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SGES 1302 INTRODUCTION TO EARTH SYSTEM LECTURE 10: Absolute/Radiometric Dating Absolute dating Relative dating places fossils in a temporal sequence by noting their positions in layers of rocks, known as strata. As shown in the diagram, fossils found in lower strata were typically deposited first and are deemed to be older. By studying and comparing strata from all over the world we can learn which came first and which came next, but we need further evidence to ascertain the specific, or numerical, ages of fossils. Absolute dating relies on the decay of radioactive elements that gives the actual number of years that have passed since an event occurred. By dating volcanic ash layers both above and below a fossil-bearing layer, as shown in the diagram, you can determine “older than X, but younger than Y” dates for the fossils. Geologists have assembled a geological time scale on the basis of numerical dating of rocks from around the world. 2 Atom An atom is the smallest particle still characterizing a chemical element. The atoms are composed of subatomic particles: electrons, which have a negative charge, a size which is so small as to be currently unmeasurable, and which are the least heavy (i.e., massive) of the three; protons, which have a positive charge, and are about 1836 times more massive than electrons; and neutrons, which have no charge, and are the same size as protons. Protons and neutrons make up a dense, massive atomic nucleus. The electrons form the much larger electron cloud surrounding the nucleus. Atoms of the same element have the same number of protons (called the atomic number). Within a single element, the number of neutrons may vary. The number of electrons associated with an atom is most easily changed, due to the lower energy of binding of electrons. Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms which have either a deficit or a surplus of electrons are called ions. 3 Isotopes Elements may exist in different isotopes, with each isotope of an element differing only in the number of neutrons in the nucleus (proton or atomic no. remains the same). Typically, the number of protons and neutrons of an atomic nucleus is the same. In an isotope, this balance is frequently broken. For example, 238U, the most common state of uranium, has three more neutrons than 235U. A neutral atom has the same number of electrons as protons. Thus, different isotopes of a given element all have the same number of protons and electrons and the same electronic structure; because the chemical behavior of an atom is largely determined by its electronic structure, isotopes exhibit nearly identical chemical behavior. The main exception to this is the kinetic isotope effect: due to their larger masses, heavier isotopes tend to react somewhat more slowly than lighter isotopes of the same element. 4 Nuclear/Radioactive Decay Protons in the nucleus are positively charged, meaning they repel each other. The presence of neutrons is necessary to separate these protons slightly, making the configuration stable. A lack of necessary neutrons makes a nucleus unstable. Some nuclides are unstable. That is, at some random point in time, such a nuclide will be transformed into a different nuclide by the process known as radioactive decay. This transformation is accomplished by the emission of particles such as electrons (known as beta decay) or alpha particles. Nuclide: Species of atom as characterized by the number of protons, neutrons, and the energy state of the nucleus. 5 Half-life While the moment in time at which a particular nucleus decays is random, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the substance in question will have decayed. Many radioactive substances decay from one nuclide into a final, stable decay product (or "daughter") through a series of steps known as a decay chain. Nuclides useful for radiometric dating have half-lives ranging from a few thousand to a few billion years. 6 Radiometric Dating In most cases, the half-life of a nuclide depends solely on its nuclear properties; it is not affected by external factors such as temperature, chemical environment, or presence of a magnetic or electric field. The half-life of any nuclide is believed to be constant through time. Therefore, in any material containing a radioactive nuclide, the proportion of the original nuclide to its decay product(s) changes in a predictable way as the original nuclide decays. This predictability allows the relative abundances of related nuclides to be used as a clock that measures the time from the incorporation of the original nuclide(s) into a material to the present. 7 Carbon-14 Dating There are a number of dating techniques that have short ranges and are so used for historical or archaeological studies. One of the best-known is the carbon-14 (C14) radiometric technique. Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years. Carbon14 is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere. The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO2). An organism acquires carbon from carbon dioxide during its lifetime. Plants acquire it through photosynthesis, and animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to intake new carbon-14 and the existing isotope decays with a characteristic half-life (5730 years). The proportion of carbon14 left when the remains of the organism are examined provides an indication of the time lapsed since its death. The carbon-14 dating limit lies around 60,000 years. 8 How to synchronize clocks: Relative & absolute time scales Main problem: not all rocks can be dated radiometrically. Radiometric dating can be used for igneous and some metamorphic rocks, but not suitable for sedimentary rocks. Igneous rocks: formed from crystallisation of magma. All the minerals in the rock are formed at about the same time and radiometric dating will give that age. Metamorphic rock: formed from pre-existing (parent) sedimentary, igneous or even other metamorphic rocks by the process of metamorphism. Radiometric ages from metamorphic rocks are difficult to interprete: they can be ages of parent rock or the time when the metamorphism took place. Sedimentary rocks: formed from materials derived from pre-existing rocks by the process of weathering, transportation and sedimentation, together with materials of organic origin. They are generally not suitable for radiometric dating. Dating will give the ages of parent rocks and not the age of sedimentation. A sedimentary rock may contain mineral grains from parent rocks of diverse ages. Exception when there are ash beds, volcanic clasts, organic materials (C14) in the sedimentary strata. 9 How to synchronize clocks: Relative & absolute time scales In 1913 Arthur Holmes published “The Age of the Earth”. He plotted radioactive ages opposite the stratigraphic time scale by studying the relationship between sediments dated by fossils and the crosscutting igneous rocks which were dated by radioactivity. Since then thousands of more accurate radiometric dating were made and the ages of various rock strata were interpreted. Ages of sedimenatry rocks were estimated by related them to radiometrically dated igneous rock (field observations required). In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) started an effort to define global references for geologic periods and faunal stages. The geologic time scale is revised every few years. The commission's most recent work is in the 2004. 10 11