One of the most frequent questions asked of paleobotanists is, "How do you date fossil plants?" Most paleobotanists are familiar with the various groups of plants that lived at different points in geologic time. Consequently, when encountering
FiguRe 1.75 Elaterocolpites castelaini (Cretaceous). Bar = 20 |im. (From Jardiné and Magloire, 1965; courtesy M. S. Zavada.)
a new assemblage of plant fossils, they usually recognize the general age immediately, but this has not always been the case. Our current understanding of the age of fossil floras is based on a long series of efforts to date the various rocks in which they are found. At the present time, the best absolute dating method involves the use of naturally occurring radioactive isotopes contained in various minerals that make up a rock unit. The inherently unstable radioactive isotopes undergo a series of complex transformations (decay) that lead to stable isotopes and, in the process, release energy. The rate of decay, A , for a radioactive isotope of a given element, sometimes called the half-life, is constant (iA = 0.693/A). Therefore, by measuring the present amount of the radioactive isotope and the present quantity of the stable product, one can calculate how much time has elapsed since the minerals in the rock formed. For example, it is known that the long-life uranium isotope 238U decays to 206Pb with a halflife of 4.5 billion years. Consequently, by measuring the relative quantities of 238U and 206Pb in a sample, it is possible to determine the length of time the decay has been going on and thus the time of formation of the rock.
A widely used technique involves the analysis of a very small amount of the relative quantities of uranium and lead contained within zircon crystals (Harrison et al., 2005). These crystals, which may be 0.1 mm in size, form as molten rock begins to cool and thus lock small amounts of uranium into their crystalline structure. This technique, which utilizes a high-resolution ion microprobe, uses a powerful beam of ions to vaporize a tiny portion (two-billionths of a gram) of a zircon crystal (Davis et al., 2003). The vapor is then passed through a mass spectrometer where the different elements are separated and analyzed. Zircon crystal geochronology has been applied widely across geologic time, including dating major earth events such as the formation of the continental crust (Harrison et al., 2005).
Other radioactive isotopes differ in their half-lives, for example, 87Rb (rubidium), 48.5 billion years; 40K (potassium), 1.25 billion years; and 235U (uranium), 0.704 billion years. One difficulty in employing these dating techniques is that radioactive isotopes occur more commonly in igneous and metamorphic rocks, whereas almost all fossils occur in sedimentary deposits. Today direct isotopic dating for sedimentary rocks is possible, but only when they contain minerals that have crystallized in the environment of deposition at or near the time they were deposited. One of these is glauconite, a silicate mineral that contains potassium (Smith et al., 1998). Since the potassium consists in part of 40K, the potassium-argon method can be used. Rubidium-strontium dating of some very fine-grained sedimentary rocks also has been successful, but the procedure is difficult and not routinely applicable.
A technique has been developed in which actual fossils can be dated. In the upper atmosphere, cosmic rays bombard 14N (nitrogen) isotopes to form an isotope of carbon (14C) that is radioactive. This carbon unites with oxygen to produce carbon dioxide (CO2). Plants take in and fix (assimilate) this carbon dioxide along with that containing the more common isotopes of carbon, 12C and 13C. Carbon dioxide is continuously assimilated during the lifetime of a plant. When the plant dies, however, it no longer exchanges carbon dioxide with the atmosphere, and thus the ratio of 14C to 13C or 12C is fixed at that time. At that point, the 14C begins to decay to 14N with its characteristic decay rate ( 4/2 of 14C is ~5730 years). For this reason, the ratio of 14C to 12C or 13C is proportional to the age of the fossil. An age limit of ~50,000 years (Balter, 2006) applies to this technique because of the short half-life of 14C. This technique obviously has somewhat limited usefulness in paleobotany because the bulk of the fossil plant record is far older. Human influence on the Earth has even altered the usefulness of the 14C dating method, because combustion of fossil fuels and nuclear testing have artificially altered the 14C content of the total carbon reservoir, and this has caused problems in maintaining reliable modern standard samples of carbon. Loss or addition of 14C to specimens and apparent fluctuations of past atmospheric 14C abundance also impose limitations on this dating method. Analytical techniques have been developed that allow direct detection of 14C atoms using high-energy accelerators. This method is especially important as it requires < 1 mg of carbon (as opposed to > 1000 mg in the conventional methods), and dates can be determined in a matter of hours rather than days.
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