du site oldowayen de Fejej FJ-1, Sud-Omo, Ethiopie : premiers résultats

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