Atomic scale transformation of bone in controlled aqueous alteration experiments

Abstract

The chemical and isotopic compositions of biogenic apatite are important geochemical markers, which can suffer modifications during fossilisation. Compared with modern ones, fossil apatites generally exhibit variations in carbonate content, enrichment in fluorine, incorporation of trace elements and an increase in crystallinity parameters. Detailed understanding of these transformations induced by fossilisation should help assess the preservation of geochemical records in apatites. In this contribution, we investigate the transformation of modern bone altered under controlled conditions. Modern bone samples were soaked in aqueous solutions of neutral to alkaline pH (9-10) for one and three weeks at various temperatures (20 and 70 °C) and experiments were duplicated in fluorine-free and in 10-2 M NaF solutions. Bone transformation was monitored through the modifications of chemical (F, Ca, P) and isotopic (δ 13 C, δ 18 Oc, δ 18 Op) composition as well as using vibrational (ATR-FTIR, Raman) and solidstate (1 H, 13 C, 19 F) NMR spectroscopies. The observed modifications sustain a transformation mechanism through partial dissolution of biogenic apatite and precipitation of secondary apatite. This transformation occurs irrespective of the presence or absence of fluorine and leads to the formation of carbonate-bearing fluorapatite or carbonate-bearing hydroxylapatite, respectively. The fraction of secondary apatite seems to be limited to ~60 %, suggesting that its formation has a protecting role against further dissolution of the primary apatite. The observation of clumped (CO 3 2-, F-) defect in the structural B-site of some samples as well as perturbation of the isotopic compositions attest to carbonate incorporation in the secondary apatite. Although the incorporation of fluoride ions can serve as a probe revealing dissolutionrecrystallisation of bone, the present study also underlines the difficulty to systematically relate mineralogical transformations to an open-system behaviour in bioapatite.

Assessing bone transformation in late Miocene and Plio‐Pleistocene deposits of Kenya and South Africa

“…These observations are consistent with those made on artificially altered modern bones by Aufort et al . () who observed the partial dissolution of biogenic apatite and precipitation of secondary apatite, in bone artificially altered in pH‐buffered and fluorinated aqueous solutions. Depending on the composition of the aqueous solution, carbonated hydroxyapatite or fluorapatite was identified as the secondary phase.…”

“…In the case of hydroxyapatite, the incorporation of carbonate in the secondary apatite also occurs during the experimental aqueous alteration of modern bone (Aufort et al . ). However, their spectroscopic signature does not significantly differ from that of the biogenic apatite.…”

“…The highest fraction observed coincides with the maximum of about 60% fraction of secondary apatite determined in modern bones altered under controlled conditions (Aufort et al . ). Accordingly, it supports the idea that the formation of secondary apatite could have a protecting role against further transformation of the original bioapatite.…”

“…The presence of bending and stretching bands related to structural OH groups also supports the formation of secondary hydroxyapatite. This is consistent with the results of bone alteration experiments (Aufort et al ., ), indicating that the formation of secondary hydroxyapatite and fluorapatite always occurs, only depending on the presence or absence of fluoride, respectively. In experimental alterations, two well‐defined trends are thus observed in a diagram reporting the F concentration as a function of the proportion of secondary phase.…”

“…), whereas the formation of secondary hydroxyapatite is less easily detected (Aufort et al . ). In the present work we analyse a series of Plio‐Pleistocene fossil bones from karstic environments of the Bolt's Farm cave system (BFCS) in the Cradle of Humankind, South Africa, and from late Miocene fluvio‐lacustrine environments of the Tugen Hills in the Gregory Rift, Kenya.…”