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What you are suggesting is it depends on which part of a felled tree is used, that the sample is rendered based on where in the rings it's taken from, that the earliest rings (of course are older) will produce an older date, that 'if' it was a 1000 year old tree when it died it will produce a range of c-14 dates that correspond to the trees age depending on which part of the tree is sampled, if this were true then c-14 dating would not be worth measuring at all, it would be a waste of time and money because we can't know which part of a 1000 year old tree the wood belonged to. I'm sorry, unless you can provide a scientific source which makes the same claim as your making I will not agree with your claim.

You have mentioned previously that you understood that a radiocarbon date must be calibrated against a radiocarbon calibration curve. The very fact that a radiocarbon calibration curve exists is proof that it is only the outer growth ring of a tree interacts with the carbon reservoir in that given year. To construct a calibration curve, the radiocarbon concentration is measured for older and older tree rings of precisely known age, i.e. trees that have been dendrochronologically dated. For most of the curve this is done in 10 year blocks of rings, with many measurements being taken on the same tree, and of course the measurements being replicated upon other trees of the same age. Recently, certain sections of the new IntCal20 calibration curve have been constructed using single year radiocarbon measurements. That is to say, the curve has been constructed by measuring the radiocarbon content of single tree rings in a contiguous fashion across many trees.

See for example the work by Pearson et al [www.pnas.org] (this study was aimed at looking at Mediterranean chronologies to aid in the dating of the bronze age eruption of Thera, Santorini, but the bristlecone pine data as well as the Irish oak data were included in the IntCal20 calibration curve). You can clearly see in the top graph of figure 2 of that paper that the radiocarbon age of single tree rings get older as the age of the tree ring increases. Note that many of these data were taken from the same tree sample of that species. Other researchers looking for so called Miyaki events (which are events characterised by sudden increases in radiocarbon concentration in single years due to cosmic events) have also measured the radiocarbon content contiguous rings from single trees, which also demonstrate that the radiocarbon age increases from the outside ring of the tree to the inside of the tree.

But you are correct regarding the issues of radiocarbon dating of wood within archaeological contexts. Take for example the dating of charcoal. This material comes from the incomplete combustion of wood. As such it is often difficult, if not impossible to identify any tree rings in the material, and so you date the sample as a whole. You have no idea whether the wood that was burnt came from the centre of the tree, or from near the outside surface. Therefore, you have no idea whether the wood was centuries older than the context it was found in. Indeed, matters can be even worse when one considers that you can have all different parts of a tree being reduced to charcoal, which effectively smears out the date over many centuries, and that is of course assuming that the charcoal is not intrusive into the archaeological horizon under consideration. This is why radiocarbon dating of artefacts from an archaeological context are not the be-all and end all of dating, and why one must consider other lines of evidence relevant to that context. For example, if you find a tree in situ with its bark intact, then fabulous, you can do a wiggle match and get a good precise and accurate date for the death of that tree (with measurement errors of the radiocarbon measurment and the uncertainties of measurement within the calibration curve). If you have a piece of wood, with no bark, or no idea from which part of the tree it came from, then in all probability, the date you get from radiocarbon will be older than it was found in.

Where radiocarbon in archaeology truly shines is in the dating of short lived samples, or single year samples, such as seeds and twigs from secure contexts. Small twigs may be only a year or two old and so you are not hampered by inherent old growth (though one must be careful since the twig may have been dead a number of years before being deposited within an archaeological context). Seeds are pretty idea to date as they only develop in a single year, but again, the archaeological context is key. For example, seeds stored in sealed jars in the volcanic destruction layer of Thera are thought to have been harvested the year of, or year prior to the Theran eruption, before being buried by the ash and Pumice.

On a related note, and returning to the radiocarbon content in old tree rings, If you are at all familiar with the attempts to date the eruption of Thera, a lot of focus has been placed upon the radiocarbon dating of an olive branch found in-situ within the volcanic pumice. the branch consisted of 72 rings, and they were radiocarbon dated in four blocks of 13 to 24 rings (thus we get a mean age for each block), and the radiocarbon age of 4 blocks was wiggle matched to the IntCal04 calibration curve to obtain teh date for the outer ring (see here [www.researchgate.net] figure 1. In graph 1b, you can clearly see that each of the radiocarbon measurements are older the deeper from the surface of the wood one goes. Also see Manning et al 2013 [www.researchgate.net].

Again, to labour the point, it is because it is only the outside ring of a tree that is "alive" (i.e. undergoing carbon exchange with the atmosphere). The internal parts of a tree no longer absorb carbon from the atmosphere, and so no longer replenish the radiocarbon concentration in subsequent years. This means that the radiocarbon concentration of the previous years growth ring begins to fall as soon as the next years growth ring begins, due to radioactive beta decay. Therefore, as soon as the next year's tree ring forms, the ratio of 14C:12C begins to decrease, and continues to do so as time moves forward. Thus when you radiocarbon date a tree ring, you are effectively measuring when that tree ring was last alive. If that tree ring is close to the centre of the tree it will give an older age than a tree ring closer to the outside of the tree, because the former "died" before the latter.

For more information on radiocarbon measurement see Radiocarbon by Bowman [books.google.co.uk] which explains the basics of radiocarbon dating. Or indeed Quaternary Dating Methods by Walker [www.wiley.com].

Jonny

The path to good scholarship is paved with imagined patterns. - David M Raup



Edited 2 time(s). Last edit at 12/19/2020 07:58PM by JonnyMcA.