CHAPTER GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS 19 A.J.T. Jull1,2 1University of Arizona, Tucson, AZ, United States, 2Institute for Nuclear Research, Debrecen, Hungary 19.1 INTRODUCTION There are several important dating methods used for studies of glacial environments. In all cases, we need a chronometer, such as radioactive decay, in-growth of a radionuclide, or some other process that has a well-defined function with time. A key to any of these geochronological methods is understanding the underlying assumptions of the method, and the time dependence of the signals studied. The purpose of all of these methods is to be able to establish the age of the material of interest. It is also important to know that the material being dated can be related to the geological feature studied. Sometimes, the need is for a precise age determination, in other cases, relative age or approximate ages based on qualitative estimates, may be sufficient. Hence, the goal of this chapter is to summarize those methods and discuss briefly their advantages and deficiencies. This chapter summarizes the various dating techniques which can be relevant to samples in a glacial environment. There are many different approaches to estimate the ages of glacial deposits. These can be a variety of methods using either the decay or in-growth of radionuclides, or the build-up of radiation dose measured in different ways. Other methods that can be used include dendrochronology, comparing the growth of annual bands in trees, and studying annually layered (varve) sediments. Chemical changes such as amino-acid racemization can also be used, with an understanding of the kinetics of the processes involved. There are other more qualitative methods using the correlation of layers of identifiable materials such as volcanic ash layers, palaeomagnetic signals and biostratigraphic changes (Fig. 19.1). These methods can be subdivided into four main categories: radioactive nuclide methods, radia- tive dosimetry methods, qualitative and comparative methods and chemical methods (Jull, 2013a). Each section is subdivided into the specific methods and cross-references to sources which direct the reader to each topic in more detail. 19.2 RADIOACTIVE NUCLIDES The most important geochronology methods for glacial materials involve radioactive decay. Some processes involve the decay of an isotope from a known initial composition, whereas in others, the build-up or ingrowth of an isotope produced by the decay, or a ‘daughter’ isotope will give useful Past Glacial Environments. DOI: http://dx.doi.org/10.1016/B978-0-08-100524-8.00020-8 © 2018 Elsevier Ltd. All rights reserved. 665 666 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS Lichenometry Tree-rings Radiocarbon Uranium/thorium Luminescence Amino acid racemization Palaeomagnetism Potassium/argon Fission track dating 1 10 100 1000 10,000 100,000 1,000,000 10,000,000 Time scale in years (logarithmic) FIGURE 19.1 Time scales of different Quaternary geochronology methods. Courtesy of Scott Elias (Royal Holloway, University of London). age information. The daughter nuclide can be radioactive itself, which leads to a more complex decay equation, since the decay of two isotopes is involved. In other cases, the in-growth may be of a stable nuclide. Since radioactive nuclide concentrations can increase or decrease with time, depending on the system of interest, we can use the concentration of the specific nuclide to estimate the age of the material. The main radioactive systems studied are given in Table 19.1. Radioactive species can be produced by three different kinds of nuclear processes: 1. Radioactive decay of a parent nuclide; 2. Production by nuclear reactions as a result of exposure to cosmic radiation; 3. In-growth of a radioactive daughter product from a longer-lived parent nuclide; 4. In-growth of a stable daughter produced from a longer-lived parent nuclide. 19.2.1 RADIOACTIVE DECAY In a simple case, such as radiocarbon dating, the number of 14C atoms will decay with time from an initial value, according to the radioactive decay equation, originally shown by Rutherford and Soddy (Dickin, 1995), following their observations on the behaviour of radioactive species: dN 5 A 52λN (19.1) dt where the decay rate of the number of atoms with time (or activity, A) is proportional to the num- ber of radioactive atoms present (N). The ‘decay constant’ λ defines the rate of decay and this defines the probability of decay of an isotope. One can also refer to the ‘mean life’ of the isotope, which is 1/λ and sometimes stated as τ. It is more common to refer to the ‘half-life’ of the radionu- clide, which is the time for one-half of the remaining atoms to decay, which is related directly to the decay constant: 19.2 RADIOACTIVE NUCLIDES 667 Table 19.1 Radioactive Dating Methods Applicable to Glacial Environments Parent Isotopic Nuclide or System Process Half-Life Time Scale Material References 14C 14C(n,p) 14N 5730 years 0À50 ka Organic Taylor (1987) material, carbonates 230Th 238U and 75,690 years 1000À500,000 years Carbonates Schwarcz (1989), 234U (230Th) Bischoff and 245,600 years Fitzpatrick (1991) (234U) 210Pb U-series 22.3 years 0À150 years Fine-grained Olsson (1986) decay sediments sequence UÀHe U isotopes Depends on 0À300,000 years Tephras Kohn et al. (2000) α-decay parent nuclides 137Cs U fission 30 years After 1950 AD Fine-grained Pennington et al. sediments (1973) 40KÀ40Ar 40K β-decay 1.25 3 109 years 104À109 years K-bearing McDougall and minerals, Harrison (1988) bones, tephras 10Be Cosmogenic 1.38 3 106 years 5À10 Myr Quartz Gosse and Phillips 26Al nuclides (2001), Lifton et al. 36Cl 7.05 3 105 years B3 Myr Quartz (2014) 3.01 3 105 years B1 Myr Calcite, basalts 5 ln 2 t1=2 λ (19.2) If Eq. (19.1) is integrated, within the limits N 5 N0 at t 5 0 and N 5 0att 5 N, the result is a more common form of the exponential decay equation: N 5 e2λt (19.3) N0 where N0 is the number of atoms at time t 5 0 and N is the number of atoms at any time t.An example is shown in Fig. 19.1 of the decay of a nuclide with a half-life of 5568 years, the value used in radiocarbon dating calculations. 19.2.2 IN-GROWTH OF A RADIONUCLIDE In-growth of a radioactive species occurs when it is produced either from the decay of another radionuclide, or produced directly in the sample by irradiation. The latter example occurs often in the case of in situ cosmogenic nuclide studies (Granger, 2014). An example is the in-growth of a radionuclide, such as in the 231PaÀ235U system. In the case of these daughter-in-growth approaches, it is useful to use the activity (e.g., the decay rate) of the 668 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS 235 231 nuclide instead of the number of atoms. U(t1/2 7.04 Ma) decays to Th, however the half-life 231 of this nuclide is very short (t1/2 25.5 h) and can be ignored, so it decays rapidly to Pa (t1/2 32.76 ka). In this case, 231Pa increases with time due to the decay of the long-lived parent nuclide 235U, which decays very slowly, so that (Ku, 1968, 2000): 2λ231t A231 5 A235ð1 2 e Þ (19.4) In this case, the activity 231Pa starts from zero and builds up to an equilibrium defined by the 231 decay rate of Pa, so that at equilibrium (t .. t1/2) the production rate of 231 is equal to the decay rate of 235U. It is important to understand that this equation works because the decay rate of the unstable daughter is much shorter than that of the parent. If the two half-lives were comparable, the decay constant (λ) would be the difference between the two values. 230 230 A more complex case occurs for Th (t1/2 75,690 years). In this case, Th increases 234 with time due to the decay of the parent nuclides U(t1/2 245,600 years), which is produced from 238U decay (Cheng et al., 2013). Because the half-life of 230Th is very low compared to the parent 238U, therefore Eq. (19.4) would be correct if the intermediate daughter product, 234U, was always in secular equilibrium with the 238U(Ku, 2000). Fig. 19.2 shows an example of the in-growth of a nuclide (in this case, 230Th) with a half-life of 75,600 years). However, 234U is often not in radioac- tive equilibrium with 238U, due to the effects of selective dissolution of 234U. When the activity 230 A234 does not equal A238 then we also have to include Th arising from the ‘unsupported’ decay of 234U. Hence, the amount of 230Th produced is a product of the decay of both the excess 234U and the 238U. The result is a more complex equation, described in detail in the section on UÀTh dating (Cohen, 2005). Finally, we have the case where we have a stable daughter product of the radioactive decay. This allows us to know the original concentration of the parent (P0), since the value is P0 5 P 1 D, where P is the current value of the parent and D is the current value of the daughter. This is valuable in a Radioactive decay 1,000,000 900,000 800,000 700,000 600,000 500,000 Number 400,000 300,000 200,000 100,000 0 0 5000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000 Time (years) FIGURE 19.2 Exponential decay of a radionuclide (14C) calculated with a half-life of 5568 years.