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Carbon-14 Dating: all viewpoints

Limitations of, & extensions
to, the dating technique

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Limitations to the C-14 measurement:

There are five main problems with this instrumental technique:

  1. If the sample died more than 50,000 years ago, it would have no measurable C14 left today. Thus, the analysis technique cannot differentiate between samples which are 50 millennia or 100 millennia BP.
  2. The ratio of C-14 to Carbon-12 in living matter has not been absolutely constant over the past 50,000 years:
    bulletThe ratio was higher before the industrial era started to release large amounts of carbon dioxide into the atmosphere. The measured age of any samples which died after the start of industrialization circa 1850 CE would appear older than they really are. This, of course, would not affect the C-14 dating of the shroud of Turin, which is the subject of hot debate between some scientists (who believe that the shroud was created in medieval times) and some conservative Christians (who believe that the shroud was used in Jesus' burial and thus is dated to the 1st century CE).
    bulletTesting nuclear bombs in the atmosphere in the 1950s increased the amount of C-14 in atmospheric carbon dioxide. The measured age of samples that were living during that time would appear younger than they really are. This has no impact on the dating of the Shroud of Turin or on material from biblical times either.
    bulletThe quantity of cosmic rays bombarding the earth affects the amount of C-14 that is created in the upper atmosphere. The level of cosmic rays varies with the sun's activity, the strength of the Earth's magnetic field, and any magnetic clouds traversed by the solar system as it proceeds around our galaxy.

    This means that the C-14 to C-12 ratio in a sample might be slightly higher or lower at the time that it died than the present value. Thus it was necessary to calibrate the technique. Samples whose ages are known are measured using C-14 dating, and a calibration curve was created. This makes minor corrections to the measured age, producing a more accurate answer than would be obtained by using the theoretical calculations alone.

  3. Libby's original estimate of the t 1/2 of C-14 was slightly in error at 5,568 years. This means that date estimates made in the very early years of the technique were 3% low.
  4. The C-14 dating system assumes that C-14 in the animal or plant matches the level in the general environment. In rare cases, plants and animals may live in very unusual environments whose C-14 content is much lower than what one would expect. This is called a "reservoir effect." For example:
    bulletIt is possible for snails to live in water that contains carbon leached out of ancient limestone which has no measurable C-14 left. As a result, the snails' shells will also be deficient in C-14 and test older than their true age.
    bulletIn a few areas of the world, seals dine on fish that in turn had eaten other fish and plants that lived in sea water that has been traveling along the bottom of the ocean for thousands of years, gradually losing its C-14 content. Again, the quantity of C-14 in their environment is deficient. They would also test older than they really are.

    According to EvoWiki.org:

    "The problem caused by the reservoir effect is well known by archaeologists, geologists, and anybody else who use radiocarbon dates; they test for it and take it into account when interpreting radiocarbon data." 1
  5. Contamination of the sample can include sufficient C-14 to make it seem newer than it really is. Porous samples can contain recently living material with a full "charge" of C-14. Sample cleaning and proper laboratory technique are critical.

Extending the calibration curve to cover older samples:

Since the time of Libby, the developer of the C-14 analysis, calibration checks have been made using  U.S. bristlecone pine and other species of trees. This pushed the calibration back beyond recorded history almost to 10,000 BP (years before the present.) One valuable source of samples of various ages came from a bristlecone pine tree called "Methuselah" in the White-Inyo mountain range of California. Counting tree rings showed that it had germinated in 2726 BCE. Samples from the tree were able to generate calibration points back to that date.

A tree creates a new tree ring each year. It is narrow or broad, depending upon whether the weather during that year was dry or wet, and whether the tree was exposed to various stressors. Bristlecone pines grow so slowly that its rings are paper thin; their width has to be studied under a microscope.

Methuselah's tree ring sequence near its core -- when it was a young tree -- was matched to the sequence found in pieces of nearby trees which had died previously. Dr. Henry Michael of the University of Pennsylvania's Museum Applied Science Center for Archaeology (MASCA) scanned the area almost every summer for over three decades. His goal was to find sections of dead trees whose rings could be pieced together to extend the samples as far back in time as possible. He found  an irregular slab from a bristlecone pine that spanned the years 3050 BCE to 2700 BCE. The tree ring sequence adjacent to the slab's bark matched the sequence near Methuselah's core.

Sometimes he was lucky. He found a piece that contained 600 rings; another contained 150 rings. Other times, he had to fit together pieces of tree like a jigsaw puzzle. According to Tom Gidwitz:

"Dendrochronologists [scientists who study tree rings] have built sequences for a number of tree species, including German, Irish and Polish oaks, Patagonian cypresses, Lebanese cedars, pine, yew, spruce, and chestnut. They've plotted rings from shipwreck timbers and roof beams, and wood from the Mediterranean, Russia, and China. The bristlecone pine series, the longest built from a single species in a single location, stretches back to 7040...BCE [9045 BP]. It is based on thousands of rings from 81 living trees and 118 dead trees, and Michael found more than three-quarters of the samples. He also recovered one older piece of wood, carbon dated to more than 10,000 years ago, whose rings cannot yet be linked to the the sequence." 2

References used:

  1. Living snails were C14 dated at 2,300 and 27,000 years old," EvoWiki.org, at: http://wiki.cotch.net/
  2. Tom Gidwitz, "Telling Time. By reading the rings of pines as old as the earliest pharaohs, Henry Michael has changed the way we look at the past," Archaeology, 2001-MAR/APR. Online at: http://www.tomgidwitz.com/ The quotation is taken from the online version.

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Copyright © 2005 to 2009 by Ontario Consultants on Religious Tolerance
Originally posted: 2005-FEB-08
Latest update: 2009-JAN-02
Author: B.A. Robinson

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