Re: C14 question for NancyB (was Re: Old rats)

From: NancyB (n.beavan_at_gns.cri-dot-nz.no-spam.invalid)
Date: 08/03/04 

Date: 2 Aug 2004 20:28:00 -0500

> Lee Olsenwrote:
On 2004-07-31 17:28:30 PST NancyB wrote: "I came here because a
> colleague told me that someone was enquiring about diet and
> radiocarbon. I am please to answer those questions, and give some
more
> background on radiocarbon and my work in general,...."
>
> I hope you will be willing to answer this one.
>
> If a single archaeological site contains a series of radiocarbon
dates
> (obtained on humus soils spanning thousands of years) that are
known
> to be wrong, would there be any reason to believe a bone found
within
> the same column would be any less likely to be contaminated than
the
> humus? Another words, would bone have special resistance or be
> armored in any particular way compared to the humus soils, or is it
> just as vulnerable to the same processes as any thing else in a
column
> that could potentially be dated?
>
> Thank you in advance.

Hello Lee:

This is a very good question, which goes to "what am I dating, and
what did they do to pretreat the sample before dating?"

If you date soils thru a sequence, it is preferable to isolate plant
fragments or charcoal in the soil. This is because humic acids are
mobile. Submitters usually send in discreet samples from fine,
easily identifiable layers ( that is, from an individual varve, or a
piece of plant or charcoal which they have carefully extracted from
an identifiable layer in their sequence.) Especially with the advent
of AMS dating which requires small ( mg sized) samples, you can do
this, rather than "bulk" soil dates which may have been taken over a
large portion of the sequence.

All materials for 14C dating are then chemically treated to extract
humic acids and other exogenous contaminants. What one wants to get
to is the specific material, well treated, which will be dated.

A bone which was extracted from a specific layer would first be
physically cleaned to remove all dirt and root hairs. Then, the bone
would be demineralised in an acid solution to remove the calcium
carbonate / apatite structure of the bone. This is because carbonates
from the soil could link with the bone apatite, and you aren't
concerned with the carbon from bone mineral -- you want to test the
bone protein.

The material left after demineralisation is called collagen. Collagen
is then further purified in a number of ways to remove any further
humics present. It is then processed to the purest form of protein,
gelatin, and is filtered and often separated by molecular weight.

You can test the purity of the protein and the possible deterioration
of same by any of the following: a Fourier transform analysis ( is
there something in the protein which indicates exogenous materials
remaining), an amino acid analysis ( does the bone protein have a
collagen like amino profile), or carbon to nitrogen ratio (carbon
nitrogen ratios of 2.8 to 3.7 as an indication of preservation, as
this is the range previous published in experiments on modern and
preserved ancient bone; see DeNiro, M J, 1985. Nature, 317, 806–9;
though this article was actually on determining best isotope ratios
for other work in stable isotope analysis.)

Read the description in the methods section of a paper to find out
exactly what an author has done re: sample preparation and testing of
protein purity.

Other ways to further test the reliability of a date:

1) The author could do paired sample analysis with two different
materials from the same layer: bone and charcoal, or bone of two
different species. How well do they agree?

2) Are the dates in the sequence that you would expect? is the
youngest from the uppermost layer, are the oldest at the bottom?

3) Is there another analysis other than 14C that could also be applied
here ( such as optically stimulated luminescence of quartz grains in
the layer; see Huntly et al 1985. Optical dating of sediments. Nature
313:105-107.)

Cheers

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