Advances in bone preservation: Identifying possible collagen preservation using sulfur speciation mapping

Abstract The complexity and heterogeneity of bone chemistry makes it difficult to discern information on physiological and taphonomic processes stored within the bone matrix. Analysis of archaeological and palaeontological bone becomes more difficult because in many cases the most pivotal specimens are too scientifically valuable for destructive analysis. This problem is further escalated by the fact that the heterogeneity of the bone may cause small “pockets” of preservation that can be missed during sampling. Therefore, a non-destructive technique that can spatially resolve such heterogeneity within the bone is needed. Here we use microfocus, non-destructive synchrotron-based X-Ray Fluorescence (XRF) imaging and X-ray Absorption Spectroscopy (XAS) to map the organic constituents within extant and fossil bovid bones. XAS analysis of sulfur allowed organic sulfur (within collagen as methionine) to be distinguished from inorganic sulfate (within bone apatite). Mapping and quantification of organic sulfur within the samples were made by setting the beam to the methionine resonance, allowing for the detection, distribution and quantification of collagen present by using organic sulfur as an internal marker. Results show organic sulfur to be distributed in small “pockets” throughout the bone matrix in both extant and fossil specimens. Significant loss of organic sulfur was seen in specimens between 100 ka and 650 ka with little organic sulfur preservation persisting after this date. Comparison of residual organic sulfur concentrations as a function of sample age revealed a second order rate law for organic sulfur oxidation (k ≈ 1 × 10 −5  y −1 ) within bone. These results show that non-destructive, synchrotron-based XRF mapping of organic sulfur is a useful tool for not only calculating rates of collagen degradation through time, but also identifying areas of potential collagen preservation for other paleobiological applications such as proteomics and stable isotope analyses.