INTRODUCTION TO DATING

Archaeomagnetic Dating
In the early to mid 1960s, a new technique for dating archaeological material emerged. this technique, known as archaeomagnetism, was introduced by Dr. Robert Dubois. As applied, archaeomagnetism relies on thermo-remnant magnetism; that is remnant of magnetic orientation is sediments that were once heated sufficiently to alter their magnetic orientation toward magnetic north. Clay, when heated, acquires a remnant magnetism with a direction paralleling the earth's magnetic field. Once the clay cools, the fired clay hold that magnetism until reheated. By knowing the date (by some other dating method) of a feature and measuring the direction of magnetism in clays from this feature, it is possible to determine the ancient pole location (called the virtual geomagnetic pole or VGP) of the earth's magnetic field at the time this clay was last fired. When a large number of these ancient VGPs are dated through this method, a composite curve of polar wandering (a VGP curve) can be reconstructed. The VGP curve can be used then as a master record against which the VGPs of samples of unknown age can be "dated". This implies that by measuring the remnant magnetism from a clay sample from an archaeological context, we can obtain a reference point relative to this curve and therefore chronometrically date the context. (See master curve in more detail.)

Measurement of the ancient magnetic field direction is made on pieces of fired clay collected at archaeological sites in an oriented fashion. In southern Arizona, people constructed what archaeologists call pithouses. Pithouses tend to be roughly rectangular and dug into in the ground in shallow pits thereby giving them their name. They typically had a clay lined hearth immediately inside the entry. These are the source for archaeomagnetic samples. Pieces of the baked clay are carefully isolated by carefully sawing them from each hearth. A nonmagnetic, cube-shaped mold (made of aluminum) is placed over the isolated column and filled with plaster. A record of magnetic north and the vertical and horizontal placement of the sample are then made and recorded on the sample recording sheet. Usually, eight to twelve of these specimens are collected from the parameter of the hearth and submitted for processing. The ancient magnetic direction and age determination for a given feature is then calculated in a lab setting on the basis of the mean direction of all specimens collected from the feature. The data are then summarized and returned to the archaeologist for interpretation relative to other data from the site.

Radiocarbon Dating
The basis for radiocarbon dating is magnificently simple. Carbon 14 is continuously produced in the upper atmosphere by the action of cosmic rays, which set free neutrons that transmute nitrogen in the air into radioactive carbon. Incorporated in carbon dioxide, the radiocarbon moves through the atmosphere and is absorbed by plants. animals in turn build radiocarbon into their tissues by eating the plants. As long as they are alive, plants and animals go on ingesting radiocarbon. When an organism dies, and ceases to take in fresh carbon, its built-in clock begins to run down. The disintegrations of its carbon-14 atoms tick away the seconds and the years: in 5,568 years (on the average) only half of its original store of radiocarbon atoms is left, and in another 5,568 years only half of those, or one quarter of the original number.

Long before that time, of course, most plants and animals have decayed into dust. but when the remains of an organism are fortuitously preserved, as a house beam, a bit of charcoal, or a seed, the age of the remains can be calculated. The amount of radiocarbon the organism possessed when it was alive is known, and so is the rate of its radioactive disintegration. It is easy to compute the age of the remains by counting the amount of radioactivity that still remains.