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Thread: How reliable is carbon dating?

  1. #1 How reliable is carbon dating? 
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    How close can we get. Lets say we have a rock we want to date. Can we get within say 100,000 years? And is carbon dating the most acurate? Thankyou. Filix.


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    Because the carbon isotopes all have half lives less than 10,000 years, there's usually not enough left to get dates much past 50,000 years. Accuracy is something like 25 years for recent events but grows to centuries by the time you get to that 50,000 year point.


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    Dating beyond 50,000 years requires other systems. Fortunately we have many. For example, to cover a time range of tens to hundreds of millions of years, we have the K40/Ar40 system. K40 (the 40 weight isotope of Potassium) decays over a long period of time to Argon 40. Argon is a gas.

    If rock becomes molten, like with an outpouring of lava, the Argon disappears into the air. The liquid rock cannot hold it. So when the lava becomes solid again, it effectively has no argon gas in it. The K40 though, will continue to decay and slowly build up Ar40 once more. By measuring the amount of K40 and Ar40, we can know how long it has been since the rock was molten.

    If you find a layer of rock in that time range, you look for some associated rock that once was melted, and use the K40/Ar40 test. If a layer of sandstone, for example, lies on top of lava, it will be younger than the lava. Date the lava and you know the maximum age of the sandstone.
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    Thankyou both. My father was a rock hound when I was a kid. That got me interested in rock and minerals. When I take a walk and see a large out croping of beautiful granite, I wish I knew how to date it. Filix.
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    Just a wee note: carbon dating is not a method one would use to date rocks and minerals. Carbon dating is typically used to date material that was once living. Examples might include things like cloth fragments or an ancient piece of furniture made from wood - both the sorts of things that an archaeologist might dig up and be interested in dating.
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  7. #6  
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    Igneous rocks, such as granite, can be dated through radioactive decay using longer half life elements.
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  8. #7  
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    C14 dating is based on the understanding that the Cosmic rays continually create C14 in the air by hitting Nitrogen atoms. So the C14 content of the air should be consistent over time, evenly distributed, and has the same chemical properties as normal C12.

    After a plant absorbs CO2 from the air and makes it part of its body, the percent of C14 in its body would initially be like that of the surrounding air, except the C14 is not being replenished by cosmic ray activity anymore. So as it decays, there will be less over time.

    Cosmic ray - Wikipedia, the free encyclopedia - C14 generation


    One thing that makes me question the process a little bit is the problem of recycled carbon. What if a plant gets eaten by an animal, that gets eaten by another animal.... etc? Or what about a tree that lives for a very long time, like several hundred years?

    Also, we can't be sure how stable the Cosmic rays have been over longer periods of time.

    Cosmic ray - Wikipedia, the free encyclopedia - Flux


    In the past, it was believed that the cosmic ray flux remained fairly constant over time. However, recent research suggests 1.5 to 2-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years
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    Any form of accurate dating requires an historically consistent half-life. This seems to be the case, as all dating methods match up with one another and stratigraphy around the world shows that certain animals are found in the same strata in places all around the world, with bacteria and invertebrates at the bottom and humans and other modern animals near the top...this fits into evolutionary theory perfectly and combined with other evidence of common descent of all animals on earth it is extremely logical.
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    This doesn't actually answer your question. I can never resist an opportunity to revisit one of my favourite youtube videos evah.

    Presenting, for your enjoyment ....... "There's no $*c%ing carbon in it!"

    Carbon dating doesn't work -- debunked - YouTube
    MeteorWayne, Strange and MrMojo1 like this.
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    Thanks for that, Adelady
    A very interesting video. I learned from it.
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  12. #11  
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    adelady, I don't usually click on links without a bit more explanation, but you've earned a pretty high reputation with me, so I gave it a shot. It was great, THANX!!
    MW
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    Quote Originally Posted by adelady View Post
    Presenting, for your enjoyment ....... "There's no $*c%ing carbon in it!"

    Carbon dating doesn't work -- debunked - YouTube
    I am usually pretty scathing about youtube but this is brilliant.
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    Love his stuff.
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    It nicely illustrates the fact that while scientific principles are often simple, their application is immensely complicated and understanding them in toto requires intellect, hard work and imagination.
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    Thanyou adelady for that link. Too funny! Could someone be so kind as too explain what a half life is? I read up on it and I'm still not getting it. Explain it like you would to a 8 year old please. Thats why I joined this forum to learn. Many thanks. Filix.
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    Quote Originally Posted by filix View Post
    Thanyou adelady for that link. Too funny! Could someone be so kind as too explain what a half life is? I read up on it and I'm still not getting it. Explain it like you would to a 8 year old please. Thats why I joined this forum to learn. Many thanks. Filix.
    The atoms in a radioactive material decay at random with a particular probability that depends only on the material. Each radioactive element decays at a different rate. It is like the atoms are continuously rolling a little dice and decide whether to decay or not. Some elements have a 6-sided dice and decay rapidly, some have a million-sided dice and decay very slowly. (Does that analogy make sense?)

    This means that in a lump of radioactive material, there is steady rate of decay which is determined by the number of atoms in the material (and the type of material) - the more atoms, the more chance there is that one is decaying at any moment. As the atoms decay their number decreases (they change into another element which is not radioactive or decays at a different rate). As the number decreases the average rate of decay slows.

    The half life is the time it takes for half of the atoms to have decayed (turned into something else) and therefore the level of radioactivity to have halved.

    If you have a million atoms, after one half-life period there will be 500,000 left. After another half-life period there will be 250,000 left. After another half-life period there will be 125,000 left.

    So by comparing the amount of radiation (or the amount of the element) currently present and comparing with the amount originally present, you can work out how many half-lives have passed; i.e. how old the sample is.

    Does that help?
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    "Reliability"

    Validating an analytical method involves determining the method's various characteristics usually referred to as: Accuracy, Precision, Specificity, Limit of Detection, Limit of Quantitation, Linearity, Range, Ruggedness, and Robustness.
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    She turns up on time but she's a bit dirty
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    I'm still trying to grasp this. When they say that carbon 14 has a half life of 5700 years, do they mean one carbon atom? Lets say I have some Olive leaves that I dug up. Now I want to do a carbon 14 test on them. I have to know how many atoms are in these leaves? How? Do I test local live olive leaves and compare the two. How do you know how many atoms are in the sample? There has to be many more than one million right? Please forgive my stupidity. The difference between the guy in the youtube video and me is I know I don't have a clue as to what I'm talking about. Filix
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  21. #20  
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    Quote Originally Posted by filix View Post
    I'm still trying to grasp this. When they say that carbon 14 has a half life of 5700 years, do they mean one carbon atom?
    The half-life only applies to a large number of atoms; it is a statistical thing. I guess the probability of a single atom decaying in 5,730 years is 50:50 but I may be wrong about that.

    Lets say I have some Olive leaves that I dug up. Now I want to do a carbon 14 test on them. I have to know how many atoms are in these leaves? How? Do I test local live olive leaves and compare the two. How do you know how many atoms are in the sample? There has to be many more than one million right? Please forgive my stupidity. The difference between the guy in the youtube video and me is I know I don't have a clue as to what I'm talking about. Filix
    I am no expert (so my explanation might be pitched at the right level - on the other hand it may not be completely accurate ) but ...

    Basically, the expected ratio of carbon isotopes in living matter is known (i.e. the proportion of C14 to all carbon); this should match the amount in the environment because the organism is constantly exchanging carbon through metabolism.

    When it dies, there is a snapshot of the ratio. Gradually, the C14 decays (half of it every 5,730 years). We come along, dig it up and measure what the ratio is now. This tells us how much of the C14 has decayed and hence how many half-lives have passed. Does that make sense?
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    Most carbon atoms have a total of 7 protons and 5 neutrons (12 in total). So it is called C12. However, if a suitable cosmic ray particle hits a nitrogen atom in the air, it turns that nitrogen atom into a carbon atom with 2 extra neutrons, called C14. This happens a lot, so that there is always a lot of C14 atoms in the air.

    The C14 atoms start to break up again, called decaying, by losing the extra neutrons, and once more become C12. It takes 5730 years for half the atoms to decay back to C12. So this time is called the half life.

    Then it takes another 5730 years for half the remaining C14 atoms (one quarter of the original lot) to decay to C12 atoms.

    So the half life is the time for half the unstable atoms to decay.
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    So then it takes another 5730 years for half of the ( one quarter of the original lot to decay? And on and on until there are no more C14 atoms? Do I have that right? If I do, so how does one determine how many atoms the sample has. Lets say we dig up a wooly mammoth bone that has not turned to stone yet. How does one go about useing the C14 test? I appreciate every ones help.
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  24. #23  
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    Yes, you seem to have cottoned onto the rate of further decay.

    The total number of C14 atoms in a sample is actually a tiny fraction of the total number of carbon atoms. Carrying out a carbon dating procedure is tricky, and requires experts, with special equipment, since it essentially requires a measurement of that tiny fraction. Knowing the ratio of C14 to C12 atoms permits a date to be essentially read off the tables. However, getting that number is the work of a genuine expert. Not me.
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    Quote Originally Posted by filix View Post
    So then it takes another 5730 years for half of the ( one quarter of the original lot to decay? And on and on until there are no more C14 atoms? Do I have that right? If I do, so how does one determine how many atoms the sample has. Lets say we dig up a wooly mammoth bone that has not turned to stone yet. How does one go about useing the C14 test? I appreciate every ones help.
    The technique is described here.
    Radiocarbon dating - Wikipedia, the free encyclopedia
    Basically, the sample is burned and the gases are collected including carbon dioxide, which is then converted to benzene. If you have a known amount of benzene, then you know how much carbon is in the sample. You then count the radiation coming off this sample. Assuming carbon-14 produces 14 disintegrations per minute per gram, you can then calculate the quantity of radioactive carbon-14 in your sample, and then determine the ratio of carbon-14 to carbon-12.
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  26. #25  
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    Quote Originally Posted by filix View Post
    I'm still trying to grasp this. When they say that carbon 14 has a half life of 5700 years, do they mean one carbon atom?
    It's the time over which one atom would have a 50% chance to decay. It's possible that the atom could go 570,000 years and still not decay, but probability of that happening is the same probability as flipping a coin 100 times and getting "heads" every time.

    If you have a lot of atoms together, the odds will almost always bear out. That is to say an approximately equal number of atoms will get "heads" as "tails", meaning that approximately half of them will decay.
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    Thankyou all very much. Filix.
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