Carbon dating question

[Mookow]

What is the mechanism behind the assumption that the ratio of C14 to C12 stays constant through time in the enviroment? Ditto with potassium.

<--- would put this in OT, but looking for a more in depth answer

 

[Particle Man]

look for spallation. This is a good question to ask your chemistry professor at college. Most of the time people will get stumped on this.

It is N2 changing to cyanide by gamma/cosmic radiation which creates 14C source. Of course, this is assuming that the gamma/cosmic radiation is constant.

 

[Mday]

Originally posted by: Mookow
What is the mechanism behind the assumption that the ratio of C14 to C12 stays constant through time in the enviroment? Ditto with potassium.

<--- would put this in OT, but looking for a more in depth answer

statistical analysis. basically, they take samples of known materials, and look. and they saw a pattern. this ratio is no longer true if you go back no less than 100 years. or so. why? cuz we've been pumping out a massive amount of carbon dioxide. while the accuracy of carbon dating can be put into doubt, it is nontheless a valid method to gauge what is older than what, that is, something dated 100 million years old probably has a 10% error (+/- 5%) which is fairly insignificant.

carbon dating is very much based on statistics and recorded data. the formulae used in general chemistry and sciences that you learn in undergrad and prior are merely approximations and extrapolations which fall apart given some other data. how the age is given with respect to the real numbers you hear around the news are much more complicated than just comparing the ratio and plugging in the number. carbon dating dates often change because new statistical data causes the forumlae to change (this does not refer to the generalized approximation used in exponential decay, which is just an extrapolation or interpolation depending on how you look at it).

if you look at one team of carbon daters, and another, they will be similar but differ enough to show you that statistics is not as simple as 1+1 = 2.

to really answer the question ,there is no actual mechanism since statistics is used to approximate and extrapolate. it's better than pulling a number out of thin air, but it's not as good as having a written record.

 

[RossGr]

while the accuracy of carbon dating can be put into doubt, it is nontheless a valid method to gauge what is older than what, that is, something dated 100 million years old probably has a 10% error (+/- 5%) which is fairly insignificant.

It is my understanding the due to the half life of C14, Carbon dating is not useful beyond about 5k yrs and best between 2k and 3k years.

As to the ratio in the air it is due to physical processes which have not changed over the life time of the earth so is considered a constant with time.

 

[rjain]

Originally posted by: RossGr

It is my understanding the due to the half life of C14, Carbon dating is not useful beyond about 5k yrs and best between 2k and 3k years.

As to the ratio in the air it is due to physical processes which have not changed over the life time of the earth so is considered a constant with time.

I think you have that backwards. The half-life of C14 hasn't changed in the past billion years, as far as we can tell. The physical processes (solar radiation) probably have varied (don't know how significantly) in the past 10,000 years.

 

[Mark R]

It is my understanding the due to the half life of C14, Carbon dating is not useful beyond about 5k yrs and best between 2k and 3k years.

It's not so much due to the half life, but the very low abundance of 14C in the natural environment. The half-life is about 6k years - but even so, there is substantial uncertainty even dating objects 6000 years old. With highly sophisticated analysis techniques using accelerators, it is possible to extend the useful range to nearly 100k years.

In the future there may be other problems - for example, the dilution of natural 14C by fossil fuel carbon which is 14C free, or the addition of 14C to the environment from nuclear accidents or weapons.

 

[RossGr]

Originally posted by: rjain

Originally posted by: RossGr

It is my understanding the due to the half life of C14, Carbon dating is not useful beyond about 5k yrs and best between 2k and 3k years.

As to the ratio in the air it is due to physical processes which have not changed over the life time of the earth so is considered a constant with time.

I think you have that back wards. The half-life of C14 hasn't changed in the past billion years, as far as we can tell. The physical processes (solar radiation) probably have varied (don't know how significantly) in the past 10,000 years.

I have what back wards? Perhaps you need to read my post a bit more carefully. Nothing you said is significantly different from what I said.

The abundance vs half life issue has to be seen as so interrelated as to be inseparable. Both factors contribute to the relatively short time range of C14 dating.

 

[rjain]

Originally posted by: RossGr

Originally posted by: rjain

Originally posted by: RossGr

It is my understanding the due to the half life of C14, Carbon dating is not useful beyond about 5k yrs and best between 2k and 3k years.

As to the ratio in the air it is due to physical processes which have not changed over the life time of the earth so is considered a constant with time.

I think you have that backwards. The half-life of C14 hasn't changed in the past billion years, as far as we can tell. The physical processes (solar radiation) probably have varied (don't know how significantly) in the past 10,000 years.

I have what back wards? Perhaps you need to read my post a bit more carefully. Nothing you said is significantly different from what I said.

The abundance vs half life issue has to be seen as so interrelated as to be inseparable. Both factors contribute to the relatively short time range of C14 dating.

The ratio in the air of C14 to N14 most definitely HAS changed over the lifetime of the earth due to physical processes involving solar radiation. The half-life is constant, and that is what makes the method useful in the first place.

 

[Mday]

Originally posted by: rjain

The ratio in the air of C14 to N14 most definitely HAS changed over the lifetime of the earth due to physical processes involving solar radiation. The half-life is constant, and that is what makes the method useful in the first place.

and to reiterate, the half life alone tells us nothing. it is that combined with ratios that gives us information.

 

[PrinceXizor]

Originally posted by: Mday

Originally posted by: rjain

The ratio in the air of C14 to N14 most definitely HAS changed over the lifetime of the earth due to physical processes involving solar radiation. The half-life is constant, and that is what makes the method useful in the first place.

and to reiterate, the half life alone tells us nothing. it is that combined with ratios that gives us information.

You measure C14. Now that you have a "measurement". How can you use that to determine how old something is? Well, you know the half-life (one part of the equation), and then you have to factor in how much C14 is "supposed" to be there. That's where it gets kind of tricky. Either answer without the other is kind of pointless. Then you factor in the minute ammounts you are measuring, the accuracy of your measurements, and you can see why C14 dating has a practical limit on age.

P-X

Edit: For clarification, I'm agreeing with Mday, just elaborating a little.