Radiometric dating how does it work

Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Researchers use data from tree rings, sediment layers and other samples to calibrate the process of carbon dating. Radiocarbon dating — a key tool used for determining the age of prehistoric samples — is about to get a major update. For the first time in seven years, the technique is due to be recalibrated using a slew of new data from around the world.

The work combines thousands of data points from tree rings, lake and ocean sediments, corals and stalagmites, among other features, and extends the time frame for radiocarbon dating back to 55, years ago — 5, years further than the last calibration update in Archaeologists are downright giddy. Archaeology: Date with history. Although the recalibration mostly results in subtle changes, even tiny tweaks can make a huge difference for archaeologists and paleo-ecologists aiming to pin events to a small window of time.

The basis of radiocarbon dating is simple: all living things absorb carbon from the atmosphere and food sources around them, including a certain amount of natural, radioactive carbon Measuring the amount left over gives an estimate as to how long something has been dead.

In recent decades, the burning of fossil fuel and tests of nuclear bombs have radically altered the amount of carbon in the air, and there are non-anthropogenic wobbles going much further back. During planetary magnetic-field reversals, for example, more solar radiation enters the atmosphere, producing more carbon The oceans also suck up carbon — a little more so in the Southern Hemisphere, where there is more ocean — and circulate it for centuries, further complicating things.

As a result, conversion tables are needed that match up calendar dates with radiocarbon dates in different regions. They will be published in the journal Radiocarbon in the next few months.

However, there are potential issues with potassium-argon dating. Argon-argon dating is an updated method, based on the original K-Ar dating technique, that uses neutron irradiation from a nuclear reactor to convert a stable form of potassium into the argon isotope 39 Ar, and then measures the ratio of 40 Ar to 39 Ar.

Argon-argon dating was used to determine that the Australopithecus Lucy , who rewrote our understanding of early hominin evolution, lived around 3.

This technique involves measuring the ratio of uranium isotopes U or U to stable lead isotopes Pb, Pb and Pb. It can be used to determine ages from 4. This method is thought to be particularly accurate, with an error-margin that can be less than two million years — not bad in a time span of billions.

U-Pb dating is most often done on igneous rocks containing zircon. The uranium content of the sample must be known; this can be determined by placing a plastic film over the polished slice and bombarding it with slow neutrons — neutrons with low kinetic energy. This bombardment produces new tracks, the quantity of which can be compared with the quantity of original tracks to determine the age.

This method can date naturally occurring minerals and man-made glasses. It can thus be used for very old samples, like meteorites, and very young samples, like archaeological artefacts. Fission-track dating identified that the Brahin Pallasite , a meteorite found in the 19 th century in Belarus — slabs of which have become a collectors item — underwent its last intensive thermal event 4.

This method involves calculating the prevalence of the very rare isotope chlorine 36 Cl , which can be produced in the atmosphere through cosmic rays bombarding argon atoms. Chlorine was also released in abundance during the detonation of nuclear weapons between and It stays in the atmosphere for about a week, and so can mark young groundwater from the s onwards as well.

However, they do use radioactive material. These methods date crystalline materials to the last time they were heated — whether by human-made fires or sunlight. Exposure to sunlight or heat releases these, removing the charges from the sample.

These methods provide valuable and valid age data in most instances, although there is a small percentage of cases in which even these generally reliable methods yield incorrect results.

Such failures may be due to laboratory errors mistakes happen , unrecognized geologic factors nature sometimes fools us , or misapplication of the techniques no one is perfect. In order to accomplish their goal of discrediting radiometric dating, however, creationists are faced with the daunting task of showing that a preponderance of radiometric ages are wrong — that the methods are untrustworthy most of the time.

Not only that, they have to show the flaws in those dating studies that provide independent corroborative evidence that radiometric methods work. This is a tall order and the creationists have made no progress so far.

It is rare for a study involving radiometric dating to contain a single determination of age. Usually determinations of age are repeated to avoid laboratory errors, are obtained on more than one rock unit or more than one mineral from a rock unit in order to provide a cross-check, or are evaluated using other geologic information that can be used to test and corroborate the radiometric ages. Scientists who use radiometric dating typically use every means at their disposal to check, recheck, and verify their results, and the more important the results the more they are apt to be checked and rechecked by others.

As a result, it is nearly impossible to be completely fooled by a good set of radiometric age data collected as part of a well-designed experiment. The purpose of this paper is to describe briefly a few typical radiometric dating studies, out of hundreds of possible examples documented in the scientific literature, in which the ages are validated by other available information.

I have selected four examples from recent literature, mostly studies involving my work and that of a few close colleagues because it was easy to do so. I could have selected many more examples but then this would have turned into a book rather than the intended short paper. In the Cretaceous Period, a large meteorite struck the earth at a location near the present town of Manson, Iowa.

The heat of the impact melted some of the feldspar crystals in the granitic rocks of the impact zone, thereby resetting their internal radiometric clocks. The impact also created shocked quartz crystals that were blasted into the air and subsequently fell to the west into the inland sea that occupied much of central North America at that time.

Today this shocked quartz is found in South Dakota, Colorado, and Nebraska in a thin layer the Crow Creek Member within a thick rock formation known as the Pierre Shale. The Pierre Shale, which is divided into identifiable sedimentary beds called members, also contains abundant fossils of numerous species of ammonites, ancestors of the chambered nautilus.

The fossils, when combined with geologic mapping, allow the various exposed sections of the Pierre Shale to be pieced together in their proper relative positions to form a complete composite section Figure 1. The Pierre Shale also contains volcanic ash that was erupted from volcanoes and then fell into the sea, where it was preserved as thin beds. Figure 1. There are three important things to note about these results. First, each age is based on numerous measurements; laboratory errors, had there been any, would be readily apparent.

Second, ages were measured on two very different minerals, sanidine and biotite, from several of the ash beds. Third, the radiometric ages agree, within analytical error, with the relative positions of the dated ash beds as determined by the geologic mapping and the fossil assemblages; that is, the ages get older from top to bottom as they should. Finally, the inferred age of the shocked quartz, as determined from the age of the melted feldspar in the Manson impact structure Meteorites, most of which are fragments of asteroids, are very interesting objects to study because they provide important evidence about the age, composition, and history of the early solar system.

There are many types of meteorites. Some are from primitive asteroids whose material is little modified since they formed from the early solar nebula. Others are from larger asteroids that got hot enough to melt and send lava flows to the surface. A few are even from the Moon and Mars. The most primitive type of meteorites are called chondrites, because they contain little spheres of olivine crystals known as chondrules.

Because of their importance, meteorites have been extensively dated radiometrically; the vast majority appear to be 4. Some meteorites, because of their mineralogy, can be dated by more than one radiometric dating technique, which provides scientists with a powerful check of the validity of the results. The results from three meteorites are shown in Table 1. Many more, plus a discussion of the different types of meteorites and their origins, can be found in Dalrymple Table 1.

There are 3 important things to know about the ages in Table 1. The first is that each meteorite was dated by more than one laboratory — Allende by 2 laboratories, Guarena by 2 laboratories, and St Severin by four laboratories. This pretty much eliminates any significant laboratory biases or any major analytical mistakes. The second thing is that some of the results have been repeated using the same technique, which is another check against analytical errors.

The third is that all three meteorites were dated by more than one method — two methods each for Allende and Guarena, and four methods for St Severin. This is extremely powerful verification of the validity of both the theory and practice of radiometric dating. In the case of St Severin, for example, we have 4 different natural clocks actually 5, for the Pb-Pb method involves 2 different radioactive uranium isotopes , each running at a different rate and each using elements that respond to chemical and physical conditions in much different ways.

And yet, they all give the same result to within a few percent. Is this a remarkable coincidence? Scientists have concluded that it is not; it is instead a consequence of the fact that radiometric dating actually works and works quite well. Creationists who wants to dispute the conclusion that primitive meteorites, and therefore the solar system, are about 4.

One of the most exciting and important scientific findings in decades was the discovery that a large asteroid, about 10 kilometers diameter, struck the earth at the end of the Cretaceous Period.

The collision threw many tons of debris into the atmosphere and possibly led to the extinction of the dinosaurs and many other life forms. The fallout from this enormous impact, including shocked quartz and high concentrations of the element iridium, has been found in sedimentary rocks at more than locations worldwide at the precise stratigraphic location of the Cretaceous-Tertiary K-T boundary Alvarez and Asaro ; Alvarez We now know that the impact site is located on the Yucatan Peninsula.

Measuring the age of this impact event independently of the stratigraphic evidence is an obvious test for radiometric methods, and a number of scientists in laboratories around the world set to work. Table 2. In addition to shocked quartz grains and high concentrations of iridium, the K-T impact produced tektites, which are small glass spherules that form from rock that is instantaneously melted by a large impact.

The K-T tektites were ejected into the atmosphere and deposited some distance away. Tektites are easily recognizable and form in no other way, so the discovery of a sedimentary bed the Beloc Formation in Haiti that contained tektites and that, from fossil evidence, coincided with the K-T boundary provided an obvious candidate for dating. Scientists from the US Geological Survey were the first to obtain radiometric ages for the tektites and laboratories in Berkeley, Stanford, Canada, and France soon followed suit.

The results from all of the laboratories were remarkably consistent with the measured ages ranging only from Similar tektites were also found in Mexico, and the Berkeley lab found that they were the same age as the Haiti tektites. The K-T boundary is recorded in numerous sedimentary beds around the world. Numerous thin beds of volcanic ash occur within these coals just centimeters above the K-T boundary, and some of these ash beds contain minerals that can be dated radiometrically. Since both the ash beds and the tektites occur either at or very near the K-T boundary, as determined by diagnostic fossils, the tektites and the ash beds should be very nearly the same age, and they are Table 2.

There are several important things to note about these results. First, the Cretaceous and Tertiary periods were defined by geologists in the early s.

The boundary between these periods the K-T boundary is marked by an abrupt change in fossils found in sedimentary rocks worldwide. Its exact location in the stratigraphic column at any locality has nothing to do with radiometric dating — it is located by careful study of the fossils and the rocks that contain them, and nothing more. Furthermore, the dating was done in 6 different laboratories and the materials were collected from 5 different locations in the Western Hemisphere.

And yet the results are the same within analytical error.