- Email: [email protected]
DATING TECHNIQUES
Cut-Marked Bone see Vertebrate Studies: Interactions with Hominids
D Dansgaard-Oeschger Events see Paleoclimate Reconstruction: Paleodroughts and Society
Dating Techniques A J T Jull, University of Arizona, Tucson, AZ, USA ã 2013 Elsevier B.V. All rights reserved.
Introduction A number of important dating methods are used for studies in the Quaternary. Many of these subjects are discussed in detail in sections of this encyclopedia. The purpose of all of these methods is to estimate the age of the material of interest. Sometimes, the need is for the age determination to be very precise; in other cases, relative age or approximate age may be sufficient. Hence, the purpose of this article is to summarize those methods and discuss briefly their advantages and deficiencies. Figure 1 summarizes the age ranges covered by different methods. These methods can be subdivided into three main categories: 1. Radioactive nuclide methods 2. Radiative dosimetry methods and 3. Qualitative and comparative methods Each of these sections is subdivided into the specific methods, and cross references to the more detailed articles that deal with these methods are also given.
production by nuclear reactions as a result of exposure to cosmic or other radiation.
Radioactive Decay For example, in the simple case of the study of a nuclide present in its maximum concentration at the creation of the material, such as radiocarbon dating, the number of 14C atoms will decay with time, according to the radioactive decay equation, originally shown by Ernest Rutherford (Dickin, 1995): dn=dt ¼ lN
[1]
where the decay rate is proportional to the number of radioactive atoms present (N). The ‘decay constant’ l defines the rate of decay. It is usual to refer to the ‘half-life’ of the radionuclide, which is the time for one-half of the remaining atoms to decay, which is related directly to the decay constant: t1=2 ¼
ln 2 l
[2]
If eqn [1] is integrated, within the limits N ¼ N0 at t ¼ 0 and N ¼ 0 at t ¼ 1, the result is a more common form:
Radioactive Nuclides The most important collection of methods for Quaternary dating involve radioactive decay, or in some cases, the buildup of radioactive species. As radioactive nuclides will increase or decrease with time, we can use the concentration of the specific nuclide to estimate the age of the material (Table 1). Radioactive species can be produced by two different kinds of nuclear processes: radioactive decay of a parent nuclide or
N ¼ elt N0
[3]
where N0 is the number of atoms at time t ¼ 0 and N is the number of atoms at any time t.
In-Growth of a Radionuclide In-growth of a radioactive species occurs when it is produced either from the decay of another nuclide or directly in the
447
448
Dating Techniques
Lichenometry Tree-rings Radiocarbon Uranium/thorium Luminescence Amino acid racemization Paleomagnetism 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 1 Time scales of different Quaternary geochronology methods. Courtesy Scott Elias (Royal Holloway College, University of London).
Table 1
Radioactive Quaternary dating methods
Isotopic system
Parent nuclide or process
Half-life
Time scale
Material
References
14
14
5730 year 75 200 year 22.3 year Depends on parent nuclides 30 1.25 109 year
0–50 000 year 1000–500 000 year 0–150 year 0–300 000 year
Organic material, carbonates Carbonates Fine-grained sediments Tephras
Taylor (1987) Schwarcz (1989) Olsson (1986) Kohn et al. (2000)
After AD 1950 104–109 year
Fine-grained sediments K-bearing minerals, bones, tephras
Pennington et al. (1973) McDougall and Harrison (1988)
C Th 210 Pb U–He
C(n,p) 14N U and 234U U-series decay sequence U isotopes a-decay
137
U fission 40 K b-decay
230
Cs K–40Ar
40
238
sample. The latter example occurs often in the case of in situ cosmogenic nuclide studies. An example of a process where the in-growth of a radionuclide is the 231Th–235U system. In this case, 231Pa increases with time due to the decay of the parent nuclides 235U, so that (Ku, 1968): [4] N231 ¼ N235 1 elt A more complex case occurs for 230Th. In this case, 230Th increases with time due to the decay of the parent nuclides 234 U, which is produced from 238U decay (see U–Th section). Because the half-life of 234U is very low compared to either the parent 238U or daughter 230Th, in principle, it should drop out of the equation. This equation would be correct if the intermediate daughter product of 238U, 234U, was in secular equilibrium with the 238U (Ku, 2000). Hence, the amount of 230Th produced is a product of the decay of both the excess 234U and the 238U. However, it is not and the result is a more complex equation, described in detail in the section on U–Th dating.
Radiocarbon Dating As discussed in the sections on radiocarbon dating, this method is widely used for reconstructing the age of various kinds of carbon-containing materials. This involves a considerable amount of pretreatment chemistry (Figure 2). Here, organic or inorganic materials in equilibrium with the
Figure 2 Sample preparation chemistry at the University of Arizona AMS Laboratory.
production of 14C in the atmosphere and its removal into the oceans, establish a consistent level of 14C. When the animal or plant dies, it is removed from this quasi-equilibrium and so the level of 14C decays according to eqn [1]. There are a vast number of applications of 14C, which can be measured either by counting the decays of the radioactive 14C or by direct atom counting by accelerator mass spectrometry (AMS). Most measurements these days are measured by AMS (see Figure 3). More details of 14C dating are given in the sections by Cook
Dating Techniques 40
Ar ¼ 40 Arinitial þ 0:10540 K 1 elt
449
[5]
where t is the age and 0.105 is the branching ratio to 40Ar. K–Ar dating has many applications, particularly during the Quaternary to the dating of bones and tephras.
Cosmogenic Radionuclides Similar equations to those for in-growth of a nuclide also apply to the case of the buildup of a cosmogenic radionuclide such as 10Be: N¼
Figure 3 The 3 MV accelerator mass spectrometer at the University of Arizona.
and van der Plicht, who discuss decay counting of 14C and Jull, who discusses AMS. Other chapters by van de Plicht and Burr discuss the causes of 14C variations over time.
U-Series Methods Studies of uranium-series nuclides (Wagner, 1995) rely on the extensive decay sequence in the uranium decay series. We can take some of these pairs such as 234U–238U, 230Th–238U, 231 Pa–235U, and 226Ra–230Th as chronometers for measurements of time on various time scales, depending on the radioactive pair chosen (Ku, 2000). Problems can arise, such as where there is some initial 230Th. Hence, calculations of the age must take into account if there is disequilibrium in the 238 U–234U system (Cohen, 2005). U-series nuclides have a wide variety of applications to dating of sediments, soil horizons, peat, bones, corals, and carbonates of all kinds.
P 1 elt l
[6]
Here, P is the production rate in atoms per year and l is the decay constant (in years1). More complex scenarios can arise where the sample is buried and re-exposed. The application of cosmogenic nuclide for dating of Quaternary surfaces and deposits has increased dramatically in the past decade (Dickin, 1995; Fifield and Fink, 2010; Zreda and Phillips, 2000), as presented elsewhere in this volume. In the case where the cosmogenically produced nuclide is stable, the number of atoms produced is simply N ¼ Pt. There have been a number of studies of stable noble gas nuclides in various rock surfaces.
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, and postbomb levels of 14C can be used to date event horizons since AD 1950. These markers are often of great use in the study of recently deposited sediments and other materials. Recent sediment studies are often done in conjunction with in-growth of 210Pb from U-series decay (Cohen, 2005).
Radiative Dosimetry Methods 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 He daughter product of U. This method is sometimes used for dating bones, mollusks, and corals (Wagner, 1995). Similarly, the U-daughter nuclide 210Pb can be used for dating of sediments in the last 200 year (Cohen, 2005; Wagner, 1995).
K–Ar Dating The K–Ar method and its companion, the 40Ar/39Ar method, are based on the radioactive decay of 40K to 40Ar (Renne, this volume). 40K undergoes a branching decay to both 40Ar and 40 Ca, 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 that of the K–Ar method, except that neutron activation is used to estimate the amount of 39K by activation to 39Ar. The in-growth of 40 Ar is a variant of the in-growth eqn [4], since there is actually a branching decay to 40Ca and 40Ar from 40K:
Radiative dosimetry methods are basically studies by which we investigate the buildup of radiation damage. This is usually measured by studying some luminescence phenomenon. Common methods are thermoluminescence, optically stimulated luminescence, and electron spin resonance (ESR) techniques. The techniques rely on the production of radiation damage in the substrate (usually, a silicate mineral). When these solids are exposed to ionizing radiation, electron/hole pairs are formed and some of these become trapped in the solid. The luminescence age appears simple enough and is constrained by the equation: ED Age ¼ P D where ED is the ‘equivalent dose’ (in units of grays, P 1 Gy ¼ 100 rad) and D is the sum of annual doses from a, b, g and cosmic radiation (Forman, 2000). ED is determined in several different ways, including estimates of the chemical composition of radioactive sources in the sample, regeneration of an additional dose of the luminescence signal in the laboratory, and partial or total bleaching (i.e., removal of the luminescence signal by exposure to light).
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Dating Techniques
Table 2
Nonradiometric Quaternary dating methods
Technique
Process
Time scale 5
9
Material
References Westgate and Naeser (1995) Wehmiller and Miller (2000)
Fission-track dating
a-particle tracks in minerals
10 –10 year
Tephras, glass
Amino acid racemization
Rate of equilibration of D and L isomers of amino acids
10–106 year
Thermoluminescence
Luminescence resulting from heating to rest electronic traps caused by radiation damage Luminescence resulting from optical stimulation to rest electronic traps caused by radiation damage Intensity of ESR signal proportional to the number of radiation-damage electron traps Annual varve deposits in sediment
100–106 year
Mollusks, ostracods, egg shells, some fine-grained sediments Fine-grained quartz or feldspar in sediments
Electron spin resonance Varve chronologies
100–106 year
Fine-grained quartz or feldspar in sediments
Stokes (1999)
100 year–50 ky
Fine-grained quartz
Lee and Schwarcz (2000)
Any age, up to 45 ka correlated with radiocarbon chronology 102–108 year
Fine-grained sediments
O’Sullivan (1983), Wohlfarth et al. (1993), and Hughen et al. (1996)
Fine-grained sediments and volcanics
Thompson and Oldfield (1986) and Verosub (1988) Westgate and Naeser (1995)
Paleomagnetism
Changes in magnetic intensity and direction
Tephrochronology
Volcanic ash layers
Varies
Biostratigraphy
Correlation of similar macrofossils
Any age
The main problem with luminescence methods is the number of possible complications which can arise due to the fact that the original sediment was not totally bleached by sunlight before the accumulation of the dose under study, or that later partial bleaching occurs, or there is an unexplained loss of the signal, called ‘anomalous fading’ (Wintle, 1973). There are two main methods which use radiative dose: fission-track dating and luminescence methods, which include thermoluminescence and optically stimulated luminescence (Aiken, 1998). Another radiative dose method which can sometimes be used is the study of the buildup of electronic ‘traps’ by ESR dating (Lee and Schwarcz, 1993). All of these methods have limitations due to the necessity of accurately estimating the levels of the source of the radiation, usually a particles (from the decay of U) and sometimes g radiation (Wagner, 1995). These topics are all discussed in various articles of this encyclopedia and the references listed in Table 2.
Tephras
600 + +
400 Age (ka)
Optically stimulated luminescence
Aiken (1998)
+ 200 + + +
0 0
0.2
0.4
0.6
0.8
D-Alloisoleucine/L-Isoleucine
Figure 4 Exchange rates of amino acids at different temperatures. Reproduced from Hearty PS and Kaufman DS (2000) Whole-rock aminostratigraphy and Quaternary sea-level history of the Bahamas. Quaternary Research 54: 163–173.
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). As 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, which is called a racemic mixture (Figure 4). The decay rates of many of these reactions are known, although they are sensitive to changes in temperature, as are nearly all chemical reactions. Hence, in order to calculate an age from an amino acid composition, it is important
to 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 D/L 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 such as 14C. The method can also be used in conjunction with independent age estimates to measure temperature (Miller et al., 1997). Otherwise, serious errors can result, such as the very early age assignments of early humans in the New World, which were later shown to be in error by radiocarbon dating (Stafford et al., 1984).
Dating Techniques
451
Environmental archives of annual resolution
Tree-rings of Huon Pine, Tasmania
Speleothem (cave calcite) from Rana, Norway
Varves from Lake Holzmaar, Germany
GISP2 Ice-core layers, Greenland
Banded coral from Papua New Guinea
Figure 5 Examples of different Quaternary archives of annual resolution. Courtesy Royal Holloway College, University of London. http://www.gg.rhul.ac.uk/cqr/MSc/courses/GG5217.html
Qualitative and Comparative Methods There are several methods used for dating of geological materials and the methods rely on what are best described as relative or incremental techniques (Figure 5). These methods rely on various observations to estimate the passage of time: a. Deposition of regular additions of sediment on a cyclical fashion, called 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. These chronologies have also been proposed as ways to cross-correlate to other chronologies. 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. b. Long varve chronologies have been established for marine sediments (e.g., Hughen, this volume) and these chronologies have been incorporated into some radiocarbon calibration schemes. These varved sediments in principle have the same problems as lake sediments, but 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. c. Lichenometry. The incremental growth of certain lichen species has been used to estimate the age of a rock surface. The technique was pioneered by Bull (2000) who used measurements of the diameter of the largest lichens on surfaces 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. d. Dendrochronology. Trees produce annual growth rings and hence, the variation of the thickness of tree rings can be cross-correlated with established chronologies (Figure 6).
Figure 6 A Bristlecone pine from the White Mountains of California. Courtesy of the University of Arizona Tree Ring Laboratory.
In cases where a piece of wood can be dendrochronologically dated, results can be precise to a few years. e. Paleomagnetic dating. The remanent magnetization in magnetically susceptible materials in sediments and ceramics can sometimes be correlated with known geomagnetic fluctuations (Butler, 1992). Both the intensity of the geomagnetic field and its direction change with time (Cohen, 2005). The sequence of geomagnetic signals in a sediment sequence can then be used to estimate the age of the sequence. f. Correlation of isotopic signals is also an important method for Quaternary dating. The long record of d18O 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 (Markgraf, 2001), through the oxygen-isotope record (Mayewski et al., 1997). g. Correlation of changes of flora and pollen can sometimes also be correlated between different sites in the same region or over wider areas (Bradbury, 2001).
See also: K/Ar and 40Ar/39Ar Dating; U-Series Dating. Cosmogenic Nuclide Dating: Exposure Geochronology. Glacial Landforms: Glaciofluvial Landforms of Erosion. Introduction: Understanding Quaternary Climatic Change. Luminescence Dating: Electron Spin Resonance Dating; Thermoluminescence. Paleoclimate: Introduction. Radiocarbon Dating: AMS Radiocarbon Dating; Causes of Temporal 14C Variations; Conventional Method; Variations in Atmospheric 14C.
References Aiken MJ (1998) Thermoluminescence Dating. New York: Academic Press. Bradbury JP (2001) Full and late glacial lake records along the PEP1 transect: Their role in developing interhemispheric paleoclimate interactions.
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Dating Techniques
In: Markgraf V (ed.) Interhemispheric Climate Linkages, pp. 265–291. San Diego: Academic Press. Bull WB (2000) Lichenometry: A new way of dating and locating prehistorical earthquakes. In: Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications, pp. 157–176. Washington, DC: American Geophysical Union. Butler R (1992) Paleomagnetism: Magnetic Domains to Geologic Terrains. Boston: Blackwell. Cohen AS (2005) Paleolimnology: The History and Evolution of Lake Systems. Oxford: Oxford University Press. Dickin AP (1995) Radiogenic Isotope Geology. Cambridge: Cambridge University Press. Fifield LK and Fink D (2010) Environmental applications of accelerator mass spectrometry. In: Beauchemin D and Matthews DE (eds.) Elemental and Isotope-ratio Mass Spectrometry. Encyclopedia of Mass Spectrometry, vol. 5, pp. 629–640. Elsevier: Amsterdam. Forman S (2000) Thermoluminescence dating. In: Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications, pp. 157–176. Washington, DC: American Geophysical Union. Hearty PS and Kaufman DS (2000) Whole-rock aminostratigraphy and Quaternary sea-level history of the Bahamas. Quaternary Research 54: 163–173. Hughen KA, Baillie MGL, Bard E, et al. (2004) Marine04 marine radiocarbon age calibration, 0–26kyr. Radiocarbon 46: 1059–1086. Hughen KA, Overpeck JT, Peterson LC, and Anderson RF (1996) The nature of varved sedimentation in the Cariaco basin, Venezuela, and its palaeoclimatic significance. In: Kemp AES (ed.) Palaeoclimatology and Palaeoceanography from Laminated Sediments, Geological Society Special Publication 116: 171–183. Kohn BP, Farley KA, PIllans B (2000) (U-th)/He and fission track dating of the Pleistocene Rangitawa tephra, North Island, New Zealand: A comparative study. In 9th International Conference on Fission Track Dating and Thermochronology, Lorne, Australia, pp. 207–208. Ku TL (2000) Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications. Washington, DC: American Geophysical Union. Ku TL (1968) Protoactinium-231 method of dating coral from Barbados Island. Journal of Geophysical Research 73: 2271–2276. Ku TL (2000) Uranium-series methods. In: Noller JS, Sowers JM and Lettis WR (eds.) Quaternary Geochronology: Methods and Applications, American Geophysical Union Reference Shelf 4: 101–114. Washington: American Geophysical Union. http://dx.doi.org/10.1029/RF004p0101. Lee H and Schwarcz H (2000) Electron Spin Resonance dating of fault rocks. In: Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications, pp. 177–186. Washington, DC: American Geophysical Union. Markgraf V (2001) Interhemispheric Climate Linkages. San Diego: Academic Press. Mayewski PA, Meeker LD, Twickler MS, et al. (1997) Major features and forcing of high-latitude northern hemisphere atmospheric circulation using a 110,000-year-long glaciochemical series. Journal of Geophysical Research 102: 26,345–26,366. McDougall I (1995) Potassium-argon dating in the Pleistocene. In: Rutter NW and Catto NR (eds.) Dating Methods for Quaternary Deposits, pp. 1–14. Ottawa: Geological Survey of Canada. McDougall I and Harrison TM (1988) Geochronology and Thermochronology by the 40 Ar/39Ar Method. New York: Oxford University Press. McSween HY, Richardson SM, and Uhle ME (2003) Geochemistry: Pathways and Processes. New York: Columbia University Press. Miller GH and Brigham-Grette J (1989) Amino acid geochronology – Resolution and precision in carbonate fossils. Quaternary International 1: 111–128.
Miller GH, Magee JW, and Jull AJT (1997) Subtropical southern hemisphere Pleistocene cooling deduced from racemization in emu eggshells from Australia. Nature 385: 241–244. O’Sullivan PE (1983) Annually-laminated sediments and the study of Quaternary environmental changes – A review. Quaternary Science Reviews 1: 245–313. Olsson IU (1986) Radiometric dating. In: Berglun BE (ed.) Handbook of Holocene Palaeoecology and Palaeohydrology. London: Wiley. Pennington W, Cambray RS, and Fisher EM (1973) Observations on lake sediments using 137Cs. Nature 242: 324–326. Schwarcz H (1989) Uranium series dating. Quaternary International 1: 7–17. Schwarcz H, Buhay W, Gru¨n R (1988) Electron spin resonance (ESR) dating of fault gouge. US Geological Survey Open File Report 87–673, pp. 50–64. Stafford TW Jr., Jull AJT, Zabel TH, et al. (1984) Holocene age of the Yuha burial: Direct radiocarbon determinations by accelerator mass spectrometry. Nature 308: 446–447. Stokes S (1999) Luminescence dating applications in geomorphological research. Geomorphology 29: 153–171. Taylor RE (1987) Radiocarbon Dating: An Archaeological Perspective. New York: Academic Press. Tuniz C, Bird JR, Fink D, and Herzog GF (1998) Accelerator Mass Spectrometry: Ultasensitive Analysis for Global Science. Boca Raton, FL: CRC Press. Verosub KL (1988) Geomagnetic and secular variation and the dating of Quaternary sediments. In Easterbrook DJ (ed.) Dating Quaternary Sediments. Geological Society of America Special Publication Paper 227: 123–128. Wagner GA (1995) Age Determination of Young Rocks and Artifacts. Berlin: Springer. Wehmiller JF and Miller GH (2000) Aminostratigraphic dating methods in Quaternary geology. In: Noller JS, et al. (ed.) Quaternary Geochronology, Methods and Applications, pp. 187–222. Washington, DC: American Geophysical Union. Westgate JA and Naeser ND (1995) Tephrochronology and fission-track dating. In: Rutter NW and Catto NR (eds.) Dating Methods for Quaternary Deposits, pp. 15–28. Ottawa: Geological Survey of Canada. Wintle AG (1973) Anomalous fading of thermoluminescence in mineral samples. Nature 245: 143–144. Wohlfarth B, Bjo¨rck S, Possnert G, et al. (1993) AMS dating of Swedish varved clays of the last glacial/interglacial transition and potential difficulties of calibrating Late Weichselian ‘absolute’ chronologies. Boreas 22: 113–128. Zreda M and Phillips F (2000) Cosmogenic nuclide buildup in surficial materials. In: Noller JS, et al. (ed.) Quaternary Geochronology, Methods and Applications, pp. 61–76. Washington, DC: American Geophysical Union.
Further Reading Hearty PS and Kaufman DS (2000) Whole-rock aminostratigraphy and Quaternary sea-level history of the Bahamas. Quaternary Research 54: 163–173. McDougall I (1995) Potassium-argon dating in the Pleistocene. In: Rutter NW and Catto NR (eds.) Dating Methods for Quaternary Deposits, pp. 1–14. Ottawa: Geological Survey of Canada. McSween HY, Richardson SM, and Uhle ME (2003) Geochemistry: Pathways and Processes. New York: Columbia University Press. Schwarcz H, Buhay W, and Gru¨n R (1988) Electron spin resonance (ESR) dating of fault gouge. US Geological Survey Open File Report 87-673, pp. 50–64. Tuniz C, Bird JR, Fink D, and Herzog GF (1998) Accelerator Mass Spectrometry: Ultasensitive Analysis for Global Science. Boca Raton, FL: CRC Press.
D Dansgaard-Oeschger Events see Paleoclimate Reconstruction: Paleodroughts and Society
Dating Techniques A J T Jull, University of Arizona, Tucson, AZ, USA ã 2013 Elsevier B.V. All rights reserved.
Introduction A number of important dating methods are used for studies in the Quaternary. Many of these subjects are discussed in detail in sections of this encyclopedia. The purpose of all of these methods is to estimate the age of the material of interest. Sometimes, the need is for the age determination to be very precise; in other cases, relative age or approximate age may be sufficient. Hence, the purpose of this article is to summarize those methods and discuss briefly their advantages and deficiencies. Figure 1 summarizes the age ranges covered by different methods. These methods can be subdivided into three main categories: 1. Radioactive nuclide methods 2. Radiative dosimetry methods and 3. Qualitative and comparative methods Each of these sections is subdivided into the specific methods, and cross references to the more detailed articles that deal with these methods are also given.
production by nuclear reactions as a result of exposure to cosmic or other radiation.
Radioactive Decay For example, in the simple case of the study of a nuclide present in its maximum concentration at the creation of the material, such as radiocarbon dating, the number of 14C atoms will decay with time, according to the radioactive decay equation, originally shown by Ernest Rutherford (Dickin, 1995): dn=dt ¼ lN
[1]
where the decay rate is proportional to the number of radioactive atoms present (N). The ‘decay constant’ l defines the rate of decay. It is usual to refer to the ‘half-life’ of the radionuclide, which is the time for one-half of the remaining atoms to decay, which is related directly to the decay constant: t1=2 ¼
ln 2 l
[2]
If eqn [1] is integrated, within the limits N ¼ N0 at t ¼ 0 and N ¼ 0 at t ¼ 1, the result is a more common form:
Radioactive Nuclides The most important collection of methods for Quaternary dating involve radioactive decay, or in some cases, the buildup of radioactive species. As radioactive nuclides will increase or decrease with time, we can use the concentration of the specific nuclide to estimate the age of the material (Table 1). Radioactive species can be produced by two different kinds of nuclear processes: radioactive decay of a parent nuclide or
N ¼ elt N0
[3]
where N0 is the number of atoms at time t ¼ 0 and N is the number of atoms at any time t.
In-Growth of a Radionuclide In-growth of a radioactive species occurs when it is produced either from the decay of another nuclide or directly in the
447
448
Dating Techniques
Lichenometry Tree-rings Radiocarbon Uranium/thorium Luminescence Amino acid racemization Paleomagnetism 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 1 Time scales of different Quaternary geochronology methods. Courtesy Scott Elias (Royal Holloway College, University of London).
Table 1
Radioactive Quaternary dating methods
Isotopic system
Parent nuclide or process
Half-life
Time scale
Material
References
14
14
5730 year 75 200 year 22.3 year Depends on parent nuclides 30 1.25 109 year
0–50 000 year 1000–500 000 year 0–150 year 0–300 000 year
Organic material, carbonates Carbonates Fine-grained sediments Tephras
Taylor (1987) Schwarcz (1989) Olsson (1986) Kohn et al. (2000)
After AD 1950 104–109 year
Fine-grained sediments K-bearing minerals, bones, tephras
Pennington et al. (1973) McDougall and Harrison (1988)
C Th 210 Pb U–He
C(n,p) 14N U and 234U U-series decay sequence U isotopes a-decay
137
U fission 40 K b-decay
230
Cs K–40Ar
40
238
sample. The latter example occurs often in the case of in situ cosmogenic nuclide studies. An example of a process where the in-growth of a radionuclide is the 231Th–235U system. In this case, 231Pa increases with time due to the decay of the parent nuclides 235U, so that (Ku, 1968): [4] N231 ¼ N235 1 elt A more complex case occurs for 230Th. In this case, 230Th increases with time due to the decay of the parent nuclides 234 U, which is produced from 238U decay (see U–Th section). Because the half-life of 234U is very low compared to either the parent 238U or daughter 230Th, in principle, it should drop out of the equation. This equation would be correct if the intermediate daughter product of 238U, 234U, was in secular equilibrium with the 238U (Ku, 2000). Hence, the amount of 230Th produced is a product of the decay of both the excess 234U and the 238U. However, it is not and the result is a more complex equation, described in detail in the section on U–Th dating.
Radiocarbon Dating As discussed in the sections on radiocarbon dating, this method is widely used for reconstructing the age of various kinds of carbon-containing materials. This involves a considerable amount of pretreatment chemistry (Figure 2). Here, organic or inorganic materials in equilibrium with the
Figure 2 Sample preparation chemistry at the University of Arizona AMS Laboratory.
production of 14C in the atmosphere and its removal into the oceans, establish a consistent level of 14C. When the animal or plant dies, it is removed from this quasi-equilibrium and so the level of 14C decays according to eqn [1]. There are a vast number of applications of 14C, which can be measured either by counting the decays of the radioactive 14C or by direct atom counting by accelerator mass spectrometry (AMS). Most measurements these days are measured by AMS (see Figure 3). More details of 14C dating are given in the sections by Cook
Dating Techniques 40
Ar ¼ 40 Arinitial þ 0:10540 K 1 elt
449
[5]
where t is the age and 0.105 is the branching ratio to 40Ar. K–Ar dating has many applications, particularly during the Quaternary to the dating of bones and tephras.
Cosmogenic Radionuclides Similar equations to those for in-growth of a nuclide also apply to the case of the buildup of a cosmogenic radionuclide such as 10Be: N¼
Figure 3 The 3 MV accelerator mass spectrometer at the University of Arizona.
and van der Plicht, who discuss decay counting of 14C and Jull, who discusses AMS. Other chapters by van de Plicht and Burr discuss the causes of 14C variations over time.
U-Series Methods Studies of uranium-series nuclides (Wagner, 1995) rely on the extensive decay sequence in the uranium decay series. We can take some of these pairs such as 234U–238U, 230Th–238U, 231 Pa–235U, and 226Ra–230Th as chronometers for measurements of time on various time scales, depending on the radioactive pair chosen (Ku, 2000). Problems can arise, such as where there is some initial 230Th. Hence, calculations of the age must take into account if there is disequilibrium in the 238 U–234U system (Cohen, 2005). U-series nuclides have a wide variety of applications to dating of sediments, soil horizons, peat, bones, corals, and carbonates of all kinds.
P 1 elt l
[6]
Here, P is the production rate in atoms per year and l is the decay constant (in years1). More complex scenarios can arise where the sample is buried and re-exposed. The application of cosmogenic nuclide for dating of Quaternary surfaces and deposits has increased dramatically in the past decade (Dickin, 1995; Fifield and Fink, 2010; Zreda and Phillips, 2000), as presented elsewhere in this volume. In the case where the cosmogenically produced nuclide is stable, the number of atoms produced is simply N ¼ Pt. There have been a number of studies of stable noble gas nuclides in various rock surfaces.
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, and postbomb levels of 14C can be used to date event horizons since AD 1950. These markers are often of great use in the study of recently deposited sediments and other materials. Recent sediment studies are often done in conjunction with in-growth of 210Pb from U-series decay (Cohen, 2005).
Radiative Dosimetry Methods 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 He daughter product of U. This method is sometimes used for dating bones, mollusks, and corals (Wagner, 1995). Similarly, the U-daughter nuclide 210Pb can be used for dating of sediments in the last 200 year (Cohen, 2005; Wagner, 1995).
K–Ar Dating The K–Ar method and its companion, the 40Ar/39Ar method, are based on the radioactive decay of 40K to 40Ar (Renne, this volume). 40K undergoes a branching decay to both 40Ar and 40 Ca, 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 that of the K–Ar method, except that neutron activation is used to estimate the amount of 39K by activation to 39Ar. The in-growth of 40 Ar is a variant of the in-growth eqn [4], since there is actually a branching decay to 40Ca and 40Ar from 40K:
Radiative dosimetry methods are basically studies by which we investigate the buildup of radiation damage. This is usually measured by studying some luminescence phenomenon. Common methods are thermoluminescence, optically stimulated luminescence, and electron spin resonance (ESR) techniques. The techniques rely on the production of radiation damage in the substrate (usually, a silicate mineral). When these solids are exposed to ionizing radiation, electron/hole pairs are formed and some of these become trapped in the solid. The luminescence age appears simple enough and is constrained by the equation: ED Age ¼ P D where ED is the ‘equivalent dose’ (in units of grays, P 1 Gy ¼ 100 rad) and D is the sum of annual doses from a, b, g and cosmic radiation (Forman, 2000). ED is determined in several different ways, including estimates of the chemical composition of radioactive sources in the sample, regeneration of an additional dose of the luminescence signal in the laboratory, and partial or total bleaching (i.e., removal of the luminescence signal by exposure to light).
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Table 2
Nonradiometric Quaternary dating methods
Technique
Process
Time scale 5
9
Material
References Westgate and Naeser (1995) Wehmiller and Miller (2000)
Fission-track dating
a-particle tracks in minerals
10 –10 year
Tephras, glass
Amino acid racemization
Rate of equilibration of D and L isomers of amino acids
10–106 year
Thermoluminescence
Luminescence resulting from heating to rest electronic traps caused by radiation damage Luminescence resulting from optical stimulation to rest electronic traps caused by radiation damage Intensity of ESR signal proportional to the number of radiation-damage electron traps Annual varve deposits in sediment
100–106 year
Mollusks, ostracods, egg shells, some fine-grained sediments Fine-grained quartz or feldspar in sediments
Electron spin resonance Varve chronologies
100–106 year
Fine-grained quartz or feldspar in sediments
Stokes (1999)
100 year–50 ky
Fine-grained quartz
Lee and Schwarcz (2000)
Any age, up to 45 ka correlated with radiocarbon chronology 102–108 year
Fine-grained sediments
O’Sullivan (1983), Wohlfarth et al. (1993), and Hughen et al. (1996)
Fine-grained sediments and volcanics
Thompson and Oldfield (1986) and Verosub (1988) Westgate and Naeser (1995)
Paleomagnetism
Changes in magnetic intensity and direction
Tephrochronology
Volcanic ash layers
Varies
Biostratigraphy
Correlation of similar macrofossils
Any age
The main problem with luminescence methods is the number of possible complications which can arise due to the fact that the original sediment was not totally bleached by sunlight before the accumulation of the dose under study, or that later partial bleaching occurs, or there is an unexplained loss of the signal, called ‘anomalous fading’ (Wintle, 1973). There are two main methods which use radiative dose: fission-track dating and luminescence methods, which include thermoluminescence and optically stimulated luminescence (Aiken, 1998). Another radiative dose method which can sometimes be used is the study of the buildup of electronic ‘traps’ by ESR dating (Lee and Schwarcz, 1993). All of these methods have limitations due to the necessity of accurately estimating the levels of the source of the radiation, usually a particles (from the decay of U) and sometimes g radiation (Wagner, 1995). These topics are all discussed in various articles of this encyclopedia and the references listed in Table 2.
Tephras
600 + +
400 Age (ka)
Optically stimulated luminescence
Aiken (1998)
+ 200 + + +
0 0
0.2
0.4
0.6
0.8
D-Alloisoleucine/L-Isoleucine
Figure 4 Exchange rates of amino acids at different temperatures. Reproduced from Hearty PS and Kaufman DS (2000) Whole-rock aminostratigraphy and Quaternary sea-level history of the Bahamas. Quaternary Research 54: 163–173.
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). As 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, which is called a racemic mixture (Figure 4). The decay rates of many of these reactions are known, although they are sensitive to changes in temperature, as are nearly all chemical reactions. Hence, in order to calculate an age from an amino acid composition, it is important
to 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 D/L 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 such as 14C. The method can also be used in conjunction with independent age estimates to measure temperature (Miller et al., 1997). Otherwise, serious errors can result, such as the very early age assignments of early humans in the New World, which were later shown to be in error by radiocarbon dating (Stafford et al., 1984).
Dating Techniques
451
Environmental archives of annual resolution
Tree-rings of Huon Pine, Tasmania
Speleothem (cave calcite) from Rana, Norway
Varves from Lake Holzmaar, Germany
GISP2 Ice-core layers, Greenland
Banded coral from Papua New Guinea
Figure 5 Examples of different Quaternary archives of annual resolution. Courtesy Royal Holloway College, University of London. http://www.gg.rhul.ac.uk/cqr/MSc/courses/GG5217.html
Qualitative and Comparative Methods There are several methods used for dating of geological materials and the methods rely on what are best described as relative or incremental techniques (Figure 5). These methods rely on various observations to estimate the passage of time: a. Deposition of regular additions of sediment on a cyclical fashion, called 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. These chronologies have also been proposed as ways to cross-correlate to other chronologies. 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. b. Long varve chronologies have been established for marine sediments (e.g., Hughen, this volume) and these chronologies have been incorporated into some radiocarbon calibration schemes. These varved sediments in principle have the same problems as lake sediments, but 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. c. Lichenometry. The incremental growth of certain lichen species has been used to estimate the age of a rock surface. The technique was pioneered by Bull (2000) who used measurements of the diameter of the largest lichens on surfaces 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. d. Dendrochronology. Trees produce annual growth rings and hence, the variation of the thickness of tree rings can be cross-correlated with established chronologies (Figure 6).
Figure 6 A Bristlecone pine from the White Mountains of California. Courtesy of the University of Arizona Tree Ring Laboratory.
In cases where a piece of wood can be dendrochronologically dated, results can be precise to a few years. e. Paleomagnetic dating. The remanent magnetization in magnetically susceptible materials in sediments and ceramics can sometimes be correlated with known geomagnetic fluctuations (Butler, 1992). Both the intensity of the geomagnetic field and its direction change with time (Cohen, 2005). The sequence of geomagnetic signals in a sediment sequence can then be used to estimate the age of the sequence. f. Correlation of isotopic signals is also an important method for Quaternary dating. The long record of d18O 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 (Markgraf, 2001), through the oxygen-isotope record (Mayewski et al., 1997). g. Correlation of changes of flora and pollen can sometimes also be correlated between different sites in the same region or over wider areas (Bradbury, 2001).
See also: K/Ar and 40Ar/39Ar Dating; U-Series Dating. Cosmogenic Nuclide Dating: Exposure Geochronology. Glacial Landforms: Glaciofluvial Landforms of Erosion. Introduction: Understanding Quaternary Climatic Change. Luminescence Dating: Electron Spin Resonance Dating; Thermoluminescence. Paleoclimate: Introduction. Radiocarbon Dating: AMS Radiocarbon Dating; Causes of Temporal 14C Variations; Conventional Method; Variations in Atmospheric 14C.
References Aiken MJ (1998) Thermoluminescence Dating. New York: Academic Press. Bradbury JP (2001) Full and late glacial lake records along the PEP1 transect: Their role in developing interhemispheric paleoclimate interactions.
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In: Markgraf V (ed.) Interhemispheric Climate Linkages, pp. 265–291. San Diego: Academic Press. Bull WB (2000) Lichenometry: A new way of dating and locating prehistorical earthquakes. In: Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications, pp. 157–176. Washington, DC: American Geophysical Union. Butler R (1992) Paleomagnetism: Magnetic Domains to Geologic Terrains. Boston: Blackwell. Cohen AS (2005) Paleolimnology: The History and Evolution of Lake Systems. Oxford: Oxford University Press. Dickin AP (1995) Radiogenic Isotope Geology. Cambridge: Cambridge University Press. Fifield LK and Fink D (2010) Environmental applications of accelerator mass spectrometry. In: Beauchemin D and Matthews DE (eds.) Elemental and Isotope-ratio Mass Spectrometry. Encyclopedia of Mass Spectrometry, vol. 5, pp. 629–640. Elsevier: Amsterdam. Forman S (2000) Thermoluminescence dating. In: Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications, pp. 157–176. Washington, DC: American Geophysical Union. Hearty PS and Kaufman DS (2000) Whole-rock aminostratigraphy and Quaternary sea-level history of the Bahamas. Quaternary Research 54: 163–173. Hughen KA, Baillie MGL, Bard E, et al. (2004) Marine04 marine radiocarbon age calibration, 0–26kyr. Radiocarbon 46: 1059–1086. Hughen KA, Overpeck JT, Peterson LC, and Anderson RF (1996) The nature of varved sedimentation in the Cariaco basin, Venezuela, and its palaeoclimatic significance. In: Kemp AES (ed.) Palaeoclimatology and Palaeoceanography from Laminated Sediments, Geological Society Special Publication 116: 171–183. Kohn BP, Farley KA, PIllans B (2000) (U-th)/He and fission track dating of the Pleistocene Rangitawa tephra, North Island, New Zealand: A comparative study. In 9th International Conference on Fission Track Dating and Thermochronology, Lorne, Australia, pp. 207–208. Ku TL (2000) Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications. Washington, DC: American Geophysical Union. Ku TL (1968) Protoactinium-231 method of dating coral from Barbados Island. Journal of Geophysical Research 73: 2271–2276. Ku TL (2000) Uranium-series methods. In: Noller JS, Sowers JM and Lettis WR (eds.) Quaternary Geochronology: Methods and Applications, American Geophysical Union Reference Shelf 4: 101–114. Washington: American Geophysical Union. http://dx.doi.org/10.1029/RF004p0101. Lee H and Schwarcz H (2000) Electron Spin Resonance dating of fault rocks. In: Noller JS, et al. (ed.) Quaternary Geochronology: Methods and Applications, pp. 177–186. Washington, DC: American Geophysical Union. Markgraf V (2001) Interhemispheric Climate Linkages. San Diego: Academic Press. Mayewski PA, Meeker LD, Twickler MS, et al. (1997) Major features and forcing of high-latitude northern hemisphere atmospheric circulation using a 110,000-year-long glaciochemical series. Journal of Geophysical Research 102: 26,345–26,366. McDougall I (1995) Potassium-argon dating in the Pleistocene. In: Rutter NW and Catto NR (eds.) Dating Methods for Quaternary Deposits, pp. 1–14. Ottawa: Geological Survey of Canada. McDougall I and Harrison TM (1988) Geochronology and Thermochronology by the 40 Ar/39Ar Method. New York: Oxford University Press. McSween HY, Richardson SM, and Uhle ME (2003) Geochemistry: Pathways and Processes. New York: Columbia University Press. Miller GH and Brigham-Grette J (1989) Amino acid geochronology – Resolution and precision in carbonate fossils. Quaternary International 1: 111–128.
Miller GH, Magee JW, and Jull AJT (1997) Subtropical southern hemisphere Pleistocene cooling deduced from racemization in emu eggshells from Australia. Nature 385: 241–244. O’Sullivan PE (1983) Annually-laminated sediments and the study of Quaternary environmental changes – A review. Quaternary Science Reviews 1: 245–313. Olsson IU (1986) Radiometric dating. In: Berglun BE (ed.) Handbook of Holocene Palaeoecology and Palaeohydrology. London: Wiley. Pennington W, Cambray RS, and Fisher EM (1973) Observations on lake sediments using 137Cs. Nature 242: 324–326. Schwarcz H (1989) Uranium series dating. Quaternary International 1: 7–17. Schwarcz H, Buhay W, Gru¨n R (1988) Electron spin resonance (ESR) dating of fault gouge. US Geological Survey Open File Report 87–673, pp. 50–64. Stafford TW Jr., Jull AJT, Zabel TH, et al. (1984) Holocene age of the Yuha burial: Direct radiocarbon determinations by accelerator mass spectrometry. Nature 308: 446–447. Stokes S (1999) Luminescence dating applications in geomorphological research. Geomorphology 29: 153–171. Taylor RE (1987) Radiocarbon Dating: An Archaeological Perspective. New York: Academic Press. Tuniz C, Bird JR, Fink D, and Herzog GF (1998) Accelerator Mass Spectrometry: Ultasensitive Analysis for Global Science. Boca Raton, FL: CRC Press. Verosub KL (1988) Geomagnetic and secular variation and the dating of Quaternary sediments. In Easterbrook DJ (ed.) Dating Quaternary Sediments. Geological Society of America Special Publication Paper 227: 123–128. Wagner GA (1995) Age Determination of Young Rocks and Artifacts. Berlin: Springer. Wehmiller JF and Miller GH (2000) Aminostratigraphic dating methods in Quaternary geology. In: Noller JS, et al. (ed.) Quaternary Geochronology, Methods and Applications, pp. 187–222. Washington, DC: American Geophysical Union. Westgate JA and Naeser ND (1995) Tephrochronology and fission-track dating. In: Rutter NW and Catto NR (eds.) Dating Methods for Quaternary Deposits, pp. 15–28. Ottawa: Geological Survey of Canada. Wintle AG (1973) Anomalous fading of thermoluminescence in mineral samples. Nature 245: 143–144. Wohlfarth B, Bjo¨rck S, Possnert G, et al. (1993) AMS dating of Swedish varved clays of the last glacial/interglacial transition and potential difficulties of calibrating Late Weichselian ‘absolute’ chronologies. Boreas 22: 113–128. Zreda M and Phillips F (2000) Cosmogenic nuclide buildup in surficial materials. In: Noller JS, et al. (ed.) Quaternary Geochronology, Methods and Applications, pp. 61–76. Washington, DC: American Geophysical Union.
Further Reading Hearty PS and Kaufman DS (2000) Whole-rock aminostratigraphy and Quaternary sea-level history of the Bahamas. Quaternary Research 54: 163–173. McDougall I (1995) Potassium-argon dating in the Pleistocene. In: Rutter NW and Catto NR (eds.) Dating Methods for Quaternary Deposits, pp. 1–14. Ottawa: Geological Survey of Canada. McSween HY, Richardson SM, and Uhle ME (2003) Geochemistry: Pathways and Processes. New York: Columbia University Press. Schwarcz H, Buhay W, and Gru¨n R (1988) Electron spin resonance (ESR) dating of fault gouge. US Geological Survey Open File Report 87-673, pp. 50–64. Tuniz C, Bird JR, Fink D, and Herzog GF (1998) Accelerator Mass Spectrometry: Ultasensitive Analysis for Global Science. Boca Raton, FL: CRC Press.