Archaeological bone porosity by micro-computed tomography as an indicator of collagen preservation

Jennifer A. Tripp§, Maria E. Squire£, Robert E.M. Hedges#, Rhiannon E. Stevens§

§ Institute of Archaeology, University College London, UK; £ Department of Biology, University of Scranton, PA, USA; # Research Laboratory for Archaeology, University of Oxford, UK.

Collagen isolated from archaeological bone is a common material for radiocarbon dating and stable isotope analysis. However, not all bones contain extant collagen, leading to unnecessary destruction of unproductive bones and wasted laboratory time and resources. In a multi-method study of variably preserved bones from Etton, Cambridgeshire, UK, we examined material properties of Neolithic cattle and sheep bones including porosity, surface area, and elemental composition. micro-Computed tomography (mCT) is an imaging technique that furnishes three-dimensional images of hard materials such as bone. In general, bones with high cortical porosity (Co.Po) as measured by mCT are unlikely to contain sufficient collagen for further analysis. Bones with apparently low cortical porosity have a more varied range of collagen preservation. Bone samples with low Co.Po and no extant collagen often contain micropores with a diameter of 10 nm or less that cannot be seen in mCT images but are apparent in pore size distributions measured by mercury porosimetry, and indicated by high surface areas measured by BET nitrogen adsorption. Furthermore, light-induced breakdown spectroscopy (LIBS) delivers elemental analysis values with minimal sample destruction. Re-evaluation of LIBS data from this same assemblage confirms that ratios of calcium to fluorine may likewise indicate the state of diagenesis. Thus, Co.Po as calculated by mCT can be used to exclude bones that are unlikely to contain collagen, while LIBS can provide further information about their diagenetic state.