Origins 22(1):4752 (1995).
WHAT THIS ARTICLE IS ABOUT
Scientists correct the raw data from radiocarbon dating determinations so as to give what they consider to be a more accurate realtime age. This is necessary because of the uncertainty about the original concentration of carbon14, which must be assumed to calculate a radiocarbon age. In order to determine what realtime age should be associated with a radiocarbon age, the radiocarbon data are often compared to historical and treering data that are considered to be more reliable indicators of time. Treering data are especially important in the correction process for dates older than 1000 BC. Extensive lists of correlation between radiocarbon data and treering data have been published.
However there is a problem. It appears that the treering chronology that has been established to adjust the raw carbon14 determinations is a fragile structure. Our oldest living trees appear to be less than five thousand years old. Radiocarbon corrections beyond that are often based on attempts to match the thickness variations of tree rings in old wood samples. If a similar pattern of variation in treering thickness is found in two pieces of wood, the two are assumed to have grown at the same time. By comparing many pieces of wood and combining matches, treering chronologies of over 11,000 years extent have been proposed for use in correcting carbon14 dates. The reliability of the system is dependent on the correctness of the treering matches, — and here there is considerable uncertainty. Statistical tests show that it is easy to get significant matches of treering patterns at various juxtapositions between samples of wood. More sophisticated statistical tests are being developed to correct for this problem. However, these tests were not used when the original dendrochronological correction scheme for carbon14 dates was established. It appears that this original scheme is subject to reevaluation.
Using radioactive carbon (carbon14 — C14) to determine age is a complex process. The method is based on the slow disintegration of C14. The less C14 present in a sample, the older it will date. To determine a date, one must have data concerning:
At the best laboratories the C14/C12 ratio can be determined to about onethousandth
of the value that characterizes contemporary plants and animals. The most recent
determination of the spontaneous C14 conversion rate indicates that, within an
uncertainty of about ± 30 years, in 5715 years half of an initial amount of C14 will
have converted into N14.^{1} At this rate of conversion approximately 57,000
years would be required for the C14/C12 ratio to diminish onethousandfold. The initial
C14/C12 ratio is not accessible to experimental determination, and must be assumed.
Accordingly, any C14 age is based on an assumption.
The simplest calibration base for the initial C14 is the assumption
that throughout all past time accessible to C14 dating, the C14/C12 ratio in the active
carbon exchange system has been the same as it is at present. With this calibration base a
specimen for which the C14/C12 ratio is 0.001 times that of corresponding contemporary
material has a 57,000 year radiocarbon "age." Radiocarbon ages obtained in this
simple, direct way may be classified as "radiocarbon isotope ages."
However, there is good evidence that the proportion of C14 has varied
over time, and a more reliable calibration base is the C14/C12 ratio found in artifacts
that have a precise and accurate historical (calendric) age. A base established in this
manner requires guessing by interpolation for C14/C12 ratios that fall between values
that have been calibrated by historical dates. Also it is insecure for extrapolation
beyond the oldest firmly established historical calibration points.
For older dates the most satisfactory calibration base is the C14/C12
ratio of wood that has been dated by dendrochronology.^{2} In temperate climates
wood cells that are produced in the beginning of the growing season are larger and have
thinner walls than the cells produced in the latter part of the growing season. The
density difference between early and late growth produces visible features known as tree
rings. Variation in the width of these rings results from yearbyyear variation in the
conditions favorable to growth of a particular portion of a tree. By assuming that a
similar variation in the pattern of ring thickness between samples represents growth
during the same period of time, the ringwidth patterns of many wood specimens can be
combined into a single master dendrochronological sequence that 1) has an average
growthring width variation pattern for periods of overlapping growth, and 2) extends the
time range beyond the time span of any one component. Extension of the time range is
accomplished by matching an upper portion of the ringwidth sequence in one specimen with
the lower portion of another specimen. The Bristlecone Pine master dendrochronological
sequence that has been foundational for C14 calibration has been based on 81 livingwood
and 118 deadwood specimens from the White Mountains of California.^{3} This basic
pattern for dendrochronological calibration of C14 age was set by C. W. Ferguson in 1969.^{4}
A calibration that falls within a time span that has been established
by wood specimens that have been dated by unquestioned historical records (usually by
crossreferencing C14 ages) can be relied on to give a high precision estimate of real
time. But because of the uncertainty in matching a wood specimen against a master sequence
only on the basis of growthring patterns, there is uncertainty regarding the validity of
a master treering sequence in a range that has been extrapolated beyond an unquestioned
historical reference point.
The magnitude of these uncertainties is indicated by treering study of
a Douglas fir log from a Mt. St. Helens pyroclastflow deposit.^{5} I am indebted
to R. M. Porter for bringing this study to my attention.^{6} The flow that
contained this log has been dated by stratigraphy (dating of rock layers) to have occurred
within the range AD 14821668. The log had 290 growth rings from core to bark. The age of
the growthring immediately adjacent to the bark is designated as the "bark
date." Segments of 20 or more treerings beginning from either edge of this 290ring
sequence were compared for possible match against the Douglas fir master treering
sequence.^{7}
Computercalculated coefficients of crosscorrelation statistically
significant at or beyond the p=0.001 level (99.9% confidence) indicated 113 possible bark
dates within the range AD 14102240 (projected bark dates that are beyond the
present are italicized). Fortythree of these matches were within the barkdate range AD
14831668, 23 within the range AD 16691771, and 47 within the future range AD 20782195.
Matches beyond the limits of the master chronology were made using a partial overlap with
the 290ring log. The AD 2195 date match had a 75 ring overlap with the AD 1980 end
of the masterring sequence. The lowest match, AD 1483, had an 87 ring overlap with the AD
1396 end of the master sequence. Matches can be evaluated using the Student'st
statistical test of probability. The 113 matches had Student'st^{8} statistical
values within the range from 3 to 7, the highest of which was 6.8 for an AD 1647 barkdate
match. All these student'st values suggest a high statistical reliability (99.9%
confidence) under the assumptions with which the matches were made. The most secure
interpretation of these data indicates treering matches that place bark dates near the
midpoints of the six AD ranges 14931510, 16421664, 17441748, 17561772, 20782098,
21722180, for which Student'st values greater than three are clustered.
To see the significance of these data, consider all the dates inverted
from AD to BC, and the "bark date" an indication of the beginning rather than
the end of a growth sequence. An investigator seeking to extend a treering master
chronology that had been developed to 1980 BC might get a match with the last 75 growth
rings of a subfossil log containing 290 growth rings. This match could provide a high
degree of statistical assurance for incorrectly extending his master chronology to 2195
BC. The investigator might not be aware of a better match possibility with a 1647 BC
terminal date (extending growth period 290 years to 1357 BC).
An individual who used C14 measurements for a guide in assembling a
treering sequence, as is often done, would be unlikely to make a single error as great as
215 years (21951980), but an accumulation of smaller errors is possible. Or, an
investigator with an unknownaged piece of wood containing 290 growth rings could with a
high degree of statistical justification chose any one of 66 matches (11347) within the
previously developed master growthring sequence, making his ultimate choice in accord
with where he had expected, or wanted, the match to occur.
Special procedures have been developed to reduce such errors. By a
mathematical technique for "whitening" a master chronology sequence, i.e.,
removing the effects of correlated ringwidth sequences within the master sequence
(repeated patterns of variation — autocorrelation), Yamaguchi^{9} was able
to eliminate 112 incorrect matches and focus on the AD 1647 bark date. After the whitening
process the crosscorrelation for the AD 1647 date had a Student'st value of 5.05
(greater than 99.9% confidence level), and a correlation coefficient of 0.29. (For a
correlation coefficient of 1.00 the relative width of each of the 290 rings in the
log would be exactly the same as the relative width of the corresponding ring in the
master chronology.) These results confirmed stratigraphic placement of the volcanic
eruption that buried the log within the AD 14821686 time range.
Whitening technique was not used in the development of the Bristlecone
Pine master dendrochronology that is the standard for calibrating C14 age. Whitening
technique analysis of the various dendrochronology master sequences that were published
prior to 1985 indicates that the master sequence developed by Ferguson has unique
autocorrelation features, and that its use is definitely questionable.^{10}
Matching a 290 growthring subfossil log to the Pacific Northwest
Douglas Fir master growthring sequence is an ideal treering dating assignment. If
crossmatching is no more certain than in this example, what confidence is justified in
the extension of a master treering sequence beyond the range that is constrained by
unquestionable historical records, since each stage in the extension of a master
chronology is a crossmatching operation? Specifically, what statistical assurance does
dendrochronology provide for presuming that C14 isotope ages relate approximately 1:1
(within 10%) with real time between 500 BC and ~10,000 BC?^{11}
ACKNOWLEDGMENTS
I am grateful for contributions to the preparation of this note made by Ariel Roth regarding content, and by Paul Giem and Maurie Evered regarding mathematical concepts.
ENDNOTES
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