Radiometric dating

Jul 23, 2013

Discussion by: Maba

I have begun to read more about how to determine the age of archeological findings. I think I understand the basic premise of radiometric dating methods but I am not sure if I got some of the details right. Maybe one of you can help me understand it better?

So, the idea is that radioactive isotopes decay over time into stable isotopes. We use the half-life of an isotope as the decay rate. By measuring the amount of stable isotopes (with mass spectrometry) in a rock or mineral one can deduce how much time must have passed for this amount to be present. Because different isotopes have different half-lifes one can cross-check the first estimate by looking at the amount of other stable isotopes that are the decay product of radioactive isotopes with a shorter half-life. This method only works when the stable isotopes can only be the result of a decay process and can not be present without said process. In this way, it is not necessary to know how much of the radioactive isotopes were present at the time when the rock was formed because we can estimate the age by "calculating backwards" from the amounts of the stable isotopes which are a direct result of the decay process that took place over hundreds of thousands of years.

Please understand that I am not a chemist or geologist. This is what I pieced together from information I found on the internet. Is this correct? Did I miss something? Do you have suggestions for good and newcomer friendly introductions to this topic?

8 comments on “Radiometric dating

  • 1
    downshifter says:

    Yes, that’s essentially correct! I would only add two details:

    1) The date is determined by using the ratio of the radioactive isotope vs. it’s decay product. (Also, some of these decay isotopes are themselves radioactive and further decay.) This is important since it means your calculation won’t be thrown off because of different amounts of radioactive isotopes in your samples.

    2) The method is used on volcanic rocks, because when they are originally formed there is a process that excludes the decayed isotopes. Vulcanism essentially “resets” the rock’s radiometric clock. You can’t use radiometric dating on sedimentary rocks, where you find fossils. Sedimentary rocks are dated by finding them between volcanic layers. (Or below for a most recent date, or above for a minimum date.) You can then use your radiometric dates to give dates for other sedimentary rocks which are not associated with igneous rock layers.

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  • 2
    AsylumWarden says:

    Hi there. You’ve got the basics correct, yes. A couple of key issues though.

    1. Because (as you said) different isotopes have different half lives it’s important to select the correct radiometric ‘clock’ for the job. After about 10 half-lives the radioactive isotope has decayed to a point where it is normally too low to measure accurately. Hence why Carbon 14 cannot be used for carbon dating for objects older than around 60,000 years due to only having a half life of 5760 years. A common creationist tactic is to use an inappropriate method for something and then cry ‘see, it doesn’t work’ when they, unsurprisingly, get poor results.

    2. You said that the method only works when the stable isotopes can only be the result of a decay process. That isn’t always the case and it only works when there is no new unstable isotopes being formed which is normally the case for very old, i.e. on the geological scale samples. For instance with carbon dating, which is more often used for archeological samples it’s that the UNSTABLE isotopes cannot have been REMOVED by any process other than decay. In a living system Carbon constantly flows in and out of it via breathing, eating etc. The ratios of Carbon 12 (stable) and Carbon 14 (unstable) are therefore relatively unchanged. When something dies however, the flow stops meaning Carbon 14 decays without being replaced and hence the 12:14 ratio and it is this ratio that is calculated backwards against that for a living system to determine how long ago it died and hence how old the sample is.

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  • The main thing you missed is Carbon 14 dating. The fortunate thing is when a living creature or plant dies, the clock starts and the Carbon 14 decays. While it is alive the Carbon 14 is refreshed. Because it is an exponential decay, the further you go back in time, the less accurate the clock becomes.

    Ever since I was a teen, I wanted to write a computer program that would given amounts of various radio active materials and their half lives and their decay patterns, would calculate the amounts of various radioactive materials and the radiation for any time in the future.

    Then I would run it backwards to see what the radiation was like in various points of earth’s prehistory. At first I presumed they would have to be extremely high, (meaning life evolved under high radiation) but I was told the decay patterns mean that is not necessarily true.

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  • It works in reverse also. You can also tell things from when it doesn’t happen.

    It’s not quite the same thing, but there was an interesting example based on these techniques where it has been recently established that neurons in the human brain regenerate normally, just as do most other cells in the human body.

    There was never serious doubt about this but it had long been a persistent myth in the general community that neurons are special and that people only have a given number, which progressively die off causing people to become feeble-minded and mentally inadequate from about the age of about 30 onwards.

    It turns out the real problem causing feeble mindedness in feeble and elderly 31 year olds is probably rock music combined with vegetarian food. Though I’m not aware of any serious research in that area. But there was a very creative experiment recently performed to demonstrate that neurons must grow normally, and would regenerate like any other cell.

    The technique involved collecting and processing brains from various people born around the time of the baby boom (details of exact technique and model of kitchen blender not relevant here). Baby boomers were born more or less around the time that rock music and hippie culture became a global epidemic. Other confounding events occurring simultaneously were the massive thermonuclear weapons explosions all over the Earth’s atmosphere as the various cold warriors rattled sabres.

    There was a pulse of radioactive materials and unusual isotopes globally dispersed throughout the atmosphere around this time. (Late 1950’s to early 1960’s.) This pulse is measurable today in particular circumstances and any organism that grew significantly around this time, including human children, would inevitably incorporate anthese relatively unusual isotopes into their molecular structures. Purely as a result of growth from breathing, drinking, and eating food that came from the global nuclear weapons environment around 1960.

    Some years following the end of atmospheric nuclear testing the rate of unusual isotopes being introduced into the atmosphere diminished and began to dissipate. So this was a once-off event. You would expect that babies made in the early 1960s and which retained neurons for life would have grown brains containing molecular traces showing the presence of this pulse of atmospheric isotopes.

    The theory is that cells which remained unchanged, or materials made from leftover cellular structures which grew at that time but which died and therefore remained unchanged (owing to their cessation of living processes – which otherwise involve cellular turnover, recycling molecular materials, and incorporation of new molecules from the environment, including the atmosphere), would have produced a fixed, but unusually high, content of unusual isotopes. The currently measured level of the slightly radioactive isotopes being a remnant indicating what proportion of molecules were originally incorporated when the organism first grew.

    According to the researchers there is no detectable molecular signal in the recently deceased brains of the rock generation. Which means that these baby boomers’ neurons must have been turning over and regenerating normally throughout their lives, dissipating and replacing the neurons that would have originally formed in early life (in the more isotope rich atmosphere).

    So the implications are that some other explanation is required for the feeble mindedness of the older generation. Another implication is that just about every neuron in one’s original brain has likely to have died and been regrown from scratch. With all its signals and interconnections lost. Which means that if one can remember anything at all in the way of Hendrix licks then it must have been relearned more recently. I suspect that some of today’s most famous rock musician performers like Keith Richards might even be obtaining their material by listening to old Rolling Stones records.

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  • 6
    Timothy McNamara says:

    I have little solid knowledge in the field, but had to comment as this topic drew my memory to when comrade Dawkins was in Lynchburg Virginia around 6-7 years ago. There was a ‘college’ nearby, named, If I recall correctly, ‘Liberty College’.
    Such was their ignorance of the field of science you’re discussing, they had dinosaur bones on display labelled as 3,000 years old. One of Dawkins’ responses, to one particular misguided pupil of said college, was one of the funniest moments I’ve ever encountered, since I started following the great man.

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  • A creationist told me that the amount of C-14 has not reached equilibrium and so there is more C-14 now than a 1000 years ago and that it proves carbon dating wrong.

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  • 8
    Tlhedglin says:

    You have the basics down, but to fully comprehend radiometric dating, I would recommend studying radioactive decay. In particular, the difference between alpha(nucleus) and beta(electron) types of decay(there is also Gamma, but it is rarely used for dating). I think it will help you understand how we can date rocks with unstable elements by cross-referencing the various amounts of other daughter nuclides.For instance

    Parent Nuclide of 238U, chain decays as follows:

    decays, through alpha-emission, with a half-life of 4.5 billion years to thorium-234
    which decays, through beta-emission, with a half-life of 24 days to protactinium-234
    which decays, through beta-emission, with a half-life of 1.2 minutes to uranium-234
    which decays, through alpha-emission, with a half-life of 240 thousand years to thorium-230
    which decays, through alpha-emission, with a half-life of 77 thousand years to radium-226
    which decays, through alpha-emission, with a half-life of 1.6 thousand years to radon-222
    which decays, through alpha-emission, with a half-life of 3.8 days to polonium-218
    which decays, through alpha-emission, with a half-life of 3.1 minutes to lead-214
    which decays, through beta-emission, with a half-life of 27 minutes to bismuth-214
    which decays, through beta-emission, with a half-life of 20 minutes to polonium-214
    which decays, through alpha-emission, with a half-life of 160 microseconds to lead-210
    which decays, through beta-emission, with a half-life of 22 years to bismuth-210
    which decays, through beta-emission, with a half-life of 5 days to polonium-210
    which decays, through alpha-emission, with a half-life of 140 days to lead-206, which is a stable nuclide.

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