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Raymond S. Bradley, in Paleoclimatology Third Edition , The fundamental assumptions in potassium-argon dating are that a no argon was left in the volcanic material after formation and b the system has remained closed since the material was produced, so that no argon has either entered or left the sample since formation. The former assumption may be invalid in the case of some deep-sea basalts, which retain ly formed argon during formation under high hydrostatic pressure. Such factors result in the sample age being overestimated Fitch, Similar errors result from modern argon being absorbed on to the surface and interior of the sample, thereby invalidating the second assumption.
Fortunately, atmospheric argon contamination can be assessed by measurement of the different isotopes of argon present. Atmospheric argon occurs as three isotopes, 36 Ar, 38 Ar, and 40 Ar. This may result from a of factors, including diffusion, recrystallization, solution, and chemical reactions as the rock weathers Fitch, Obviously, any argon loss will give a minimum age estimate only.
Fortunately, some assessment of these problems and their effect on dating may be possible. Twyman, in Encyclopedia of Quaternary Science , Like the conventional method, argon—argon dating relies on the measurement of 40 Ar produced by the radioactive decay of 40 K, but unlike conventional potassium—argon dating, it is unnecessary also to measure the amount of 40 K in the sample. Instead, the 39 Ar produced in the nuclear reactor is used as a substitute for 40 K, and the age is calculated from the ratio or argon isotopes. This can be determined in a single experiment, rather than the two separate measurements required for conventional potassium—argon dating.
As is the case in the conventional method, certain assumptions need to be made and correction factors need to be calculated to ensure accurate . For argon—argon dating, an important factor is the conversion of 39 K to 39 Ar. The amount of 39 Ar produced in any given irradiation will not only depend on the amount of 39 K in the sample, but also the duration and intensity of irradiation, the latter expressed as the neutron flux density.
It is virtually impossible to calculate these factors from first principles so the approach used is to simultaneously irradiate a control sample known as the flux monitor, whose age has already been determined. The argon ratios from the flux monitor are used to calculate a flux constant, J , which is applied to the age calculation for the experimental sample as follows:. Corrections must be made for atmospheric argon and for interfering argon isotopes produced by neutron reactions with calcium and other potassium isotopes.
These reactions are shown in Table 1. Table 1. The principle of argon-argon dating is to measure the amount of 39 Ar produced from 39 K as a proportion of the amount of 40 Ar. However, 39 Ar and 40 Ar can both be produced in competing reactions involving various isotopes of calcium, chlorine, potassium and argon present in the same sample.
The reactions causing the most interference are identified by sad faces in the table, and correction factors must be introduced to for them. Such interfering reactions are monitored using laboratory salts and glasses which are irradiated along with the sample. Other reactions identified by happy faces are beneficial, because they provide a means to calculate correction factors.
For example, the production of 38 Ar from 37 Cl allows the amount of chlorine in a sample to be determined. If an irradiated sample is completely melted, the argon in the sample is released in a single stage and provides an age comparable to that derived from conventional potassium—argon dating, at least when correction factors have been taken into . Ages calculated for each temperature increment can be plotted on an age-spectrum diagram, which for an undisturbed sample will yield a horizontal line i. Although potentially more accurate than potassium—argon dating and more applicable to smaller samples, the method does have some drawbacks.
One is the reliance on an age standard which must be dated by an alternative method usually the potassium—argon method in order to derive the neutron flux J constant. Another is the inherent error in extrapolating the J constant between samples differing in structure and homogeneity. This error can be minimized as much as possible by closely matching the irradiation conditions of the sample and control, and by using a series of control samples.
Some materials are also subject to a phenomenon known as argon recoil, in which the kinetic energy gained by 39 Ar during the neutron bombardment is sufficient to eject it from the sample. This is a ificant problem in grained materials like clay, and may also result in the redistribution of argon in inhomogeneous samples.
Bethan J. This chapter has summarized the application of radiometric methods cosmogenic nuclide dating, radiocarbon dating, optically stimulated luminescence dating, argon-argon and potassium-argon dating to glacial environments. These numerical radiometric techniques can be used in conjunction with archival methods, relative dating methods such as morphostratigraphy and Schmidt hammer dating, and incremental methodologies such as lichenometry, varve counting and dendroglaciology to date glacial landforms across a wide range of different glacial environments and different timescales.
The rapid pace of developments in radiometric dating glacial landforms provides glacial geologists a powerful toolbox for fixing in time past glacier-climate interactions. Dating features such as moraines allows the timing of ificant stabilizations of outlet glaciers to be characterized. All of these dating techniques rely upon a sound understanding of the regional glacial geomorphology and geology, and must be underpinned by a thorough geomorphological mapping campaign that seeks to understand the morphometric and stratigraphic relationships between landforms.
Chronologies should be constructed with adherence to quality assurance protocols, which also allow tools for the analysis and comparison of new datasets to legacy data. These methods together can be applied to derive highly confident mean age estimates for glacial landforms. Careful application of these methodologies, together with improving understanding of their assumptions and limitations, and improving protocols for sample selection, laboratory analysis, age calculation and identification and treatment of outliers, has resulted in large datasets for every ice sheet.
Compilations of these geomorphological and chronological data, together with an understanding of their age reliability, now allow an unprecedented view into past ice-sheet behavior through time Batchelor et al. These datasets highlight key gaps in knowledge and can emphasize future research priorities, evaluate regional disparities, and calculate regional rates of horizontal and vertical recession Davies et al. Robust glacial chronologies using numerical ages that can be used to carefully correlate ice extents over wide regional areas are vital to understanding past ice sheet or glacier response to palaeoclimate.
Integration of these empirical datasets with numerical simulations requires a robust treatment of uncertainties. These empirical datasets are increasingly used to calibrate numerical simulations Albrecht et al. These exciting developments herald a new understanding of ice mass response to external drivers of change ocean and atmospheric temperatures compared with internal drivers, such as ice divide migration, topography, calving, or ice-dammed proglacial lakes.
These efforts will help to understand likely drivers of change in current ice masses, and future rates and magnitudes of sea level change, mountain glacier recession and meltwater supply, and changing glacier-related hazards. Large empirical datasets of geomorphology and carefully collected and analyzed chronological data, grounded in a thorough understanding of glacial process, is critical to this effort. Prior to , much paleoclimatological research focused primarily on climatic reconstructions that described what happened, with studies involving a variety of different proxy data types Wendland, Radiometric dating techniques, such as radiocarbon and potassium-argon dating , provided a quantitative means to date past climatic change.
Paleoclimatic research was propelled by the establishment of numerous research centers that specialized in particular proxy data and dating methods. For example, dendroclimatology, the study of tree-rings, accelerated after the establishment of the Laboratory of Tree Ring Research at the University of Arizona, USA in Similarly, prominent research centers in Quaternary paleoceanography emerged as well, with some notable centers being based at Cambridge University, Brown University, and Columbia University.
As time progressed, improvements and new techniques in data analysis were developed at prominent research centers such as the Quaternary Research Center in the United States, and the Xian Laboratory of Loess and Quaternary Geology at the Chinese Academy of Sciences. Newly trained academics that graduated from these research centers started research centers of their own, building up paleoclimatic databases. Beginning in the early s, the development of high-speed computers fostered a new type of paleoclimatology that specializes in analyzing large paleoclimatic datasets Wright and Bartlein, Some interpretive tools in paleoclimatic analyses are qualitative in nature, which continue to this day and can involve analyses from local to hemispheric scales Figure 1.
Earlier quantitative studies applied basic transfer functions to convert proxy variables into climate variables, which also involved the calibration of modern climatic data with modern environmental data. The modern relationships were applied to fossil environmental data to quantitatively reconstruct past climate Webb and Bryson, As the datasets grew, so did the sophistication of quantitative interpretive tools for analyzing large-scale paleoclimatic datasets Mann et al.
For example, the recently compiled North American Drought Atlas, which provides geographical maps of drought severity by year, is based on a geographical network of tree-ring sites Figure 3 ; Cook and Krusic, The computer revolution also created a paleoclimatic perspective for dealing with GCMs.
These models are similar to those used in daily weather forecasting but instead the principles are applied to simulate large-scale climate patterns of the past. Earlier attempts focused mostly on the atmosphere, but paleoclimatic modeling has evolved to link atmospheric models with detailed feedbacks as they relate to processes in the biosphere, lithosphere, and hydrosphere Kohfeld and Harrison, ; Kutzbach et al. Considerable attention has been paid to ocean-atmospheric feedbacks. GCMs have been used to simulate paleoclimates ranging from a few hundred to millions of years ago Kutzbach, , as well as selected timeframes and phenomena of interest in the past LeGrande et al.
Increased resolution in dating techniques and the growing body of paleoclimatic evidence, particularly from ice cores and marine sediments from the North Atlantic, indicate that abrupt decadal-centennial scale changes in climate occurred in the distant past, that are much different in magnitude and character than those observed in the modern instrumental record Clark et al.
These changes are ificant for society, for we now know that such abrupt climatic changes may occur within a single human lifetime. Paleoclimatic records thus offer the only means to test whether our predictive models can simulate such future changes. Modeling attempts have been made to simulate the causes and nature of these abrupt changes.
These model runs are enabling scientists to conduct detailed data-model comparisons at the hemispheric and global scales Clark et al. However, to date, most modeling studies on these events are still focusing on providing sensitivity tests to assess potential forcing mechanisms. We are only beginning to document and understand the controls and causes of these abrupt changes, and such questions will continue to be important for the paleoclimatology community for years to come.
A large of techniques have been developed that can provide a timescale for processes that take place during the Quaternary. Many numerical dating methods had their early roots in the enthusiastic study of radioactive decay as a means of obtaining the age of the Earth. Some of these, such as radiocarbon and potassium argon dating, have been around for more than 50 years and are widely accepted. Other methods, such as those based on cumulative effects of radioactive decay, have been around for somewhat less time. The most recent addition to the battery of techniques is cosmogenic nuclide dating, which has been around for only two decades.
Each technique is limited in the type of material that can be dated and the time range over which it can be applied. Together, the methods contribute to the production of a timescale for events throughout the Quaternary, whether they cover a few million years or a few decades. These methods are briefly summarized in terms of their initial inception and key advances in technique development, and they are illustrated by a limited of applications. Turekian, M. Bacon, in Treatise on Geochemistry Second Edition , Volcanic debris from explosive volcanism occurring at convergent plate boundaries can be deposited in deep-sea sediments.
Hence, volcanic-ash layers provide the opportunity of dating strata by a of radiometric methods. Macdougall used fission-track dating of glass shards to determine the ages of volcanic layers in deep-sea sediments. Table 2. Comparison of fission track ages and potassium—argon ages of volcanic material in deep-sea sediments. Source: Macdougall D Fission track dating of volcanic glass shards in marine sediments. Earth and Planetary Science Letters — The volcanic layer dating has been extended by stratigraphic correlation of diagnostic chemical imprints of volcanic ash dated on land adjacent to the deep-sea sediments of eastern Africa Brown et al.
Davis, K. As mentioned in the historical introduction Section 1. In current practice, a standard and sample are irradiated together, and the I—Xe age of the sample is measured relative to that of the standard. The standard most widely used since the s is enstatite from the Shallowater enstatite chondrite. The absolute Pb—Pb age of Shallowater enstatite, Both parent and daughter are mobile elements, and coupled with the relatively long half-life, this means that closure effects on the I— Xe system likely limit its utility to parent-body processes e.
New analytical techniques that enable the investigation of single mineral phases Crowther et al. Brazzle et al. At another extreme, Whitby et al. David P. Gold, in Encyclopedia of Geology Second Edition , Reconstructions of relative sea-level change are based on empirical data that estimate sea-level height during a particular period or range of geological time. The reconstructed sea-level history is produced by measuring the bathymetry of the sea-level indicator with respect to a modern datum such as global mean sea-level Kemp et al. Sea-level indicators are physical, chemical or biological proxies that can be shown to occur at a particular bathymetry.Potassium 40 radiometric dating
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