Earth Materials. John O'Brien

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СКАЧАТЬ ratio. As a result, most Pliocene–Pleistocene samples have a negative δ¹⁸O, with small negative numbers recording maximum glacial ice volumes and larger negative numbers recording minimum glacial ice volumes. Because different organisms selectively fractionate¹⁸O and¹⁶O, a range of organisms must be analyzed and the results averaged when determining global changes in¹⁸O/¹⁶O. Nothing is ever as easy as it first seems.

      It should be noted that many δ¹⁸O analyses have used a different standard. This standard is the average¹⁸O/¹⁶ ratio in ocean water known as standard mean ocean water (SMOW). Of course, that has been complicated by global warming, that generally increases evaporation rates, changing the ratios in natural waters.

      Because the original SMOW and PDB standards have been used up in comparative analyses, yet another standard, Vienna standard mean ocean water (VSMOW), is also used. This name is misleading as the Vienna standard is actually a pure water sample with no dissolved solids. There is currently much discussion concerning the notion of which standards are most appropriate and how δ¹⁸O and other isotope values should be reported.

      Oxygen isotope analyses, as well as carbon isotopes discussed below, contribute essential data regarding Earth's paleoclimate as well as dynamic climate changes currently affecting Earth.

       Carbon isotopes

      Three isotopes of carbon occur naturally in Earth materials: carbon‐12 (¹²C), carbon‐13 (¹³C) and the radioactive carbon‐14 (¹⁴C), used in radiocarbon dating. Each carbon isotope contains six protons in its nucleus; the remaining mass results from the number of neutrons (six, seven or eight) in the nucleus.¹²C constitutes >98.9% of the stable carbon on Earth, and¹³C constitutes most of the other 1.1%.

      When organisms synthesize organic molecules, they selectively utilize¹²C in preference to¹³C so that organic molecules have lower than average¹³C/¹²C ratios. Enrichment of the organic material in¹²C causes the¹³C/¹²C in the water column to increase. Ordinarily, there is a rough balance between the selective removal of¹²C from water during organic synthesis and its release back to the water column by bacterial decomposition, respiration, and other processes. Mixing processes produce a relatively constant¹³C/¹²C ratio in the water column. However, during periods of stagnant circulation in the oceans or other water bodies, disoxic–anoxic conditions develop in the lower part of the water column and/or in bottom sediments. These conditions inhibit bacterial decomposition (Chapters 11 and 14) and lead to the accumulation of¹²C‐rich organic sediments. These sediments have unusually low¹³C/¹²C ratios. As they accumulate, the remaining water column, depleted in¹²C, develops a higher ratio of¹³C/¹²C. However, any process, such as the return of vigorous circulation and oxidizing conditions, that rapidly release the¹²C‐rich carbon from organic sediments, is associated with a rapid decrease in¹³C/¹²C. By carefully plotting changes in¹³C/¹²C ratios over time, paleo‐oceanographers have been able to document both local and global changes in oceanic circulation. In addition, because different organisms selectively incorporate different ratios of¹²C to¹³C, the evolution of new groups of organisms and/or the extinction of old groups of organisms can sometimes be tracked by rapid changes in the¹³C/¹²C ratios of carbonate shells in marine sediments or organic materials in terrestrial soils.

¹³C/¹²C ratios are generally expressed with respect to a standard in terms of δ¹³C. The standard once again is the¹³C/¹²C ratio of the Pee Dee Belemnite, or PDB. δ¹³C is usually expressed in parts per thousand (mils) and calculated from:

      Box 3.1 illustrates an excellent example of how oxygen and carbon isotope ratios can be used to document Earth history, in this case a period of sudden global warming that occurred 55 million years ago.

      3.3.2 Radioactive isotopes

Radioactive isotopes possess unstable nuclei whose nuclear configurations tend to be spontaneously transformed by radioactive decay. Radioactive decay occurs when the nucleus of an unstable parent isotope is transformed into that of a daughter isotope. Daughter isotopes have different atomic numbers and/or different atomic mass numbers from their parent isotopes. Three major radioactive decay processes (Figure 3.12) have been recognized: alpha (α) decay, beta (β) decay, and electron capture.

      Alpha decay involves the ejection of an alpha (α) particle plus gamma (γ) rays and heat from the nucleus. An alpha particle consists of two protons and two neutrons, which is the composition of a helium (⁴He) nucleus. The ejection of an alpha particle from the nucleus of a radioactive element reduces the atomic number of the element by two (2p⁺) while reducing its atomic mass number by four (2p⁺ + 2n⁰). The spontaneous decay of uranium‐238 (²³⁸U) into thorium‐234 (²³⁴Th) is but one of many examples of alpha decay.

      Beta decay involves the ejection of a beta (β) particle plus heat from the nucleus. A beta particle is a high‐speed electron (e⁻). The ejection of a beta particle from the nucleus of a radioactive element converts a neutron into a proton (n⁰ − e⁻ = p⁺) increasing the atomic number by one while leaving the atomic mass number essentially unchanged. The spontaneous decay of radioactive rubidium‐87 (Z = 37) into stable strontium‐87 (Z = 38) is one of many examples of beta decay.

      Electron capture involves the addition of a high‐speed electron to the nucleus with the release of heat in the form of gamma rays. It can be visualized as the reverse of beta decay. The addition of an electron to the nucleus converts one of the protons into a neutron (p⁺ + e⁻ = n⁰). Electron capture decreases the atomic number by one while leaving the atomic mass number unchanged. The decay of radioactive potassium‐40 (Z = 19) into stable argon‐40 (Z = 18) is a useful example of electron capture. It occurs at a known rate, which allows the age of many potassium‐bearing minerals and rocks to be determined. Only about 9% of radioactive potassium decays into argon‐40; the remainder decays into calcium‐40 (⁴⁰Ca) by beta emission.

      The heat released by radioactive decay is called radiogenic heat. Radiogenic heat is a major source of the heat generated within Earth. It is an important driver of global tectonics and many of the rock‐producing processes discussed in this text, including magma generation and metamorphism.

The time required for one half of the radioactive isotope to be converted into a new isotope is called its half‐life and may range from seconds to billions of years. Radioactive decay processes continue until a stable nuclear configuration is achieved and a stable isotope is formed. The radioactive decay of a parent isotope into a stable daughter СКАЧАТЬ