Radiocarbon has quickly become the reliable workhorse behind archaeological dating, used in almost every post-excavation analysis. In this month’s ‘Science Notes’, however, we are going to explore how its effectiveness in certain scenarios may be changing as CO2 emissions continue to rise.
In addition to dating organic samples from archaeological sites, radiocarbon dating is also frequently used on samples from unknown contexts in order to try to flesh out their origins. It is particularly helpful for law enforcement in being able to detect fake goods or illegally traded wildlife. For example, it is an extremely useful tool in determining whether ivory artefacts were made before or after elephant poaching became illegal, or for detecting whether chance-discovered human remains represent an archaeological site or a crime scene.
To do this, however, radiocarbon dates need to be incredibly accurate, to within a couple of years – and, as we know from archaeological examples, depending on where on the curve the samples fall, sometimes dates can be quite broad (see ‘Science Notes’ CA 335 and the Hallstatt Plateau). Modern samples, however, benefit from the ‘bomb-curve’, which is the most recent part of the radiocarbon curve, characterised by the large spike in atmospheric 14C (carbon-14) that occurred between 1952 and 1962 when nuclear weapons-testing around the globe doubled the amount of radiocarbon present in the atmosphere. Since reaching a peak in the mid 1960s, 14C levels have largely been declining in a steady pattern. Because of this well-documented ‘boom’, so to speak, samples from between 1960 and 2020 have a very predictable 14C/C ratio, which can often be precisely matched onto this section of the curve – usually providing date ranges to within a few years.
The mixing of bomb 14C into the ocean, and into plants and soils on land, first led to the decreasing trend in the 14C/C ratio. While this decreasing trend is continuing, now it is being driven by the ever-increasing burning of fossil fuels. Since radiocarbon has a half-life of 5,700 ± 30 years, however, fossils fuels – which are millions of years old – do not contain any 14C, so, when they are combusted they emit only 12C and 13C stable isotopes into the atmosphere as CO2. This means that as fossil fuel emissions continue to increase at a much greater rate than 14C levels, the earth’s atmosphere – based on its 14C/C ratio – is beginning to appear artificially old.
In a recent ‘Correspondence’ in Nature, scientists Heather Graven from Imperial College London, Ralph Keeling from the Scripps Institution of Oceanography, and Xiaomei Xu from the University of California Irving, highlighted a worrying milestone that was hit last year: several measurements taken from both La Jolla in California and Mauna Loa in Hawaii show that, for the first time in modern history, the atmospheric 14C/C ratio was lower than pre- industrial levels. While this reversal has not yet happened in the southern hemisphere, it is probably only a matter of a year or two away.
This change means that objects today will have similar radiocarbon ratios to objects from before 1890, making them virtually indistinguishable in age if there is not enough context to rule out whether they are modern or not. And, as this phenomenon continues (since we are still a way away from seeing fossil fuel emissions start to decline), by 2050 Heather Graven predicts that the 14C ratios will be similar to those seen between the 5th and 15th centuries AD.
It appears that, starting from now, it will become extremely difficult to accurately date certain objects based solely on 14C levels: for organic samples discovered from unknown contexts, such as human remains or ivory, determining whether they are historical or modern will become a much more difficult, if not impossible, task. There may be hope, however, that other isotopes could potentially be used instead. But these techniques will take time to develop and may not be helpful for the most recent samples, which will need dating in the interim.
For the ‘Correspondence’ in Nature, see: https://doi.org/10.1038/d41586-022-01954-y. And for more information on the modern 14C and CO2 measurements used to identify this trend, see: https://scrippsco2.ucsd.edu.