How Old Is It? the Story of Dating in Archeaology

Part 1

HOW OLD IS IT?

THE STORY OF DATING IN ARCHAEOLOGY

MUSEUM OF NEW MEXICO PRESS POPULAR SERIES PAMPHLET NO. 2

HOW OLD IS IT? DATING IN ARCHAEOLOGY by James Schoenwetter

There is a whole field of science devoted to the invention and development of dating methods—or “clocks” as we may think of them. It is called geochronology, the science of dating events. There are relatively few geochronologists, scientists trained in the use of all kinds of dating methods and in the theories upon which these methods are based.

Geochronologists tell us that there are two major types of clocks: those that tick at an absolute rate of speed which can be measured, and those which tick only once in a while. A clock of the first type yields what is called an absolute date, revealing the number of hours, days, years, centuries or millennia since an event occurred. A clock of the second type yields what is called a relative date, placing an event as before or after another event, but does not tell us exactly how far they are apart in time nor how long ago they occurred.

Depending upon how accurate his date must be to solve the problem he has set for himself, the archaeologist will select absolute or relative dating methods. Often, of course, the type of clock he wishes to use is not available, and he must use the next best type. Probably he will try to use a number of clocks of different kinds on the problem since each clock will act as a check on the others.

The absolute clock utilized most widely in archaeology is the historical record. Men have used calendars for a long time, and have often left records with written dates. On tombstones at a site in old Virginia, on the pedestals of statues and other monuments from classical Greece and Rome, on the walls of the tombs of Egyptian kings, dates are clearly inscribed which can be related to the sites dug into by the archaeologists. These dates must often be recalculated in terms of the Christian calendar which we use. Most calendars in use in the Mediterranean, the Near East and China during classical antiquity have been successfully correlated with the one we use today, and a date inscribed or noted on such sites can be considered in our own terms. Other calendars, such as those developed in the ancient cultures of the Maya on the Yucatan Peninsula, have yet to be accurately correlated with our own. Such calendars can be used on their own terms of course, and a site which has an inscription in the Mayan calendar is known to be so many years older or younger than another one with a different date in that calendar. We speak of such a situation as a floating chronology. That is, the sequence of events and the number of years which separate them are known, but the dates of those events in absolute time are unknown.

Tree rings afford another kind of absolute clock, the dendrological method. Each year a tree adds a growth ring. Depending on the amount of water the tree has available to it for cell growth, the ring will be wider or narrower. Certain trees whose water requirements are high live near streams or other places where their roots can tap a constant supply of water. Such trees, referred to as complacent, have annual rings which are all of about the same width. Other sensitive trees live in places where they must depend almost wholly on rainfall for their water supply, as on the slopes of hills or in the clefts of rocks. Such trees have annual rings which vary in width depending upon the amount of rain they receive. In any given area, especially in arid and semiarid regions, some years have more rainfall than others. The sensitive trees will produce wider rings during years when there is more rainfall and narrower rings in years when there is less. Often there are periods of a decade or so when all of the sensitive trees will produce the same pattern of ring growth; for example, three years of narrow rings, one year of wide, two more of narrow and three more of wide. Such a pattern is called a signature.

Signatures are the basis of tree ring chronologies. All the trees in a region did not begin growing at the same time of course, but every time there is a series of years which will produce a signature, all the sensitive trees still alive will have that signature. Let us say we cut down a sensitive tree in 1960 and, by counting back the rings, find signatures at 1940-45, 1910-14, 1880-89, 1821-27, 1795-1800 and 1750-58. Next we recover a beam from an abandoned Spanish Mission built in 1810. It happens to be from a sensitive tree, and we can spot the 1795-1800 and the 1750-58 signatures near the outer rings. Now we have two records which can be said to be crossdated. Let us assume that the Spanish Mission log give us signatures as far back as 1350. An abandoned Indian pueblo produces a log with signatures crossdating those of the Spanish Mission log and continuing the record back to 1250. Older and older archeological sites will yield older and older signatures with each log crossdating some of the signatures of younger logs.

At present we have a tree ring calendar for certain species of trees extending back to about 100 B.C. This is called a master tree ring chronology. A log from an archaeological site may contain only one or two signatures and be, in itself, a floating chronology, but by comparison with the master chronology one can determine the cutting date. Having a series of cutting dates for the construction timbers in an archaeological site yields the probable dates at which the site was built and occupied.

C THIS BEAM CAME FROM AN OLD HOUSE B THIS BEAM CAME FROM A HOUSE A THIS WAS A LIVING TREE WHEN CUT BY US THE RING PATTERNS MATCH AND OVERLAP BACK INTO TIME SPECIMENS TAKEN FROM RUINS, WHEN MATCHED AND OVERLAPPED AS INDICATED, PROGRESSIVELY EXTEND THE DATING BACK INTO PREHISTORIC TIMES.

Not all kinds of trees can be used for dendrochronology. Pine, fir and pinyon are the most useful; juniper can sometimes be used. Oak, cottonwood, willow and others are very difficult to date and frequently cannot be used at all.

Another widely used absolute clock is that based upon the orderly decomposition of carbon. This is the radiocarbon or C-14 method of dating. Molecules of various substances are made up of atoms. We know now that not all atoms of a substance are precisely the same. We speak of isotopes of an atom. To clarify this let us think of the atoms of carbon as being made up of a mass of ping-pong balls. Some atoms will have more ping-pong balls than others, but all will have enough and in the proper order to be carbon atoms. Each of the atoms with 14 ping-pong balls we shall refer to as the carbon-14 isotope of carbon. There will be other isotopes of carbon atoms too.

The carbon-14 isotope is about average in life span. It has been determined that in any group of C-14 isotopes half of them will lose two of their ping-pong balls and become C-12 isotopes in 5,730 years, plus or minus 40. This is known as the half-life of the C-14 isotope. The “plus or minus 40” allows for laboratory error in terms of years.

Now all living things contain carbon atoms, and some of those atoms are C-14 isotopes. The amount of C-14 isotopes in a living organism quickly reaches a stable percentage after which there is no increase or decrease while the organism is alive. After it dies, the C-14 supply is not replenished, and with the passing of 5,730 plus or minus 40 years, it has half the number of C-14 isotopes it had when alive. In 11,460 plus or minus 80 years, it will have one quarter as many as it had when alive.

The geochronologist takes a certain weight of carbon-bearing matter from an organism which once lived. With simple chemistry he can determine the number of carbon atoms in the material, usually charcoal, wood, bone or shell. He places the material in a chamber equipped with geiger counters and records the number of C-14 isotopes converting to C-12 isotopes within a certain number of hours or days. Since he knows how many carbon atoms there are in the specimen, he knows how many C-14 isotopes there would be if the specimen were alive. He also knows that as the number of C-14 atoms decreases the number of clicks on the geiger counter will decrease too. For example, if there are 2,000 C-14 isotopes, the decomposition of half of these over a period of approximately 5,730 years would register 1,000 clicks on the counter. In the next 5,730 year period there would be 1,000 isotopes left, and only 500 of them would decompose to register as geiger counter clicks.

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