carbon dating

[Jdeutsch]

I have a question about carbon dating.  Seems like it is useful for about 10x T1/2  of C14  

 

During fossilization, I assume there is exchange or replacement of molecules as "bone turns to stone"which might include carbon (in carbonates).

 

How does one account for carbon exchange when looking at isotope degradation? 

[ynot]

I think t has to do with the type of carbon that is used in the dating test. IE carbon 14

Carbon 14 is a biologically created carbon compound and is different than the carbon in carbonates.

[FossilDAWG]

Carbon-14 is not biologically created, it is generated by cosmic rays colliding with nitrogen in the upper atmosphere, especially at high latitudes where the Earth's magnetic field curves down to the surface and allows cosmic rays to penetrate the atmosphere more effectively.  Subsequently C¹⁴ can react with oxygen, initially to form carbon monoxide but that oxidizes to for CO2 which becomes dispersed in the atmosphere.  Plants use CO2 to produce sugars and subsequently other biomolecules, so plants (while alive) are at equilibrium with atmospheric C¹⁴-CO2, as are animals that eat those plants.  This equilibrium is maintained as long as the plants (or animals) are alive, as (1) those biomolecules turn over, which is to say they are recycled and replaced with newly synthesized biomolecules, and also (2) the lifespan of almost all plants and animals is very short compared to the half-life of C¹⁴.  When plants or animals die, they cease taking in CO2 (or fixed CO2 in the form of plant or animal food) so their C¹⁴ content is not being replenished.  From that point on, their C¹⁴ content will decrease according to the rate of radioactive decay, which generates a half-life of 5,730 (+ or - 40) years.  After about 10 half-lives the content of C¹⁴ becomes too low to measure accurately, which puts a limit of about 60,000 years for useful dating by C¹⁴ content.  Note that this process is a bit more complicated in actual practice, as it is known that the rate of C¹⁴ production varies somewhat over time due to solar activity and the generation of cosmic rays.  This can be detected and corrected for by measuring C¹⁴ in ancient tree rings of precisely known age, and applying that correction factor to other types of samples such as bone.

 

The half-life of C¹⁴ is very very short compared to the age of almost all carbonates.  For example, a 50 million year old Eocene limestone will have gone through over 8,726 half-lives, so the C¹⁴ content remaining would be 1/2^8,726th what it was when the limestone was deposited.  This is far below any possible detection limit, and in practice it is doubtful that any C¹⁴ would make it from a carbonate deposit into a piece of organic material with which it had contact (say via ground water).

 

Don

[ynot]

@FossilDAWG Thanks for the tutorial! Very nice!

 

Guess I flunked this one.

[jpc]

FossilDAWG... that was one of the best explanations I have seen in a while.  

[Jdeutsch]

Thanks for explanation

 

So back to my original question-in a younger fossil- say a 50,000 year old something- if left in a box, it's native 14C would have gone through nearly 10 half lives and be depleted.

But as it fossilizes,  is it now reaching some new equilibrium because minerals in the specimen are exchanging with minerals in the environment.  (if replacement was being done with environmental calcium carbonate then you would be adding "high 14C" and making the specimen seem younger than it was)

 

Does that happen, and if so is there a way to account for it?  

 

I guess I have a much better feel for isotopes than the process of fossil formation.

[FossilDAWG]

I thought I had covered this.  The carbon in your calcium carbonate was "fixed" when the rock was deposited.  Rock does not breath, so atmospheric CO₂ is not continuously incorporated into limestone deposits, for example.  The carbon in an Ordovician limestone has been there since the calcium carbonate was deposited about 500 million years ago.  In a Cretaceous chalk deposit, the calcium carbonate is formed of the minute shells of foraminiferans that lived in the Cretaceous (about 80 million years ago in the case of the White Cliffs of Dover for an example), died, and their shells settled to the sea floor to form an ooze that was subsequently compacted and dewatered to form chalk.  The carbon in those shells was not exchanged with atmospheric CO2, nor have the shells grown and added new material, since the foraminiferan that made the shell died 80 million years ago.  Any ¹⁴C that was present in that carbon has gone through so many half lives that (mathematically speaking) none of it remains.

 

I think perhaps you are confused by experience with long-lived radioisotopes that are present as mineral deposits or an original component of the planet.  Every atom of Uranium-235, which has a half life of ~704 million years, was formed in the core of a giant star and dispersed during a supernova, and so it has been a constituent of the planet since the Earth's formation.  The conditions to make ²³⁵U do not exist on the Earth.  This is totally different from ¹⁴C, which is continuously produced in the upper atmosphere by cosmic rays colliding with nitrogen.  There is no source of ¹⁴C within the Earth, it is all generated in the atmosphere.  Any ¹⁴C that may have been present when the Earth condensed from the cloud of gas and dust orbiting the sun is long gone, as that event was over 842,000 half-lives ago.  The only possible way it can get into calcium carbonate is by the action of the plants or animals that pulled it out of the atmosphere and added it to their shells.  Once the shell-maker is dead it ceases adding carbon to the shell, and the ¹⁴C that was put there begins its inexorable path to decay.  Your comment about environmental calcium carbonate adding high ¹⁴C to the mineralizing fossil could only be true if the environmental calcium carbonate had been deposited within the last half-life or two of ¹⁴C, which is to say the organisms that generated the calcium carbonate were alive within the last 10,000 years at most.  I cannot think of an actual geological context that this scenario would apply to.  

 

Moreover, if a fossil is buried in calcium carbonate (limestone), surely it is the same age as the limestone?  How could limestone generate an anomalously young age for, say, a shell that is encased in that limestone, unless the shell is somehow much older than the limestone it is embedded in?

 

¹⁴C is only very rarely used to date fossils, such as unmineralized very late Pleistocene bones, because the half life is so short.  Dating older fossils or rocks relies on other isotopes/radioactive decay series with much longer half lives.  In such cases it is true that care must be taken to avoid sources of environmental contamination.  One tactic is to use different elements, with different decay rates, to independently generate dates for formation of a rock deposit.  If two or three different isotope series give congruent dates, there is a good likelihood that the date is correct.  If ground water, for example, percolates through a deposit and leaves behind (or removes) certain isotopes, it is very unlikely that it will affect multiple different elements in exactly the same way, considering the differences in chemical reactivity, solubility, etc between different elements.  In that case different radioisotopes will give divergent dates.  These days, a lot of radioisotope dating is based on elements lock in zircon crystals, which are chemically inert and so are resistant to contamination.

 

Don

 

 

[Jdeutsch]

 

 

I'm finding your answers to be very interesting.

 

When doing carbon dating (I'd ask about any other dating but I'll start with carbon)  How do you process a specimen?  How long do you collect counts?  I assume the cpm must be pretty low.

[doushantuo]

QUDAT

[doushantuo]

There are three principal isotopes of carbon which occur naturally - 12C, 13C (both stable) and 14C (unstable or radioactive). These isotopes are present in the following amounts 12C - 98.89%, 13C - 1.11%, and 14C - 0.00000000010%. Thus, one carbon-14 atom exists in nature for every 1,000,000,000,000 or (1 in a trillion) carbon-12 atoms in living material. The radiocarbon method is based on the rate of decay of the radioactive or unstable carbon isotope 14 (14C), which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen-14. The reaction is: 14N + n => 14C + p (Where n is a neutron and p is a proton) The 14C formed is rapidly oxidized to 14CO2 and enters the earth's plant and animal life ways through photosynthesis and the food chain. The rapidity of the dispersal of 14C into the atmosphere has been demonstrated by measurements of radioactive carbon produced from thermonuclear bomb testing. 14C also enters the earth's oceans in an atmospheric exchange and as dissolved carbonate [the entire 14C inventory is termed the carbon exchange reservoir (Aitken, 1990)].

 

This information of course overlaps somewhat with Fossildawgs excellent explanation

14C is of course very handy in recharge hydrology studies.We like to know where our groundwater is coming from

 

a bit of "literature archeology" turned up this:

 

halflife

 

dating scene

 

short review

NICE!

 

fudger

 

a word of caution:there are indications from quantum mechanics that decay could be something other than strictly exponential

 

 

[FossilDAWG]

I believe the measurement of relative abundance of isotopes is done by very high resolution mass spectrometry, not by counting cpms (counts per minute for those readers who are not into radiochemistry.  Each decay of a radioactive nucleus generates some sort of particle or energy that can be measured).  In theory one "count" corresponds to the decay of one nucleus, but that assumes 100% efficiency in detecting decay products such as relatively low energy beta particles produced by ¹⁴C decay; in reality cpms are an inefficient way of counting decay events, most scintillation counters detect only a fraction of the total decays.  They work well if you are using radioisotopes in an experiment, as you can put enough ¹⁴C (or tritium, or ¹³⁵I, or ³⁵S, or whatever isotope you prefer) into your experiment to be able to detect a large number of "counts".  If you are dealing with a sample that may produce a single decay at intervals of several minutes they are hopelessly insensitive.

 

On the other hand, high resolution mass spectrometry can detect both ¹⁴C and ¹²C (and also ¹³C which is not radioactive).  The ratio of ¹⁴C to ¹²C will be an accurate measurement of how much ¹⁴C has been lost to radioactive decay, as we know what the starting ratio should be (all living organisms have the same ratio, which is the ratio of ¹⁴C to ¹²C present in the atmosphere) and modern mass spectrometers can measure incredibly tiny proportions of ¹⁴C.

 

Don

[ashcraft]

I think the original poster was wondering how to carbon date a bone that has petrified.  As I understand it, you cannot use carbon dating on such a specimen, it has to be original material, as some or all of the carbon has been removed during the petrification (permineralization ) process.  It is not usually an issue with material within the working limits of dating with C-14, as most remains are still remains during that time period.

 

Please correct me if I am wrong in any or all of my assumptions.

 

Brent Ashcraft

[FossilDAWG]

Hi Brent,

 

I think you are correct.

 

Don

[jpc]

2 minutes ago, FossilDAWG said:

Hi Brent,

 

I think you are correct.

 

Don

Yes... 

 and for those interested,there are very few place in the US that actually do thiswork.  We had some Wyoming samples sent to a lab in Florida.  Costs about 700 bucks per sample.  

[ashcraft]

I did a little reading.  Mass specs are the preferred method of measuring, and mass specs are relatively common.  The problem comes in sample introduction to the mass spec, which basically separates the components so they can be measured.  The mass specs I have used were attached to a gas chromatograph, others are coupled to an inductively coupled plasma, but for C-14 work, they use a mass accelerator.  I have been out of the business for 16 years, and I don't even know what that is.

 

Brent former mass spec operator Ashcraft

[Ptychodus04]

My brain hurts...

[ynot]

14 minutes ago, Ptychodus04 said:

My brain hurts...

-- So does Mine.

[Jdeutsch]

Doushantuo provided some information regarding sample preparation (see his links)- for those interested, at least in some systems, the carbon in the sample is oxidized to CO2 and then reduced to Benzene as benzene is more than 90% carbon (72/78) whereas CO2 is perhaps 30%  carbon (12/44).  I looked up a few articles and it looks pretty straight forward in principle.

 

Others use direct absorption of the generated CO2.   Most of the articles I've seen so far referred to liquid scintillation.

eg  http://www.sciencedirect.com/science/article/pii/S0969804309000414

 

 I still haven't broached the MS literature and am still a long way from looking at Fossil Hounds "element lock" used in dating older specimens