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A radiocarbon dating question


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#1    Riaan

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Posted 15 July 2013 - 05:14 PM

I hope that one of you will be able to explain the following.  

As I understand, RC dating depends on the environmental conditions that existed during the life of a plant, and specifically the ratio of C14/C12 isotopes found in the plant. When the plant dies, the C14 starts decaying, while the C12 content remains unchanged. What I do not understand is why the calibrated RC dates diverge increasingly with age.

For example: a plant grew for a couple of days in a specific environment and died 1 000 years ago. The same plant had grown and died under identical conditions 10 000 years ago. The C14/C12 ratio of both plants at the time of death should therefore be identical. The only difference in the RC dates should then be 9 000 years as determined by the C14 rate of decay. No correction should be required (is this assumption wrong?). However, the difference between the calibrated dates is nearly 10 500 years.

I am sure it is simply due to my lack of understanding about how the process works.

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#2    Insanity

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Posted 15 July 2013 - 05:44 PM

The source of the C12 and C14 for plants is the atmospheric CO2 that they absorb via photosynthesis.  The ratio of C12 and C14 in the atmosphere has and does fluctuate with time, and thus two plant specimens of the same species in the same locale but separated by a thousand or more years can indeed have a different C12/C14 ratio at the time of death.  The C12/C14 ratio has been recorded naturally such as in tree rings and certain mineral deposits, so we can sort of adjust or calibrate findings base on this record.

"We see things only as we are constructed to see them, and can gain no idea of their absolute nature. With five feeble senses we pretend to comprehend the boundlessly complex cosmos, yet other beings with wider, stronger, or different range of senses might not only see very differently the things we see, but might see and study whole worlds of matter, energy, and life which lie close at hand yet can never be detected with the senses we have." - H.P. Lovecraft, "From Beyond" Published 1934

#3    questionmark

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Posted 15 July 2013 - 05:48 PM

View PostRiaan, on 15 July 2013 - 05:14 PM, said:

I hope that one of you will be able to explain the following.  

As I understand, RC dating depends on the environmental conditions that existed during the life of a plant, and specifically the ratio of C14/C12 isotopes found in the plant. When the plant dies, the C14 starts decaying, while the C12 content remains unchanged. What I do not understand is why the calibrated RC dates diverge increasingly with age.

For example: a plant grew for a couple of days in a specific environment and died 1 000 years ago. The same plant had grown and died under identical conditions 10 000 years ago. The C14/C12 ratio of both plants at the time of death should therefore be identical. The only difference in the RC dates should then be 9 000 years as determined by the C14 rate of decay. No correction should be required (is this assumption wrong?). However, the difference between the calibrated dates is nearly 10 500 years.

I am sure it is simply due to my lack of understanding about how the process works.

C14 is not a "natural" element, it is nitrogen that has been bombarded with cosmic rays, by which it acquired 2 extra neutrons and suddenly chemically works exactly as if it would be carbon. That is called a isotope (or "a element similar to"). These isotopes are not stable and will, sooner or later, loose those two neutrons again, and it is again nitrogen that reacts like nitrogen.

There are variation in the C14 concentration of the earth, that to start with, therefore it is necessary to use calibration equivalents, normally sourced from dateable strata such as layers of lakes.

As for the medium life, the isotopes decay continuously. It is not like suddenly half the isotopes disappear suddenly after 5,500 years. If you put a Geiger counter to anything radioactive every time it clicks a isotope has just decayed. It is just that statistically after 5500 years half of the C14 should be back to nitrogen. And, so far, within a percent or two  it happens every time.

So, basically you are right, within a small variation the same organism will accumulate the same amount of carbon under same conditions, just, the longer the time, less of it will still be in the remains.

Edited by questionmark, 15 July 2013 - 06:15 PM.

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#4    Riaan

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Posted 15 July 2013 - 08:01 PM

View PostInsanity, on 15 July 2013 - 05:44 PM, said:

The source of the C12 and C14 for plants is the atmospheric CO2 that they absorb via photosynthesis.  The ratio of C12 and C14 in the atmosphere has and does fluctuate with time, and thus two plant specimens of the same species in the same locale but separated by a thousand or more years can indeed have a different C12/C14 ratio at the time of death.  The C12/C14 ratio has been recorded naturally such as in tree rings and certain mineral deposits, so we can sort of adjust or calibrate findings base on this record.

My question is this: assuming that they had absolutely identical C12/C14 ratios at death (the environmental conditions just happened to be identical), but died 9 000 years apart, calibrated RC dating would place their deaths 10 500 years apart. Can't figure out what I am missing here.

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Thera and the Exodus, published February 2013, details here
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#5    questionmark

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Posted 15 July 2013 - 08:16 PM

View PostRiaan, on 15 July 2013 - 08:01 PM, said:

My question is this: assuming that they had absolutely identical C12/C14 ratios at death (the environmental conditions just happened to be identical), but died 9 000 years apart, calibrated RC dating would place their deaths 10 500 years apart. Can't figure out what I am missing here.

That is quite simple, if it is 9000 years apart the older sample will have only 1/4 (giver or take a little) of the C14 ratio the younger sample has, because the C14 of the older sample had a longer time to reconvert to nitrogen.

Once the organism is dead is accumulates no more carbon and therefore no more carbon 14.

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#6    Riaan

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Posted 15 July 2013 - 08:27 PM

View Postquestionmark, on 15 July 2013 - 08:16 PM, said:

That is quite simple, if it is 9000 years apart the older sample will have only 1/4 (giver or take a little) of the C14 ratio the younger sample has, because the C14 of the older sample had a longer time to reconvert to nitrogen.

Once the organism is dead is accumulates no more carbon and therefore no more carbon 14.

I am having trouble explaining myself, it seems. I assume that when an organism dies, its carbon composition is frozen. The C14 isotopes then start decaying and from this decay, which can be measured (this may be where I am going wrong), the age of organism can be calculated. Let's say the carbon reservoir and carbon absorption of a plant is constant and has never changed throughout history - will there be a need for calibration?  One would simply determine the C14 decay and calculate the age of the organism. In my hypothetical example I assume this to have been the case at those two moments in time. To me it looks like RC calibration makes no provision for this example.

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#7    questionmark

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Posted 15 July 2013 - 08:32 PM

View PostRiaan, on 15 July 2013 - 08:27 PM, said:

I am having trouble explaining myself, it seems. I assume that when an organism dies, its carbon composition is frozen. The C14 isotopes then start decaying and from this decay, which can be measured (this may be where I am going wrong), the age of organism can be calculated. Let's say the carbon reservoir and carbon absorption of a plant is constant and has never changed throughout history - will there be a need for calibration?  One would simply determine the C14 decay and calculate the age of the organism. In my hypothetical example I assume this to have been the case at those two moments in time. To me it looks like RC calibration makes no provision for this example.

But C14, as I tried to explain to you above (and a few others a dozen times) IS NOT CARBON, it is nitrogen that acquired two extra neutrons through cosmic radiation and therefore NOT STABLE, it will decompose sooner or later, whether the organism is alive or dead, capiche? If the organism is alive it will accumulate new C14 for the decomposed, while if it is dead it cannot accumulate any carbon anymore and therefore no C14 either.

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#8    Riaan

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Posted 15 July 2013 - 08:48 PM

View Postquestionmark, on 15 July 2013 - 08:32 PM, said:

But C14, as I tried to explain to you above (and a few others a dozen times) IS NOT CARBON, it is nitrogen that acquired two extra neutrons through cosmic radiation and therefore NOT STABLE, it will decompose sooner or later, whether the organism is alive or dead, capiche? If the organism is alive it will accumulate new C14 for the decomposed, while if it is dead it cannot accumulate any carbon anymore and therefore no C14 either.

My mistake - could someone please fix the Wikipedia article on radiocarbon dating? Should it rather be called nitrogen dating? Of course it is not stable. Given the same starting point, the rate of decay of the two samples will be constant along an exponential curve. This rate of decay will presumably no longer be affected by the external environment, as the plant no longer absorbs any carbon/nitrogen from its surroundings (is this true?). The raw RC date will then reflect the exact time difference of 9000 years, because of the fact that the two samples had the same starting date and that the only change in the C12/C14 ratio is due to the C14 decay. Calibration of this date would in my opinion be incorrect.

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Thera and the Exodus, published February 2013, details here
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#9    questionmark

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Posted 15 July 2013 - 09:00 PM

View PostRiaan, on 15 July 2013 - 08:48 PM, said:

My mistake - could someone please fix the Wikipedia article on radiocarbon dating? Should it rather be called nitrogen dating? Of course it is not stable. Given the same starting point, the rate of decay of the two samples will be constant along an exponential curve. This rate of decay will presumably no longer be affected by the external environment, as the plant no longer absorbs any carbon/nitrogen from its surroundings (is this true?). The raw RC date will then reflect the exact time difference of 9000 years, because of the fact that the two samples had the same starting date and that the only change in the C12/C14 ratio is due to the C14 decay. Calibration of this date would in my opinion be incorrect.

Basically yes, as soon as an animal/plant dies it no longer uses carbon as combustible and whatever it has in it at that point in time is what will be found in its remains.

Calibration is more complicated than the ratio, because radiation varies. In fact at this point in time we have about 100% more C14 on the planet than there ever was before due to atomic testing. That is why I mentioned lakes above. Just like tree rings undisturbed lakes layer sediment of mineral and organic origin year after year. So to determine what is normal samples are taking from these layers and tested. To this point we have samples that go as far back as 40,000 years and therefore a good idea what the natural radiation incidence was until then. Taken that in consideration the measurements are corrected giving a date within a few percent of error.

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#10    Insanity

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Posted 15 July 2013 - 09:00 PM

View Postquestionmark, on 15 July 2013 - 08:32 PM, said:

But C14, as I tried to explain to you above (and a few others a dozen times) IS NOT CARBON, it is nitrogen that acquired two extra neutrons through cosmic radiation and therefore NOT STABLE, it will decompose sooner or later, whether the organism is alive or dead, capiche? If the organism is alive it will accumulate new C14 for the decomposed, while if it is dead it cannot accumulate any carbon anymore and therefore no C14 either.

C14 is carbon.
You are correct that is it produced from nitrogen being struck by cosmic rays, and that it is not stable, but the nuclear equation is 147N + 10n --> 146C + 11p.
In short N14 gains a neutron, forming N15, then loses a proton becoming C14.

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#11    questionmark

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Posted 15 July 2013 - 09:09 PM

View PostInsanity, on 15 July 2013 - 09:00 PM, said:

C14 is carbon.
You are correct that is it produced from nitrogen being struck by cosmic rays, and that it is not stable, but the nuclear equation is 147N + 10n --> 146C + 11p.
In short N14 gains a neutron, forming N15, then loses a proton becoming C14.

If it was carbon it would stay carbon until radiated and acquiring neutrons to be something else, it does not, sooner or later it will loose those neutrons and is what it always was: nitrogen. It just assumed the role of carbon for some time (albeit between a second and a billion years). You can irradiate lead until it becomes a gold isotope, it still is not gold.

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#12    Riaan

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Posted 15 July 2013 - 09:25 PM

View Postquestionmark, on 15 July 2013 - 09:00 PM, said:

Basically yes, as soon as an animal/plant dies it no longer uses carbon as combustible and whatever it has in it at that point in time is what will be found in its remains. ... Calibration is more complicated than the ratio, because radiation varies.

This is exactly what bothers me - here you refer to the radiation that varies. I assume you mean the radiation (C14) absorbed by the organism when it was still alive. In my hypothetical example the radiation and everything else is exactly the same for the two samples. If you meant a varying rate of decay in the organism, that is also the same in my example. In other words, the level of decay can be calculated accurately from the exponential curve, which is by definition the 'raw' RC date. If you now apply the calibration curve, you get a totally incorrect answer.

On calibration - the need for calibration arose because of the variation in the level of C14 in the atmosphere. I am reasonably familiar with the processes followed for the creating the calibration curves (as you refer to). However, the calibration curves do not seem to make provision for my hypothetical case, which could actually occur. I really don't know what to think. Do you agree that if you have the same starting point (C12/C14 ratio, etc), calibration cannot be done? If not, why not?

PS: Nearly midnight in South Africa, will check again tomorrow.

Edited by Riaan, 15 July 2013 - 09:26 PM.

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Posted 15 July 2013 - 09:30 PM

View PostRiaan, on 15 July 2013 - 09:25 PM, said:

This is exactly what bothers me - here you refer to the radiation that varies. I assume you mean the radiation (C14) absorbed by the organism when it was still alive. In my hypothetical example the radiation and everything else is exactly the same for the two samples. If you meant a varying rate of decay in the organism, that is also the same in my example. In other words, the level of decay can be calculated accurately from the exponential curve, which is by definition the 'raw' RC date. If you now apply the calibration curve, you get a totally incorrect answer.

On calibration - the need for calibration arose because of the variation in the level of C14 in the atmosphere. I am reasonably familiar with the processes followed for the creating the calibration curves (as you refer to). However, the calibration curves do not seem to make provision for my hypothetical case, which could actually occur. I really don't know what to think. Do you agree that if you have the same starting point (C12/C14 ratio, etc), calibration cannot be done? If not, why not?

PS: Nearly midnight in South Africa, will check again tomorrow.

yes, and one died 9000 years before the other (or almost twice the half life), therefore there should be about 1/4 of the C14 as found in the younger sample. And that is how you measure the age of the sample.

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#14    Swede

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Posted 16 July 2013 - 12:47 AM

View Postquestionmark, on 15 July 2013 - 08:32 PM, said:

But C14, as I tried to explain to you above (and a few others a dozen times) IS NOT CARBON, it is nitrogen that acquired two extra neutrons through cosmic radiation and therefore NOT STABLE, it will decompose sooner or later, whether the organism is alive or dead, capiche? If the organism is alive it will accumulate new C14 for the decomposed, while if it is dead it cannot accumulate any carbon anymore and therefore no C14 either.

QM - With all due respect, and as Insanity pointed out, 14C (and 13C) are both isotopes of 12C. From a metabolic perspective, lifeforms utilize 14C in the same manner(s) as 12C, hence their incorporation. Due to the metabolic incorporation aspect, induced 14C is utilized as a marker in current medical research.

While the isotope 14C does decay to 14N, prior to decay it is considered to be a carbon.

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#15    Swede

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Posted 16 July 2013 - 01:14 AM

View PostRiaan, on 15 July 2013 - 09:25 PM, said:

This is exactly what bothers me - here you refer to the radiation that varies. I assume you mean the radiation (C14) absorbed by the organism when it was still alive. In my hypothetical example the radiation and everything else is exactly the same for the two samples. If you meant a varying rate of decay in the organism, that is also the same in my example. In other words, the level of decay can be calculated accurately from the exponential curve, which is by definition the 'raw' RC date. If you now apply the calibration curve, you get a totally incorrect answer.

On calibration - the need for calibration arose because of the variation in the level of C14 in the atmosphere. I am reasonably familiar with the processes followed for the creating the calibration curves (as you refer to). However, the calibration curves do not seem to make provision for my hypothetical case, which could actually occur. I really don't know what to think. Do you agree that if you have the same starting point (C12/C14 ratio, etc), calibration cannot be done? If not, why not?

PS: Nearly midnight in South Africa, will check again tomorrow.

Actually, they do. The calibration curves are based upon proxies that reflect the variations in atmospheric 14C. If you choose to run the IntCal or OxCal programs on a series of dates, you will find that the spread of the intercepts can vary depending upon the atmospheric 14C of a given period. However, you will generally find that as your raw data begins to exceed circa 3-4 kya, the disparity between the raw data and the calibrated date will increase. In the case of disparities, the calibrated dates will generally be earlier than the raw data.

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