Early diagenesis of elephant tusk in marine environment
Résumé
Three elephant tusks recovered from a 18th century Dutch shipwreck in Brittany seawaters were analyzed to study early diagenetic changes of ivory in a marine environment. Micro-small angle X-ray scattering (μSAXS) analysis revealed sub-microscopic structural modifications of ivory (hydroxyapatite particle thickness, mineralized collagen fibers organization and orientation). Chemical compositional changes of the organic fraction (nitrogen and carbon
loss) and of the mineral part (magnesium loss, carbonate, calcium, chlorine and strontium uptake) were investigated by non-invasive micro-Particle Induced X-ray Emission/ Gamma-ray Emission/ Rutherford and Elastic Backscattering Spectrometry analysis (μPIXE/PIGE/RBS/EBS). These chemical and structural markers can serve as proxies for the early diagenesis of elephant tusk in marine settings. The results show that the macroscopic preservation state of the tusk surface does not reflect its microscopic preservation state. A diagenetic sequence is proposed, depending on the local environment of each tusk. First, collagen hydrolysis and ion adsorptions from the marine environment are proposed to occur. Second, a tusk buried in the sand was protected from Ca enrichment of the surface and the formation of a flaky white concretion whereas tusks out of the sand were subjected to these latter phenomena as well as sea-urchin attacks. Third, ion leaching and uptake, increased crystallinity of the hydroxyapatite crystals, and the modifications of the mineralized collagen fiber network are assumed to happen for all three tusks with different level of completion.