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Calibrating carbon dating

Published: 2 February 2013 (GMT+10)

Anthony P. from the United States writes:

I read the scientific article on the carbon dating done on the Jericho site written by Bruins and Van Der Plicht. When I did the math from their results section of the YBP, they all turned out to be right around the year 1400 BC. But in their abstract and conclusion they told how the date was around 1550 BC. I understand calibration might have something to do with this, but then in the article it says in italicized words that the uncalibrated date “Must Always Be Mentioned”. But when I read articles about the results, they never mention the uncalibrated data, which could actually be correct. Please clarify for me where I err. Thank you very much.

CMI’s Dr Rob Carter responds:


As a fan of biblical archaeology, I was asked to address your question. I am not an expert in every subject that impinges on the discussion, but I will do my best.

There are two reasons uncalibrated dates must be mentioned: 1) this prevents people from making up any number they please, and 2) it is for the sake of posterity, where future scientists can check the results and apply new ideas of calibration.

Why is calibration necessary? Radiocarbon dates are affected by many outside factors. The accuracy of the machines is not in question (especially modern ones, which are astoundingly accurate when properly zeroed in). The rate of decay is also not in question. But, any source of old carbon in the ancient environment can affect the amount of C-14 in a sample.


  1. During the end stages of the Ice Age, it has been reasoned that the Mediterranean Sea had a sheen of fresh water on top (it would have been a lake at that point, not a sea), and that lots of old carbon may have built up in the salty waters below (this is the situation in the Black Sea today). When the fresh water flow petered out, the lake suddenly turned over, the surface became salty once again, and massive amounts of old carbon were dumped into the atmosphere. Any plants growing downwind would have been affected. Today, they have northerly Scirrocco winds in the winter and southerly Mistral winds in the winter, but all bets are off back in Ice Age times. It should be expected that this turnover could have affected C-14 dates in downwind areas by decades at least, and perhaps more, but nobody really knows. This was only a few thousand years ago (≤4k for the creationist, ≤10k for the evolutionist), well within the range of radiocarbon dating and perhaps on the edge of the modern historical era.
  2. There has been debate about whether or not Europeans brought syphilis back with them from the Americas, or whether it was already there before Columbus. Radiocarbon dates of syphilitic skeletons were used as evidence for pre-Columbian syphilis in Europe, until someone realized that every example came from coastal areas. A diet rich in seafood affects radiocarbon dates due to the incorporation of old carbon in the marine food chain. When a person eats lots of fish, they are eating ‘older’ carbon (lower C-14) and so the dates were off. This is quite controversial.
  3. The man who invented C-14 needed something that was historically dated to calibrate the techniques. He used Egyptian coffin lids, but noticed a systematic error: the older lids were dating several hundred years too old and the error decreased on younger samples. Why? Perhaps (I believe) Egyptian chronology is way off and needs to be adjusted by a couple of hundred years. Or, perhaps (I also believe) the atmosphere was flooded with old carbon during the Flood (through volcanism). Thus, any plant alive in the early post-Flood centuries would have less C-14 content and would then date ‘older’ when analyzed today.

For all of these, and more, reasons, calibration is needed in C-14 dating. Thus, reports generally specify the ‘raw’ numbers and the ‘fudged’ numbers. This does not mean that recalibration is bad, indeed it is necessary, but it should make one more soberly assess any reported dates as being tentative. The problem is that most people reporting on these issues fail to report the initial number along with the calibrated date. There is simply too much faith in fudge.

The Jericho controversy is soundly rooted in C-14 calibration. The first excavations were performed prior to WWII, and supported the biblical chronology. When Kathleen Kenyon came away from her study in the 1950s and essentially announced ‘I see no evidence for the destruction of Joshua here,’ she was basing her opinions, in part, on the new field of radiocarbon dating.

See: The walls of Jericho, The story of Jericho, and Q and A pages on the Ice Age and radiometric dating. See also this useful offsite resource: Jericho chronology dispute.

Dr. Robert Carter

Readers’ comments

Michael F.
Yes, I read the article, but I still find it strange that there are multiple observations which point to billions of years of time, yet the Bible is the only thing which points to a few thousand years of time. I will give you the benefit of the doubt and say that carbon dating is not accurate at dates longer than a few thousand years, but there are so many other observable things which point to billions of years of time rather than a few thousand. As Einstein said "A man should look for what is, and not for what he thinks should be." Every time you try to debunk a dating method by using a reference to the flood, I get the impression that you are looking for what you think should be, rather than what is.
Robert Carter
Einstein also said something like, "A thousand experiments cannot prove me right. A single experiment can prove me wrong." Please consult our Radiometric Dating Q&A section for answers to many of the questions you are asking.
Paul N.
Dr. Carter incorrectly states "The rate of decay is also not in question.". On this site alone there have been statements disputing the constancy of radioactive decay. One such is http://creation.com/national-geographic-plays-the-dating-game FYI
Robert Carter
Indeed, as can be found in several more articles here: radiometric-dating-questions-and-answers. However, I will stand by my statement with this defense: First, we do not need changing decay rates to explain 14C dating. There are enough uncertainties in the physical history of earth to throw great uncertainty on the early dates. A variable rate would only make the case worse for secular archaeology. Second, while we have discovered in recent years that certain radiometric decay rates do vary, the measured effect is slight, so far. Third, some creationists like the members of the RATE group theorize there was a pulse of accelerated radioactive decay around the time of the Flood, but this would not apply to the post-Flood era. Fourth, while it is true that we cannot know the past (this is the great limitation of experimental science), it is sometimes convenient to use the opposition's numbers against them. In the end, though, it seems to me there is little debate about the rate of decay in the historical era. Most of the debate centers around Creation Week and the Flood, so I do not think my statement was made in error, at least in the context of what is being discussed.
Paul S.
If you read articles like [note: link deleted as per our fedback rules], it is clear that the Egyptian dates don't always follow the dig. Note the clear references to a "plateau in the calibration curve" from 2500 to 2900BC, which would be due to the flood. C14 was originally calibrated using Egyptian artifacts of "known" age on the "standard" chronology. If that chronology is wrong, as many think, the calibration is wrong. Dendrochronology is used to determine variations in the C14/C12 ratio, but dendrochronology has assumptions that are not always valid (see bristlecone pine dating). They even miss the flood when it is staring them in the face.
Robert Carter

Sadly, I could not include the URL in your reply, but the article you cited was interesting nonetheless. However, the "plateau" certainly does not equate to the Flood, for that would put the Flood in the middle of Egyptian history, the archaeological evidence of which is sitting on top of kilometers of Flood-deposited sediments. They also brought up the question of "old wood" (the fact that any wood used in an archaeological context must have been growing prior to when it was harvested), which affects my point #3, and warned against using organic material from an aquatic context, corroborating my point #2.
Michael F.
Excellent article. Carbon dates can be used to tell the age of organic materials up to around 50,000 years. And uncalibrated dates are usually only off by less than 20%. But, lets be extremely conservative and say a 50,000 years old date is off by half. That still puts the earth at over 20,000 years old. So, why continue beating the 6,000 year old earth drum?
Robert Carter
How certain are you that carbon dating is reliably able to give us dates back to 50KYA? With all that was said about the assumptions behind the measurements, about non-linear forcing functions (like an expected pulse of non-radioactive carbon at the time of the Flood), and add the two prior comments about the demonstrably changing magnetic field strength of the earth, and I submit there is a lot more "art" than "fact" when generating such dates. We beat this "drum" because of the straightforward historical claims of Scripture. We address areas of science that supposedly refute the historical claims of scripture because we are commanded in places like 1Pet3:15 to be ready always "to make a defense to anyone who asks you for a reason for the hope that is in you." We believe our faith is reasonable and comports to a greater historical reality than that which adherence to the uniformitarian straightjacket provides. Did you read the article?
Aleksandar K.
What about the weaking of the magnetic field?
Robert Carter
See my comment on magnetic field strength above. I did not get it posted before you asked the question again.
Daniel B.
We have, in the Masoretic chronology, the best information possible for calibrating C-14 data. Anyone who rejects this source, or worse yet, tries to use a C-14 paradigm based on unknowable levels and rates to 'correct' the historical record found on the pages of Scripture, will be unable to arrive at the truth and will find a later generation scoffing at such purposeful ignorance.
Robert Carter
Yes, we have a reliable biblical chronology, but it sure is nice when scientific interpretations of data line up with historical reality. Evolutionists have been beating us over the head with radiometric dating for over 100 years. Many people have struggled with the faith because of the age-of-the-earth issue, and many other have rejected the faith based on a perceived lack of answers to these questions. Therefore, it behooves us to attempt to answer the challenge of naturalistic science whenever and where ever we can.
Mac H.
Anthony P's question deals with a 100-year difference - 1450 BC vs 1550 BC. That seems to be a small difference, regardless of whether using C-14 dating or a different method, and not pertinent to a Creationist vs Evolutionist discrepancy.
Robert Carter
Yes, a 100-year discrepancy is minor when one is talking about estimated radiocarbon ages for events so long ago, but I answered as if the discrepancy was much larger because that is the way the question is most often worded. In essence, I wrote for posterity. Yet, a 100-year difference would be a major argument for or against, say, an artifact that pointed to the existence of the Davidic dynasty only 500 or so years after the battle of Jericho.
John S.
Here are a few references on 14C calibration for Anthony:

Bard, E., Hamelin, B., Fairbanks, R.G., and Zinder, A., (1990), Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals, Nature, 345, 405-410.

Becker, B., Kromer, B., and Trimborn P., 1991, A stable-isotope tree-ring timescale of the Late Glacial/Holocene boundary:Nature, vol. 353 (17 Oct 1991), 647-649.

Dickin, A. P. (1995), Radiogenic Isotope Geology, Cambridge University Press.

Dalrymple, G. Brent, (1991), The Age of the Earth. California: Stanford University Press, ISBN 0-8047-1569-6.

Edwards, R. L., Beck, J. W., Burr, G. S., Donahue, D. J.,
Chappell, J. M. A., Bloom, E. R. M., Druffel, E. R. M.,
Taylor, F. W., 1993, A large drop in atmospheric 14-C/12-C and reduced melting in the Younger Dryas*, documented with 230-Th ages in corals: Science, vol. 260 (14 May 1993), 962-967

Furgeson, C. W. (1970), Dendrochronology of bristlecone pines, Pinus aristata. Establishment of a 7484-year chronology in the White Mountains of eastern-central California, USA.In: I. U. Olsson (Ed.), Radiocarbon Variations and Absolute Chronology, Proc. 12th Nobel Symp. Wiley, pp. 629-40.

Kitagawa, H., and van der Plicht, J., (1998), Atmospheric radiocarbon
calibration to 45,000 yr B.P>: Late glacial fluctuations and
cosmogenic isotope production, Science, v. 279, 20 Feb 1998.

Libby, W. F. (1952) Radiocarbon dating, University of Chicago Press.

Libby, W. F. (1970) Ruminations on radiocarbon dating
In: I. U. Olsson (Ed.), Radiocarbon Variations and
Absolute Chronology, Proc. 12th Nobel Symp. Wiley, pp. 629-40.

Lowe, J. John, ed. (1991) Radiocarbon Dating: Recent Applications and Future Potential, Quaternary Proceedings, Number 1, 1991, Wiley
Robert Carter
Even though this is not my field of study, I happen to have several of these in my files already. But don't forget to compare to what is already available on creation.com: radiometric-dating-questions-and-answers.
Peter B.
The dates could be out too because of the expotenially reducing magnetic field total energies ie less protection from uv changing the ratio of nitrogen, carbon(14) and carbon(12).
Robert Carter
Yes, a decreasing magnetic field strength would allow for more cosmic rays to enter the atmosphere over time, which would induce increased rates of 14C production and throw off any ancient measurement with respect to modern values. Good point. See the-earths-magnetic-field-evidence-that-the-earth-is-young.

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