Radiocarbon Dating - Part Two - Reporting

Figure One - A handful of Folsom dart points, surface found on private land in the high plains. 

Age is around 10,900 to 10,200 BP. John Branney Collection.    

Welcome to Part Two of my article on Radiocarbon Dating. In Part One, I discussed how the radiocarbon date process worked. In Part Two, I explore how radiocarbon dates are reported in archaeological papers and journals. If you missed Part One, click the link to that article; Radiocarbon Dating - Part One - Process, and don't forget to return and read Part Two! 

         

“Radiocarbon dates indicate that Folsom may have had a relatively long residence in the Rocky Mountains and adjacent Plains from about 10,900 to 10,200 BP.

“At the Hell Gap site the investigators defined a Midland level with dates estimated between 10,700 and 10,400 RCYBP (Irwin-Williams et al. 1973).”

                                 -   Marcel Kornfeld, The First Rocky Mountaineers, page 46

When you read the above passage from Dr. Kornfeld’s archaeological book The First Rocky Mountaineers, do you get the impression that both Folsom and Midland are a tad north of 10,000 years old? I did. In fact, the first book I wrote I used 10,500 years ago for the age of the Folsom Complex (figure two). I was wrong. I was using an uncalibrated radiocarbon date. The calibrated radiocarbon date for Folsom is much older. After calibrating the raw radiocarbon date, we find that Folsom occurred around 12,500 years ago during the Younger Dryas climate event! 

Do you know what the BP refers to in the above passage from Dr. Kornfeld? Do you know what RCYBP stands for? By the time you read this article, you will be able to answer these questions.   

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Shadows on the Trail. 

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In Part One of my article on Radiocarbon Dating, I mentioned certain “gotchas” that archaeologists and scientists must be aware of when using the radiocarbon dating process. One of the bigger assumptions scientists must account for is that the amount of carbon-14 isotope in the atmosphere has not been constant throughout prehistoric time. At different times during Earth's prehistory, plants absorbed different amounts of the carbon-14 isotope and since the carbon-14 isotope is the basis for measuring in the radiocarbon dating process, this became a major challenge. To determine age, the radiocarbon model must know how much carbon-14 isotope was available in the atmosphere at a specific time so it can calculate how much radioactive carbon-14 isotope has decayed.  

Scientists recognized this problem early on in the radiocarbon process and went about solving it. They used dendrochronology, or the study of tree rings, to tie tree rings to levels of carbon-14 isotope levels. For example, using 4,500-year-old bristlecone pines in the Sierra Mountains, scientists determined carbon-14 isotope levels at the time the tree rings were formed. The scientists then correlated the tree rings from living bristlecone pines to dead stumps,  allowing the scientists to calibrate carbon-14 isotope levels back to around 8,200 years ago. With this information, scientists created the first pass at “calibrating" raw radiocarbon ages to corrected calendar ages.   

Figure Three - Paleoindians waiting
their turn at the cafeteria. 

 

To accommodate the lack of certainty in radiocarbon measurements, scientists added an error factor to the measured radiocarbon age. For example, an archaeological report might read that the age of a site is 10,000 ± 160 B.P. This means that there is a 67 percent chance (one standard deviation) that the real age of the site is 160 years plus or minus the 10,000 years before present (B.P.). Whenever you see Before Present or B.P. or BP or bp, it means the age is measured from the baseline year of 1950. For the above example, if we want 95 percent accuracy, we must use two standard deviations, so we add two times 160 years, or 320 years to either side of 10,000 years before present (B.P.).         

Archaeologists and scientists oftentimes publish uncalibrated or uncorrected radiocarbon dates instead of calibrated radiocarbon dates. Knowing the difference between calibrated and uncalibrated radiocarbon dates can be misleading and confusing for those who don't know the difference. Raw radiocarbon dates do not tie up well to calendar years. For example, an uncalibrated date of 9,000 radiocarbon years is approximately 10,200 calendar years old. That is a 1,200-year swing! See and play around with the correction curve in figure four. The error between uncorrected and corrected grows substantially as the age of the samples increase! As an example, 11,000 uncalibrated radiocarbon years correct to approximately 13,000 calendar years. That is a 2,000-year difference! So when you read a report where the author quotes the Clovis Complex at 11,000 RCYBP, remember that the author is referring to an uncalibrated date. Most scientists currently believe that Clovis was around 13,000 years old. 

Most of us think in calendar years, not radiocarbon years. We want to know how old a site or an artifact is based on our day-to-day paradigm of calendar years, not something as esoteric as radiocarbon years. 

Figure Four - One representation of radiocarbon age conversion to calendar age. 

Enter the vertical axis with an uncalibrated radiocarbon age 

and read the calibrated calendar age on the horizontal axis.   

 

Why do archaeologists and scientists report uncalibrated radiocarbon dates instead of calibrated and/or corrected dates? I asked that same question to a practicing archaeologist and he told me, "Because archaeologists write reports for other scientists and archaeologists, not for laypeople."

Okay...I thought his answer was a tad bit snobbish, but pretty much what I expected. I mentioned to him that most of the sites he and other archaeologists excavate came from "laypeople" and that many of us "laypeople" are just as interested in prehistoric cultures as archaeologists. I saw his response as more evidence for the broad chasm and lack of respect that exists between professional archaeologists and us "laypeople".     

Another reason that archaeologists and scientists might report uncalibrated radiocarbon age is that they might not trust the calibration or correction model. After all, models are only representations, not reality. And in this case, the models assume a certain amount of carbon-14 isotope in the atmosphere at different times in prehistory. That is a big assumption. I remember working on my undergraduate degree in geology and a professor told us that models were only as good as the assumptions behind them, i.e., "garbage in, garbage out."  

I became interested in archaeology over fifty years ago and since then I have seen the proposed ages of certain prehistoric cultures change over time. I remember when Clovis technology was reported at around 11,000 years old and now scientists are reporting the more precise age of 13,000 years for Clovis. I am sure these reporting discrepancies come from improvements in technology, modeling, sampling methodology, and computer power.   

When you read archaeological reports, be sure to note the radiocarbon dating nomenclature the authors use. It is not always clear. Uncalibrated radiocarbon dates are reported as bp, RCYBP, C14 ka BP, 14C ka BP, ¹⁴C ka BP, radiocarbon years, c14 years before the present, rcbp, carbon-14 years before the present, and CYBP. In all cases, BP and bp is referenced from the baseline year of 1950. Again, if you see this nomenclature it means the estimated radiocarbon date is from uncalibrated data. It is my experience that most authors of archaeological site reports don’t explain whether they are using calibrated or uncalibrated radiocarbon dates, they assume the readers will figure it out. Beware! The difference between uncalibrated and calibrated radiocarbon dates can be huge, especially when the samples get older.  

Calibrated radiocarbon dates are reported as cal B.P., cal yr. BP, B.P., BP or the one I like the most; Site XYZ is blankety-blank-blank years old (even though George Frison's fantastic books often used 'years old' but used uncalibrated radiocarbon dates much of the time). 

Figure Five - A handful of Midland dart points surface found on private land in the high plains. According to the radiocarbon age nomenclature used by Marcel Kornfeld, Midland is close to two thousand years older than Folsom. We know better, Folsom and Midland technologies were contemporaries. John Bradford Branney Collection.      

Let me now return to the original passage at the top of this article from The First Rocky Mountaineers by archaeologist Marcel Kornfeld. 

"Radiocarbon dates indicate that Folsom may have had a relatively long residence in the Rocky Mountains and adjacent Plains from about 10,900 to 10,200 BP.

"At the Hell Gap site the investigators defined a Midland level with dates estimated between 10,700 and 10,400 RCYBP (Irwin-Williams et al. 1973)."

                                 -   Marcel Kornfeld, The First Rocky Mountaineers, page 46

On page 46, Dr. Kornfeld discussed the ages of the Folsom and Midland prehistoric cultures. Most knowledgeable people contend that the Folsom and Midland prehistoric cultures were contemporaneous, or nearly so. Many people, including me, believe that the people who made Folsom points also made Midland points.    

If we use the appropriate nomenclature I presented earlier in the article, the 10,900 to 10,200 BP in Dr. Kornfeld's upper passage referred to a calibrated radiocarbon date, but this is not the case. Folsom is much older than 10,900 to 10,200 years ago. The author used common calibrated radiocarbon nomenclature for an uncalibrated radiocarbon date!  

The lower passage, 10,700 and 10,400 RCYBP, uses uncalibrated radiocarbon date nomenclature. This is perfectly fine but is in direct conflict with the nomenclature in the upper passage. Dr. Kornfeld mixed uncalibrated radiocarbon age nomenclature with RCYBP with calibrated radiocarbon age nomenclature with BP. By doing this, he is stating that the Midland culture was around 12,620 calendar years old while the Folsom culture was only around 10,500 calendar years old. We know better than that, both cultures were pretty much the same age. Dr. Kornfeld should have used bp instead of BP in the first passage to indicate an uncalibrated radiocarbon date. This just shows that even the experts get confused.   

Be careful when you read radiocarbon dates in archaeological journals and articles, even if written by experts! The dates are not always what they seem. I hoped you learned something from this article. I did when I did the research.   

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