It is safe to say that the development of radiocarbon dating has changed our understanding of the world. It has not only revolutionized our approach to archaeology and anthropology, offering new insights into the lives of ancient peoples and cultures, but has also offered new perspectives on the planet’s geological and atmospheric past. But have you ever wondered how this important scientific technique works and where it came from?
Carbon-14: Cosmic rays and dead things
Radiocarbon dating, or carbon dating (and sometimes carbon-14 dating), is a method used to determine the age of certain objects of a biological nature. This groundbreaking technique relies on the natural decay of one important radioactive carbon isotope: carbon-14. By measuring the amount of carbon-14 in a sample, scientists can calculate its age with remarkable precision. But to understand how that is achieved, we first need to know how carbon-14 gets into “things” in the first place – and the answer has a lot to do with outer space.
When cosmic rays – usually from the Sun, but sometimes other celestial bodies like distant exploding stars or black holes – enter the atmosphere, they can collide with other atoms and create secondary cosmic rays in the form of energetic neutrons. These neutrons can then collide with nitrogen-14 atoms which, in turn, become carbon-14 atoms (and hydrogen atoms). The carbon-14 atoms then combine with oxygen to form carbon dioxide, which is absorbed by plants and incorporated into them through carbon exchange (photosynthesis). Animals (including humans) then eat the plants and take carbon-14 into their bodies.
As such, all animals, including you and me, have a small but constant percentage of carbon-14 atoms in our bodies, alongside carbon-12 atoms, which make up the vast majority of our carbon content, and carbon-13.
Carbon-14 "is the rarest of the three naturally-occurring isotopes of carbon”, Dr Maarten Blaauw, the Director of 14CHRONO Centre, told IFLScience. “[I]n living tissues such as plant leaves or human teeth, only around 1 in a million million carbon atoms will be [Carbon-14]”,
Throughout our lives, we accumulate quantities of carbon in our bodies – but when we die, this process stops. Over time, the carbon-14 starts to decay, while the carbon-12 and 13 remain constant.
Carbon-14 "is an unstable, radioactive isotope of carbon”, Blaauw added. “[O]ver time it will decay back into nitrogen. The rate of this decay is known and constant; its average life-span has been found to be around 8,033 years. This is handy, because when organisms are alive they will take up [Carbon-14] through their metabolism; once they die they no longer top up their carbon, and the radiocarbon content of their tissues starts to decay until after around 50,000 years no measurable [Carbon-14] is left.”
This is the key to carbon dating and what makes it so useful for understanding the past.
Counting evidence from the past
The technique was first developed at the University of Chicago in the late 1940s by Willard Libby, a chemistry professor, and former researcher on the Manhattan Project. Earlier in the decade, other researchers had discovered carbon-14 isotopes for the first time and observed that they had a half-life of approximately 5,730 years. This means that, after 5,730 years, half of the initial number of carbon-14 atoms in a sample will have decayed and turned back into nitrogen (here’s a handy visual representation of the decay process provided by Dr Blaauw). Libby had the insight that, if you could count the amount of carbon-14 in a sample, then you could figure out how old it is by how much has since decayed.
Over the next few years, Libby and colleagues published ideas about the potential of carbon dating and continued to refine their thoughts. One key development came from his graduate student, Ernest C. Anderson, who found that organic materials contained pretty much the same amount of carbon-14 at all latitudes on the planet.
With all this work, Libby and his colleagues set about developing the first carbon-14 technique by testing it on artifacts from museum collections. This meant the team could establish the technique’s accuracy as archaeologists already knew the ages of these objects through other evidence, such as tree-ring dating (Dendrochronology). Needless to say, it worked, and by the 1960s the technique was being employed in over 30 radiocarbon labs across the world. Today, it is still one of the most significant methods for dating the past.
It’s “[v]ery important”, Blaauw explained, “radiocarbon dating is the most-used tool to date material back to c. 50,000 years ago, a period full of important events in past environments as well as in human development”.
As powerful as this new tool was, it was quite slow, as Blaauw added. “Initially, the radiocarbon decay events themselves were measured. This required large amounts of material and long counting times of weeks to months. Since the 1990s, the method of counting the [carbon-14] atoms themselves, through accelerator mass spectrometry, has become more popular.”
“Radiocarbon ages can now be measured in minutes, and on much smaller samples down to, for example, single seeds.”
The “Radiocarbon Revolution”
Carbon dating had a huge impact on the archaeological and geological sciences. Before this technique was developed, efforts to date objects relied on relative factors, such as comparing the layers of an archaeological site in which something was recovered, with the assumption that the layers were laid down chronologically. This approach merely established the order of events rather than offering a precise measurement of their age.
But the arrival of carbon dating changed this – and, in doing so, it helped disprove various established but erroneous beliefs. For instance, it debunked the idea that civilization started in Europe and then diffused outwards. The examination of artifacts from Asia, Africa, the Americas, and Oceania showed that civilization developed independently in various places. By not needing to spend as much time trying to determine the age of artifacts, archaeologists were now able to ask new questions about human evolution and culture in prehistoric ages.
Other disciplines have also benefited from this technique. In particular, geologists, sedimentologists, and lake studies have utilized carbon dating techniques. In addition, palaeobotanists and palaeoclimatologists can use it to date things like pollen found in sediment sequences, along with other traces of plant material or charcoal. By accurately dating these samples, they can use the information to establish the ages of correlating strata in other locations.
Obviously, despite its power, carbon dating has its limitations. The most obvious is the hard limit of dating objects that are more than 50,000 years old. This is because any carbon-14 content for such artifacts would be too low to be of use.
But there are other limitations too.
One other limitation relates to isolating the correct carbon source to be measured. As Blaauw explained, “[o]ften, samples will contain mixtures of carbon sources, for example, an old wooden item from a museum might have been treated with oils to preserve it, and then we'll have to try and separate the wood from the oils in order to carbon-date the wood itself.”
This question of contamination is significant, “especially for samples that reach the 50,000 year limit of radiocarbon dating - since in such samples only very low amounts of radiocarbon remain, contamination with even minute amounts of modern carbon could severely throw off the date. For example, a 65 million-years-old dinosaur bone with only 1 [percent] modern carbon contamination would result in a radiocarbon age of around 37,000 [carbon-14] years!”
To address this, archaeologists and other researchers attempt to estimate “the effects of any background contamination by not only measuring the samples themselves but also standards and background samples.”
The only time researchers will attempt to date something known to be older than the limit is to establish background measurements.
Addressing Creationist criticisms
It is important to be clear on how carbon dating works as, in recent years, certain creationists and Young Earth advocates have relied on poor understandings of carbon dating techniques, as well as misinterpretation of Libby’s original work, to argue for a chronology that fits biblical scripture. There are, however, solid explanations on why creationists have gotten things muddled up.
In particular, claims that ancient fossils have traces of carbon-14 in them have been used to suggest that certain specimens that predate the biblical narrative are actually much younger than they are (a dinosaurs at the Flood kind of thing). However, as Blaauw explained, contamination is extremely easy. Beta radiation caused by cosmic rays, the type of radiation that turns nitrogen-14 into carbon-14, forms a large part of the natural background radiation surrounding us at all times. As such, it is not possible to completely exclude it from a laboratory where it may make an older artifact or fossil appear younger.
There are many other criticisms from Young Earthers which have been explained by experts currently working in the field. Contrary to their claims, the evidence for carbon dating and its accuracy is superlatively solid. It seems their objections rely on citing other creationist writers who all quote the same original misunderstanding.
The future for the past
Today, carbon dating remains a powerful and widely used tool for determining the age of historical objects or events. Increasingly, researchers are finding new ways to refine their methods and to eliminate contaminations.
“[W]e have an oven that we can put at a range of temperatures and then we isolate and measure the carbon coming off at different temperatures”, Blaauw said, “this gives us better insights into the different carbon sources within more complex samples. Labs also try out new chemical pretreatment methods to better get rid of contamination.”
Clearly, archaeologists and other scientists are aware of the limitations surrounding their work, but they are not the types of issues creationists like to imagine. As with so many things, the evidence is there, you just have to know how to look at it.
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