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123rf.com/Sergei Popov

Calibrating carbon dating

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

Published: 2 February 2013