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Throughout the Quaternary, variations of the insolation received over Africa have governed the monsoon dynamics in this region, generating a recurrence of intense rainfall periods. These African Humid Periods (AHP) are characterized by a major transformation of the Saharan hydrological cycle, favouring the development of vast fluvial systems and tropical humid ecosystems in the currently hyper-arid Sahara Desert. In the current context of global warming, the mechanisms as well as the environmental responses associated with these periods of rapid changes between two extreme climatic contexts remain crucial to understand. Many studies have investigated the mechanisms associated with the last AHP that occurred in the early Holocene (9 to 5ka), and more particularly its initiation and termination. Despite all these efforts, these climatic transitions remain highly debated (e.g. influence of high latitudes versus regional forcing, vegetation feedback). Here, we propose to improve our understanding of the Holocene AHP by studying Saharan dust deposited in Lake Bastani (Corsica, western Mediterranean) during the last 12ka. Indeed, as dust emissions are function of the aridity of their sources, among other parameters such as wind intensity, Saharan dust fluxes recorded over and out of Africa may represent an indirect way to reconstruct Sahara past hydrological changes. Bastani Lake is a high elevation system with a very restricted watershed and has been described as a natural Saharan dust trap during the last 3ka (Sabatier et al., accepted). In this study, we present a Holocene multi-proxy characterization of the fine-grained sediments recorded in Bastani lake. We develop a multiproxies approach based on mineralogy and major elements composition of the clay fraction as well as microscopic observations and quantification of the biogenic silica, which complicates Saharan dust supply estimation in this system. This effort to decipher the Bastani lake sediments composition will allow us to qualify and quantify the Saharan dust signal from the bulk sediment record (watershed erosion/alteration, biogenic silica productivity) in order to discuss, to our knowledge, the northernmost aeolian response of the Sahara desert hydrological changes of the termination of this key climatic transition. Reference: Sabatier et al., Past African dust inputs in Western Mediterranean area controlled by the complex interaction between ITCZ, NAO and TSI, Climate of the Past, accepted.Keywords: Saharan dust, Saharan hydrological cycle, Paleoclimatology, Holocene, clay mineralogy, geochemistry, biogenic silica.

International audience

International audience; Siliceous banded and nodular iron ores intercalated in mica schists and marbles of Precambrian age, were studied from the NW part of Iran (Takab region). The iron ore is composed of magnetite, which was partly transformed into maghemite and hematite along fractures and cleavages during a major oxidizing deformation event. Quartz shows undulose extinction. Both quartz and iron oxides partly form 120° triple junctions, but essentially grain boundary migrations. In the nodular ore, grain boundary migration is more pronounced leading also to grain isolation quartz is partially annealed. Iron oxides are agglomerated, elongated and disrupted. These finding indicate a longer recrystallization episode in the nodular ore. The iron oxide grains contain inclusions of euhedral, zoned and corroded zircons and euhedral monazite. Magnetite contains traces of Al. Hematite and goethite show higher Al, Si and Ca contents. Later hydrothermal solutions precipitated goethite surrounding the magnetite-hematite-maghemite grains and replacing hematite. Barite and uraninite occur in fractures of iron oxides, Mn-Ba-Pb oxi-hydroxides and scheelite occur interstitial to iron oxides.

International audience; Banded iron formations (BIFs) comprise complex textures and mineralogy, which result from fluid-rock interactions related to high and low temperature alteration. The initial iron oxy hydroxide mineralogy and associated phases such as carbonates, quartz, apatite and phyllosilicates were transformed leading to an upgrading of these BIFs into the world's largest source of iron ore. In low-grade BIFs, a large part of the iron is related to micro-and nano-metric iron-bearing inclusions within micrometric quartz and/or carbonates (mainly dolomite). We studied laminated jaspilitic BIF samples from a drill core containing 26.71 wt.% total iron, 0.2 wt.% SiO 2 , 0.32 wt.% MnO, 15.46 wt.% MgO, 22.32 wt.% CaO, 0.09 wt.% P 2 O 5 , < 0.05 wt.% Al 2 O 3 , 0.15 wt.% H 2 O and 34.08 wt.% CO 2 (Àguas Claras Mine, Quadrilátero Ferrífero, Brazil). Bright rose coloured dolomite and quartz bands alternate with massive specular hematite bands. Raman spectroscopy, X-ray diffraction and FIB-TEM analyses reveal that the micro-and nano-metric inclusions in dolomite are mainly hematite and minor goethite, partly occurring as clusters in voids. Curie Balance analyses were carried out at different heating steps and temperatures on whole rock samples and a synthetic mix of decarbonated sample and pure dolomite. X-ray diffraction on the products of the heating experiments shows that that hematite is stable and new phases: magnesioferrite (MgFe 2 O 4), lime (CaO), periclase (MgO), portlandite (Ca(OH) 2) and srebrodoskite (Ca 2 Fe 2 O 5) were formed between 680°C and 920°C. These finding gives hints to optimizing the beneficiation process, as the presence of hydroxyl ions bearing goethite micro-and nanoinclusions lowers the sintering temperature. Prior separation of coarse hematite and barren dolomite and quartz, followed by lower temperature sintering of the inclusion-bearing dolomite/quartz leads to transformations into phases with higher magnetic susceptibilities (such as hematite and magnesioferrite). The entire Fe and Fe/Mg oxide feed (the priorly separated fraction added to the sintered fraction) can then pass through wet-high intensity magnetic separation after crushing.

International audience

International audience; Fossil materials that contain iron sulfide are well known for their instability when exposed to oxygen and humidity. This term however combines a great variety of materials showing different types of damages. Most of them consist of crystal efflorescence appearing on the surface and inside the matrix. In this work, a methodology was determined for the analysis of these damages by the use of Raman and infrared spectroscopy. The infrared and Raman signatures of a large set of iron sulfates were characterized. Specific attention was paid to sideronatrite and ferrinatrite, which are two associated sodium/ iron(III) sulfates, and their infrared and Raman bands were partially assigned. Analysis performed on a selection of 11 damaged fossils showed a great variety of degradation products: besides one case that appeared to be a synthetic resin close to polyvinylchloride acetate, which was applied with a brush on the fossil surface, all degradation products belong to the sulfate group. However, many iron-free sulfates, such as gypsum, halotrichite, epsomite, or pentahydrite were found, often in association with iron sulfates. In one case, despite the presence of iron in the matrix, no iron sulfate could be detected. This shows that the term 'pyritic fossil', commonly used by collection managers, is not appropriate as it oversimplifies the reality. A name such as 'sulfide-containing fossil' would be more suitable.