Texas A&M University College Station and Qatar
A technique based on cold argon and oxygen plasmas
permits radiocarbon dates to be obtained on paintings that
contain inorganic pigments. (To listen to a podcast about
this feature, please go to the Analytical Chemistry website
at http://pubs.acs.org/journal/ancham.)
Rock art images are among the most enigmatic and personal
artifacts studied in paleoarchaeology. Rock paintings (pictographs)
left by ancient prehistoric cultures are found all over the
worldsvirtually everywhere there are rocks. One example of a
polychrome painting in the lower Pecos River region of southwest
Texas (where thousands of other impressive painted images are
found on the limestone shelter walls) is shown in Figure 1. Two
frequently asked questions are, without regard to location, how
old is it and what does it mean? This review deals with attempts
to answer the question of age.1
Rock art is an important, irreplaceable part of our heritage.
When there is inadequate ethnology for a region, as is most often
the case, it can be argued that rock art is the most important
evidence available for discerning the thought processes and the
aesthetic, symbolic, and religious ideas of the prehistoric cultures
of the peoples who painted them.2 But to incorporate rock art
into mainstream archaeological thought, one must be able to
assign it to a specific ancient culture and time span.
Archaeology gained an important technique for dating archaeological artifacts in the 1940s when Willard Libby and his colleagues
developed radiocarbon dating,3,4 a revolutionary method for which
he was awarded the 1960 Nobel Prize in Chemistry. Thus, when
we were asked about two decades ago by anthropologist Harry
Shafer to date a piece of a pictograph that had been picked up
from the floor of an ancient painted shelter in southwest Texas,
we primarily considered radiocarbon dating while trying to devise
a means of dating that pictograph.
The principle of radiocarbon dating is deceptively simple.
Radioactive 14C, or radiocarbon, is continuously produced in
the earth’s upper atmosphere by interactions of secondary
cosmic ray neutrons with the most common atmospheric
isotope, 14N. The 14C produced is rapidly oxidized to 14CO2 and
quickly distributed throughout the atmosphere, thus becoming
incorporated into the earth’s biological carbon cycle. For this
paper, we assume that most living matter has a very similar
14C/12C level. When an organism dies, the contemporary 14C
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