Bone Pretreatment by Ultra-Filtration: A Report on Unintended Age Offsets Introducted by the Method
Author(s): Bronk Ramsey, Thomas Higham, J A Pearson
Bone samples for radiocarbon dating undergo pre-treatment to extract original proteins from the bone and to exclude contaminants. Ultrafiltration is an extra stage in this process which further purifies the proteins and was first developed in 1988 (Brown et al 1988). In 2000 the Oxford Radiocarbon Accelerator Unit (ORAU) adopted this method for the pre-treatment of bone in order to improve the accuracy of its dating. Diagnostics of protein purity, including CN ratio, and background measurements all indicated that the method was working well. However, in 2002 it became apparent that the accuracy of dates, particularly those younger than 10,000 years old, was in some instances, being affected by the technique because of traces of contaminants from the filters themselves. A new method of preparing and testing the filters was developed (Bronk Ramsey et al 2004) and a programme of re-dating started. Despite the purity of protein resulting from the original method, the remaining filterderived contaminants (averaging 1.5%) were very different in age from the samples and this produced age offsets averaging 120 years. The effect was size dependent and greatest for samples yielding <10mg collagen. However, even samples >10mg collagen had an average age offset of 100 years and only above about 30mg does the average offset fall to 55 years. The effect is also seen to be very variable. The original ultrafiltration method used between 2000 and 2002 represented a regressive step in terms of the accuracy in dating bones < 10,000 years old. The method did, probably, produce accurate results in comparison to other methods for bones between 10,000 and 15,000 years old, although producing slightly older rather than slightly younger dates. The only significant improvement was apparent in bones in excess of 15,000 years old. The implementation of new cleaning protocols (Bronk Ramsey et al 2004; Brock et al 2007) has resulted in a significant improvement in the routine application of the technique at ORAU.
- Report Number:
- 91/2011
- Series:
- Research Department Reports
- Pages:
- 23
- Keywords:
- Radiocarbon Dating