Neogene desertification of Africa

International audienc

Long-term dynamic topographic support during post-orogenic crustal thinning revealed by stable isotope (δ18O) paleo-altimetry in eastern Pyrenees

Altres ajuts: CERCA Programme/Generalitat de CatalunyaUnderstanding the geodynamic and Earth surface processes at the origin of post-collisional surface uplift in mountain ranges requires reconstruction of paleo-elevation. Here, we focus on the topographic evolution of the Cerdanya Basin in the eastern Pyrenees formed by post-orogenic extension during the Late Miocene. Stable isotope (δ18O) analyses of small rodent teeth and biogenic carbonates show the basin uplifted by 500 m since 6.5 Ma. These new paleoaltitudes constraints when combined with the regional geology and geophysical data reveal the anomalously high topography of the region is the result of density changes in the sublithospheric mantle associated with crustal thinning and then opening of Gulf of Lion during the Chattian-early Burdigalian

Modeling the attenuated total reflectance infrared (ATR-FTIR) spectrum of apatite

International audienceAttenuated total reflectance (ATR) infrared spectra were measured on a synthetic and a natural fluorapatite sample. A modeling approach based on the computation of the Fresnel reflection coefficient between the ATR crystal and the powder sample was used to analyze the line shape of the spectra. The dielectric properties of the samples were related to those of pure fluorapatite using an effective medium approach, based on Maxwell–Garnett and Bruggeman models. The Bruggeman effective medium model leads to a very good agreement with the experimental data recorded on the synthetic fluorapatite sample. The poorer agreement observed on the natural sample suggests a more significant heterogeneity of the sample at a characteristic length scale larger than the mid-infrared characteristic wavelength, i.e., about 10 micrometers. The results demonstrate the prominent role of macroscopic electrostatic effects over fine details of the microscopic structure in determining the line shape of strong ATR bands

Environmental significance of Upper Miocene phosphorites at hominid sites in the Lukeino Formation (Tugen Hills, Kenya)

The Lukeino Formation contains an important sedimentary and fossiliferous record of the late Miocene (6.09–5.68 Ma), which has in particular yielded the fossil remains of the oldest East African bipedal hominid called Orrorin tugenensis. This fluvio-lacustrine sedimentary succession crops out in the Kenyan part of the East African Rift. It is mainly composed of clay to sandy clay deposits intercalated with volcanic ash horizons, and localized layers of carbonates and diatomites. A detailed sedimentological and mineralogical study of the Lukeino Formation was conducted to throw light on the environmental conditions in which the hominids lived. Several centimetric, relatively continuous and indurated phosphatic horizons, of sedimentary origin, were identified at two sites (Sunbarua and Kapcheberek). Mineralogical (XRD) and geochemical analyses as well as observations by SEM, which was coupled with an energy dispersive spectroscopy (EDS) microprobe, indicate that the autochthonous phosphate layers are composed of a micritic matrix of francolite (38–93%), with incorporation of silicates in variable proportions from one layer to another. The phosphate matrix contains very well preserved and abundant diatom frustules in the basal phosphate layer. These diatoms are identified as Aulacoseira granulata, implying a pH of 7.8–8.2 for freshwaters of the Palaeolake Lukeino. Calcitic tubular structures, linked to a possible bacterial origin, are also observed locally. Phosphate layers occur abruptly within a thick clay–sandy series, associated with an intense runoff phase during the deposition of this interval of the Lukeino Formation. The massive and cyclic input of phosphorus to the lake promoted productivity to the stage where it caused a diatom bloom. The establishment of several phosphate horizons testifies to successive phases of eutrophication of Palaeolake Lukeino. The diatom cells provided some of the organic matter, which was decomposed by bacterial activity at the bottom of the lake in suboxic conditions, but in insufficient quantities to fully form the phosphatic materials. The rest of the organic matter needed for phosphogenesis came from terrigenous supply (plant debris), suggesting the presence of dense vegetation in the catchment of the Palaeolake Lukeino, during this well-drained interval of deposition of the Lukeino Formation

Site-specific equilibrium isotopic fractionation of oxygen, carbon and calcium in apatite

International audienceThe stable isotope composition of biogenic apatite is an important geochemical marker that can record environmental parameters and is widely used to infer past climates, biomineralization processes, dietary preferences and habitat of vertebrates. In this study, theoretical equilibrium isotopic fractionation of oxygen, carbon and calcium in hydroxyapatite and carbonate-bearing hydroxyapatite is investigated using first-principles methods based on density-functional theory and compared to the theoretical isotopic fractionation properties of calcite, CO 2 and H 2 O. Considering the variability of apatite crystal-chemistry, special attention is given to specific contributions of crystal sites to isotopic fractionation. Significant internal fractionation is calculated for oxygen and carbon isotopes in CO 3 between the different structural sites occupied by carbonate groups in apatite (typically 7‰ for both 18 O/ 16 O and 13 C/ 12 C fractionation at 37 °C). Compared with calcite-water oxygen isotope fractionation, occurrence of A-type substitution in apatite structure, in addition to the main B-type substitution, could explain the larger temperature dependence of oxygen isotope fractionation measured at low temperature between carbonate in apatite and water. Theoretical internal fractionation of oxygen isotopes between carbonate and phosphate in B-type carbonated apatite ($8‰ at 37 °C) is consistent with experimental values obtained from modern and well-preserved fossil bio-apatites. Concerning calcium, theoretical results suggest a small fractionation between apatite and calcite (À0.17‰ at 37 °C). Internal fractionation reaching 0.8‰ at 37 °C occurs between the two Ca sites in hydroxyapatite. Furthermore, the Ca isotopic fractionation properties of apatite are affected by the occurrence of carbonate groups, which could contribute to the variability observed on natural samples. Owing to the complexity of apatite crystal-chemistry and in light of the theoretical results, measurements of site-specific isotopic fractionation properties could improve our understanding and the interpretation of isotopic records in apatites

Bone diagenesis and origin of calcium phosphate nodules from a hominid site in the Lukeino Formation (Tugen Hills, Kenya)

International audienceThe Lukeino Formation (6.09-5.68 Ma; Kenya) is a fossiliferous fluvio-lacustrine sedimentary sequence (~100 m thick), which yielded the fossil remains of the oldest East African bipedal hominid, called Orrorin tugenensis. At one of the hominid sites (Aragai), centimetric to pluri-centimetric, indurated calcium phosphate nodules occur in a specific sedimentary layer. Mineralogical and geochemical analyses, coupled with SEM, optical microscopy and cathodoluminescence observations, were performed on four calcium phosphate nodules and one bone showing differential alteration, with characteristics locally similar to that of the nodules, in order to determine the nature and origin of these nodules. A crystallinity index (CI) and histological index (HI) were also assigned to these samples to characterize their degree of transformation. Analyses reveal that all nodules and the studied bone have a similar mineralogical and chemical composition. Nodules show no internal structure and are composed of a carbonate-fluorapatite matrix, including feldspars, clays and spheres of iron and manganese oxides. Their high CI indicates significant recrystallization compared to modern bone. The altered bone shows infilling of the trabecular cavities, characterized by a clay deposit along the walls, oxide spheres with a silica center and carbonate-fluorapatite baguettes. In the highly altered area of the bone, a phenomenon of dissolution and dismantlement of bone structures adjacent to an area characterized by the presence of a phosphate matrix is observed. The genesis of these calcium phosphate nodules seems to result from an advanced stage of transformation of bones, due to a combination of physical, chemical and biological (bacterial) processes. The different stages of diagenesis are characterized by the incorporation of components from the sedimentary layer (including diatoms) and by neoformations of clays and oxides in the bone pores. The bone structure is affected by fracturing and dissolution of carbonate-hydroxylapatite, which reprecipitated in bone cavities under the form of carbonate-fluorapatite automorphic crystals. Then, the bone structures and mineral neoformations were affected by a dismantlement phenomenon, which increased the dissolution of the bone apatite. This resulted in a carbonate-fluorapatite matrix, in which no trace of the initial bone structure is visible. This particular mechanism of genesis of calcium phosphate nodules most probably takes place by action of fluid circulation in a confined environment such as a lake edge or marsh

Des volcans éteints et des fossiles par millier en Ouganda

International audienc

Bone diagenesis in arid environments; An intra-skeletal approach

International audienceBone trace element content and isotopic composition are closely related to human nutrition. The investigation ofarchaeological bone geochemistry can help us to better understand the relationship between past populationsand their environment alongside cultural practices as inferred from dietary reconstruction. However, dietary in-formation may be altered post-mortem by diagenetic processes in soil. In this study, bone mineralogy (Ca/P, sec-ondary minerals, organic matter content and bone apatite crystallinity), histology, element content (Mg, Na, F, Sr,Ba, Mn, Fe, La, Ce and U) and stable isotope composition (δ13Candδ18O carbonate) were investigated at the intra-individual scale in order to understand the effects of diagenesis on skeletons buried in different saharo-sahelianenvironments. Between 10 to 18 bone samples were taken from 4 Neolithic skeletons excavated in theMauritanian Dhar Oualata and Néma and in the Daounas, Mali. Additionally, the enamel of two third molarteeth was also analysed from each skeleton for comparison with bone.The results show that the four skeletons, buried in the same desert climate area, all exhibited different degrees ofdiageneticmodification,relatedto localtaphonomic conditions. Highly drainedand periodically flooded environments generated substantial bone bacterial damage, low to moderate apatite crystallinity and secondary minerals in bone pores. Bone trace element content and isotopic composition were more diagenetically affectedthan in bones from skeletons buried in a drier environment, which display little bioerosion, high apatite crystallinity and the absence or late precipitation of secondary minerals in their bone pores.Intra-skeletal variability of the geochemical composition, and the comparison of geochemical data from bonesand teeth, enables the approximation of ante-mortem bone trace element and stable isotope compositionsusing the best preserved bones from each skeleto