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Why is carbon dating not affected by atmospheric CO2 levels?

  1. Jul 13, 2018 #1
    When I read about carbon sinks and the production of CO2, I am puzzled by the trust we have in carbon dating. If I understand correctly, C14 is produced by cosmic rays and nitrogen and is considered a very steady process thus carbon dating is trusted. But if C14 is brought into biological systems as a percentage of C14 in the atmosphere, why does the C14 percentage not change with atmospheric CO2 levels and thus give conflicting dating information. If CO2 levels have doubled in the last several hundred years, is the C14 level not half of what it would have been? Do I misunderstand how C14 is incorporated into biological systems? Is there some mechanism in the biosphere that keeps a steady ratio of C12 to C14? Once the C14 is captured, the whole thing makes sense but I do not see how the capture can be steady over time.
     
  2. jcsd
  3. Jul 13, 2018 #2

    jim mcnamara

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    Staff: Mentor

  4. Jul 13, 2018 #3
    That article seems to support the problem. It measured production rates as lower than previously thought but does not pose that the ratio of C14 to C12 remains constant or that production rates have changed. It states that C14 production from nuclear testing masked natural production. If nuclear testing sources masked natural production, would not high CO2 production rates also mask the natural production? It does not connect the high rates of CO2 generation from human activity to changes in ratio. Does it not support the possibility that C14 dating must take into account the historic CO2 levels in the atmosphere to be reliable. This may be done by the professionals but I am unaware if that is the case.
     
  5. Jul 13, 2018 #4
    the carbon dating actually takes into account the variation in atmospheric carbon 12 and carbon14

    just see the article in wiki-

    The method uses measurement of the amount of 14C in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died.


    The half life of C14 is about 5700 years and this method can be reliable up to 50 thousand years.


    Investigations has been going on to determine what the proportion of 14C in the atmosphere has been over the past fifty thousand years.


    A calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.


    Other corrections which are made to account for the proportion of 14 C in different types of organisms, and the varying levels of 14C throughout the bio system on earth (reservoir effects).


    Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s.


    Fossil fuels contain almost no 14C, and as a result there was a noticeable drop in the proportion of 14C in the atmosphere beginning in the late 19th century.


    On the other hand nuclear testing increased the amount of 14C in the atmosphere, which attained a maximum in about 1965 of almost twice what it had been before the testing began.


    In the early years of using the technique, it was understood that it depended on the atmospheric 14C/12C ratio having remained the same over the preceding few thousand years.


    To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects.


    Over time, however, discrepancies began to appear between the known chronology for the oldest Egyptian dynasties and the radiocarbon dates of Egyptian artefacts. Neither the pre-existing Egyptian chronology nor the new radiocarbon dating method could be assumed to be accurate, but a third possibility was that the 14C/12C ratio had changed over time.


    The question was resolved by the study of tree rings:[37][38][39] comparison of overlapping series of tree rings allowed the construction of a continuous sequence of tree-ring data that spanned 8,000 years.[37] (Since that time the tree-ring data series has been extended to 13,900 years.)[29] In the 1960s, Hans Suess was able to use the tree-ring sequence to show that the dates derived from radiocarbon were consistent with the dates assigned by Egyptologists. This was possible because although annual plants, such as corn, have a 14C/12C ratio that reflects the atmospheric ratio at the time they were growing, trees only add material to their outermost tree ring in any given year, while the inner tree rings don't get their 14C replenished and instead start losing 14C through decay. Hence each ring preserves a record of the atmospheric 14C/12C ratio of the year it grew in. Carbon-dating the wood from the tree rings themselves provides the check needed on the atmospheric 14C/12C ratio: with a sample of known date, and a measurement of the value of N (the number of atoms of 14C remaining in the sample), the carbon-dating equation allows the calculation of N0 – the number of atoms of 14C in the sample at the time the tree ring was formed – and hence the 14C/12C ratio in the atmosphere at that time.[37][39] Equipped with the results of carbon-dating the tree rings, it became possible to construct calibration curves designed to correct the errors caused by the variation over time in the ratio.


    Reference- https://en.wikipedia.org/wiki/Radiocarbon_dating#Atmospheric_variation
     
  6. Jul 13, 2018 #5
    Thanks. I should have looked there first.
     
  7. Sep 5, 2018 #6
    I also am puzzled by the assumptions made in setting the "constants" for carbon dating. Might there not be also other factors that change over time and location? As the weather can not be foretold accurately, so the atmosphere is not the same in any given place at any given time. Even the amount and effect of cosmic rays in the atmosphere is subject to influencing factors, maybe even some cosmological factors we have not experienced in the 8000 years for which we have been able to get somewhat reliable data with the tree rings. Why are we so sure this has been the same for tens of thousands of years? And say, what if there was a time when only half the amount of cosmic rays reached the atmosphere as reach it today?
     
    Last edited: Sep 5, 2018
  8. Sep 5, 2018 #7
    After only 100 years of observation, how can we be so sure that radioactive decay in minerals and carbon 14 has always been the same for thousands or even millions of years? Sure it's pretty clear that in the world as we know it, it is always the same and does not seem to be influenced by any factor known to us. But isn't science constantly proving that there is a lot we don't know yet?
     
  9. Sep 5, 2018 #8
    Please don't get me wrong, I love science and the improvements it brings. I just also love keeping an open mind to new directions and observations, and exploring possibilities where maybe what we strongly believe are constants could be in reality variables. Like after Copernicus, suddenly the earth became a "variable" instead of a "constant"
     
  10. Sep 21, 2018 #9
    To answer befree, the tree ring data provides a calibration curve that is independent of the amount of C12 and C14 added to the atmosphere in any year. All we care about is the ratio, and that is "as measured" rather than based on calculations. Technically, the decay rate can change, or more to the point be changed in the lab. Doesn't matter. Same for variations from cosmic ray intensities, and from CO2 production rates changed by climate. In effect, the C14/C12 ratio is measured and matched to the year where the starting ratio has decayed to match the final ratio. If there were some weird change in decay rates, tree rings and other plant matter from the same year would still have the same measured ratios. It would be nice if all those factors were well enough behaved so that each year had a unique ratio, but in practice, if two years match, they are almost certainly within a decade of each other.
     
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