Refining Radiocarbon Calibration for the Southern Levant

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.

Sources:

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

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7 Responses to Refining Radiocarbon Calibration for the Southern Levant

  1. Geoff Hudson says:

    Brent, if the age of a third century manuscript was younger by nineteen years, how would that affect your views?

    • It’s not quite so simple. The issue isn’t 19 calendar years, it’s 19 radiocarbon years. A difference of 19 radiocarbon years will produce different ranges of calendar years depending on which point in the calibration curve we’re talking about. So, what is needed are similar kinds of experiments for early periods (like the early Christian era). But it would be difficult to carry out such experiments because testable objects from that era whose ages are known are scarce, valuable, and unlikely to be subjected to destructive testing.

      • Geoff Hudson says:

        Brent, a number of DSS manuscripts have been the subjected to destructive radio carbon dating with various estimates for the spreads in their ages. Why can’t the same be done for the mass of material from Oxyrhynchus? It does seem to me, from the research that you quote, that nineteen years is an approximate order of magnitude for the reduction in age of any manuscript, say for example from the third century. So, how could this affect your views? What would be the effect if it was calculated to be 30 or 40 years?

      • Many institutions that own papyri have total non-destruction policies; no testing that involves the loss of even the smallest fragment of the artifact is entertained. In this context, I’m not sure what you mean by “order of magnitude.”

  2. This is fascinating. I am very glad to hear that there are more tests going into calibration of c14 dates. I really do wonder how the chemical warfare of the early 20th century, and the use of nuclear weapons and fuel has changed the atmospheric measurements. I look forward to reading both of the articles mentioned here!

    • I don’t know about the effects of chemical warfare, but certainly the nuclear tests of the 1950s caused a giant spike in the amount of 14C in the atmosphere. If you want a good general book on the topic of radiometric dating, I would check out Doug Macdougall’s book, Nature’s Clocks (2008). It’s very informative and extremely well written.

      • Thank you! I have almost zero science background since high school, so for me to have an informed statement/ opinion on such matters requires me to do some good research. Thanks!

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