My explanation of Carbon Dating

[Langston]

Here's the explanation I just typed up for someone in a PM regarding the carbon-dating process. Please note the LAST SENTENCE. I'm aware that this is not a perfect explanation... no need to nit-pick it. For general purposes, though, it's adequate to explain what Carbon dating is and how it works.

Feel free to comment as you like... but I'm not going to argue with anyone that is going to point out something minute and try to make some flame about it. If you want to make this more technical, that's fine by me... keep it civil.

This has nothing to do with religion, so keep out of the discussion entirely if you can't avoid bringing religion into it.

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Every element on the periodic table (with the exception of the Nobel gases) have "variants" called isotopes. Each of these isotopes retain the same basic chemical properties of the element, but have varying radioactive properties.

For example:

Hydrogen in it's natural "inert" form has a single Proton particle for a nucleus. Tritium (an isotope of hydrogen) is still hydrogen, but it has 2 neutrons in the nucleus, as well. These tritium atoms can bond with oxygen to make water (called "tritiated" or "heavy water"), or with any other group of elements that would normal form chemical compounds with hydrogen. The difference between H1 and H3 (tritium) is that H3 has radioactive properties. It's primary use is to be the neutron producer in a "hydrogen" atomic weapon. Tritium is the "hydrogen" that gives the device it's name.

What many people don't understand is that when something is radioactive, it emits particles/waves of radiation when an atom "decays". You see, any isotope by it's very nature is unstable. The "extra" neutrons in the nucleus make the atom unstable and it "breaks down" - releasing some particles/waves to drop it to some lower "energy" level - meaning that it sheds neutrons or radiation (which makes neutrons change into protons) and it becomes a whole new atom with potentially very different properties. Many of these new atoms are now inert... some are not and will begin their own decay cycle; regardless, the original material is now forever changed and will no longer give off the same kind of radiation any more (if any at all).

To keep the same tritium example:

When a tritium atom decays (remember, it's 1 proton and 2 neutrons), it releases a beta particle (essentially a rogue electron) and one of the neutrons becomes a proton - and the atom is now He3 (Helium-3 - 2 protons and 1 neutron). The atom now is no longer hydrogen at all - it has all of the properties of helium, instead. It has DECAYED to become a helium atom.

Now, every isotope which is radioactive has a "half-life". This half-life is the period of time which it takes the radioactive material to decay to 1/2 of it's original quantity. In other words, if you had 100 radioactive isotopic atoms, a half-life would be the period of time that would transpire for 50 of those atoms to decay as I explained in the H3 -> He3 example. Every isotope is different and has different half-lives. Tritium has a 9 year half life, Uranium-236 is thousands of years, and there are some isotopes created in the fissioning core of a nuclear reactor which have half-lives measured in milli-seconds.

Many of these isotopes exist naturally in the world. We know precisely what percentage of a given quantity of a particular element found in the "wild" would be of each isotopic variety. Furthermore, with carbon, we know that it is created by the oxydation (burning/chemical reaction/etc) of organic compounds (living creatures/plants/etc). For example, when you burn wood (oxydation via burning), the complex compounds that make up the wood are broken down and you are left with a large resulting quantity of raw carbon. The chemical reaction of the burning results in the creation of carbon - some known percentage of which is Carbon-14.

Now, where Carbon dating comes into play is that they will measure what quantity of the remaining carbon in a material is still Carbon-14. They know how long it takes for Carbon-14 to half-life, so by comparing how much is left and reversing the formula, they can make a very good estimation of when the carbon was first created by a chemical reaction (thus starting the "life" of the carbon in the first place). This gives them a "starting point" for the life of the organism/material for which they are trying to determine a date of origin.

Understand, this is not a perfect explanation, but it's one that's a little more understandable than most of the gobbledy-gook you'll find on googled sites... but it does lose some information in the translation.


edit: yeah, I found a major glaring mistake already... fixed it

[mofish]

Excellent ugz

[Gidan]

Looks like a great exlination without trying to read a text book to people.

[DangerPaul]

did god create carbon dating ?

[Maeya]

He's so dang smart ^_^


I have a question though honey -

You said that Noble gases don't have isotopes? If I'm remembering correctly, when the Atomic Mass (I think?) is listed on the Periodic Table, it's the weight of the protons + the neutrons, but it's always listed out to several decimal places to take in account the average weight of the various isotopes... Why do the Noble Gases not have perfect atomic weights? Argon for example is 39.948. Are the isotopes just really rare? Or what accounts for the missing .052?

[xaoshaen]

Noble gasses can have isotopes, their stable external electron shell configuration makes them largely chemically inert, but doesn't necessarily preven them from gaining or losing neutrons: witness Helium-3.

[Maeya]

That makes sense, thank you.

[The Kizzy]

My head is spinning.

[Narrock]

It's actually quite simple, Kizzy. You're just intimidated by the jargon. Ugz did explain it well though.

[Langston]

He's so dang smart ^_^


I have a question though honey -

You said that Noble gases don't have isotopes? If I'm remembering correctly, when the Atomic Mass (I think?) is listed on the Periodic Table, it's the weight of the protons + the neutrons, but it's always listed out to several decimal places to take in account the average weight of the various isotopes... Why do the Noble Gases not have perfect atomic weights? Argon for example is 39.948. Are the isotopes just really rare? Or what accounts for the missing .052?

Yeah - I made an error there. Xao's response is correct.

The atomic weight of an element is an aggregate weighted value based upon the distribution of the isotopic variants of the element. For example:

If Element_01 has the following isotopes and "populations" per isotope:

Isotope1 - Atomic mass of 10 - 50% occurrence
Isotope2 - Atomic mass of 15 - 25% occurrence
Isotope3 - Atomic mass of 20 - 25% occurrence

The Atomic Weight for Element_01 would be:

(.50 x 10) + (.25 x 15) + (.25 x 20) = AW
(5) + (3.75) + (5) = AW
13.75 = AW

For Argon:

There are six isotopes:

Ar36
Ar37
Ar38
Ar39
Ar40
Ar41

I don't have their relative distributions handy - but when I get back from Dallas I have a book at home of all known isotopes and their distributions and I can show you where the atomic weight was derived from this.

[Maeya]

I'll take just your word for it =)

I'm pretty sure I know roughly how they get the Atomic Weights, I was just confused why the Nobel Gasses had imperfect weights if there were no isotopes.... but there are, so I'm happy now =)

[Langston]

You already marked the calendar with a "Langston was wrong today!" sticker, didn't you.

[Maeya]

Several times.

Right underneath the "omfg I am going to shoot my boss" stickers.

[Langston]

I think you should actually use stickers to denote days that you do NOT want to shoot your boss.

Would be less clutter on your calendar that way.

[Maeya]

Or else get one of those big desktop calendars with the 4 inch square for the dates. And use small stickers.

[Langston]

Can you call American Express for me? I lost my card... again - oh and I need you to check the availability on flights to Zimbabwe - I'll be using frequent flyer miles... Delta flies to Zimbabwe right? And I need you to change the website to purple... no blue... no purple... and then can you call the day care for my kids and let them know that Katie called me and told me she didn't want milk today... also, have the FBI sweep the building for microphones again - I SWEAR my ex-husband is spying on me...

I'll be in my office planning my next plastic surgery and signing up for internet dating services.

Thanks.

[Phlegm]

Carbon 14 (C14) is an isotope of carbon with 8 neutrons instead of the more common 6 neutrons. It is unstable, and scientists know that it radioactively decays by electron emission to Nitrogen 14, with a half life of 5730 years. This means that given a statistically large sample of carbon 14, we know that if we sit it in a box, go away, and come back in 5730 years, half of it will still be carbon 14, and the other half will have decayed.

Or in other words, if we have a box, and we don't know how old it is but we know it started with 100 carbon 14 atoms, and we open it and find only 50 carbon 14 atoms and some other stuff, we could say, 'Aha! It must be 1 carbon 14 half-life (or 5730 years) old.' This is the basic idea behind carbon dating.

So in the real world, looking at a sample like say a bone dug up by an archaeologist, how do we know how much carbon 14 we started with? That's actually kind of cool. It's a semi-long story, so bear with me. In the atmosphere, cosmic rays smash into normal carbon 12 atoms (in atmospheric carbon dioxide), and create carbon 14 isotopes. This process is constantly occurring, and has been for a very long time, so there is a fairly constant ratio of carbon 14 atoms to carbon 12 atoms in the atmosphere. Now living plants 'breathe' CO2 indiscriminately (they don't care about isotopes one way or the other), and so (while they are living) they have the same ratio of carbon 14 in them as the atmosphere. Animals, including humans, consume plants a lot (and animals that consume plants), and thus they also tend to have the same ratio of carbon 14 to carbon 12 atoms. This equilibrium persists in living organisms as long as they continue living, but when they die, they no longer 'breathe' or eat new 14 carbon isotopes Now it's fairly simple to determine how many total carbon atoms should be in a sample given its weight and chemical makeup. And given the fact that the ratio of carbon 14 to carbon 12 in living organisms is approximately 1 : 1.35x10-12, we can figure out how many carbon 14 atoms were in the sample when it ceased to replenish it's supply.

In actually measuring these quantities, we take advantage of the fact that the rate of decay (how many radioactive emissions occur per unit time) is dependent on how many atoms there are in a sample (this criteria leads to an exponential decay rate). We have devices to measure the radioactivity of a sample, and the ratio described above translates into a rate of 15.6 decays/min per gram of carbon in a living sample. And if you play with the exponential decay equations, you can come up with the nice formula (1/2)n=(current decay rate)/(initial decay rate), where n is the number of half lives that have passed. Voila, now you can tell how old a sample of organic matter is.

Some notes:

1) Obviously, this technique only works for dead organic material.

2) This technique is best for dating items which died between on the order of 1000 to on the order of 1,000,000 years ago. Carbon 14 dating is not great for dating things like a year old because if much less than 1 half-life has passed, barely any of the carbon 14 has decayed, and it is difficult to measure the difference in rates and know with certainty the time involved. On the other hand, if tons of half-lives have passed, there is almost none of the sample carbon 14 left, and it is really hard to measure accurately how much is left. Since physics can't predict exactly when a given atom will decay, we rely on statistical methods in dealing with radioactivity, and while this is an excellent method for a bazillion atoms, it fails when we don't have good sample sizes. However it is possible, when dating very old rocks for instance, to use longer lived isotopes for dating on a longer time scale.

3) The assumption we based this on (that the ratio of carbon 14 in the atmosphere and thus in living organisms is constant) is a decent one for ballpark figures, but this method will not be able to give results accurate to, say, a couple of minutes.

[Maeya]

With company money.


And then harrass the consultants to bill more for me because I think I want to buy a new car and take a vacation to Europe. Because I'm so over worked. And do my taxes for me too. Oh, and I forgot to bring in my electric bill from 2 months ago, and they're threatening to cut off my service. Why didn't you pay it? You should have known it was due. Call them and make sure it doesn't show up on my credit - if it does it's your fault. Oh, and I'm bringing my kids to the office even though they scream and yell and don't behave, I need you to clean up after them and fix computers they wreck. And I'm bringing the dog too. Go ahead and clean up after him too. And while you're at it, come to my house and fix my personal computer in your free time.

*sigh*

Sorry to hijack your thread. Boss-hating is at an all time high right now. I ranted to you in an e-mail instead of continuing to take up space here.

[Tikker]

That's a nice simple explanation too

[Langston]

After seven half-lives, the material has effectively depleted itself almost entirely.

This is why many people say that Carbon-dating isn't very useful beyond 60,000 year-old samples - at that point you've actually had more than 10 cycles and are left with 1/2^10 (less than .1%) of the material left. At a million years, you're dealing with only 4.2x10^-51% of the item remaining. That number looks like this:

.00000000000000000000000000000000000000000000000000042%

(I think I counted the right number of zeroes there....)

That's pushing the limits of machine and man to measure and test. Can it be measured? Yes, it can, given sufficient quantity of the sample. A single decay can be measured and when you're dealing with radioactive materials you're looking at massive quantities of atoms (measured in moles, typically) - so finding one decay isn't unreasonable... but you open yourself up to a much larger chance of statistical anomaly at that degree of minimalization.

[ClakarEQ]

I'm just asking and not trying to shoot holes in the assumptions and not being a "scientist", but this quote:

that the ratio of carbon 14 in the atmosphere and thus in living organisms is constant

This would imply the atmosphere we exist in today is EXACTLY the same atmosphere that existed 1,000,000+ ago. I mean the world was still evolving just as the organisims on it. Does this not seem like a big assumption to make?

I realize you could throw all kinds of what if's at this. However wouldn't just a minor adjustment to the formula "1 : 1.35x10-12" into "1 : 1.355x10-12" screw the resulting number extensively?

What stops / reduces cosmic rays (if anything, ie Ozone /shrug)?
Were there more or possibly greater/more intense cosmic rays say 2million years ago vs today?

Again not trying to debunk this just don't like the "assumption + science = fact" formula.

[ClakarEQ]

OK I just read Phlem's post in the LAWL thread.

So my questions above could be a bit moot. Are the formulas and other topics above related to only carbon 14 or does argon or the other minerals/isotopes used for dating get subjected to the same variables (cosmic rays, etc)?

[Langston]

Obviously, the calculations that the mass-spectrometers use don't round off at 2 decimal places.

Regarding the amount of radiation from the sun being greater/less... I don't know. But I can tell you this much: the type of radiation we're talking about here is proton radiation generated by the fusion reaction taking place in any star. Protons are highly energetic particles that are readily shielded by our atmosphere. Thinning of the ozone layer would have only the most slightest of effects on the quantity of this type of radiation and the number of proton absorptions/interactions would be almost perfectly constant.

You could argue that events like enormous sun spots will cause anomalies... but as with all complex theories/formulas, they can't be "dumbed down" and still be explained in the most technical of terms at the same time. These explanations we've posted are extraordinarily simplistic and by no means should they be taken as the end-all, be-all of how isotopic dating processes work.

So to answer your questions:

- Our planet's atmosphere shields us from the overwelmingly vast majority of the cosmic radiation.

- It's not just a possibility - it's a probability that the amount of cosmic radiation has varied any number of times in the last two million years. I can't answer to whether or not the dating processes take this into account... but if they did not, then one would have to come to the conclusion that there would be a very large number of scientists crying out against this obvious "gap" in the formulae.

[Langston]

OK I just read Phlem's post in the LAWL thread.

So my questions above could be a bit moot. Are the formulas and other topics above related to only carbon 14 or does argon or the other minerals/isotopes used for dating get subjected to the same variables (cosmic rays, etc)?

Ninja poster!

All sources of radiation - cosmic and terran affect the quantity of these isotopes. Everything from cosmic to man-made to naturally occuring uranium interacts with particles and can cause isotopic change.

[ClakarEQ]

Thanks Langston. I realize the formula is more extensive then the dumb-down version your posting for us

My mind begs the question how science is able to come across with such certainties even though there appears to be some, but accepted flaws / variables in the formula. (e.g. the ozone layer may have been 1000 times stronger or weaker then today, the lowest point on the planet a million years ago could have been like the tip of Mt. Everest)

I suppose science would ask the same questions and my takeaway from this is close to my entry point, Science makes the best educated guess they can with the tools and instruments they have at their disposal, today. Yet we continue to learn and adjust science and it's theories as time progress's.

Eitherway very insightful stuff.

Appreciate the spark.