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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.
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.
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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.
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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.
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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!
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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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Sand
and Sage by the
Author. A horse of a different color! |
