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:
- 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.
-
The ratio of C-14 to Carbon-12 in living matter has not been absolutely constant over the
past 50,000 years:
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The 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).
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Testing 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. |
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The 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.
- 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.
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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:
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It 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. |
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In 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
- 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:
-
Living snails were C14
dated at 2,300 and 27,000 years old," EvoWiki.org, at:
http://wiki.cotch.net/
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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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