Hey, DU Science Geeks!

[ProfessorGAC]

I have a question based upon something i saw yesterday.

I was watching something on Discovery or Science or something, and they were talking about Carbon 14 dating. For the first time ever, (a little embarassing that i didn't come to this dilemma sooner), i started wondering why that works.

I have an advanced degree in quantum chemistry, so I understand the concept of low level nuclear decay and the transition from 14C to 13C to 12C. However. . .:

Essentially all the carbon on earth is the same age, or about 6.2 billion years old. We haven't added any carbon to the ecosystem, save the stray meteor or a thousand. Plants were eaten by dinosaurs, turned into animal matter, they died, became petroleum, etc. etc. etc., but the carbon is all the same carbon.

Chemical processes (combustion, digestion, biodecay, and other organic rearrangements) are not capable of introducing new neutrons to a nucleus. There are no known quantum chemical interactions from which a neutron would be added to a nucleus over time. And besides, where would the new neutron come from. The earth is pouring out streams of neutrons and the consequent decay radiation.

So, why isn't all the carbon showing the same distribution of 12C through 14C, and why does carbon dating even work? I always just assumed it to be valid, and since it's not really in my field of work (although 13C NMR is widely used), i never gave it any thought. Now, i'm wondering why anything, no matter it's age, wouldn't appear to be 6.2 billion years old by 14C decay.

Yeah, i know i could look it up. I'm too lazy today to do that, so i'm looking for the other science geeks to hit me with an explanation

Any thoughts?
The Professor

[Bowline]

1. Pixie dust? n/t

[trotsky]

2. Carbon-14 is always being created.

I think from gamma rays or something! (Been awhile.)

At any rate, while an organism is living, it's absorbing and using carbon, some of which is always going to be Carbon-14.

When the organism dies, it stops taking in carbon, and the C-14 begins to decay. No "fresh" C-14 replaces it, and so we can determine the age of a dead organism based on what percentage of the original C-14 is still there.

Does that help?

[Kellanved]

5. exactly

The C-14 content of the atmosphere is constant (Nitrogen N14 gets converted to C14).

[ProfessorGAC]

11. Thanks For Trying

But, i'm trained as a quantum chemist, originally. Gamma rays don't explain where the extra matter comes from. In order to replenish 14C, you would need an extra (or two) neutrons. The gamma rays may have the ability to shift the SNF energy level to allow the agglomeration of a new neutron, but it doesn't provide the neutron.

That's what has me questioning why this works. Where is the extra matter coming from?
The Professor

[TXlib]

13. They're almost right

But it's not gamma rays -- it's cosmic rays consisting of ionised particles.

[AngryAmish]

3. The way I remember it from TV

Carbon 14 becomes "expressed" when an organic molicule is oxidized (burning or used in biological process). I think. I may be wrong.

[Commie Pinko Dirtbag]

4. Carbon in the atmosphere gets bombarded by cosmic rays

and thus maintains a constant C14 proportion. As soon as a living being stops feeding on its environment, decay begins.

This is a rough explanation, maybe the details differ. Maybe it's nitrogen that gets mutated into C14, I don'k know for sure.

[truhavoc]

6. By no means am I an expert

My fiance is an anthropologist and she made an attempt to explain this once. I believe that the carbon in our bodies (or any carbon based lifeform) while we are living is a newly formed, stable isotope at that point (chemistry behind this I have no idea), which then decays when our body stops maintaining it?

[Massacure]

7. Carbon-14 has a half life of 5,730 years

So if someone digs up some bones in greece egypt, and they have 50% of the Carbon-14 of a modern day human, then it is abou 5,700 years old.

[ProfessorGAC]

9. That's Not What I Meant

Thanks, but that doesn't answer my question. I already knew what you explained.
The Professor

[EstimatedProphet]

8. Solar energy

There is always a small amount of C14 being created from C12 because of exposure to solar radiation. After an organism dies and gets buried (one way or another), it is no longer exposed to the sun, and all the C14 that is present is all there is going to be. So, C14 dating examines this amount.

[ProfessorGAC]

16. But, What Is the Mechanism That Creates The New Matter?

That's the part that has me scratching my head. And, gamma and other cosmic rays go right through the earth, so why does being buried matter.

And, how about things that aren't buried, like the Shroud of Turin, which is carbon dated both fabric and paint. The paint is from surface pigments (not mined) and the fabric is from plants.

Those weren't buried and it works, but if it's buried it works?

I think you see why i am questioning it, and searching for the mechanism that makes it work.
The Professor

[EstimatedProphet]

25. I think you're thinking of neutrinos that go through the earth

Don't quote me on this, but gamma rays will go about a foot into things.

[Waistdeep]

10. Read this

http://science.howstuffworks.com/carbon-14.htm/printable

[ProfessorGAC]

12. That's Somewhat Helpful, But. . .

. . .i have never heard (despite several grad courses in quantum theory) of this secondary cosmic ray decaying into a neutron. The energy level of this secondary ray would have to be enormous to inverse decay into a neutron.

So, this is a good start, but it doesn't explain the mechanism by which new 14C is formed.

I'm not disputing it, though! I am more interested in understanding this quantum mechanical phenomenon.

Of course, at the very beginning i realized that the only way it works is if new 14C is being formed. But, i can't recall any quantum mechanism in which that would happen. Doesn't mean i'm right, but it does mean i have a conundrum.
The Professor

[TXlib]

18. The cosmic rays knock neutrons out of atoms

They also produce alpha and beta particles, and more exotic, short-lived particles.

The secondary cosmic ray doesn't decay into a neutron; it was initally a neutron kicked out of a nucleus.

[ProfessorGAC]

20. That's Better!

The only thing now, is to understand the distribution of those rays and the probabilities of those occurances.

The problem i have NOW is that the exact concentrations of atmospheric gasses, and surface particles is not constant and the rays hitting earth are distributed differently over time as certain cosmologic events occur. (Solar flares, and like that.)

I understand the particle ejection mechanism, and that makes sense, but it's still hard to fathom why this is reliable mechanism.
The Professor

[trotsky]

23. Here's a bit on that.

From baldguy's link:

Fourth, the ratio of C-14 to C-12 in the atmosphere is not constant. Although it was originally thought that there has always been about the same ratio, radiocarbon samples taken and cross dated using other techniques like dendrochronology have shown that the ratio of C-14 to C-12 has varied significantly during the history of the Earth. This variation is due to changes in the intensity of the cosmic radiation bombardment of the Earth, and changes in the effectiveness of the Van Allen belts and the upper atmosphere to deflect that bombardment. For example, because of the recent depletion of the ozone layer in the stratosphere, we can expect there to be more C-14 in the atmosphere today than there was 20-30 years ago. To compensate for this variation, dates obtained from radiocarbon laboratories are now corrected using standard calibration tables developed in the past 15-20 years. When reading archaeological reports, be sure to check if the carbon-14 dates reported have been calibrated or not.

[Beetwasher]

14. I Believe It Has To Do W/ Cosmic Ray Bombardment

New 14C is created in the atmosphere and it filters down and is ingested....

[Rabrrrrrr]

15. Others have answered well, but I add: it only works for organics

So you can't grab a lump of rock, for instance, and carbon date it. Only organics. Which is why it works, since after something dies it stops taking in new carbon 14.

[ProfessorGAC]

17. From Where?

I understand that part of it, even if only on faith. What i'm trying to grasp is two things:

1) If gamma and other cosmic rays are responsible, then why does being buried have anything to do with it? Those go right through the earth anyway.

2) Where does the added matter come from? Neutrons are heavy atomic particles. The energy level required to form a new neutron is darned high. (>6GeV IIRC).

I get the basic concept, but i'm caught in a quantum mechanical conundrum.
The Professor

[baldguy]

19. Google is your friend:

Nitrogen (7 protons, 7 neutrons) is bombarded with cosmic radiation in the upper atmosphere changing a proton to a neutron, resulting in Carbon 14 (6 protons, 8 neutrons). Since the level of cosmic radiation is relatively constant, and the level of Nitrogen in the atmosphere is relatively constant, the c12/c14 ratio is constant.

Even though the c14 is always decaying, it is also always being replenished while the organism is alive. Once the organism dies, the c14 is not replenished and begins the decay. We can tell how long something has been dead by comparing the normal c12/c14 ratio to the ratio in the remains of the body.

http://www.archserve.id.ucsb.edu/Anth3/Courseware/Chronology/08_Radiocarbon_Dating.html

[trotsky]

21. Hey, there ya go.

Explains why it doesn't affect buried organisms - the reaction to create C-14 only takes place in the upper atmosphere.

Thanks, baldguy!

[ProfessorGAC]

22. Thanks

Still doesn't completely answer my question, but it's a great start.

Since all the N2 is the same age, the same question applies. Eventually, we should be running out of "free" neutrons to "steal". I know its trillions of trillions of atoms, but there are still some isssues that don't seem to jibe mechanically.

Thanks, though.
The Professor

[Aristus]

24. Carbon-14 is an isotope, and isotopes are inherantly unstable.

They are always giving off neutrons and electrons, which causes them to decay. Some isotopes decay faster than others. For example, the rare element Francium has a half-life of something like fifteen minutes and is itself formed from the decay of other isotopes. The neutrons and electrons given off by C-14 and others are most easily absorbed by other isotopes. Which is why heavy water, or H2O2, is used to make fissionable material. It absorbes stray electrons before a chain reaction can start.

[jpak]

26. Check this out...

http://hypertextbook.com/physics/modern/half-life/

Cosmic rays constantly liberate neutrons during collision interactions with atmospheric gas molecules.

When 14N absorbs one of these neutrons, it is converted into 14C.

14C is in equilibrium within the atmospheric system and has a "constant" concentration.

Small inter-annual variations do occur, however, requiring calibration against old-growth tree ring chronologies.

that's it more or less....