Diagenèse osseuse et genèse de nodules phosphatés, associés aux sédiments fluvio-lacustres de sites à hominidés miocène (Kenya)

audience: researche

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

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