INTRODUCTION TO ARCHAEOLOGY Nancy White - Touro Institute

What is AMS radiocarbon dating? The letters stand for accelerator mass spectrometry, and this
technique is a wonderful refinement that allows the physicist with a big particle accelerator
machine to count the actual carbon atoms. Therefore it needs far less material for dating, such as
a piece of charcoal as big as a pencil point tip. This can be a great advantage when there is not
good organic preservation at your site. From our USF excavations of a shell midden in Florida,
we obtained a sherd of fiber-tempered pottery that broke open to show a couple fibers of Spanish
moss still unburned and undecayed in the clay. These were dated by the AMS method—one of
the rare times that we COULD date a potsherd in this fashion. The date returned was about 4,000
years before the present.
There is yet another disadvantage of radiocarbon dating that has lately been corrected by
utilizing a totally independent chronometric method, known as dendrochronology. What does
this word mean? Tree ring dating, using sequences of thick and thin rings from trees with
overlapping lifetimes and working backward in time. We can even find living trees that are
several thousand years old, and your book shows a picture of one of these, the gnarly old
bristlecone pine from the southwestern U.S. Ring characteristics depend on annual weather
patterns, so this method works best where there is great variability, as in the southwestern desert.
Disadvantages of the method are that you have to have enough wood, and the wood the artifact
was made of may have been old already when used, or used for a long time, as in a house beam.
How has dendrochronology changed radiocarbon dating? The great gift of tree-ring dating has
been to help correct the errors in radiocarbon dates and provide a cautionary tale of science. The
assumption was that uniform amounts of radioactive carbon were present every year in the
atmosphere and absorbed by all living things. When someone decided to check this by
radiocarbon dating individual tree rings with known ages, it became clear that past levels of
carbon 14 have fluctuated a lot. Now whenever you get a radiocarbon date, it is corrected or
calibrated along a curve produced by tree ring dates to give a more accurate calendrical age.
What other dating methods are important for archaeology? Several other chronometric methods
are based on measuring time elapsed since some physiochemical change. Most important is
potassium-argon dating which again measures decay of a rare radioactive isotope of potassium
that results in its becoming argon gas. Here the half-life is 1.31 billion years, so much older
things can be measured. But you need something that has the requisite minerals. Luckily, there is
potassium in volcanic deposits that have contained early human fossil forms in east Africa, so
many of those finds have been so dated.
What is archaeomagnetism? Another chronometric dating method that is based on the fact that
the earth’s magnetic fields shift over time. When soil or clay is heated, the iron particles in it
align themselves with the poles. Since north moves over time at a known rate, we can measure
the magnetic alignment in the ground and compare it with today’s. The method is often used in
the eastern U.S. when we encounter hearths or fire pits with burned clay. You isolate a small
cube of soil in the ground and cover it with plaster, on which you mark the current compass
north. Then you cut it out of the ground and send it for archaeomagnetic dating, and they will see
how the alignment of the iron particles in this soil cube compares with the current alignment of
north. Again, the disadvantages are that you have to have burned soil with iron content. The
advantage is that it is not difficult or too expensive.