Forbidden Archeology_ The Full Unabridged Edition - Michael A. Cremo [252]
5.4.4.1 The Uranium Series Dating of the Hueyatlaco Site
The principal technique used for dating materials from Hueyatlaco and El Horno was the uranium series method. The tests were performed by Barney J. Szabo of the U.S Geological Survey (Szabo et al. 1969). In this section, we will discuss Szabo’s results in some detail to show they support his dates. In particular, Szabo’s data suggest that leaching of uranium from the sample could not have produced erroneously old dates, as some have hypothesized. Readers uninterested in the technicalities may proceed to the next section.
The uranium series technique is based on the fact that each of several isotopes of uranium spontaneously breaks down into a distinct series of byproducts. At the Hueyatlaco and El Horno sites, Szabo was concerned with uranium 238 and uranium 235.
Uranium 238 decays to uranium 234, with a half life of 4.51 billion years, and uranium 234 decays to thorium 230, with a half life of 248,000 years. Thorium 230 in turn decays to radium 226, with a half life of 75,000 years (Considine 1976, p. 1866).
Uranium 235 decays to protactinium 231, with a half life of 707 million years, and protactinium 231 decays to actinium 227, with a half life of 32,500 years (Considine 1976, p. 1868).
The concept of a half life can be explained as follows. Say you start out with one pound of uranium 234, with a half life of 248,000 years. After 248,000 years, you would have a half pound of uranium 234, along with some thorium and radium. After another 248,000 years, you would have a quarter pound of uranium 234, with more thorium and radium, after another 248,000 years, an eighth of a pound of uranium and still more thorium and radium, and so on.
Small amounts of the uranium isotopes that form the starting points of our two series (uranium 238 and uranium 235) occur naturally in water, yet their decay products, thorium and protactinium respectively, are not found in water (Gowlett 1984, p. 86). Certain types of rocks (such as travertines, tufas, and concretions) form when inorganic carbonates precipitate out of water. During this precipitation, small quantities of uranium are included within the rock, but no thorium or protactinium. Hence, under ideal conditions, all of the thorium and protactinium found within such rocks comes from the decay of uranium isotopes. Also, bones that are soaked in uranium-bearing water tend to absorb uranium, which decays and produces byproducts.
Since the half lives of uranium, thorium, and protactinium are known, scientists say that by measuring the amounts of these elements present within a sample they can calculate the age of the sample. The more decay products present in the sample, the older it is. Determining the exact age of the sample is complicated by the fact that uranium and its byproducts may migrate in or out of the sample. An open system is one in which such migration occurs; a closed system is one in which migration does not occur.
Uranium series tests were applied to samples from Hueyatlaco and the nearby site of El Horno (Szabo et al. 1969). In obtaining these dates, both the uranium 234/thorium and the uranium 235/protactinium series were used, and they yielded results that were in substantial agreement with each other.
Calculations yielded dates of about 245,000 years b.p. for sample MB3 (a camel pelvis) from Unit C of the Hueyatlaco site. Unit C is the uppermost layer at Hueyatlaco, and was found to contain highly sophisticated stone tools. This layer is underlain by Unit E, which contained similar tools, and Unit I, which contained tools of a simpler mode of manufacture. Unit E and Unit I are separated by a stratigraphic discontinuity, which suggests that Unit I is considerably older than Unit E. In other words, 245,000 years is a minimum age for the site, the lower levels of which could be substantially older.
The uranium series method gave open and closed system