Calibration tests: Olduvai Gorge

Olduvai Gorge, a ravine and archaeological site in northern Tanzania, is known as one of the best dated hominid sites in the world. It is a steep sided gorge that was created by earthquake activity. The deposits exposed in its walls were formed in an ancient lakebed and are more than 2 million years old. Archaeological findings include stone tools, building sites and bones of early hominids.

Valerie Lothian, a Williams College student, began the study in June 1999.

Method:

  1. The tooth was first measured, labeled, and photographed as shown in Figure 1 below.
    Figure 1: Photograph of CT 13

  2. Next it was represented in multiple diagrams, from all sides, as shown below in Figures 2 and 3.
    Figure 2: Diagrams of CT 13

    Figure 3: Diagrams of CT 13

  3. A diamond tipped drill was used to segment the tooth in such as way as to produce multiple samples on which to carry out the experiment. The enamel and dentine of each sample was separated and stored in glass vials.
  4. The enamel of each sample was cleaned to ensure that all the dentine was removed. It was then ground into a powder and sent for irradiation.
  5. The dentine and any associated sediment were sent for neutron activation analysis (NAA) to discover the uranium, thorium and potassium content.
  6. The irradiated enamel for each sample was run through the ESR spectrometer.
  7. The enamel samples were then sent for NAA.
  8. Computer programs written expressly for the purpose of dating teeth, calculated the accumulated dose (AD) by way of a graph, Figure 4, as well as the age of the tooth, Table 1.
    Figure 4: Graph showing the accumulated dose for CT 13

    Table 1: Ages obtained for CT 8
  9. In this experiment, three teeth were used: CT 13 was diagrammed, drilled, and powdered; CT 11 was run through the spectrometer, and preliminary ages were obtained; and CT 8 was dated and isochrons were done. In this way, all the steps in the entire process were experienced.

Results:

The age obtained for CT 11 was low (i.e. young), considering that it was recovered from upper bed II of the gorge (see Figure 5 below), and was therefore expected to be about 1.3 Mya (millions of years ago). This was obtained using some data for CT 8. However, the actual values of the data belonging to CT 11 would have to be vastly different to obtain an age close to the one expected.

CT 8 was used to compare two dating methods: direct dating using the gamma content of the sediment, and isochrons. The former gives an age for each sub-sample of the tooth, while the latter gives a single age for the whole tooth. The age obtained for CT 8 also seemed to be lower that expected, but since it is not presently known from which bed CT 8 was recovered, it cannot be confirmed if the dating of CT 8 was accurate or incorrect. The tooth CT 13 was recovered from lower bed II or upper bed I (see Figure 5), which means that it has an expected age of 1.7 Mya.

Figure 5: Diagram showing placement of Olduvai beds

Figure 5: Diagram showing the configuration of the Olduvai Gorge beds

Discussion:

The principles of ESR is based on the equation:

Age = AD / dose rate (I)

with the AD and the dose rate being calculated by the computer programs, Visual Fit and Data-hps respectively. The isochron graph, shown in Figure 6, is based on the equation:

Total AD = ADext + (t) (internal dose) (II)

which satisfies the general equation y = a + bx.

The isochron graph was not considered particularly accurate. This was because the points on the graph, of which there were only six, had a narrow spread, which meant that the extrapolation to find the external dose, ADext (the point on the graph at which the internal does is equal to zero). One possibility for the younger ages obtained, is that the tooth was actually from a younger part of the site, and was washed down into the older beds of the site. This would be relatively easy considering the configuration of the site. If this problem persists for all six of the Olduvai teeth in the ESR lab, then the error will be attributed to uranium leaching.

It would almost be impossible to imagine that a tooth would be sitting in the soil for millions of years, absorbing uranium, without returning some of it to the soil. If equilibrium was set up between the uranium content of the soil and the tooth, then eventually the tooth would become saturated, and the equilibrium would shift, releasing uranium into the soil.

Figure 6: Isochron Graph

Figure 6: Isochron Graph

Conclusion:

The concept of uranium leaching is one error that plagues the technique of ESR dating. This is because the technique is built on the concept of discerning the uranium content of the tooth; if the uptake method can be judged, then the age of the tooth can be deduced. But if the present uranium content of the tooth is not the highest the tooth has ever absorbed, error will be introduced in the calculation of the age. It will remain a problem until more is known about it. Presently, other labs are testing whether or not it does occur. Until this is discovered, very little can be said about it, other then that it probably does happen. This means that there will always be error in ESR dating when dealing with teeth older than 1 Mya.

References:

http://encarta.msn.com/index/conciseindex/2F/02F0B000.htm

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