Yeah it will generally remain very same, but there are factors that will alter it to a certain degree. For example; if it rains more than usual for that time of year one particular month, it will show in the carbon levels, because water influences vegetative growth. Then we can assume that this will affect the diet of certain animals, such as cows and sheep, which tallow comes from.
I've read dissertaions detailing how, a certain animal went out of it's domain and travelled 15 miles, stayed there for a week, then returned to where it came from. Working from the animals fat, skin, hair, etc. So it can get pretty accurate!

I'm really interested in this as I just assumed that C14 was "may as well say" static. 🙂

I think that covers most archaeology 😉

I'm really interested in this as I just assumed that C14 was "may as well say" static. 🙂

A fairly safe assumption. As far as I'm aware the radioactive form of carbon, C14, is produced through the bombardment of our atmosphere by very energetic particles from outer space, called cosmic rays. (apologies if this is granny and eggs time 🙂 ).It decays with a half-life of 5600 years. But the constant flow of cosmic rays into our atmosphere insures that it gets replaced as fast as it decays so there is a constant amount of this radioactive carbon in our atmosphere at all times. As many are produced per second as decay per second. Once removed from the atmosphere it of course continues to decay..................

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I'm really interested in this as I just assumed that C14 was "may as well say" static. 🙂

A fairly safe assumption. As far as I'm aware the radioactive form of carbon, C14, is produced through the bombardment of our atmosphere by very energetic particles from outer space, called cosmic rays. (apologies if this is granny and eggs time 🙂 ).It decays with a half-life of 5600 years. But the constant flow of cosmic rays into our atmosphere insures that it gets replaced as fast as it decays so there is a constant amount of this radioactive carbon in our atmosphere at all times. As many are produced per second as decay per second. Once removed from the atmosphere it of course continues to decay..................

I'm afraid it's not a safe assumption as far as C14 dating goes - the rate of production of C14 in the atmosphere fluctuates dependent on solar flare activity and there's been 40+ years of work to factor this into dating. If you take a very broadbrush view then you can say that over time, the amount of C14 in the atmosphere remains the same but this was the mistaken assumption Libby made when he first developed the technique in the 1940s, but by the 1960s it was realised that there are fluctuations in the atmospheric C14 content which affected the apparent C14 date from a sample. As a result, much work was done to develop the calibration curves for converting an absolute C14 date (i.e. the one calculated directly from the sample) into a calendar date.

Another important thing to remember about such dates is that there's statistics involved. The basic premise is that for a sample of mass x you observe over period y z number of radioactive decays from the C14 and from that reckon what percentage of your sample is C14 now and based on the half-life, work out how much has been lost and hence the date. However, radioactive decay is a random process and so there will be fluctuations in the rate of decay which must be factored in using statistical mathematics to cover as much as possible for an anomalous peak or trough in the rate of decay during the observation period. So, when you see a C14 date quoted with its +/- range of dates, that's generally the 95% confidence interval. In other words, you can be 95% certain that the actual date of the sample lies within that range. However, there's still the 5% possibility that it's outside this, maybe quite massively so. I suspect that the "wrong" C14 dates which creationists so proudly wave about as "proof" that radiocarbon dating doesn't work beacuse the date it gave is demonstrably wrong actually show that a particular sample test happened to pick up an anomalous period of radioactive decay and thus fall into the 5% outside the confidence range....

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This is getting too complicated for me to handle :confused:

Noooo... I cant wait to see the response... ::)

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Noooo... I cant wait to see the response... ::)

Explain?


In relation to the topic though, what AR says seems to make sense.

Thank you everyone - the replies both here and on UKCaving have been useful and encouraging. Now I just need the time and help to get on with a plan!

AR
Thanks for your concise posting. I admit I was looking at it more from a sun-climate change point of view rather than carbon dating although the basic points still applie. A little more detail to clarify what I mean and a wee question at the end.

Radioactive isotopes record solar magnetic activity and related climate change for thousands of years. During periods of increased activity on the Sun, when the Earth was presumably warmer, the magnetic fields in the solar wind had a larger shielding effect on cosmic rays. This prevented the energetic charged particles from entering the Earth's atmosphere and producing radioactive isotopes. In contrast, high amounts of the radioactive elements were produced when the Sun was inactive and the climate was cold.
Carbon-14, dubbed radiocarbon and designated C14, is the first radioactive isotope to be used to reconstruct past solar activity. Radiocarbon is produced in the Earth's atmosphere by a nuclear reaction in which energetic neutrons interact with nitrogen, the most-abundant substance in our air. The neutrons are themselves the products of interactions between cosmic rays and the nuclei of air molecules.
Radiocarbon can be found in annual tree rings dating back to eight thousand years ago. Each radiocarbon atom, C14, joins with an oxygen molecule, O2, in the air to produce a form of carbon dioxide, designated by 14C02 , that is assimilated by live trees during photosynthesis, and deposited in their outer rings. The time of assimilation can be determined at any later date from the age of the annual tree ring. Just count the number of tree rings that have been subsequently formed at the rate of one ring per year.
The radiocarbon records confirm that the Maunder Minimum corresponded to a dramatic reduction in solar activity, and show that such prolonged periods of inactivity are a fairly common aspect of the Sun's behavior.

This time scale can be extended. Cores taken from the polar ice caps extend the tree-ring evidence for sun –climate connections. The ice contains the radioactive isotope of beryllium, Be10, that has been deposited there by snows. Like radiocarbon Be10 is produced reactions between energetic neutrons and the molecules of the air, a consequence of cosmic rays entering the atmosphere.

The point of all this is that it can be determined when the sun was active or inactive which then, presumably, affect carbon dating calculations. This is where I have a question. I understand that the rate of decay is vital but I'm not totally clear in my mind why the amount of C14 in the atmosphere has a bearing on this. In other words why do fluctuations in the atmospheric C14 content affect the apparent C14 date from a sample? What's the correlation between amount and rate of decay? I'm assuming from your post that you can't calculate the loss by decay without knowing the starting amount, thus the calibration curves.

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The point of all this is that it can be determined when the sun was active or inactive which then, presumably, affect carbon dating calculations. This is where I have a question. I understand that the rate of decay is vital but I'm not totally clear in my mind why the amount of C14 in the atmosphere has a bearing on this. In other words why do fluctuations in the atmospheric C14 content affect the apparent C14 date from a sample? What's the correlation between amount and rate of decay? I'm assuming from your post that you can't calculate the loss by decay without knowing the starting amount, thus the calibration curves.

The fluctuations in the atmospheric C12/C14 ratio affect the starting amount of C14 in a sample, the base principle being that living things take up atmospheric carbon either directly (through photosynthesis) or indirectly (by eating plants or things that have eaten plants) but on death, that uptake stops and the "clock" starts. The correlation between age and rate of decay works by reckoning the difference between the observed rate of decay of your sample and the rate of decay you would expect from a sample of fresh organic carbon of similar mass. This will give you an approximate date but as I've previously mentioned, radioactive decay is a random process which has peaks and troughs hence the need for statistical analysis to minimise the chance you happened to measure during a period of unusually high or low decay. Obviously, if there are variations in the initial C12-C14 ratio these also need to be taken into account as the amount of radioactivity you're measuring is pretty small and so it can have a significant effect on the calculated dates.

The calibration curves you use to factor this in have been calculated so that you can take a date from an uncalibrated C14 result and convert it to a calendar date, they were originally built up using dendrochronology samples (get your carbon sample from rings fom the relevant year and see what the C14 date comes out to be, Californian Bristlecone pines were a great help in this!) but a much wider variety of data has now been used and at the heart of calibration is the idea that ultimately, you need to allow for the variations in the C12-C14 ratio.

There are still a few problems, the middle Iron Age is notoriously difficult to calibrate dates for and as Vanoord mentioned, recent stuff is difficult to do - the rate of decay is so similar to a fresh sample of organic carbon that it's hard to give a realistic date, plus the range variation may include the future!

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Radioactive decay sure is a random process, which is why it's poor form to use any data arising from a very small sample with a long half life.

However, if we are looking at C ratios via mass spec, that inaccuracy disappears (to a fair degree).

The above posts will take a while to digest. :thumbup:

Edit:- Carnkie, regarding solar variation, do you know the order of size of the errors which are created?

In answer to your question Stuey, sorry, but I'm afraid I don't.

Just been reading about radiocarbon dating, phasing, and chronology done at the excavations at Trethurgy Round.
The six radiocarbon age determinations obtained on samples from Trethurgy were processed by the Queen's University, Belfast, Radiocarbon Research Laboratory.

For analysis and interpretation of the chronological data they used the OxCal version 3.5. (web site given). I see the current version is 4.1. http://c14.arch.ox.ac.uk/embed.php?File=oxcal.html 

As AR suggests this sure is complicated. 🙂

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Talking about dendrochronology, I was given a piece of timber which was obtained from right by the Chernobyl power station. I'll dig it out and whack a photo up, it's pretty incredible. Anyway, that might skew the data a tad! (of you were using a counter)

Regarding C14, I would have thought it would be possible to factor in solar variation from a number of angles.

Surely, it can't be THAT complicated?!?!?!

It certainly would skew the data but it doesn't appear to show up on C14 atmosphere readings at Schauinsland, Germany. Of course taking a single station is extremely dangerous as you well know. http://cdiac.esd.ornl.gov/trends/co2/graphics/cent-scgr.gif 

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