Elemental composition and diagenetic alteration of dinosaur bone: Distinguishing micron-scale spatial and compositional heterogeneity using PIXE

Proton Induced X-ray Emission (PIXE) analysis of thirteen Late Cretaceous (hadrosaurid, tyrannosaurid, and Tyrannosaurus) and Early Jurassic (Dilophosaurus) dinosaur bones, two Late Cretaceous crocodylians, one plesiosaur, and three modern archosaurs (Rhea and Caiman), documents micron-scale diagenetic patterns of elemental uptake and loss. PIXE detected Al, Si, P, S, Cl, K, Ca, Ba, Mn, Fe, Sr, Y, and Nd at levels > 1000 ppm; and Ti, Cr, Ni, Zn, As, Zr, and U at trace amounts < 1000 ppm. PIXE interrogation with a 1.5 μm diameter beam revealed a diagenetic pattern of significant Fe (180,000 ppm) and Mn (13,000 ppm) enrichment, with a corresponding loss of Ca and to a lesser degree P, within micron-scale regions in the Haversian system of compact dinosaur bone. Elemental composition of bone mineral immediately adjacent to the annuli surrounding a Haversian canal was depleted by approximately 90% in Fe and Mn, but showed a 50% increase in Ca ppm. Calcium levels in the fossils vary from 57% to 95% concentration of modern bone. Phosphorous levels in some dinosaur bone bioapatite are > 100% of modern. This enrichment may reflect the migration of Ca and P between the hydroxylapatite matrix and the surrounding sediment after death, resulting in early diagenetic loss followed by uptake and recrystallization on crystallite surfaces. The elemental composition of fossil bone from the Early Jurassic dinosaur Dilophosaurus is distinguished by elevated As ppm, generally not present in the Late Cretaceous dinosaurs. We suggest that As is adsorbed from the reductive dissolution of As-rich iron oxyhydroxides in the local aquifer and is not reflective of Mesozoic atmospheric conditions or arsenic toxicosis in dinosaurs. PIXE allows us to reaffirm that the histological structure, tissue density, and high surface area of the vascular canals of dinosaur Haversian bone affect postmortem elemental enrichment and depletion and to quantify these differences at scales as small as 1.5 μm.