Page 126 - Radiochemistry and nuclear chemistry

112 Radiochemistry and Nuclear Chemistry

14Ccorr = 14C { 1 --2(~13C + 25)/1000 } (5.4)
The value of 813C differs for various substances; e.g. -35 to -20 for terrestrial organic
matter, -8 to -7 for atmospheric CO 2, etc. Figure 5.4 illustrates the use of various (~13C
values for biological matter. These corrections lead to considerable adjustments of
conventional 14C ages; e.g. for the period 7 000 to 2 000 years ago, corrections increase
ages by up to 1 000 y.
Another necessary correction has to be made for changes in the cosmic ray irradiation,
which, a priori was assumed to be constant, but is not over long geological periods. By
counting the number of annular rings on old trees such as the Sequoia gigantea, which can
be almost 4000 years old, and determining the 14C content of each ring, the variation of
cosmic radiation through thousands of years can be studied accurately, see Figure 5.5.
Obviously there is a periodic cycle of some 10 000 y, which coincides with the variation
in warm periods and small ice ages.
A large number of important age determinations have been made with the 14C dating
method. It was believed that a large ice cap had covered parts of the North American
continent until about 35 000 years ago. Dating of wood and peat by the 14C method has
shown that the ice must have lasted until about 11 000 years ago. Moreover, several
hundred pairs of sandals found in a cave in Oregon have been shown to be about 9 000
years old, indicating that tribes with significant cultures had developed soon after the
withdrawal of the ice cap from North America. By analyzing inorganic carbon in bone
apatite, it has been determined that hunters in Arizona killed mammoths there 11 300 years
ago. Another example of the use of 14C dating which has attracted widespread attention
involves the Dead Sea scrolls. There was considerable controversy about their authenticity
until 14C dating showed their age to be slightly more than 1 900 years.
Some researchers claim that 14C-ages lag behind with as much as 3 500 years for times
close to 20 000 y, when compared to U-Th dating (w Such adjustments play a large
role in the debate about cultural migrations. It is interesting to note that adjusted 14C-dates
now have led to the conclusion that e.g. the stone monuments in Camac, Brittany, are
believed to be > 6 000 years old, i.e. older than the cultures in Egypt and Babylon.
Similarly, grave mounds near the ancient edge of the inland ice in southern Norway and
in Newfoundland have been found to be > 7 000 years old.

5.8.2. Dating by K-Ar method

Potassium is the eighth most abundant element in the earth's crust and occurs in many
important rock-forming minerals. The radioactive isotope 4~ is present only to 0.0117 %
in natural potassium. It has a branched decay as follows:
EC, 10.7 %
......... > 4~ (99.6 % of natural At)
4~ 1.28 x 109 y) ( (5.5)
......... > 40Ca (96.8 % of natural Ca)
~, 89.3%