Chemistry Reference
In-Depth Information
(a)
(b)
FIGURE 65
Radiocarbon calibration.
Radiocarbon dates are always reported in
terms of years before present (
B
.
P
.). The actual values of these dates are calculated on
the assumption that the relative concentration of radiocarbon in the atmosphere has
always been constant. The concentration of radiocarbon in the atmosphere however,
has not always been the same; significant variations have taken place in the past. Con-
sequently, there is no fixed relationship between radiocarbon dates and calendar dates;
radiocarbon dates have to be corrected or “calibrated” into calendar dates. The con-
version process, which is known as radiocarbon calibration, is not straightforward:
it is based on comparing radiocarbon dates with dates derived with other dating
methods. The most reliable and relevant of these is tree ring counting (den-
drochronology) that involves measuring the relative amounts of radiocarbon in tree
rings, calculating their radiocarbon dates, and then comparing these dates with the
dendrochronological date of the rings. Calibrated calendar dates are generally followed
by the suffix “Cal
B
.
P
.” or simply
B
.
P
. The two graphs show the relationship between
radiocarbon dates and calendar dates. Had the concentration of radiocarbon in the
atmosphere been constant over time, the relationship between the dates would have
been linear and the radiocarbon and dendrochronological dates would be on a straight
line. As can be seen in the two graphs, there are significant mismatches: in graph
(a)
are shown dates between 9.000 and 13.000 years
BP
and in
(b)
dates between 0
and 1.500 years
BP
. A radiocarbon age of 11.000 years bp, as in
(a)
for example, cor-
responds to a calibrated (or calendar) age of about 13.000 bp and a radiocarbon age
of about 200 years bp in
(b)
corresponds to a calibrated age of between 300 and under
500 years bp (Hughen et al. 1998, 2004).
The amount of information gathered over the decades since the radio-
carbon dating method was developed is very extensive, so that even the
briefest of reviews on the subject is out of the question here. The interested
reader is referred, therefore, either to
Radiocarbon
, the periodical dedicated
to the regular publication of new radiocarbon data, issued by the University
of Arizona, or to one of the excellent reviews on the subject such as those by
Hedges 2001 and by Lowe 1997.
