Calibrations of radiocarbon
age determinations are applied to convert BP results to calendar years. The shortterm
difference between the two is caused by fluctuations in the heliomagnetic modulation of
the galactic cosmic radiation and, recently, large scale burning of fossil fuels and
nuclear devices testing. Geomagnetic variations are the probable cause of longer term
differences.
The parameters used for the corrections have been
obtained through precise analyses of hundreds of samples taken from known-age tree rings
of oak, sequoia, and fir up to about 10,000 BP. The parameters for older samples, up to
about 20,000 BP, as well as for all marine samples, have been inferred from other
evidence. Good calibrations are presently provided for samples to about 10,000 BP . From
10,000 to 20,000 BP approximate calibrations are possible, but subject to change in the
future. Our calibrations are calibrated
using the newest calibration data as published in Radiocarbon, Vol. 40, No. 3, 1998
using the cubic spline fit mathematics as published by Talma and Vogel, Radiocarbon,
Vol. 35, No. 2, pg 317-322, 1993: A Simplified Approach to Calibrating C14 Dates. It is
always important to quote the data source and mathematics with your interpretations.
The program chosen for these dendrocalibrations uses
splines through the tree-ring data as calibration curves, which eliminates a large part of
the statistical scatter of the actual data points and gives a better "real"
approximation of the sample's calendar equivalence using the measured correlation curve.
The spline calibration allows adjustment of the average curve by a quantified
closeness-of-fit parameter to the measured data points.
Each result is provided in text form and graphic
form on a single hard-copy (example below). On the calibration curve print-outs, the solid
bars represent one sigma statistics (68% probability) and the hollow bars represent two
sigma statistics (95% probability). Marine carbonate samples that have been corrected for dC13 and have also been
corrected for both global and local geographic reservoir effects (as published in
Radiocarbon, Volume 35, Number 1, 1993) prior to the calibration. Marine carbonates that
have not been corrected for dC13, have been adjusted by an assumed value of 0 % in addition to the reservoir
corrections. Reservoir corrections for fresh water carbonates are usually unknown and are
generally not accounted for in those calibrations. In the absence of measured dC13 ratios, a typical value of
-5 % was assumed for freshwater carbonates. Variables used in each calibration are listed
below the title of each calibration page.
(Caveat: the calibrations assume that the
material dated was living for exactly ten or twenty years (e.g. a collection of 10 or 20
individual tree rings taken from the outer portion of a tree that was cut down to produce
the sample in the feature dated). For other materials, the maximum and minimum calibrated
age ranges given by the computer program are uncertain. The possibility of an "old
wood effect" must also be considered, as well as the potential inclusion of some
younger material in the total sample. Since the vast majority of samples dated probably
will not fulfill the ten/twenty-year-criterium and, in addition, an old wood effect or
young carbon inclusion might not be excludable, dendro-calibration results should be used
only for illustrative purposes. In the case of carbonates, reservoir correction is
theoretical and the local variations are real, highly variable and dependant on
provenience. The age ranges and, especially, the intercept ages generated by the program
must be considered as approximations.)
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