Uranium-Thorium dating is based on the detection by mass spectrometry of both the parent U and daughter Th products of decay, through the emission of an alpha particle. The decay of Uranium to Thorium is part of the much longer decay series begining in U and ending in Pb. With time, Thorium accumulates in the sample through radiometric decay. The method assumes that the sample does not exchange Th or U with the environment i. The method is used for samples that can retain Uranium and Thorium, such as carbonate sediments, bones and teeth. Ages between and , years have been reported. Journal of Quaternary Science
Piggot and Urry found in , that Radium excess corresponded with an excess of Thorium. It took another 20 years until the technique was applied to terrestrial carbonates speleothems and travertines. In the late 80's the method was refined by mass spectrometry. After Viktor Viktorovich Cherdyntsev 's landmark book about uranium had been translated into English, U-Th dating came to widespread research attention in Western geology. U-series dating is a family of methods which can be applied to different materials over different time ranges.
Each method is named after the isotopes measured to obtain the date, mostly a daughter and its parent. Eight methods are listed in the table below. Using this technique to calculate an age, the ratio of uranium to its parent isotope uranium must also be measured. U-Th dating yields most accurate results if applied to precipitated calcium carbonate, that is in stalagmites , travertines, and lacustrine limestones. Bone and shell are less reliable.
Mass spectrometry also uses smaller samples. From Wikipedia, the free encyclopedia. Department of Geosciences, University of Arizona. Retrieved 24 October Issues, News, and Reviews. External links [ edit ] Shakhashiri, Bassam Z. Attempts to date cave paintings illustrate the difficulties of radiometric dating, and also show evidence of a young earth. A recent article about U-series dating of Paleolithic art in 11 caves in Spain 1 contained some frank discussions about the wild assumptions that had to be made to date the paintings, and raised some interesting questions about the scientifically accepted age of the Earth.
Although Paleolithic art has nothing to do with evolution, the article does give us an opportunity to talk about dating techniques in general, and U-series dating in particular. Furthermore, the measured levels of uranium isotopes are nowhere near what the Old Earth model predicts. All dating methods depend upon measurement of something that varies with time. The simplest shape is a straight line, like the one below.
Normally we look first at the horizontal axis of a graph like this because we know the TIME, and then look up Y at that point because we want to see what the value of Y is at that TIME. But we could work backwards. When the slope is flat like this, contamination can be a very big problem.
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Just a small measurement error in Y results in huge errors in the calculated age. Suppose it is a cyclic graph like the wave in the graph below. Imagine it is something like the plot of average daily temperature over several years. If you measure a Y value of 1.
Uranium series dating problems
The point of all these graphs is to show that, for a process to be useful for determining time, it must be monotonic and have a moderate slope in the region of interest. If the slope is too flat, contamination is a problem. We, however, are interested in it because it describes radioactive decay. It is a number like pi that describes natural physical processes, like the decay of voltage on a capacitor, radioactive decay, and lots of other natural things.
Engineers and mathematicians talk about exponential decay in terms of tau because it makes the math easy. The half-life is the time it takes for half of the material to decay.
There is a simple relationship between tau and the half-life. The half-life is equal to the natural logarithm of 2 which is 0. So, although radioactive decay is generally specified in terms of half-lives, calculations are always done in terms of tau. The age of cave paintings has traditionally been determined by measuring carbon 14 levels.
One obvious problem with this is that one has to scrape part of the painting off the wall in order to analyze the carbon content, destroying part of the painting. The other problem is that the paintings are presumed to be tens of thousands of years old, which is on the very flat part of the carbon 14 exponential curve, where contamination could be a problem.
It would be very difficult to tell a contaminated 30, year-old painting from an uncontaminated 3, year-old painting.
Paleolithic cave art is an exceptional archive of early human symbolic behavior, but because obtaining reliable dates has been difficult , its chronology is still poorly understood after more than a century of study. Where suitable material exists e. Discrepancies between multiple 14 C determinations on a single painted motif have been common , as are discrepancies between the dates of different chemical e.
Our U-series ages ranged from 0. All we care about is that the measurements ranged from years to 40, years, which hardly inspires confidence in the method. We should also explain the difference between calendar years and radiocarbon years. So, correction coefficients have been developed for historic times to convert radiocarbon years accurately to calendar years.
Those correction coefficients have been extrapolated to prehistoric times based on presumed prehistoric CO 2 levels. They use a tricky method we will explain in a moment.Radiometric or Absolute Rock Dating
The tricky method they use tells more about the age of the Earth than it does about the age of the paintings. Back to their method of dating the paintings: Anyone who has been to a cave knows stalactites, stalagmites, columns, and draperies form gradually in caves.
These cave formations are simply differently shaped deposits of minerals such as calcite on the floor, walls, and ceiling of caves. Ancient cave art is painted on one layer of these deposits, and covered by another layer. If one can tell how old the layer under the painting is, and how old the layer over the painting is, then one can set upper and lower bounds on the age of the painting.
This means one can date the painting by taking samples from the wall near the painting without damaging the painting itself. The method is described in the supplementary material on the Science website, but not published in the journal itself. The Uranium-series disequilibrium method. Differential solubility between uranium and its long lived daughter isotope Th means that calcite precipitates e. Over time, there is ingrowth of Th from the radioactive decay of U until radioactive equilibrium is reached where all isotopes in the series are decaying at the same rate.
An additional problem is the incorporation of detritus in the precipitating calcite.
This can be from wind-blown or waterborne sediments. Detrital sediments will bring U and Th and usually will result in the apparent age of a contaminated sample to be an overestimate of the true age. However, the presence of the common thorium isotope, Th, indicates the presence of contamination, and there are several methods to correct the U-series date for it. Note the conservative error on this assumption. While the date obtained using measured detritus values agrees within error in both cases with the date using an average crustal silicate, we must be cautious in using dates corrected using the insoluble detritus.
To be cautious therefore, we base our interpretation of the dates for samples O and O on dates corrected with our assumed rather than measured detrital value. First of all, notice the assumption that when water seeps through the ground and dissolves minerals, it dissolves some uranium, but no thorium.
Therefore, when the water evaporates in the cave, leaving the minerals behind on the cave wall, there is some uranium but no thorium when the flowstone first formed. Later, they presume, some of the uranium decays to produce all the thorium in the flowstone. Is that assumption reasonable? Can one really assume that no thorium was present in the water that evaporated to form the flowstone?
How much thorium would it take to produce a false old age for a modern formation? Not much, as we will see, later. Second, the method depends upon equilibrium or lack thereof.
The assumption is that the newly formed flowstone contains uranium but no thorium. As time goes by, the uranium will decay resulting in less uranium and more thorium.