Radiocarbon dating the 40-60 kya interval

Jeff Pigati and colleagues have a paper in press in Quaternary International about a new protocol that enables radiocarbon dating of very old samples (i.e., between 40-60 kya in age). This should come as very good news for anyone interested in the chronology of the late Middle Paleolithic and early Upper Paleolithic, since this period has always been very tricky to date accurately. Between this, the ultrafiltration methods now being used at the Oxford lab, and advances in radiocarbon age calibration past 26 kya, it can be hoped that the chronology of that period will come into sharper focus in the next few years.

From the abstract:
"At the University of Arizona’s Desert Laboratory, we recently constructed new low background vacuum extraction and graphitization systems that are dedicated to preparing old (40-60 ka) samples for 14C dating. These systems are designed to minimize the amount of contaminant carbon, specifically atmospheric carbon, that is introduced to a sample during laboratory processing. Excluding contaminants is particularly important for 14C dating of old samples because the impact of contamination increases with sample age...

Based on the AMS results, the background level of our system is characterized by a nonlinear inverse relationship with sample mass (adjusted R2=0.75; n=24). For a 1 mg graphite target, the total procedural blank, including chemical pretreatment, combustion, cleanup, graphitization, storage, and AMS measurement, is 0.05±0.01 pMC (2σ), equivalent to a 14C “age” of 61.1±1.8 ka. This should not be taken as the upper limit of our system, however, because if the 14C activity of a sample is statistically indistinguishable from the appropriate mass-dependent blank value at the 95% confidence level (2σ), then its age is considered to be “infinite”. Thus, for a 1 mg target, the practical limit of our system is actually ~55 ka; for a 0.5 mg target, the practical limit is ~50 ka. Although our extraction system can accommodate inorganic samples (e.g., calcite, aragonite), the above limits are only applicable to geological graphite, charcoal, and organic samples that are processed via combustion. Future work will be directed toward determining the appropriate background levels for inorganic materials."

References

Pigati, J. S., J. Quade, J. Wilson, A.J. Timothy Jull, and N. A. Lifton. (in press) Development of low-background vacuum extraction and graphitization systems for 14C dating of old (40-60 ka) samples. Quaternary International: doi:10.1016/j.quaint.2006.12.006