Chemistry Reference
In-Depth Information
oscillations in the earth, and a short-range period, only a few years long,
caused by the electromagnetic activity of the sun. Both cycles, combined,
seem to be the cause of systematic errors of several hundred years that are
generally allowed for when dating with radiocarbon.
A mass of evidence seems to confirm that the mixing rate of radiocar-
bon in the atmosphere is rapid, and that with respect to its radiocarbon
content the atmosphere can be considered as a homogeneous entirety. The
contamination
of samples with matter from an extraneous source can never-
theless invalidate this assumption. Two types of contamination can be dif-
ferentiated:
physicochemical contamination
and
mechanical intrusion
. There are
two forms of
physicochemical contamination
. One is due to the dilution of the
concentration of radiocarbon in the atmosphere by very old carbon, practi-
cally depleted of radiocarbon, released by the combustion of fossil fuel, such
as coal and oil. The other is by the contamination with radiocarbon produced
by nuclear bomb tests during the 1950s and later in the twentieth century.
The uncertainties introduced by these forms of contamination complicate the
interpretation of data obtained by the radiocarbon dating method and
restrict its accuracy and the effective time range of dating.
Mechanical intrusion
is the penetration of the matter to be dated by carbon
of a different age from that of the sample itself; if not taken into account,
mechanical intrusion, too, leads to erroneous ages. The penetration of
rootlets from growing plants into buried specimens, the infiltration of wind-
blown organic matter, and the accidental insertion of fibers from brushes or
other instruments used to clean a sample are examples of likely
modern
carbon intrusions
into prospective samples, which lead to assigning to a
sample later dates than the true ones;
old carbon intrusions
, such as those
caused by the penetration of carbonate minerals from groundwater, or of
petrol or oil from excavating tools, on the other hand, are conducive to
assigning earlier dates than the true ones.
Fortunately, however, the deviations from “true dates” caused by all
these factors are usually small for dates falling within the last three millen-
nia.
Correction curves
are used to correct dates that fall between several mil-
lennia B.C.E. and the present day, so that the dates determined with
radiocarbon are concordant with historical dates (Pearson et al. 1989; Suess
1965).
Radiocarbon Dates.
Historical dates are usually expressed in calendar
years, but dates determined by the radiocarbon method are expressed as
radiocarbon years
. This is done on the assumption that during the past the rel-
ative concentration of radiocarbon in the atmosphere has been constant;
