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A dating method based on the time dependent, radiation induced accumulation of electrons and holes in the crystal lattice of certain minerals. From: Encyclopedia of Archaeology, Raymond S. Bradley, in Paleoclimatology Third Edition , Accurate dating is of fundamental importance to paleoclimatic studies.
Without reliable estimates on the age of events in the past, it is impossible to investigate if they occurred synchronously or if certain events led or lagged others; neither is it possible to assess accurately the rate at which past environmental changes occurred. Strenuous efforts must therefore be made to date all proxy materials, to avoid sample contamination, and to ensure that the stratigraphic context of the sample is clearly understood.
It is equally important that the assumptions and limitations of the dating procedure used are understood so that a realistic interpretation of the date obtained can be made. It is often just as important to know the margins of error associated with a date as to know the date itself. In this chapter, we discuss the main dating methods widely used in Quaternary studies today. Further details can be found in Geyh and Schleicher , Noller et al.
Dating methods fall into four basic Figure 3. Depending on the time period of interest, different dating methods will be more suitable than others Figure 3. Figure 3. Principal dating methods used in paleoclimatic research. Schematic diagram showing the age ranges for which different dating methods are suitable. Not all dating methods provide a reliable numerical age, but may give an indication of the relative age of different samples.
Thus, there is a spectrum of approaches to dating: numerical age methods, calibrated age methods, relative age methods, and methods involving stratigraphic correlation Colman et al. In this and the following chapter, all of these approaches are discussed, beginning with numerical age methods. Dating methods based on concentration measurements of cosmogenic nuclides, with surface exposure dating SED at the forefront, have evolved considerably over the last three decades e.
Among the wide diversity of geomorphological settings in which they were applied, cosmogenic nuclides allowed successful dating of various landforms in coastal environments. While flights of marine terraces were often targeted, with sampling of either sediment-bearing marine platforms e. However, unlike other similar landforms, such as clasts from glacial moraines that are routinely dated by cosmogenic nuclides to infer the timing of deglaciation worldwide e. The challenge primarily arises from methodological difficulties induced by the use of SED in this specific environment.
This also emphasizes the fact that obtaining robust chronologies by any methods for the transport and deposition of supratidal coarse-clast sediments remains a difficult task Paris et al. In this chapter, the theoretical background related to cosmogenic nuclides and SED is first briefly presented. The main text section is devoted to the current difficulties and limitations of dating supralittoral coarse clasts and how these, to a certain extent, can be overcome this is exemplified by a successful case study on the Cape Verde Islands; Ramalho et al.
Several future perspectives of this dating method are briefly tackled at the end of the chapter. Indeed, SED of coarse clasts can unravel extreme event histories over both the Holocene Rixhon et al. Importantly, it also has the potential to pinpoint the timing of the event, i. The latter are, among others, prone to reworking issues of the dated material, or are more susceptible to alterations that interfere with their isotopic ratios.
Murray-Wallace, in Treatise on Geomorphology , Isotopic dating methods are those that measure the isotopic composition of a material as a result of radioactive decay. These are isotopes produced as a result of the bombardment of the Earth by high-energy gamma or cosmic rays. However, unstable isotopes undergo changes in the atomic organization to become a more stable atomic form. Thus, the isotopic composition of a material and the concentration of radioisotopes change through time. If the rate of this change to a more stable atomic form is known, then the age of the material being dated can be established Lowe and Walker, ; Noller et al.
Numerous elements undergo radioactive decay over a variety of time scales including U-series. Radiocarbon dating 14 C is probably the most commonly used numeric dating method for geomorphological research and is outlined in more detail below. The isotopic dating method can also be based on certain cosmogenic nuclides e. However, the accumulation of cosmogenic nuclides at the surface and concentration in the upper few meters is controlled by the balance between the production rate of the isotope, its decay, and the rate of erosion Zreda and Phillips, ; Duller, This dating method enables age determinations to be made on the time that a particular surface has been exposed to cosmic rays.
This provides the time frame in which the timing and rate of geomorphic processes can be assessed. However, interpretation of cosmogenic nuclide becomes very complex in environments where a surface may have a history of exposure, burial, and subsequent reexposure. For a more detailed review of cosmogenic dating and application geomorphic problems, see Cockburn and Summerfield Bethan J. Incremental dating methods rely on the constant, or incremental, growth or constrained growth rate of an organism lichenometry, dendrochronology or incremental sedimentation into a water body annually laminated lake sediments, or varves Lowe and Walker, The incremental growth may result in annual layering, allowing counting to produce an absolute time scale.
Annually resolved incremental dating methods have been critical in the calibration of numerical radiometric dating technologies such as radiocarbon dating. Other incremental dating techniques that are not covered here, as they have less relevance for dating glacial landforms, include annual layers in ice cores, annual increments in speleothems, annual banding in corals and molluscs Lowe and Walker, , and annual layers in mumiyo Berg et al. Reimann, in Developments in Quaternary Sciences , Luminescence dating methods provide a new tool to gain a quantitative understanding of vertical and lateral soil mixing processes.
This requires measurement of luminescence als from single grains. Where most investigations were based on quartz OSL, we recently showed the great potential of single-grain feldspar luminescence methods for this application. This method determines what fraction of grains has reached the surface at least once during soil formation the NSF as well as information on mixing rates from the luminescence ages of individual grains.
By combining both measures, the mixing intensity and possibly residence time of soil particles can be quantified. We expect this tool to be valuable for improved understanding of soil mantle responses to environmental conditions. Scott A. Elias, in Encyclopedia of Geology Second Edition , Incremental dating methods have benefitted greatly from technological advancements since the s, from microprobes that can detect the chemical composition of minute tephra samples to the most recent generation of ice-coring drills that can successfully extract several kilometers of ice from sites in Greenland and Antarctica.
These technological improvements are allowing earth scientists to reconstruct Quaternary environments and date events recorded in these archives with ever-improving precision and accuracy. In some cases, a new piece of equipment will be developed commercially, and then adopted by researchers. But the researchers themselves drive many technological advances. Their thirst for knowledge is a powerful motivator to build or modify instruments that will allow them to get the data they need to address research questions.
Twyman, in Encyclopedia of Quaternary Science , Radiometric dating methods measure the decay of naturally occurring radioactive isotopes, and are used to determine the ages of rocks, minerals, and archaeological artifacts. Of the 84 elements found in nature, there are 70 naturally occurring radioactive isotopes whose half-lives vary from a few thousand years e. Potassium—argon dating is based on measuring the decay of potassium 40 K to argon 40 Ar , a process with a half-life of 1. Radioactive decay is a nuclear process and is independent of chemical and physical conditions found in geological processes.
It has been shown that radioactive decay is constant over the wide range of temperatures and pressures encountered in geological events. Consequently, potassium—argon dating is one of the most widely used geological dating methods. Argon—argon dating is a derivative method in which samples are bombarded in a fast neutron reactor, converting a proportion of the abundant and stable isotope 39 K into 39 Ar. This article describes the principles and practical aspects of these two forms of radiometric dating.
Radiometric dating methods were developed in the twentieth century, and have revolutionized Quaternary Science. In , physicists Ernest Rutherford and Frederick Soddy had discovered that radioactive elements broke down into other elements in a definite sequence or series, through the process of nuclear fission.
The possibility of using this radioactivity as a means of measuring geologic time was first discussed by Rutherford in In , Rutherford began calculating the rate of radioactive decay of uranium. This decay process uranium decaying to lead has since been discovered to go through multiple steps, with intermediate daughter products. It is now possible to use various uranium-series decay processes to derive age estimates for uranium-bearing fossils and sediments, back many millions of years [cross-ref.
Numerical dating methods based on natural radioactive phenomena were initially developed as a means of determining the age of the Earth Dalrymple, Arguments on this topic raged in the second half of the 19th century, a time when the effects of recent glaciation were also being debated. However, it was not until the discovery of radioactivity in the s that consideration was given to radioactive isotopes for use as natural clocks.
As early as , Rutherford suggested that ages could be obtained based on the production of helium by the decay of uranium in rocks. The first calculation of the age of the Earth based on the amount of radium in the Earth's crust was made by Russell in However, it was not until the s, when Nier brought together the understanding of natural isotopes in the uranium and thorium decay chains and the construction of the first mass spectrometers, that it became possible to measure a range of isotopes.
Also at this time, the potential of several different decay series that could be used for dating was proposed, based on a better understanding of the atomic structure of elements in the periodic table. The relationship between these early radiometric dates and the evolution of the geological timescale has been covered in a history of the work of Arthur Holmes Lewis, Dating Method A dating method based on the time dependent, radiation induced accumulation of electrons and holes in the crystal lattice of certain minerals.
Set alert. About this . Dating Methods I Raymond S. Bradley, in Paleoclimatology Third Edition , 3. View chapter Purchase book. Cosmogenic nuclide dating of coarse clasts Gilles Rixhon, in Geological Records of Tsunamis and Other Extreme Waves , Introduction Dating methods based on concentration measurements of cosmogenic nuclides, with surface exposure dating SED at the forefront, have evolved considerably over the last three decades e. Methods in Geomorphology C. Reading the Soil Archives J.
Reimann, in Developments in Quaternary Sciences , 4. Volume 6 Scott A. Elias, in Encyclopedia of Geology Second Edition , Conclusions Incremental dating methods have benefitted greatly from technological advancements since the s, from microprobes that can detect the chemical composition of minute tephra samples to the most recent generation of ice-coring drills that can successfully extract several kilometers of ice from sites in Greenland and Antarctica. Twyman, in Encyclopedia of Quaternary Science , Introduction Radiometric dating methods measure the decay of naturally occurring radioactive isotopes, and are used to determine the ages of rocks, minerals, and archaeological artifacts.
Wintle, in Encyclopedia of Quaternary Science Second Edition , Early Developments Numerical dating methods based on natural radioactive phenomena were initially developed as a means of determining the age of the Earth Dalrymple,Reliability of dating methods
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Dating Samples: Some Limitations of Dating Methods