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6. Dating the Fossils
- Michigan State University Press
- Chapter
- Additional Information
Dating the Fossils The most scientifically important Pleistocene vertebrate fossils are those that have absolute dates to go with them. Today, there are several methods of absolute dating of Pleistocene events. Dates based on a time scale arrived at by counting cyclic events such as the formation of yearly varves (layers of dark sediment deposited in a lake or pond) or yearly tree rings have been successful in northern regions or in areas where trees have lived to be very old, such as in Arizona (bristlecone pine) and California (giant redwoods). Unfortunately, trees have not been found that have lived to be old enough to date Pleistocene events. Radiocarbon dating (the carbon-14 method) is the most commonly used method of dating Great Lakes region vertebrate sites. This method is accurate back to about 50,000 B.P. Radiocarbon dates are based on the fact that plants incorporate small amounts of the unstable radioactive isotope carbon-14 in their tissues. Plants, of course, are producers at the base of the food chain, and plant carbon-14, therefore, is incorporated in all the higher trophic (feeding) levels. When any organism dies, carbon-14 no longer is incorporated into its tissues. On the other hand, the residual carbon-14 is lost through the process of radioactive decay. Thus, if the amount of carbon-14 in a sample of previously living tissue is measured, the time that has elapsed since the death of the organism may be calcu30 lated. Radioactive decay decreases the carbon-14 by half at regular intervals, so there is usually not enough left to measure after about 50,000 years. All sorts of organic material may be used to obtain radiocarbon dates, but the most common tissues used in Great Lakes region Pleistocene vertebrate assemblages are wood and bone. Historically, wood has been used more than bone, but in the last decades, bone dates have become rather common because of better radiocarbon dating techniques. The use of wood for carbon-14 dating can be problematic in certain cases. Pleistocene wood usually has such a high organic content that it may attract other organisms such as tiny roots and mold, especially if the fossil wood lies near the top of the deposit. Abnormally recent dates may be obtained where carbon has been added to the wood by the invasion of these organisms. Moreover, in kettle bog and shallow basin sites, where fossil mastodonts and mammoths are often found, the probability is that the large mammals sank into older sediments where the fossil wood was already in place. For bone radiocarbon dates, two components, collagen and bone apatite, have been used. Collagen, the complex protein that gives bone its tensile strength, is the most dependable component, although the tendency of collagen to weather out of bone in certain situations creates problems. In cases where no collagen remains, bone apatite has been used. This method, however, is of questionable accuracy because carbon in the groundwater may have replaced the original collagen in the apatite. Even with these problems, the recent dating of bone is generally much more accurate than the bone dates of a few years ago because methods of extracting collagen and removing organic contaminants have improved. Methods using the decay of potassium to argon have been used to date Pleistocene sites in places with potassium-rich volcanic ash layers, and these dates go back much farther than 50,000 years. But the Great Lakes area lacks definable layers of Pleistocene volcanic ash, and the potassium to argon method has not been used here. Obtaining carbon-14 dates is rather expensive. It costs well over $200 to get a standard radiocarbon date for a sample that contains about 1 gram (1 ounce = 28.4 grams) of carbon after the pretreatment process by the company (which, unfortunately, destroys the sample ). The following table indicates the minimal amount of material in dry weight one needs to get a normal radiocarbon date. Of course, it is better to send more. If it is important to obtain dates and the organic material from the site is not large, special accelerator mass spectrometer (AMS) techniques are available that use much smaller amounts of material. Obtaining these charcoal - 3-5 grams (optimum 10-50 grams) wood - 5 grams (optimum 30-100 grams) shells - 15 grams (optimum 50-100 grams) bone - 150 grams (optimum 200-500 grams) peat - 150 grams (optimum 100-200 grams) dates, however, is much more expensive and for bone may cost well in excess...