BASICPRINCIPLES – MAKING THEMOST OF THECLUES 25
Tree-ring dating can give very precise results in opti-
mum circumstances, perhaps even to a season in a single
year. However, once the sample has been matched with a
point in the sequence it is important to understand what
the resulting date actually means. Does the sample come
from a context where a date would be useful to the
understanding of the site? The wood could have been intro-
duced on site any time between the site being formed and
its discovery. Similarly, wood is often re-used. Ships’ tim-
bers are frequently found forming part of waterfront
structures and other buildings, so care must be taken in
how dating information gained from such elements is used.
Ships and wooden structures in general require frequent
repairs. A wood sample taken from a repaired area might
be significantly later in date than the rest of the structure.
The sample may have been found in a ship in UK waters,
but the wood may have come from elsewhere. Also, it
is very important to remember that the date provided
by dendrochronology relates to the growing period of the
tree (the period in which the rings were formed) and not
its arrival on site. Timber can be stored for long periods
to allow for seasoning before it is used. One also has to
bear in mind that not all wood samples will be datable,
even when a suitable number of growth rings exist.
Sometimes a match cannot be found and dating is there-
fore unsuccessful.
Despite these problems, dendrochronology is suffi-
ciently accurate to be used to check, or calibrate, other
dating methods such as radiocarbon dating. Recom-
mended procedures for dendrochronological sampling
are given in chapter 15.
Radiocarbon dating: Radiocarbon dating is based on
the known rate of decay of a radioactive carbon-14
isotope, which occurs in very small quantities in all
living things. For the dating process, all that is needed is
a sample of carbon that can be directly related to the
evidence that requires a date. During their lives all living
things absorb the naturally occurring carbon-14 isotope.
The amount of isotope absorbed depends on its level of
concentration in the organism’s surroundings. When the
organism dies, the carbon-14 level starts to reduce as
the isotope decays. Since the rate at which this particular
isotope decays is known, specialists can make an accu-
rate assessment of how long ago the organism died by
measuring the remaining amount, assuming that the
levels of the isotope in the organism’s surroundings have
remained constant.
The method is generally less precise than dendrochron-
ology but can be used to date much earlier material.
Radiocarbon measurement is normally effective back
to 30,000 – 40,000 bp(before present; i.e. before 1950) and
up to 60,000 bpis possible. However, as with dendro-
chronology, radiocarbon dating relates to the living period
of the organism, not to its arrival or use on site, so the
same degree of care has to be used when the dates are
interpreted.
One of the basic assumptions with radiocarbon dating
has been that the amount of radiocarbon in the atmosphere
has remained constant throughout time. Calibration
work using techniques such as dendrochronology suggests
that the level has in fact fluctuated and that calibration
of all radiocarbon dates is now necessary to give an
approximate date in calendar years.
Radiocarbon dating results include the following in-
formation that identifies uncertainties in the date given:- Radiocarbon dates are normally quoted giving an
error margin. For example, a date may be quoted
as 1764 ± 100 years. This is a statement of
Figure 4.6 Tree-ring sequence. By using years of growth shown in the rings of individual trees from the same area, a
year-by-year sequence can be constructed back into the past. (After Eckstein et al., 1984:10; drawing by Graham Scott)