At the website for the Proceedings of the National Academy of Sciences, a new article has been posted in pre-print format: “Fluctuating radiocarbon offsets observed in the southern Levant and implications for archaeological chronology debates.”
While the title isn’t exactly catchy, this is an interesting article that is potentially significant for the dating of ancient manuscripts that may have been produced using materials native to this area. Essentially, the authors set out to test how accurate the standard calibration curve is for artifacts from the southern Levant. What is the calibration curve? When scientists measure the amount of the radioactive isotope 14C (“carbon-14”) in an object, they use an equation based on the rate that 14C decays. This equation produces a certain number of 14C years (“radiocarbon years”) before the present (BP), where the “present” is understood to be the year 1950. These results are based on the assumption that the amount of 14C in the atmosphere is constant, but we know this is untrue. So, to translate these radiocarbon years into ranges of calendar dates, scientists have tested objects of known age, usually trees, whose exact ages can be known through dendrochronology—counting the growth rings. By testing these objects with known ages, scientists are able to determine how the levels of 14C in the atmosphere have fluctuated over the centuries and create a calibration curve that helps them adjust the results of their equation accordingly. It’s really a brilliant solution.
But the process is always being refined. One tricky part is determining whether the trees used to establish the calibration curve, which come from North America and northern Europe, accurately represent the level of 14C in the atmosphere in other areas as well. That is the aspect of the process that the authors of this article set out to test.
The authors used AMS radiocarbon analysis to test the dates of Juniper trees from Jordan. The tree samples had calendar dates from AD 1610 to 1940. As the authors write, “Our data reveal an average offset of ~19 14C years, but, more interestingly, this offset seems to vary in importance through time. While relatively small, such an offset has substantial relevance to high-resolution 14C chronologies for the southern Levant, both archaeological and paleoenvironmental.”
By “offset of ~19 14C years,” the authors mean that the radiocarbon analysis yielded an age that is older than the actual (known) age of the objects. The material tested for this article, however, is relatively recent (no more than about 500 years old). Nevertheless, the results may be significant for more ancient materials from the Levant, such as the Dead Sea Scrolls, a few of which have been subjected to AMS radiocarbon analysis. The article does not address the scrolls but does briefly refer to the potential for this offset to affect current debates of the chronology of the Late Bronze and Iron Age Levant: “we may note that every shift is to “lower” or more recent calendar age ranges (whichever adjustment is considered), which is significant when considering recent debates over absolute dates for the Iron Age archaeological periods in the southern Levant: “…we may note that every shift is to ‘lower’ or more recent calendar age ranges (whichever adjustment is considered), which is significant when considering recent debates over absolute dates for the Iron Age archaeological periods in the southern Levant.”
It is also interesting to compare these results with those of a 2010 article that ran similar tests on short-lived plant material from Egypt of known ages in historical collections (this material is thus also relatively recent—AD 1700-1900). This study found a similar offset of 19 14C years (again indicating that radiocarbon analysis yields an age that is older than the actual age of the object). The fluctuation that the authors of the 2018 study note suggests that further testing is needed to see more precisely how this offset might apply to other more ancient sections of the calibration curve.
Dee, M.W. et al., “Investigating the likelihood of a reservoir offset in the radiocarbon record for ancient Egypt.” Journal of Archaeological Science 37, 687–693.
Manning, Sturt W. et al., “Fluctuating radiocarbon offsets observed in the southern Levant and implications for archaeological chronology debates.” Proceedings of the National Academy of Sciences. 2018. www.pnas.org/cgi/doi/10.1073/pnas.1719420115