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 0 50 ka Organic Taylor (1987) material, carbonates 230Th 238U and 75,690 years 1000 500,000 years Carbonates Schwarcz (1989), 234U (230Th) Bischoff and 245,600 years Fitzpatrick (1991) (234U) 210Pb U-series 22.3 years 0 150 years Fine-grained Olsson (1986) decay sediments sequence U He U isotopes Depends on 0 300,000 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) 40K 40Ar 40K β-decay 1.25 3 109 years 104 109 years K-bearing McDougall and minerals, Harrison (1988) bones, tephras 10Be Cosmogenic 1.38 3 106 years 5 10 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 231Pa 235U 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 U Th 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. 19.2 RADIOACTIVE NUCLIDES 669
case such as K Ar dating, where we can measure the concentration of both the parent and the daugh- ter product. This gives a general equation: 1 D t 5 ln 1 1 (19.5) λ P
19.2.3 RADIOCARBON DATING Radiocarbon dating is one of the most widely used methods for understanding the age of organic materials and it is widely used for reconstructing the age of various kinds of carbon-containing materials (Jull, 2013b). Sample preparation involves a substantial amount of pretreatment chemistry (Jull and Burr, 2014a). For 14C dating to work, we must assume that organic or inorganic materials were in equilibrium with the production of 14C in the atmosphere and its removal into the oceans, to establish a consistent level of 14C. This is not a radioactive equilibrium in the sense discussed in Section 21.2.2. When the animal or plant dies, it is removed from this steady-state equilibrium and so the level of 14C will decay according to Eq. (19.1) and as shown in Fig. 19.1. However, it turns out that the initial value of 14C can vary with time, so that results are usually calibrated either against tree-rings of known age (dendrochronology) or, for older samples, against other dated mate- rial. There is a very large number of applications of 14C, which can be measured either by counting the radioactive decays of the isotope or by direct atom counting by accelerator mass spectrometry (AMS) (Tuniz et al., 1998). Most measurements today are measured by AMS (Fig. 19.3). As of 2016, there are over 100 AMS laboratories operating around the world. Sample selection of the appropriate material is important (Hatte´ and Jull, 2013). For the decay-counting approach, Cook and van der Plicht (2013) have an excellent summary, while Jull and Burr (2014b) discuss AMS. The 14C signal varies over time, due to gradual (van der Plicht, 2013) and rapid changes in the
In-growth of a radioactive daughter isotope 1,000,000 900,000 800,000 700,000 600,000 500,000
Number 400,000 300,000 200,000 100,000 0 0 50,000 100,000 150,000 200,000250,000 300,000 350,000 400,000 Time (years)
FIGURE 19.3 Exponential in-growth of a radionuclide (230 Th) calculated with a half-life of 75,600 years. 670 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS
cosmic-ray flux (Burr, 2013). Indeed, there are some variations occurring on a scale of years which could affect some dating results (Miyake et al., 2012). However, the time-variability can also be useful, since it allows us to date events more precisely by matching 14C fluctuations in tree rings, a process called ‘wiggle matching’. This method, developed in 1949, has been used to date geological samples, as well as archaeo- logical samples and other materials. Radiocarbon dating is crucial in understanding ocean circula- tion, chronology during the last glacial, the Younger Dryas, Glacial-Holocene transition and the Holocene (Thornalley et al., 2011). Kaiser (1994) combined radiocarbon dating and dendrochronol- ogy to date the Two Creeks event contemporary with the Older Dryas in Europe. Clark et al. (2009) summarized an enormous number of 4271 14C and other dates to constrain the maximum extent of glacial ice sheets and also demonstrated that sea-level rise at 14.5 ka is coincident with the deglaciation of the West Antarctic Ice Sheet. Similarly, Dyke et al. (2002) and Dyke (2004) has summarized the large number of radiocarbon dates used to constrain the extent of the North American ice sheets at different periods of time. In an application to megafaunal extinctions, Veltre et al. (2008) showed by radiocarbon dating that some mammoths survived well beyond the end of the Pleistocene to 5700 years on the Pribilof Islands in the Bering Strait. There are many archaeo- logical examples of the use of radiocarbon dating (Taylor, 1987). Beck et al. (2003) demonstrated the use of 14C to date corals from the statues on Easter Island. Chatters et al. (2014) studied radio- carbon and U Th ages related to the dating of some of the earliest New World skeletons. In an interesting example of combining radiocarbon dating and dendrochronology (see Section 21.2.6), Wacker et al. (2014) were able to date beams from a Swiss church to a single year, combining both 14 dendrochronology and radiocarbon, to show the presence of a large C excursion at 774 AD.
19.2.4 U-SERIES METHODS Studies of Uranium-series nuclides (Wagner, 1995) rely on the extensive decay sequence in the ura- nium decay series. We can take some of these pairs, such as 234U 238U, 230Th 238U, 231Pa 235U and 226Ra 230Th as chronometers for measurements of time on various time scales, depending on the radioactive pair chosen (Ku, 2000). The full equation for the U-series dating system was given by Kaufman and Broecker (1965): ! ! 230 234 Th Th 2Tλ 5 3 ð1 2 e 230 Þ 234U 234U ( ) ( ) 238U 1 λ =ðλ 2 λ Þ 3 1 2 (19.6) 230 230 234 234U ( )
2Tðλ 2λ Þ 3 1 2 e 230 234
In this equation, the activities (not number of atoms) of 230Th, 234U and 238U are needed, t is time and λ is the various decay constants for these three nuclides. This equation is useful since one can observe that the first part of the equation is the ‘supported’ 230Th from decay of 234U, whereas the second part is the decay of the ‘excess’ 234U where the 234U is not in radioactive equilibrium with 238U. Since it is not possible to solve this equation analytically, the equation can only be 19.2 RADIOACTIVE NUCLIDES 671
solved by an iterative approach by hand or usually today by a spreadsheet program (Ludwig, 2012), since there are more variables than the number of equations. There are several complications to bear in mind. First, problems can arise, such as where there is some initial 230Th. Second, in many aqueous systems, the activity ratio of 234/238 is .1 and for many soils, it can be ,1. Third, seawater systems have a constant initial 234/238 of 1.15, which, in fact, can be used to our advantage since we can use the simple decay of this 234/238 ratio, similar to Eq. (19.3). Hence, calculations of the age must take into account whether there is disequilibrium in the 238U 234U system (Cohen, 2005). In order to deal with the initial 230Th problem, one approach (called the Bischoff Rosholt plot) is to use the equation: 0 1 0 1 230 230 Th Th 2 λ @ A 5 @ A 3 ðe t 230 Þ 232Th 232Th t 0 i1 (19.7) 234 U 2 λ 1 @ A 3 ð1 2 e t 230 Þ 232Th i this then allows one to define this equation in the form of a plot of 230/232 against 234/232 (this can also be done for 238/232), (Bischoff and Fitzpatrick,1991). In this case, the intercept is the first part of the equation and the slope is: m 5 ð1 2 e2λtÞ (19.8) In the general case, where 234/238 is in disequilibrium, the ratios of 230/234 and 230/238 derived from the slopes should be applied to the general ‘Kaufman Broecker concordia’ diagram, which plots 234/238 versus 230/234, as shown in Fig. 19.4 (Luo and Ku, 1991). A second graphical approach is to use a different system, to plot 230/238 versus 232/238 (or analogously, 230/234 vs 232/238). This approach is called an ‘Osmond plot’. In this case, the intercept gives the value of 230/238 for the solution with no initial 230Th—since it is assumed the detrital 230Th will follow the 232Th concentration. These calculations can also be done using the open-access Isoplot software package (Ludwig, 2012). A third approach is normally applicable only to samples in seawater, where we know the initial 234U/238UisB1.15 (Chengetal.,2000) and where we have corals or carbonates growing in ocean water. Here, we can estimate age just by looking at the decay of the excess 234/238, using the equation:
234 234 2λ234t δ Ut 5 δ U0e (19.9) where δ234U is the excess of 234, δ234U 5 1000ð234U=238U 2 1Þ (19.10) 234 234 234 where δ Ut is the value of δ U at time t, and δ U0 is the initial seawater value. U-series nuclides have a wide variety of applications to dating of sediments, soil horizons, peat, bones, corals and carbonates of all kinds. Uranium-series dating has been particularly important in understanding carbonates, whether ocean corals, cave deposits or other types of carbonate systems. For example, Gallup et al. (2002) and Thomas et al. (2009) explore the U Th ages of the penulti- mate deglaciation and find that it is not synchronous with northern hemisphere forcing, as is the case of the last deglaciation. U Th has been successfully used to date pedogenic carbonates (Sharp 672 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS
2.5 20,000 YRS.
2.0 40,000 YRS. 300,000 YRS.
400,000 YRS. 60,000 YRS. 200,000 YRS. 500,000 YRS.
100,000 YRS. U activity ratio
238 1.5 U/
INFINITE AGE 234
1.0 0.00 0.50 1.00 230Th/234U activity ratio
FIGURE 19.4 Evolution diagram of 230 Th/238U versus 234U/238U as defined by Kaufman and Broecker (1965). The initial 234U/238U ratio is given on the vertical axis, and 230Th/234U is given on the horizontal axis. Samples of the same age will plot on the time lines shown. Note that 230Th/234U can exceed 1.0 in cases where the initial 234/238 ratio was .1, due to the decay of ‘excess’ 234U. et al., 2003). It has limited usefulness for other types of materials where the U-series may not be a closed system, due to loss or addition of U, or addition of detrital Th. For example, bones cannot be dated with U Th since fossil bones absorb U over time. In a review of the difficulties of dating palaeolake deposits, Placzek et al. (2006) discuss various approaches that can be used to deal with the problem of detrital Th and U mobility.
19.2.5 OTHER DATING METHODS BASED ON U-SERIES DECAY Although sometimes listed as a separate method, in U He dating one observes the in-growth of the stable 4He daughter due to α-decay of U-series isotopes. This method has become useful in many tectonic and geomorphic studies, such as detrital zircons (Boyce et al., 2006; Reiners and Shuster, 2009) and can be used for dating of apatites (Zeitler et al., 1987). Similarly, the U-daughter nuclide 210Pb can be used for dating of sediments in the last B200 years (Wagner, 1995; Cohen, 2005).
19.2.6 K Ar DATING The method of K Ar dating is useful for the dating of young volcanics, tephras and sometimes bones (Coe et al., 2004). The K Ar method and its companion the 40Ar/39Ar method are based on the radioactive decay of 40Kto40Ar ratio (Wijbrans and Kuiper, 2013). 40K undergoes a branching 19.2 RADIOACTIVE NUCLIDES 673
decay to both 40Ar and 40Ca, so it is important to revise the simple radioactive decay equation to take into account the decay via different processes. The 40Ar 39Ar process is based on the same idea as the K Ar method, except that neutron activation is used to estimate the amount of 39Kby activation to 39Ar. The in-growth of 40Ar is a variant of the in-growth equation (19.4), since there is a branching decay to 40Ca and 40Ar from 40K:
40 40 40 2λt Ar5 Arinitial 1 0:105 Kð1 2 e Þ (19.11) where t is the age and 0.105 is the branching ratio to 40Ar and the number of atoms of 40Ar and 40K should be measured, and λ is the decay constant of 40K. K Ar dating has many applications, particularly during the Quaternary in dating of bones and tephras. When Ar Ar dating is used, the nuclide 39Ar is produced in a nuclear reactor and there is a need to calibrate the amount of produc- tion by this irradiation. The various parameters of the irradiation are usually packaged together into the ‘J factor’ (Wijbrans and Kuiper, 2013), which is determined from a sample of a known-age standard. Several different standards have been used and the exact value of this standard is determined from independent age-determinations. The most common standard is the Fish Canyon sanidine (Jourdan and Renne 2007).
ðλtÞ 40 39 J 5 ðe 2 1Þ=ð ArÃ= ArkÞ (19.12) Here, J represents a factor defined by the age (t) of the standard material, 40Ar is the amount of 39 39 39 the daughter isotope measured in the standard and Ark is the Ar derived from K during the irradiation of the standard. J neatly includes the duration of the irradiation, the production effi- ciency of 39Ar, the branching ratio and other factors during the irradiation (Phillips et al., 2017). The J factor then allows the definition of a simple equation for time, which is a variant of Eq. (19.5) already discussed (Wijbrans and Kuiper, 2013).
40 39 t 5 ð1=λÞ 3 ln½J 3 ArÃ= Ark 1 1Þ (19.13) Because there is always the possibility of the presence of atmospheric 40Ar, it is common to use an isochron technique to remove the atmospheric component from the signal, since this would be independent of the other signal. 40Ar dating is useful over a wide range of ages from the very young (Turrin et al., 2007) to bil- lions of years. It is most usefully applied to K-rich volcanic rocks and sometimes to bone samples. Turrin et al. (2007) were able to show the use of Ar Ar dating for very young volcanics. Coe et al. (2014) studied magnetic reversals in lava flows from Maui (Hawaii) and Ton-That et al. (2001) reported on the ages of tephra layers at about 41 ka in the Mediterranean region. 40Ar dating can be used for determining the age of sequences of lava flows which overlie glacial moraines, for example in the case of glaciations in Patagonia (Singer et al., 2004).
19.2.7 COSMOGENIC RADIONUCLIDES The in-growth of a nuclide occurs in materials which have been subjected to cosmic radiation close to the surface of the Earth. The production of these nuclides is dependent on the composition, loca- tion (due to geomagnetic effects), altitude and other parameters. A detailed summary of these pro- cesses is given by Gosse and Phillips (2001) and Granger (2013, 2014). A large international calibration programme refined the production rates of some of these nuclides (Phillips et al., 2016). 674 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS
14 10 26 1 1 Nuclides of interest for these studies include C(t/2 5730 years), Be (t/2 1.38 Myr) and Al 36 1 1 (t/2 705,000 years), which are normally studied in quartz-rich materials and Cl (t/2 300,000 years) which is more useful for volcanics and calcite (Martin et al., 2014). Similar equations to those already discussed can be applied to the in-growth of a nuclide and also apply to the case of the build-up of a cosmogenic radionuclide such as 10Be or 26Al:
5 P ð 2 2λtÞ N λ 1 e (19.14) Here, P is the production rate in atoms/year and λ is the decay constant (in years21) of the nuclide of interest. This in-growth behaves the same way as in Fig. 19.2, but with the appropriate half-life. In this case, though, the production rate (P) is influenced by the cosmic-ray flux, which varies with the geomagnetic latitude of the locations (Gosse and Phillips, 2001). Production also depends on the sample composition of the material, since the production of a given nuclide depends on the elements present. In addition, production depends on the depth in the material since the sec- ondary cosmic-ray particles are attenuated as a function of depth. Production can be reduced by geometric factors which can be influenced by the topography of the site, if the sample is not exposed to the entire sky, for example, if the sample is on a cliff face or deep valley. In these cases, the sample is not exposed to the entire hemisphere of the sky and a geometric correction may need to be applied. However, this is usually not a large correction (Gosse and Phillips, 2001). In addition, this equation only works if there is no erosion. In practice, a more general equation is: P 2ðÞλ1ρε N 5 ρε 1 2 e Λ t (19.15) λ 1 Λ Here, we include terms for the bulk density of the rock (ρ), erosion (ε) and Λ, which is the mean attenuation length of neutrons in the sample, usually B150 g cm22. Since the production rate (P) is a function of the depth in the material, it is important to understand the depth dependence. A full equation for the production rate would include the contributions from fast and slow muons. These particles have little effect at the surface (B2%) but rapidly become important for deep sam- ples (Lifton et al., 2014).
2d=Ln 2d=Ls 2d=Lf P 5 Pne 1 Pse 1 Pfe (19.16) where Pn, Ps and Pf are the production rates for neutrons, slow muons and fast muons at the surface (Heisinger et al., 2002; Ivy-Ochs and Kober, 2008). In the case of 10Be at sea level and high lati- tude, these are approximately 4.4, 0.078 and 0.087 atoms g21 year21 for neutrons, slow muons and fast muons, respectively. The attenuation lengths for muons is much longer than for neutrons, so that the attenuation length for slow muons is B1510 g cm22 and for fast muons is B4320 g cm22, compared to 150 g cm22 for neutrons. More complex scenarios can arise where the sample is bur- ied and re-exposed. The application of cosmogenic nuclide for dating of Quaternary surfaces and deposits has increased dramatically since about 2000 (Zreda et al., 2000; Lifton et al., 2015; Phillips et al., 2015, 2016). When both 10Be and 26Al are combined, this is a powerful method, as we can obtain information about the exposure history of the material, erosion and burial. When 19.2 RADIOACTIVE NUCLIDES 675
26Al/10Be is plotted against 10Be, in a so-called ‘banana’ plot, this shows these features in a graphi- cal form (Fig. 19.5). The 10Be 26Al system is the most developed use of cosmogenic radionuclides. Its production rates are well-defined (Lifton et al., 2014, 2015). It can be useful for several important topics: sur- face exposure dating, testing spatial scaling models, deciding between single or multiple exposure, burial dating and bedrock erosion-rate estimates. This has been formalized in the CRONUS calcula- tor, two versions of which are available. The older and user-friendly version of Balco et al. (2008) can be found at http://www.hess.ess.washington.edu. A newer program with many more options is a result of the CRONUS program, which sought to improve cosmogenic production rates and other parameters (Marrero et al., 2016). These calcula- tors are available at: http://web1.ittc.ku.edu:8888/2.0/. A special case arises where the samples are dominated by erosion, as is often the case. In this situation, time disappears from the above equation, and we are left with a simpler system, where: P N 5 ρε (19.17) λ 1 Λ
1000 m/My 100 m/My 10 m/My 7 1 m/My 6 Constant 5 exposure 1 My Steady prosion 3 Secular 2 My equilbrium
Al/ 3 My 26
1 4 My
0.7 Radioactive decay 5 My
0.4 10–4 10–3 10–2 10–1 100 N * 10
26 10 10 Exposure-burial dating diagram for Al/ Be versus Be (given as N10 ). Here, the solid lines represent continuous exposure (top) or continuous erosion (second line). Lines for constant burial are given in decreasing 26Al, which would decay preferentially due to its shorter half-life. The ‘secular equilibrium’ point is where 26Al and 10Be are both in radioactive equilibrium. Adapted from Granger, D.E., 2014. 14.7 Cosmogenic nuclide burial dating in archaeology and paleoanthropology. In: Turekian, K.K., Holland, H. (Eds.), Treatise of Geochemistry, vol. 14. Elsevier, Amsterdam. pp. 81 97. 676 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS
This can also be rewritten in terms of erosion, so that: PΛ ε 5 (19.18) ρN This equation is often applied to the study of erosion of sediment over a wide areas, such as a catchment area or basin, and in this case the assumption is that the average number of atoms in the sample, given a known production rate (P) can give an average erosion rate, ε. There are many such studies (Granger et al., 1996; Kirchner et al., 2001). In the case where the cosmogenically produced nuclide is stable, the number of atoms produced is simply N 5 Pt, although again here the problem of erosion arises, as this will reduce the number of atoms present. Since the nuclide is stable, erosion cannot be directly assessed using a stable nuclide alone. We need a shorter-lived radionuclide to constrain the erosion rate as the stable nuclide will give us no independent information. There have been several studies of stable noble gas nuclides in various rock surfaces (Fenton and Niedermann, 2014). There are many other examples of the use of diverse cosmogenic nuclides. An early example is the classic study of the glacial moraines of the Wind River Range in Wyoming (Gosse et al., 1995; Phillips et al., 1997) and the Bloody Canyon moraines in the eastern Sierra Nevada (Phillips et al., 1990). Goehring et al. (2011) and Hippe et al. (2014) demonstrated the usefulness of in situ 14C, which, as a short-lived nuclide, can be used for exposure dating in the Late Glacial and Holocene periods. As already noted, Fenton and Niedermann (2014) discuss the use of the stable nuclides 3He and 21Ne to date young volcanics. Ackert et al. (2003) studied production rates of 3He. Granger et al. (1996, 2013) have pioneered the use of 10Be in basin-wide sedimentary studies. McHargue et al. (2010) used atmospheric 10Be in a deep lake core to date the core sequence. A wide variety of studies using 10Be and 26Al have been applied to glacial geochronology in the last and penultimate glaciations (Ivy-Ochs and Kober, 2008). An excellent example of the case of burial dating using a 26Al 10Be isochron for samples of the same age, but different concentrations derived from the same eroding surface, was applied to the burial of early human fossils of 3.67 Myr age (Granger et al., 2015). Some recent studies have emphasized the importance of Arctic terrains. Gjermundsen et al. (2015) showed erosion and burial history of rocks at Alpine peaks in Svalbard. Most recently, Schaefer et al. (2016) were able to show that bedrock in Greenland has a substantial cosmic-ray exposure history, indicating that the Greenland Ice Sheet was not always present. Indeed, their results suggest the ice sheet was absent for up to B300,000 years in one or more periods, which ended no longer than the rocks being buried under the ice for the last 1.1 Myr. Beryllium-10 has been used extensively to date glacial moraine sequences, for example, Kelley et al. (2014) showed results from the Southern Alps in New Zealand. Schaefer et al. (2006) demon- strated the time synchroneity of deglaciations in the northern and southern hemispheres. Other examples of the extensive use of cosmogenic-nuclide methods are studies in Argentina (Kaplan et al., 2005).
19.2.8 POSTNUCLEAR TRACERS The radionuclides produced as the result of anthropogenic nuclear activities can be used as tracers for recent processes. Nuclides such as 137Cs, 90Sr, 210Pb and post-bomb levels of 14C can be used to date event horizons since 1950 AD (Pennington et al., 1973; Olsson 1986). These markers are 19.3 RADIATIVE DOSIMETRY METHODS 677
often of great use in the study of recently deposited sediments and other materials. Recent sediment studies are often done in conjunction with the in-growth of 210Pb from U-series decay (Noller et al., 2000)
19.3 RADIATIVE DOSIMETRY METHODS Radiative dosimetry methods are basically studies by which we investigate the build-up of radiation damage. These are summarized in Table 19.2. The dose of radiation exposure is usually measured by studying some luminescence phenomenon (Aiken, 1998). Common methods are thermolumines- cence (TL), optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL). Electron spin resonance (ESR), which studies the electronic damage due to radiation has limited usefulness as the electron damage is not well-retained (Lee and Schwarcz, 2000; Jull, 2013c). All the techniques rely on the production of radiation damage in the substrate (usually a silicate min- eral). In the case of luminescence, this is displacement of atomic or molecular electrons to higher energy levels. These are often referred to as ‘traps’ in the literature (Murray and Wintle, 2000). The signal is detected by stimulating these excited states to decay back to the ground state. It is also important that the sample was initiating free of the radiation-damage signal, through some ‘bleaching’ event, usually exposure to sunlight. Fig. 19.6 shows how luminescence dating works. When these solids are exposed to ionizing radiation, electron/hole pairs are formed and some of these become trapped in the solid (Table 19.3). The luminescence age is constrained by the equation: ED Age 5 (19.19) DR
Table 19.2 Radiation-Damage Dating Methods Technique Process Time Scale Material References Fission-track dating α-Particle tracks in minerals 105 109 years Tephras, Westgate and glass Naeser (1995) Thermoluminescence Luminescence resulting from 100 106 years Fine-grained Aiken (1998) heating to rest electronic traps quartz or caused by radiation damage feldspar in sediments Optically stimulated Luminescence resulting from 100 106 years Fine-grained Stokes (1999) luminescence optical stimulation to rest quartz or electronic traps caused by feldspar in radiation damage sediments Electron spin Intensity of ESR signal 100 years 50 kyr Fine-grained Lee and resonance proportional to number of quartz Schwarcz (2000) radiation-damage electron traps 678 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS
200 Exposure to light
Age = ED/dose rate 100
80 Dose rate = Grays/time ED
Luminescence (Grays) 40
FIGURE 19.6 A simple explanation of how luminescence methods work. It is assumed there is an initial bleaching event due to exposure to sunlight, that resets the luminescence clock. Then, the luminescence signal should build up with time due to radiation damage from U, Th and K decay, as well as cosmic rays. The age can then be calculated from the effective dose (ED). In reality, the evolution curve may not be linear, which means that the ED is usually estimated using a regenerative-dose approach.
Table 19.3 Nonradiometric Dating Methods Technique Process Time Scale Material References Amino-acid Rate of 10 106 years Molluscs, ostracods, Wehmiller and Miller racemization equilibration of egg shells, some (2000), Miller et al. D and L isomers of fine-grained (2013) amino acids sediments Dendrochronology Counting of 1 15 ka Wood Fritts (1976), annual tree rings Schweingruber (1989) Varve Annual varve Any age, up to 45 ka Fine-grained O’Sullivan (1983), chronologies deposits in correlated with sediments Wohlfarth et al. sediment radiocarbon (1993), chronology Hughen et al. (1996) Palaeomagnetism Changes in 102 108 years Fine-grained Verosub (1977), magnetic intensity sediments and King et al. (1983), and direction volcanics Butler (1992) Tephrochronology Volcanic ash Varies Tephras Westgate and Naeser layers (1995) Biostratigraphy Correlation of Any age similar macrofossils 19.4 AMINO ACID RACEMIZATION 679
where ED is the ‘equivalent dose’ (in units of grays, 1 Gy 5 100 rad) and DR is the sum of annual doses from α, β, γ and cosmic radiation (Forman, 2000). ED is determined in several different ways. It is first important to understand the dose rate DR which can be determined from estimates of the chemical composition of radioactive sources in the sample from the concentrations of K, U and Th in adjacent material, or by direct measurement of the local dose rate. To determine ED, the equivalent dose is usually determined by regeneration of an additional dose of the luminescence signal in the laboratory after measurement of the original signal, and then re-irradiating the sample to model the previous conditions. This is done to correct for the nonlinear response of the samples and can be achieved in several ways. In most cases, a standard OSL technique is to use a purpose- built device that performs these various tasks (Murray and Wintle, 2003). Two common methods are the single-aliquot regenerative dose method (SAR) and multiple-aliquot regenerative dose method (MAR). Further details can be found in Murray and Wintle (2000, 2003) and an example of a comparison with the MAR approach is given by Rosenberg et al. (2011). In general, the luminescence signal can be easily removed by exposure to sunlight, which is the resetting event prior to the accumulation of the radiative dose under study. Other issues can arise due to later partial bleaching occurring or if there is an unexplained loss of the signal, called ‘anomalous fading’ (Wintle, 1973). In newer methods using the automated OSL systems, multiple samples of grains can be measured and statistical tests are applied to reject individual grains (Galbraith et al., 2005). Another radiative dose method which can sometimes be used is the study of the build-up of electronic ‘traps’ by ESR (electron spin resonance) dating. All of these methods have limitations due to the necessity of accurately estimating the levels of the source of the radiation, usually α particles (from the decay of U) and sometimes γ radiation (Wagner, 1995). Luminescence methods have been found to be particularly useful in the dating of aeolian depos- its (Wright et al., 2011) and alluvial sediments, for example, Craddock et al. (2010) have applied OSL methods to rapid fluvial retreat in the Yellow River system.
19.4 AMINO ACID RACEMIZATION The technique of amino acid racemization relies on the kinetics of changes in the symmetry of amino-acid molecules (Miller and Brigham-Grette, 1989). Since amino acids can have two different optically active isomers, there is one biologically preferred form (usually the L-form). After initial deposition, these amino acids react over time to approach an equal mixture of D and L isomers, a process called racemization. Racemization means that an initially pure component of one optical isomer (enantiomer) will convert to the other isomer, through several chemical methods which can interact with the molecules to create a nonoptically active intermediate which then has an equal probability of reforming the D or L form. A racemic mixture is the resulting equal abundance of both D and L forms. In practice, the ratio of the optical isomers of D-alloisoleucine and L-alloisoleucine are commonly used (referred to as A/I), and in natural systems, the ratio will increase from 0 to about 1.3 over time in nature (Hearty and Kaufman 2000). The decay rates of many of these reactions are known, although they are sensitive to changes in temperature, as are all chemical reactions. Hence, to calculate an age from an amino acid composition, it is important to 680 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS
understand the reaction rates and the dependence of the reaction rates on temperature and other parameters (Wehmiller and Miller, 2000). The amino-acid composition is usually expressed as the ratio of the D/L optical isomers (or A/I) ratio of the amino acid of interest. If the goal is absolute dating, it is usually necessary to compare the rates of change with some independent dating method. The method has also been used in conjunction with independent age estimates to measure temperature (Miller et al., 1997, 2013). Some early studies did not take this into account resulting in overestimates of age, such as some early work on age assignments of early humans in the New World, which were later shown to be inconsistent with radiocarbon dating (Stafford et al., 1984).
19.5 COMPARATIVE METHODS There are several methods used for dating of geological materials which rely on what are best described as relative or incremental techniques. These methods rely on various observations to esti- mate the passage of time: 1. Deposition of additions of sediment on a cyclical fashion, termed rhythmites. These are often called ‘varves’ if annually deposited. Lake varve chronologies were originally proposed as a dating technique by the Swedish geologist De Geer (1912). These chronologies have also been proposed as ways to cross-correlate to other chronologies (Wohlfarth et al., 1993). In general, there is a degree of scepticism about some of these chronologies since it is difficult to establish a priori that the deposition is annual. 2. Long varve chronologies have been established for marine sediments (Hughen et al., 1996, 2004) and some lakes (Ramsey et al., 2012). These lake chronologies have been incorporated into recent radiocarbon calibration schemes (Reimer et al., 2013) and some sections of the marine record from the Cariaco Basin were included (Reimer et al., 2013). In the case of the Cariaco Basin chronology (Hughen et al., 2004) there is good evidence that the annual deposition assumption can be taken as valid over certain parts of the time scale, although it cannot always be assumed in varved sediments. Varve chronologies in Lake Sugiestu appear to have an excellent deposition record and it is currently incorporated in the radiocarbon calibration (Ramsey et al., 2012). 3. Lichenometry. The incremental growth of certain lichen species has been used to estimate the age of a rock surface. The technique was pioneered by Beschel (1950, 1961), who used measurements of the diameter of the largest lichens on a surface as a way of estimating the age of the surface. This approach appeared to give approximate ages for some surfaces which could be confirmed by other methods (Bull, 2000). 4. Dendrochronology. Trees produce annual growth rings and, hence, the variation of the thickness of tree rings can be cross-correlated with established chronologies (Fig. 19.7). In cases where wood can be dendrochronologically dated, results can be very precise, within a few years and in an ideal case, to one year (Fritts, 1976). Dendrochronology is also an important basis of the calibration of the radiocarbon time scale (Reimer et al., 2013). 5. Palaeomagnetic dating. The remanent magnetization in magnetically susceptible materials in sediments and ceramics can sometimes be correlated with known geomagnetic fluctuations (Verosub, 1977; King et al., 1983; Butler, 1992). Both the intensity of the geomagnetic field and its direction change with time (Cohen, 2005). The sequence of geomagnetic signals in a 19.6 SUMMARY 681
FIGURE 19.7 A Bristlecone pine from the White Mountains of California. Copyright of A.J.T. Jull.
sediment sequence can then be used to estimate the age of the sequence. Correlation of isotopic signals is also an important method for Quaternary dating. The long record of δ18O in ice cores and marine sediments is often used to cross-correlate between different marine and ice core records, and therefore establish definite time markers. An excellent example is the ability to cross-correlate dramatic changes in climate from glacial to interglacial, or a return to glacial-like conditions, such as the Younger Dryas event at about 13,000 years ago in the Northern Hemisphere (Markgraf, 2001), through the oxygen-isotope record (Mayewski et al., 1997). 6. Correlation of changes of flora and pollen can sometimes also be correlated between different sites in the same region, or over wider areas (Bradbury et al. 2001).
19.6 SUMMARY There are many different methods for acquiring information on the age of material in glacial depos- its and environments. The most important task is to make sure that the material is datable by one of the methods discussed in this chapter and to understand the assumptions and limitations of each technique. Some methods might work well in one environment but not another. Hence, it is impor- tant to be aware of these limitations (Fig. 19.7). 682 CHAPTER 19 GEOCHRONOLOGY APPLIED TO GLACIAL ENVIRONMENTS
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