Thermodynamic stabilities of apatite phases in a wetland system: Implications for bone diagenesis

Abstract

Evaluating the fate and stability of apatite minerals, the largest phosphorus (P) reservoir on Earth's surface, has direct consequences for understanding our vertebrate fossil record as well as biogeochemical cycling. Bone (bioapatite) deposited following the death of an animal represents a potential source of phosphorus in terrestrial and aquatic ecosystems for micro and macrofauna. Exposure of bone to ambient geochemical conditions also represents the first step of the fossilization process. However, the thermodynamic stability of bone in these ecosystems, which are frequently phosphorus-limited, is unknown. The goal of this study was to approach questions related to recrystallization, diagenesis, and fossilization from a thermodynamics perspective, focusing on the role of geochemistry on bioapatite (bone) preservation. By developing thermodynamic models, run using PHREEQC, the stabilities of a range of apatite and other phosphorus-bearing phases were considered under physiochemical conditions measured in a Louisiana wetland and an adjacent river. The models integrated variable temperatures (5–40 °C), concentrations of phosphorus ([PO43-] from 0.0006 to 0.0484 mmol/L), and pH (6.5–9.25) from representative waters collected over 3 years at the wetland. In all models, waters are undersaturated with respect to hydroxylapatite, except under the highest temperatures, [PO43-], and pH conditions. In contrast, fluorapatite approached stability under lower pH and temperatures, and carbonated fluorapatite exhibited a broad range of stable conditions. These results suggest that if bioapatite (bone) were deposited under these present-day conditions, solutions are predicted to be undersaturated with respect to many apatite phases, leading to mineral dissolution. However, if bioapatite undergoes substitutions forming a fluorinated and/or carbonated phase, stability may shift towards preservation. These results have implications for identifying potential biases and mechanisms driving bone preservation over geologic time in a wetland system. The modeling results demonstrate that preservation of bone is favored under subsurface (burial) rather than surface conditions, aligning with long-held views on the formation of our vertebrate fossil record.