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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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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Radiocarbon Dating - Part One - The Process

Figure One - Three-inch long Montana Clovis point made from a multi-colored jasper and around 13,000 years old. How do we know who old Clovis points are?  Radiocarbon dating John Bradford Branney Collection.

Radiocarbon dating is a controversial subject. People either believe it works or they don't, and a lot of people including myself have assumptions about the way the process works and how scientists report its results. I am not here to convince you that radiocarbon dating works or does not work. That you must decide on your own after doing your own investigation. The process has its advantages and its limitations, as does any process. I will share what I learned during my investigation on the subject. 

In Part One of my two-part series on radiocarbon dating, I present an overview of the radiocarbon dating process and some of its pitfalls. It is not my intent to cover every nuance and detail of the radiocarbon dating process but to supply readers with enough information for a basic understanding. During my research, I found that the radiocarbon dating process is a lot like making sausage; we all have a general idea of how the sausage-making process works, we just don’t want to know the ingredients that went into making the sausage.  

 

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Over the decades, I have read many books and reports on archaeological sites, especially those related to my stomping grounds, the high plains of North America. I read these books and reports because I have hunted prehistoric artifacts for five decades plus and I have a passion for learning about the people who made my prehistoric artifacts. Reading these archaeological reports also helps me to write good stories for my historical fiction book series titled the SHADOWS on the TRAIL Pentalogy.  

I have always been curious about how and why archaeologists report radiocarbon dates the way they do. There does not appear to be any standard as to how archaeologists and scientists report the dates. Archaeologists use a plethora of confusing terms and phrases when reporting radiocarbon dates. For example, these are radiocarbon dates from two reports: 10,000 RCYBP and 10,000 years BP. Does this mean both sites are 10,000 years old? For someone unfamiliar with the terminology, they might say yes, but the answer is no. The first date reports an uncalibrated radiocarbon date while the second date is a calibrated radiocarbon date. Either the reader or the author must correct the uncalibrated radiocarbon date to get something meaningful like calendar years old.

Radiocarbon dating is indispensable in archaeology. Every archaeologist uses radiocarbon dating in one way or another. If we understand the basics of the radiocarbon dating process, it helps us to better understand archaeological reports and the limitations to the process. As consumers of these reports, we need to have a basic understanding of the process and its strengths and weaknesses. 

Figure Two – Variations in Cody Complex artifacts surface found on private land on the High Plains of North America. From left to right; Eden (Colorado), eccentric Cody Complex knife form (Wyoming), Eden? (Colorado), Scottsbluff knife form (Colorado), Cody knife (Wyoming), and Cody knife (Colorado). Eden on left is two inches long. Based on several radiocarbon dates (Knell and Muñiz 2013:13). Cody Complex's age is 11,600 to 8785 cal BP. John Bradford Branney Collection.  

Radiocarbon dating is one of the most widely used methods for scientists to figure out the relative ages of biological samples, such as wooden artifacts or bones. The process uses a natural phenomenon occurring in the Earth’s atmosphere (figure three). When cosmic rays from the sun bombard nitrogen atoms in our upper atmosphere, it creates an unstable, radioactive carbon isotope called carbon-14. This radioactive carbon-14 isotope oxidizes into a carbon-14 dioxide isotope which settles in the lower atmosphere. Plants and algae take in the carbon-14 dioxide isotope at the same ratio that exists in the atmosphere for that specific time. Other living organisms exchange carbon with the atmosphere through respiration and by eating plants and organisms that have the carbon-14 isotope in their tissue.

 

Figure Three - How the radiocarbon dating process works by Pass My Exams. 

When an organism dies, its carbon intake stops, and the radioactive carbon-14 isotope in its tissue begins to decay into a stable carbon-12 isotope. Radioactive decay is the process by which an unstable atomic nucleus loses energy (in terms of mass) by emitting radiation. The carbon-14 isotope has a known radioactive half-life of approximately 5,730 years. This means that in 5,730 years half of the carbon-14 isotope in the dead organism will decay into the stable carbon-12 isotope. To figure out the age of the dead organism, archaeologists and scientists test the remains for the presence of the radioactive carbon-14 isotope and its ratio to the stable carbon-12 isotope. Wood and charcoal are the best materials to use in the process, but other previously "alive" materials also work.

 

Figure Four - Paleoindians from my book Shadows on the Trail.

Radiocarbon dating is one of the most reliable means in the toolbox for dating archaeological sites, but the process has a few challenges that we should be aware of. The first challenge for the field archaeologist is to find a reliable and uncontaminated sample within the cultural material desired. The archaeologist must ensure that the sample tested is associated with the cultural material in question and that unrelated organic deposits have not contaminated the sample. As an example,  testing a piece of charcoal from a prehistoric fire hearth in a geologic formation where there are clinker coal deposits from an earlier geological episode presents a problem. The radiocarbon date might turn out too old.   

The second challenge is that the site must be less than 50,000 to 60,000 years old. As I previously mentioned, the half-life of the radioactive carbon-14 isotope is 5,730 years. If you cut the amount of carbon-14 isotope in half several times through radioactive decay, there comes a point where there is not enough carbon-14 isotope left to supply a statistically reliable answer. Over the years, advances in measurement technology have improved the ability to measure smaller amounts of carbon-14 isotope , but it still is a challenge.   

The third challenge is the biggest hurdle of all. In my fourth paragraph, I explained how cosmic rays from the sun bombarded nitrogen atoms in our upper atmosphere and created the radioactive carbon-14 isotope which is the key element needed for radiocarbon dating. We know the half-life of carbon-14 and we know the amount of carbon-14 currently produced in the atmosphere. Therefore, we should be able to calculate the amount of carbon-14 isotope left in our sample and therefore figure out its age. Unfortunately, it is not that easy.

The big snafu is that the amount of bombardment of cosmic rays from the sun has not been consistent through time. Therefore, the production of the radioactive carbon-14 isotope has not been consistent through time. The inconsistency in the production of radioactive carbon-14 throws a wrench in the monkey works. Throughout geologic time, there were peaks and valleys in the production of the carbon-14 isotope in the atmosphere. One example of this is between 11,300 to 11,600 years ago; scientists believe the atmosphere produced less carbon-14 isotope. A reduction in the production of carbon-14 isotope in the atmosphere results in a difference between measured radiocarbon years and actual calendar years. The reduction resulted in a flat plateau on the radiocarbon calibration curve and a compression of the true or calendar age.   

To correct for these discrepancies, scientists tied radiocarbon dating to tree rings (dendrochronology) and how much carbon-14 is there. This is challenging at best and probably why so many archaeologists cite uncalibrated radiocarbon years in their reports and not calibrated radiocarbon years. Readers of these reports must be aware of how the author or authors are reporting dates. There is a monstrous difference between uncalibrated and calibrated (corrected) radiocarbon ages, especially in the older Paleoindian sites!  

Figure Five – Generic example. The difference between uncalibrated radiocarbon dates          (vertical axis) and calibrated radiocarbon dates (horizontal axis).

Figure five is a simplified version of a radiocarbon date calibration curve and illustrates my point. The vertical axis (left-hand side) shows years in uncalibrated radiocarbon years. This stands for the raw radiocarbon measurement for my example. The horizontal axis (across the bottom) shows years in calibrated calendar years from the baseline year of 1950. The red curve illustrates a simplified calibration curve for the data. In this example, you enter the uncalibrated radiocarbon date on the vertical axis and where it crosses the red line you read the calendar years on the horizontal axis. For example, a site yielded an age of 10,000 radiocarbon years calibrates to 11,400 calendar years, a correction of 1,400 years! If the author of the data reports the uncalibrated age of 10,000 radiocarbon years, an unsuspecting reader might think this is how old the site is in calendar years. It is not! 

It is easy to see how this can lead to confusion for those not living archaeology every day. While some authors report uncalibrated radiocarbon years others report calibrated radiocarbon years. 

In Part Two of my two-part series on radiocarbon dating, I will cover some of the different ways archaeologists report radiocarbon dates from archaeological sites. 

Reference Cited

Knell, Edward J., and Mark P. Muñiz

2013 Paleoindian Lifeways of the Cody Complex. The University of Utah Press. Salt Lake City.   

 

 

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