Phosphate uptake is an essential process for rapid bone mineralization during early diagenesis – evidence from bone alteration experiments

Bones and teeth are often the only fossil remains of vertebrates that are preserved over geological time in sedimentary rocks. They render valuable archives for geochemical proxies which are commonly used for paleo-reconstructions. However, the fossilization mechanisms of bone are not yet well understood. Crucial processes are the transformation of bioapatite into and the replacement of collagen by thermodynamically more stable apatite phases, such as fluorapatite. In the present study, aqueous alteration experiments on cortical bone samples were performed under simulated early diagenetic conditions in order to investigate whether and how an external phosphate (PO₄) source affects post mortem bone mineralization. Additionally, abiotic oxygen isotope exchange mechanisms between bioapatite and aqueous solutions were assessed by using either ¹⁸O-enriched water or PO₄ as a tracer. The presence of an external sedimentary ¹⁸O-labeled PO₄ source led to a rapid formation of new fluorapatite crystallites at the sample's margin that was highly enriched in ¹⁸O. Meanwhile, in the interior of the samples carbonate-poor hydroxylapatite formed through a dissolution-precipitation process without incorporating significant amounts of the ¹⁸O tracer. These two processes appear to act independently from each other. In samples exposed to ¹⁸O-labeled aqueous solutions lacking a PO₄ source, no newly grown apatite crystallites were found, however in the interior of these samples, nano-crystalline carbonate-poor or -free hydroxylapatite precipitated. A comparatively low but uniform ¹⁸O-enrichment was measured from the sample's margin towards its interior, which is assumed to have resulted from the adsorption of H₂¹⁸O onto crystallite surface sites and collagen. Overall, our results suggest that a fast incorporation of ¹⁸O-doped PO₄ from the sediment source accelerated bone mineralization and consequently changed the oxygen isotope composition of the PO₄ group in the bone mineral phase more rapidly and to a greater extent than in a diagenetic setting lacking additional external PO₄. Dissolved PO₄ from the taphonomic setting thus seems to be an important factor fostering bone fossilization and preservation as well as oxygen isotope alteration.