Technique Developed to Date Bone Collagen

Illinois State Geological Survey (ISGS) researcher Hong Wang and colleagues in the ISGS Radiocarbon Laboratory have developed a technique to date bone collagen using a pyrolysis to separate volatile components from larger molecules in organic samples submitted for radiocarbon dating. Only a few radiocarbon (14C) dating laboratories perform high-resolution bone collagen dating. Radiocarbon dating of bone collagen is challenging because its preservation is influenced by many environmental factors, including temperature, moisture, pH, and microbial activity. Bone organic matter could be altered physically and chemically through absorbing dissolved organic carbon and "fusing" humic substances with collagen compounds through humification processes. These contaminants could cause 14C dates of bone collagen to be either too young or too old, depending on variations of local environment and soil chemistry. To completely remove molecular contaminants is cumbersome and expensive.

Since 2003, the ISGS Radiocarbon Dating Laboratory has applied the high-temperature (800°C) pyrolysis combustion technique to separate pyrolysis-volatile (Py-V) or low molecular weight compounds and pyrolysis-residue (Py-R) or high molecular weight compounds for 14C dating of wood, peat, cave sediments, and soil organic matter. This technique also has been applied to human bone collagen to examine the significance of age differences between fractions.

The preliminary results from the ISGS analysis of large numbers of human bones collected from Illinois show that Py-V and Py-R fractions yield identical 14C dates for recent prehistoric and earlier Woodland Period, but significantly different 14C dates for Archaic human bones. The results suggest that in relatively recent archeological burial sites, human bones are not significantly altered chemically or contaminated by mobile soil organic compounds that differ in 14C content from the collagen. However, at the older Archaic sites, the thermally volatile fraction of human bone collagen is apparently contaminated by older, probably humic, contaminants.

An older age for contaminants seems counterintuitive if one assumes that the humic contaminants that combine with collagen came mainly from soils that formed after burial of the bones. However, because these bones were obtained from human burials, the bones are likely to have been interred well below the penecontemporaneous soil surface in soils that were formed before burial. The skeletons would, therefore, have been exposed to older soil organic carbon compounds than the bones that were buried by natural sedimentation processes. Experiments by others show that collagen absorbs humic acids very rapidly, becoming saturated within four days. Therefore, intentionally buried human bones would most likely absorb contaminants that were mainly controlled by the age of the soil horizon in which they were buried.