The defense of the Ph.D. of Philip Kenneth Cook
« Microfocused X‐ray methodologies
for the biogeochemical study of archaeological and modern otoliths »
will take place on 22 Septembet 2015 at 2pm in the Salle de conférences du Pavillon d’accueil at Synchrotron SOLEIL.

The jury will be composed of Karin LIMBURG (State University of New York), Ina REICHE (UPMC), Sylvain RAVY (Université Paris Sud), Claire E. LAZARETH (IRD), Jean-Denis VIGNE (CNRS/MNHN), Loïc BERTRAND (IPANEMA), Élise DUFOUR (MNHN), Marie-Angélique LANGUILLE (CRC), et Jean-Pierre CUIF (ancien professeur Université Paris Sud).

Abstract

Otoliths, aragonitic growths in the inner ear of teleost fishes, can be used as proxies for the water conditions experienced by an individual over its lifetime. A set of archaeological Sciaeni- dae and Ariidae otoliths and modern counterparts was studied with the objective of improving palaeoenvironmental reconstruction methodologies using otoliths and other incremental carbonate biominerals. The incorporation of strontium, the most accessible environment-related element, was studied by X-ray absorption spectroscopy (XAS) point analyses and mapping. A fast multivariate chemical environment mapping approach was implemented to deter- mine the mode of Sr incorporation over an area of 0.25 mm2 with micrometric resolution. XAS results demonstrate for the first time with a micrometric lateral resolution over millimetric distances that strontium randomly substitutes for calcium in the aragonite lattice, independent of strontium concentration, or individual or geological age. Elemental maps on areas up to 2.6 mm2 were produced with micrometric lateral resolution X-ray emission techniques (Particle-induced X-ray emission (PIXE) and synchrotron X-ray fluorescence (SR-μXRF)). These maps permit the detailed examination of an individual’s life history and sample taphonomy with a high temporal resolution while also identifying defects or abiogenic elements. Synchrotron X-ray diffraction (SR-XRD) was used to map the crystal texture on complete otolith sections and may deepen understanding of otolith internal structure and growth processes, as well as providing a valuable tool for future studies of biominerals and advanced materials.

The selection and implementation of methods were carried out with a view to maxi- mise the potential contribution to the study of stratified biocarbonates, considering and seeking to complement existing approaches in aspects including information depth, lateral resolution, sensitivity, and beam damage. This work demonstrates the stability and homogeneity of Sr incorporation by random substitution for Ca in biogenic aragonite in both modern and archaeological otoliths. Multielemental maps were collected using SR-μXRF in a reasonable time scale of a few to several hours, with the ability to distinguish contamination and defects in the sample, as well as to correlate the maps to microscopic observations of the sections to provide temporal resolution. The preferential orientations of crystallites composing the otolith sections were analysed using the rapid acquisition ‘flyscan’ method, which reduces measurement time to minutes rather than hours.