Carbon dating

[Onceler]

isin't based on the asumption that carbon 14 is present at the exact same level in the past?
I would think that the amounts would be different for different time periods for various reasons

 

[nismotigerwvu]

Theres plenty of evidence to support it staying constant. My biggest worry about the concept is that it measures absolutely minuscule amounts, requiring super accurate instruments and even the smallest error/contamination could shift the calculated date by millions of years.

 

[silverpig]

Originally posted by: nismotigerwvu
Theres plenty of evidence to support it staying constant. My biggest worry about the concept is that it measures absolutely minuscule amounts, requiring super accurate instruments and even the smallest error/contamination could shift the calculated date by millions of years.

Carbon dating isn't used to date things that old.

If you're talking about radiometric dating with other isotopes, well sure, but you can perform a lot of measurements...

 

[silverpig]

Originally posted by: Onceler
isin't based on the asumption that carbon 14 is present at the exact same level in the past?
I would think that the amounts would be different for different time periods for various reasons

It may have changed slightly due to the sun's intensity, but the sun doesn't change much over 10k years, and the small change is incorporated into the errors.

 

[QuixoticOne]

IIRC it is harder to do certain kinds of dating since the first nuclear tests because they did skew the isotopic ratios over much of the world somewhat.

There was a recent article about looking at the world wide isotopic richness shifts of meteorite origin metals to estimate the size of the impactors, for instance.

http://www.sciencedaily.com/re...08/04/080411160239.htm

There are some naturally occuring reactors that create anomalous isotope concentrations near them, and of course the reactor in the planetary core, but overall the isotopic decay on a world wide basis is pretty predictable since over even the last hundreds of millions of years the earth's mantle / crust hasn't changed isotopic composition much other than due to purely natural and very predictable rate decay processes.

There are various error sources and corrective calibration options mentioned / discussed
a bit under here:
http://en.wikipedia.org/wiki/Carbon_dating
q.v. "Carbon exchange reservoir".

 

[johnpombrio]

Radioactive dating occurs because living tissue creates and Ionized version of carbon while it is alive. They are not found in nature otherwise. They start to decay ata constant rate after death. I think you can only go back 10-20 thousand years with this technique. quite accurate tho in that range.

 

[LostUte]

Originally posted by: johnpombrio
Radioactive dating occurs because living tissue creates and Ionized version of carbon while it is alive. They are not found in nature otherwise. They start to decay ata constant rate after death. I think you can only go back 10-20 years with this techique. quite accurate tho in that range.

That's not how it works at all. C-14 is continuously created on earth. Living things are ingesting carbon (through food, photosynthesis, etc.), so their C-14 concentration is the same as the atmospheric concentration. Once the plant or animal dies, it is no longer ingesting carbon so the C-14 concentration begins to fall. The technique is accurate for much longer than 10-20 years (more on the order of thousands or tens of thousands of years).

 

[BrownTown]

Originally posted by: johnpombrio
Radioactive dating occurs because living tissue creates and Ionized version of carbon while it is alive. They are not found in nature otherwise. They start to decay ata constant rate after death. I think you can only go back 10-20 years with this techique. quite accurate tho in that range.

are you talking about something other than carbon-14 dating? Becasue if not then your wrong.

 

[jagec]

Radiometric dating constantly gets recalibrated and cross-calibrated as new papers are published. The cool thing about science is that if you keep good enough records, your work can actually be re-evaluated and gain accuracy as new discoveries are made and better analysis methods come online.

 

[gorcorps]

Originally posted by: jagec
Radiometric dating constantly gets recalibrated and cross-calibrated as new papers are published. The cool thing about science is that if you keep good enough records, your work can actually be re-evaluated and gain accuracy as new discoveries are made and better analysis methods come online.

Exactly. Actually the constants for carbon dating are pretty easy to analyze on a case-by-case basis, depending on the equipment available and the number of samples needing to be tested.

The equation for carbon dating (generally) is as follows:

ln ([C14]/[C14°])=k*t

where:
[C14]= concentration of C14 at a given time -OR- decays/(gram*time) in live organic matter
[C14°] = INITIAL concentration of C14 when sample 'died' -OR- decays/(gram*time) measured in sample
k=constant
t = time

What would change is the constant. But you can figure out the k by measuring the decay rate of C14, which is done and hasn't really changed.

Whats measured is the HALF LIFE of C14, or how long it takes for half of the C14 present to decay. That's measured to be 5730 years. Now that we know that after 5730 years, [C14] = (1/2)[C14°] and solve for k:

ln ((1/2)[C14°]/[C14°])= k (5730yrs)

**[C14°] cancels out**
ln (.5) = k (5730 yrs)
k = 1.21*10^-4 years^-1

 

[soccerballtux]

also assumes the rate of decay has been the same yes?

 

[f95toli]

Originally posted by: soccerballtux
also assumes the rate of decay has been the same yes?

Yes it does. But the decay rate is a property of the nucleus and has -as far as we know- been constant ever since the universe cooled down enough to allow the formation of carbon nuclei.
This means that the only way for there to be a significant error due to this would be if it turns out that one or more of the natural constants of the universe (e.g. the fine structure constant) has changed A LOT over time thereby affecting the strong/weak nuclear forces. There is NO evidence for any large change during tha past few billion years and definitly no evidence for a change over the very short time scales we are talking about here.

 

[soccerballtux]

For instance we know the speed of light is decaying
take a look at what happens when you reverse the rate of decay and interpolate

 

[f95toli]

Originally posted by: soccerballtux
For instance we know the speed of light is decaying
take a look at what happens when you reverse the rate of decay and interpolate

No, we don't. There are SOME experimental evidence (from rather crude astronomical data) that suggest that alpha (the speed of light is constant by definition, so what is perhaps is changing is the fine structure constant, there is litterarly no way to measure a change in the speed of light) might have changed a tiny bit over the past few billion years.
However, the interpretation of the data is somewhat controversial.

Moreover, alpha is as far as we know not changing now; at least not at a rate where it can be detected and the experiments that try measure changes in alpha over e.g a period of 12 months are many orders of magnitude more sensitive than the experiments used by the astronomers that published the initial data (which is why 12 months is enough).

Anyway, even if alpha is changing the rate of change would be so small that it would be of no consequence for carbon dating.

 

[KurskKnyaz]

Originally posted by: Onceler
isin't based on the asumption that carbon 14 is present at the exact same level in the past?
I would think that the amounts would be different for different time periods for various reasons

The ratio of C-14 to C-12 is constant in the environment. Since all life gets its carbon from the environment the ratio is also constant inside your body. When you die you stop eating so your body no longer absorbs carbon 14. So thousands of years from now someone can uncover your corpse and tell when you died based on c14/c12 ratio in a sample of your body.

The only event that I can think of that would change the c12/c14 ratio would be a colossal nuclear reaction that would cause the carbon nucleus to undergo neutron capture. C13 would have a slightly greater probability of capturing a neutron since the nucleus is larger and so the c14/c12 ratio would change. I can't image any geological event that would cause such an enormous neutron release.