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

The 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.