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Hans
2008-Apr-14, 10:12 PM
Came across this explanation of C-14 dating that seems ....odd

Quote:
The answer may be as simple as the way in which a tree absorbs c-14. Trees grow from the centre outwards, rendering the centre or 'heartwood' of the tree effectively dead tehreby unable to absorb further c-14 isotopes. The youngest part of the tree are the outer rings which will be larger than the inner rings thus absorbing larger quantities of c-14 isotopes than the inner heartwood. A single tree then - depending on its age - can provide an array of dates. If a 1,000 year old tree was felled today it will produce c-14 dates ranging from 2008AD to 1008AD. Thus we would find more c14 data with 2008 dates because the rings of the tree are larger. We would also find lower quantities of 1008 c14 dates because the rings would have been smaller whent he tree was much younger. So, it is entriely possible for one tree to produce a wide range of c14 dates.

End quote

I thought the entire tree would have the same C-14 age through out? Is this a valid explanation or is it out of whack?

Jeff Root
2008-Apr-14, 11:28 PM
Hans,

I probably don't know anything about it that you don't. In particular, I
don't know whether everything in the article you quoted is correct. But
it should be obvious that the inner rings of a tree which began growing
1000 years ago should date to about 1008 AD, while rings which just
grew this year should date to about 2008 AD. Different parts of the
wood are different ages.

And I agree that there will generally be more wood produced as the
tree gets larger, so there will be more wood from recent years than
from long ago. There will also be more wood from years of good growth
than from years of poor growth. The quote seems OK to me.

-- Jeff, in Minneapolis

Acolyte
2008-Apr-14, 11:38 PM
There's an assumption here that C14 is absorbed ONLY from the outer layer of the tree. I don't know if that's necessarily so. Seems to me leaves would play a part & the transport of nutrients through the tree would distribute it around.

Also is the centre of a tree necessarily dead? I've cut down trees with heart wood that is unlike the timber part of the trunk, but it's moist & seems a connected, living part of the tree.

Also the logic - yes the outer rings will be larger but the C14 isn't measured by testing the whole layer, it's from a sample of the wood. Measuring the inner ring would be using the same sample size so the location of the ring shouldn't have an effect.
ie. the total amount of C14 in a ring might be higher due to a larger ring but the amount in the sample size would vary only due to the incidence of C14 in the tree's environment. (including the leaves as source)

Jeff Root
2008-Apr-14, 11:53 PM
Acolyte,

The leaves absorb carbon dioxide from the air, convert it to sugars, and
move the sugars containing carbon down to where they will be used to
build the new, outermost ring.

I agree about the sizes of the samples.

-- Jeff, in Minneapolis

Delvo
2008-Apr-15, 12:47 AM
Dendrochronology was one of the things I learned about while I was getting my bachelor's degree in the science of forestry. This particular aspect of it involves tree physiology, which was also part of the deal. (I did work in forestry for a few years after getting my degree, but I quit several years ago.)

It is correct that the middle of a tree is not alive, and the deeper it is from the outside, the longer it's been since it was. If you look at a cross-section of a trunk or branch, the only living tissue is a thin layer between the wood and the bark, called the "cambium". The wood and bark are like your hair, fingernails, epidermis, and most of the mass of your bones: produced by cells but not undergoing any of the processes of life and thus not exchanging carbon or anything else with the environment. The only exception to this is if there's been an injury that exposes the wood to the outside again. Then it doesn't come back to life, but some fungi can start to consume it or some animals can start to excavate it.

That makes it also correct that different parts of the tree yield different ages, because they really are different ages. If you didn't separate the rings and test them separately, you'd get a date somewhere between the true dates of the oldest and youngest parts (but more recent than the halfway time because more recent circles of the wood are larger).

People using carbon dating always knew that carbon dates could miss by several years because they had to treat the ratio of carbon isotopes in the atmosphere as a constant but didn't actually know whether or how much it might vary. Then tree rings' carbon ratios showed how atmospheric carbon ratios have actually varied and gave us very precise carbon ratios for every single year we have tree ring records for, because the ratio in a specific ring could be compared with the "assumed" value for that ring's year. As a result, some previous carbon dates that had been made before that was possible had to be adjusted by a few years apiece to account for the atmospheric ratio changes as recorded year by year in tree rings. Thus, not only has the separateness of carbon ratios in every tree's separate rings allowed tree ring dates and carbon dates to confirm each other, it's also refined the precision with which carbon dating can be used.

PS: This only works for gymnosperms and dicots (one of the two big groups of angiosperms). It doesn't work for monocots (the other big group of angiosperms). Monocot physiology and growth patterns are different in a way that isn't conducive to this... but that isn't much of an issue because it also isn't conducive to reaching a sufficient size to be called a "tree". Most monocots are grasses, grains, and sedges. The only monocots that grow big enough to be relevant here are the palms (big overgrown woody grass!), and they're hollow or full of only some loose stringy fibers like you might use for shipping-box padding.

mugaliens
2008-Apr-15, 04:01 AM
Carbon 14 is continuously created by cosmic rays which impact Nitrogen, kicking out a proton, turning it into C 14. It's a nuclear reaction.

C 14 is unstable, having a half-life of 5730 years. Nearly all dating methods using decaying isotopes are good for about 10 times the half-life. Thus, C 14 dating methods can be used to date things up to around 60,000 years ago. Beyond that, it's just not all that useful.

Levels of C14 in the atmosphere have varied over time, but not by much. There are many ways of checking those levels over time, including gas trapped in ice cores.

The beauty of C14 dating is that for short-lived organisms (less than 100 years) it gives a pretty precise date of when they lived. For longer-living organisms (1,000-year old trees), it's usually measured when they died (outer edge of the tree, the cambium).

Then, it's easy to find out when the tree was born - just count the rings and subtract!

grant hutchison
2008-Apr-15, 09:40 AM
I think the whole thing is trying to be a digest of what Delvo has just explained, but that it is potentially misleading here:
The youngest part of the tree are the outer rings which will be larger than the inner rings thus absorbing larger quantities of c-14 isotopes than the inner heartwood.The size of the ring is essentially irrelevant: it incorporates carbon as it grows, and that carbon will have the same ratio of carbon isotopes whether it is in a small tree ring or a large tree ring. What's important is the time at which the wood stops growing and therefore stops incorporating fresh carbon from the atmosphere: that's the moment at which the C-14 clock starts counting, as C-14 decays and is not replaced.
So the heartwood clock is already running, while the outer, growing ring hasn't started its clock yet.

If you have an intact tree, or a beam with the outer growth ring intact, you can sample the outer ring and tell when the tree was felled. If you have nothing but charcoal (common enough in archaeology), then you're going to end up with a date that's somewhere during the tree's lifetime, depending on the mix of heartwood and growth rings represented in the charcoal. Although the outer rings may originally contain a greater mass of carbon, because of their greater circumference, there's no guarantee that they'll be present in our sample in the same proportion they existed in the original tree: just thinking about how planks and posts are made should make that clear.

Grant Hutchison

geonuc
2008-Apr-15, 11:13 AM
Grant,

What you've posted is correct, except I don't see anything particularly misleading about the quote in the OP.

Neverfly
2008-Apr-15, 11:44 AM
Grant,

What you've posted is correct, except I don't see anything particularly misleading about the quote in the OP.

This misleading statement quoted suggests that there is a difference in the absorption.


The over-all effect remains the same, though.

geonuc
2008-Apr-15, 11:56 AM
This misleading statement quoted suggests that there is a difference in the absorption.

I don't read it that way. It says there is a difference in absorption (which is true) but does not say there's a difference in rate (which would be untrue).

Neverfly
2008-Apr-15, 11:58 AM
I don't read it that way. It says there is a difference in absorption (which is true) but does not say there's a difference in rate (which would be untrue).

mmmm...
I think I saw jlhredshift replying to the thread- but we'll let him address it too.

From how I read it- that is exactly why it was worded "misleading" rather than "wrong".

I know that sometimes I word things all wrong too. So I think we all know it wasn't intentionally misleading. But we still need to be reminded so we can learn how to word things better:)

jlhredshift
2008-Apr-15, 12:04 PM
The attached image is from Twilight of the Mammoths by Paul S. Martin 2007 pg 46.

This is a calibration of C14 to 22Kya which gives a pictorial sense of how C14 has varied through time.

jlhredshift
2008-Apr-15, 12:22 PM
I don't read it that way. It says there is a difference in absorption (which is true) but does not say there's a difference in rate (which would be untrue).

Ahh, I had to read the original four times to see it both ways. I agree with GEONUC. A larger area would contain more C14 in absolute quantity but not a greater ratio than what existed at the time of growth. That wasn't spelled out.

But, I still haven't figured out how to include tone inflection in my typing!:boohoo:

jlhredshift
2008-Apr-15, 01:04 PM
There is still uncertainty in the absolute accuracy of C14 dating.
From T.-C. Chiu et al. / Quaternary Science Reviews 26 (2007) 1836


An under-estimate of the 14C half-life is a possible explanation for excessively elevated D14C values. If the calorimetry estimated 14C half-life, 6030 years, is validated by new half-life measurements, it could explain much of the linear component of the radiocarbon calibration curves and the discrepancy between D14C values derived from corals and modeled D14C values based on paleointensity combined with a range of carbon cycle scenarios. We conclude that the variation in paleointensity and a possible offset in the absolute value of 14C half-life together control the overall shape and amplitude in the D14C record for the past 50,000 years. A re-determination of the 14C half-life is urgently needed for radiocarbon-based research. After the 14C half-life is accurately measured and replicated by multiple techniques, our coral data will provide an opportunity to examine subtler carbon cycle influences on the younger half of the D14C record.

grant hutchison
2008-Apr-15, 01:18 PM
What you've posted is correct, except I don't see anything particularly misleading about the quote in the OP.
I think it's potentially misleading both because of the way it's phrased, and because of the implications that are taken from it later:
The youngest part of the tree are the outer rings which will be larger than the inner rings thus absorbing larger quantities of c-14 isotopes than the inner heartwood.While this is true, it would have been better stated as "... larger quantities of carbon isotopes ...": it's not just C-14 which is absorbed in larger quantities, and the text can be read as suggested some sort of preferential uptake.
Then, later, we get:
Thus we would find more c14 data with 2008 dates because the rings of the tree are larger. We would also find lower quantities of 1008 c14 dates because the rings would have been smaller whent he tree was much younger.Again, it seems that size is being emphasized over age. In fact we find a lower fractional content of C-14 in the inner rings because they're older. Although we would also find a smaller overall content of carbon because the inner rings have a smaller volume, that's not usually a problem, or part of the technique: we analyse an appropriate small-volume sample taken from the wood, not the entire cross-section of a thousand-year-old tree.

I guess it depends on the context from which the piece was taken. It may be that the meaning is very clear when read along with the supporting text.

Grant Hutchison

geonuc
2008-Apr-15, 04:08 PM
OK, I can accept that some might see it as misleading.

Now, about that second geonuc 'quote' ... :mad:

grant hutchison
2008-Apr-15, 04:12 PM
OK, I can accept that some might see it as misleading.

Now, about that second geonuc 'quote' ... :mad:Oops, sorry, I thought I'd fixed that. :doh:A dodgy bit of cut-and-paste. I'll fix it properly now.

Grant Hutchison

geonuc
2008-Apr-15, 04:16 PM
Ta. ;)

mugaliens
2008-Apr-16, 03:56 PM
I think the whole thing is trying to be a digest of what Delvo has just explained, but that it is potentially misleading here: The size of the ring is essentially irrelevant: it incorporates carbon as it grows, and that carbon will have the same ratio of carbon isotopes whether it is in a small tree ring or a large tree ring.

Bingo. It's not how much C-14 there is. (Could be a lot, could be a little). It's the ratio of C-14 to C-12. Since that's fairly constant throughout nature, and C-14 radioactively decays to C-12, the lower the ration of C-14 to C-12, the older the object.

BioSci
2008-Apr-16, 04:19 PM
... and C-14 radioactively decays to C-12...

to be "correct" C-14 undergoes beta decay to become N-14 (not C-12) :)

This then does result in a reduced ratio of C-14 to C-12 in the material over time.

Hans
2008-Apr-16, 05:39 PM
So guys, you lost me here.

So as I understand the comments above: We take a 1,000 year old tree and when it dies we take samples after it been laying around in a nice dry environment for 2,000 years. A sample from the outer ring and most inner ring is used.

The dates from those will show what? The outer will show an earlier age and the inner a 1,000 year older age? Or the same age for both?

Jeff Root
2008-Apr-16, 09:19 PM
Hans,

I'm surprised that you are confused by this. It seems quite straightforward.

If you remove a portion of a ring from a tree trunk or branch, carbon dating
will tell you approximately how long it has been since that ring was alive.
The rings near the center grow, die, and are replaced by new rings farther
from the center, which grow and die in their turn. So wood from rings near
the center of a trunk or branch will measure as older than wood from rings
closer to the surface.

If a hunk of wood is analyzed without separating it by its rings, the different
ages of the rings will all affect the measurement, and the result will be less
precise. If the wood is a round post made from a whole trunk or branch, it
will likely have more wood made in later years than early in its life, because
the rings get larger and larger from year-to-year as the tree grows. That
biases the measurement toward a younger age. If the wood is a board or
timber which includes the heartwood but has had the outer layers mosly
cut away, the measurement will be biased toward an older age.

A thorough analysis would measure the age of each ring separately, which
would provide a good deal of confirmation (or possibly dis-confirmation)
that the measurements are correct (or incorrect, as the case may be).

-- Jeff, in Minneapolis

Hans
2008-Apr-17, 05:09 AM
Thanks Jeff

I needed that summary for, er, ah lets say more challenged individuals

Thanks

mugaliens
2008-Apr-17, 07:56 PM
Good explanation, Jeff Root. Perhaps I was being a bit obtuse.