IMAGING FOSSIL CHEMISTRY IN THE SYNCHROTRON

Fossil morphology and trace metal biomarkers have shown that discrete elemental inventories can be identified for fossil organisms that correlate to specific biosynthetic pathways. Quantitative chemical analyses are rarely undertaken on fossils, despite the potentially valuable information. Ideally such analyses measure and map the chemistry of bone, soft tissue structures, and embedding rock matrix. Mapping fossils in situ helps place constraints on mass transfer between the embedding matrix and fossil, aiding in distinguishing taphonomic processes from original chemical zonation remnant from the fossil itself. Conventional non-destructive analytical methods face serious problems in this case and most recent technological advances have been targeted at developing nanometre-scale rather than decimetre-scale capabilities. However, the development of Synchrotron Rapid Scanning X-ray Fluorescence (SRS-XRF) at the Stanford Synchrotron Radiation Lightsource (SSRL) permits large paleontological specimens to be non-destructively analyzed and imaged using major, minor, and trace element concentrations. Beam line 6–2 at SSRL can operate at an energy range from 2.4 to 17 keV and for each study the excitation energy is optimized. For light-element (Low-Z) XRF images [P, S, Cl, and K] of fossils, an excitation energy of 3.15 keV was chosen; for heavy-element (High-Z) images [Ca, Fe, Ni, Cu, Zn, Se, Ba, and Pb], excitation energies from 9 to 13.5 keV were chosen. Flux at the sample surface varied between approximately 10¹⁰ and 10¹¹ photons s⁻¹, depending on the specific analytical conditions. Fossil and extant samples are mounted in a purpose-built sample chamber held on a computerized x-y-z translational stage. For light-element XRF imaging, an X-ray-transparent ∼30-μm thick polyethylene film was placed on the sample chamber, and purged of air with He to minimize scattering and increase signal to the detector. Pivotal to the mapping is spectroscopy that constrains oxidation state and coordination chemistry of key elements such as Cu, S, Fe, K, etc., providing insight to the endogeneity of chemistry being mapped to PPM sensitivity. This is a precise, repeatable and quantitative technique building upon decades of research at synchrotron facilities, shedding new light on the chemistry of life in the 21^st century.