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C14 Carbon Dating, not precice ? Please help!

  1. Sep 20, 2006 #1
    Hello, this is my first post on yours forum, I thing it’s great. I’m reading it for some time.

    But I’ve registered because of problem. I’m an archeologist. Within archaeology C14 Radiocarbon dating, a radiometric dating method is considered an absolute dating technique. But in discussion with one physicist, in very tense discussion he tried to persuade me that C14 technique can’t be taken as relevant, but I can’t understand it because of my low math knowledge, to understand it he suggested me an advanced course of partial differential equations (it is true that I don’t know partial d.). And he sad to me that any mathematician or physicist knows that C14 can’t be absolute, because of “some fundamental” problems, and he is not giving me partial differential lectures for free. So I’m asking you, because I’ve seen here many smart and good people physicist and mathematicians, to help me to understand this correctly. I can’t find peace since that discussion.

    Thank you very much!
    Marko
     
  2. jcsd
  3. Sep 20, 2006 #2

    ZapperZ

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    Unfortunately, unless you can get your "physicist friend" to come online and clearly outline the issue, there's not a whole lot that can be done. Simply pointing to the PDE itself is not helpful, because one needs to know what exactly goes into the PDE in terms of the parameters or any assumption made. If we are simply using the standard exponential decay, then there aren't any issues in the first place. So obviously your physicist friend has something else in mind and it isn't the mathematics here that is the issue.

    It would help if you or someone can come up with a reference to any papers that deal with the "problem" regarding C14 dating. If not, then I don't see anything strange that would nulify C14 dating simply from the PDE itself.

    Zz.
     
  4. Sep 20, 2006 #3

    Gokul43201

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    There are some issues with knowing the half-life accurately. To my knowledge, this number has been revised a few times (by at least a couple of percent) in the last couple of decades or so. There are also some issues with assumptions of initial conditions. I'm pretty sure some of the regulars at the Earth Science subforum are familiar with the problems involved.
     
  5. Sep 20, 2006 #4

    russ_watters

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    That is a strange thing to say. Archeology is a science and sciences don't deal in absolutes. Radiocarbon dating, like all scientific measurements, is subject to uncertainties and limitations. Specifically, range and precision - there is a limit to how old of an object it can accurately date (about 60,000 years) and to what precision (not sure about that).

    The acheology community is aware of these issues, as they are absolutely critical to accurate dating of objects and understanding how relevant the results are. HEREis an article I found on an archeology website about it, but I'm certain you can find actual professional journal articles about it if you look.
     
  6. Sep 20, 2006 #5
    Thank You russ_watters, Gokul43201 and ZapperZ for replying ! That physicist isn’t really my friend, if he was we would debate it normally without his “metaphysic” arguments… I’m much better now because I thought that there could be large obvious fundamental mistake in this technique and I’m not aware of it, and all mathematicians and physicist are. Now when I see that It doesn’t ring a bell to you, it’s ok. All problems with C14 that you presented, I’m aware of, and those are classical problem s with this dating method. Technical, engineering, physical details of dating techniques aren’t my field of interest, but of course I have knowledge about them.


    Russ_watters of course absolute is not meant in common way, it's just.... well let wikipedia explain it, with better English than mine:

    “Absolute dating is the process of determining a specific archaeological date. Such dating is opposed to relative dating based on stratigraphy, which is the realization that deposits closer to the surface of the ground are generally younger than more deeply buried deposits. Some archaeologists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies a certainty and precision that is rarely possible in archaeology. Absolute dating techniques usually involve the physical or chemical properties of the raw materials used to create artifacts, buildings, or other items that have been modified by humans. Absolute dates do not necessarily tell us when a particular cultural event happened, but when taken as part of the overall archaeological record they are invaluable in constructing a more specific sequence of events”
     
  7. Sep 20, 2006 #6

    Danger

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    I was under the impression that C14 and K(?) decays are both measured when possible, and some sort of average worked out. Are there other substances as well that can add to the data pool?
     
  8. Sep 20, 2006 #7

    russ_watters

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    Well that just makes it sound like he isn't really a physicist.
    Oops, my mistake. I didn't realize they used the word that way. That's the problem with English - a word can have more than one definition and sometimes they are very different. You meant 'independent/not relative' (definition 6) - I thought you meant 'infallible' (definition 1). http://dictionary.reference.com/search?q=absolute
    Yes, I certainly tripped over that one.
     
  9. Sep 20, 2006 #8

    Mk

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    Radiocarbon dating before the 1980s required large samples of carbon (several grams), and much more than found in seeds or bones they were dating. They resorted to finding carbon that they associated with the material in question. You can see the problem, because they depended on the materials of all being from the same time.

    The second problem is that the carbon ration fluctuates over time. Uh oh, so they have to calibrate with the time period, using dendrochronology. Most often, if dates are uncalibrated, they won't say. If they do, probably in the form of saying bc, instead of BC, to signify uncalibrated dates.

    Bias is also discussed in this thread: https://www.physicsforums.com/showthread.php?t=129123
     
  10. Sep 20, 2006 #9

    Astronuc

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    Nice little overview of C-14 dating
    http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/cardat.html

    The half-life of an element is constant, and any fluctuation may be due to the experimental method.

    That is another and different problem, and I was going to get around to that in the discussion with Andre where he cites a different half-life.

    Plants absorb CO2 from the atmosphere and animals in turn consume plants. Carbon in plants is in the form of cellulose, sugars, nucleotides, etc, and in animals in proteins, sugars, fats, . . . . etc. When the plant or animal dies, the exchange of the forms of carbon from living processes stops.

    Now in nature, carbon is largely C-12 (98.89%), C-13 (~1.11%), both of which are stable, and traces of C-14, which has a half-life of ~5730 years. During each half-life period, half of a radionuclide decays (transforms) into a different element, which in the case of the decay of C-14 is N-14. C-14 is actually produced by the collision of high energy neutron with N-14 nuclei (n,p reaction) high in the atmosphere, and the resulting C-14 diffuses into the atmosphere combining with oxygen to form CO2.

    http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm

    Let's assume that the half-life of C-14 is well-known and constant, and let us assume that one can measure the C-14/C-12(C-13) ratio with reasonable accuracy. Well, there are two things that can affect the ratio in the earth's biosphere (atmosphere and oceans). One is the production rate of C-14 by cosmic radiation. If the reactions which produce the high energy neutrons, which produce the C-14, increase, then the ratio of C-14 to C-12 will increase. Conversely if the production of high energy neutrons decreases, then C-14 production will decrease. The other factor is the release of CO2 from volcanos or the exchange of carbon from carbonates or carbonic acid in the ground. If the volcano's CO2 or carbonates have been around for 10's millenia, or millions of years, they will have very little C-14 since it will have decayed awayed. After ~57300 yrs (10 half-lives), the C-14 will have been reduced by a factor of 210 or ~1000.

    So one has to be careful regarding the environmental conditions in which organic fossils are found, as well as the environment in which the organism (plant or animal) existed, and how the environmental conditions might affect the C-14/C-12 ratio.

    The C-14 half-life is experimentally determined quite independently of radiocarbon dating technique and those who use it.
     
  11. Sep 20, 2006 #10

    Astronuc

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    40K has a half-live of 1.248 billion years. It decays into Ar-40 which is trapped in the rocks.

    Here is an overview of Radioactive Dating, which mentions C-14 and K-40 methods.

    http://www.physics.vanderbilt.edu/astrocourses/AST101/radioactivity.html
     
  12. Sep 20, 2006 #11

    ken

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    Hi

    Well from what I've read I think C14 dateing has some bigger problems in it's base assumptions.

    Assumption 1 : The level of C14 in the environment has been stable for a very long time. That is to say that the rate of production form cosmic radiation in the upper atmosphere is the same as the decay rate in the environment and has been so for a very long time. That mean that you assume that no matter how old a sample is, that at the time the organism died it had the same C12/C14 ratio as exists in the general environment today. On that basis, you can get very old dates where little C14 exists in the sample and this gives results consistant with the 1st assumption. However I read there is evidence that the C14 ratio is still increasing. So if we pick a date, any date 100m, 100k, 10k or even just 100 years etc as our datum for zero environmental C14 and extrapolate a steady increase from then, all the dates compress and the results afirm the new starting assumption however rideculous it may be.

    So the problem is that there is no real check on the starting assumption and evidence is that the standard assumption above is probably incorrect.


    Assumption 2 : That all orgaisms absorb the general environmental C14/C12 ratio until death. I have read that some living organisms having been tested show this assumption is not correct either.

    However this is not my field of expertise or study, just some things I have come across over the years though I should add that my memory of it is that it was written by a physicist some decades ago. So I'd be interested to know if there is more recent work on validating these assumptions.


    Ken
     
  13. Sep 20, 2006 #12

    Danger

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    Thanks, Astro. This isn't an area that usually interests me... until there's a discussion about it. :rolleyes:
    My knowledge, therefore, is restricted. That's a nice link.
     
  14. Sep 21, 2006 #13

    Chronos

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    The only significant variable in carbon dating is the sample purity. The method is otherwise extremely precise. The soundness of the science behind carbon dating has not been seriously challenged for, I would guess, over 50 years.
     
  15. Sep 21, 2006 #14

    ken

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    Yes I understand that.

    I realise you may suspect I have another agenda so I'd like to explain I sometimes like to play devils advocate and push such points to see if people really can give an adequate answer to things like this.

    Just saying it has not been seriously challenged in 50 years is not an explanation of the reasons it's considered so precise.

    I just wanted to know if there has been more recent work on validating or modifying the base assumptions than at the time the information was written. It seems to me some serious questions were being raised and I'm interested to know if they were ever resolved. You know sometimes things just get accepted in the absence of anything more definitive.

    How can you independantly verify the precision of the method?

    Of course logically half life is very accurate. But if environmental C14 is increasing, then samples with low levels would have had low levels to start with and the results would be less dependant on half life and more dependant upon the rate of increase of C14 in the environment over that time and the older the sample, the exponentially greater the error. The faster that rate of increasing environmental C14 the more true this is. Therefore you can't use the internal logic of the method for verification. You need an independant check but that is also tricky because the error if one exists would be exponential with time so unlikely to show very significantly against recorded history.

    The verification methods I've heard of, take confirmation from history then the extrapolate many orders of magnitued ouside the confirmation sample range. In statistical analasys this is just well a really unprofessional thing to do. Just 1 standard deviation beyond the sample range generally shows significant errors.

    The method may yield good relative results but in terms of absolute time frame, I have to suspect we have no real idea of its precision. Don't you think these are important issues?

    I do and I'd really like to know if it's resolved and how.


    Ken
     
  16. Sep 21, 2006 #15

    Chronos

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    Last edited: Sep 21, 2006
  17. Sep 22, 2006 #16
    There are a variety of element isotope decays used in archeaology, C14 being the most commonly thrown around. The half lives of these decays vary in time period and you should look them up to find the exact numbers, but I will tell you that the accuracy of their datings have a sweet spot at around thier half lives. For example, if C14's hl = 5700 years
    your dating accuracy will be greatest around 5300-6000 years. I'm surpised you asked this question frankly, I thought you'd have a text or database access to this type of information for you archeaology studies.
     
  18. Sep 26, 2006 #17

    Chronos

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    No harm in asking well intended questions, Chaos, which I believe this one is. I think an informative reply, without judgement, is most appropriate in such cases. I have no disagreement with your comment, just the tone. User friendly is the operative term here.
     
  19. Sep 26, 2006 #18
    The actual HL of 14C is probably more like 6030 years. If the link that Mk had provided was clicked and that thread was studied then it would been clear that this discussion has been done before, actually with some more depth.

    Furthermore, it's not the accuracy of determining the Δ14C and from that the "age" but the preciseness. The actual Δ14C of an organism at death is not only depending on the greatly variable atmospheric Δ14C in CO2 but also the fractination processes in the following carbon Cycle. For instance there are two photosynthese processes, we have C3 and C4 plants. C3 causes about the double depletion in 13C and quadrupple that of 14C compared to the C4 plants, mostly warm grasses. So depending on the diet of the animal, the initial Δ14C can differ significantly. The actual δ13C can help to assess that problem but it introduces another (small) error.

    On the balance, yes correct carbon dating is more a complicated art but all the abbarations appear to be under control.

    Not under control are the incorrect conclusions from early carbon datings without the proper corrections. And that could be far fetching.
     
    Last edited: Sep 26, 2006
  20. Sep 27, 2006 #19
    Agreed. My last sentence wasn't too nice. Its just odd to me to see a professional ask a question so implicate to his profession. We don't see many doctors here asking how to use a stephoscope.
     
  21. Sep 27, 2006 #20

    Has your value of 6030 years been adapted as the official number? All my books say 5700 years, but they are at least 3 years old.

    Also, do you have a link to a page comparing C3 and C4 plants, I'm curious as to how one plant depletes more than the other. Thanks.
     
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