Thursday, 20 November 2008

Why Turnover Rate Matters

Hedges, REM; Clement, JG; Thomas, CDL & O'Connell, TC. 2007. Collagen turnover in the adult femoral mid-shaft: modeled from anthropogenic radiocarbon tracer measurements. American Journal of Physical Anthropology 133:808-816.

* Due to the testing of nuclear weapons, the amount of atmospheric 14C almost doubled in the 1960s and then began to decrease slowly.
* Collagen turnover is important in archaeological science because it affects both the precision of 14C dates and the accuracy of dietary reconstructions.
* Terrestrial food sources have the same ratio of 14C to the other carbon isotopes as the atmosphere, though the signal can take as much as a year to appear.
* d13C and d15N were measured as well as 14C. The d-values for males and females were significantly different.
* Between the ages of 20 and 80 years, the turnover rate of collagen decreases from about 4 to 3%/year for females and from 3 to 1.5%/year for males.
* "Human femoral bone collagen isotopically reflects an individual's diet over a much larger period of time than 10 years, including a substantial portion of collagen synthesised during adolesence."

The reason I'm interested in this paper is precisely because the turnover rate is important in palaeodietary studies, which is what I'm doing. Now that last conclusion, the one I quoted directly is particularly important to me. Our current dietary reconstructions are fairly broad and spotting changes that occurred in later life is particularly difficult. Which is the entire point of my project. If we can separate out the different stages, then those dietary changes should be fairly easy to see.

Shin, JY; O'Connell, TC; Black, S & Hedges, REM. 2004. Differentiating bone osteonal turnover rates by density fractionation: validation using the bomb 14C atmospheric pulse. Radiocarbon 46:853-861.

* Using bone density fractionation [bdf] to separate older, denser bone from yougner, lighter bone.
* The method can be used to remove diagenetically altered bone from archaeological samples.
* Adequate milling is essential for the method's success.
* Results show that the lighter fractions have 14C values that correspond wtih more recent parts of the bomb curve.
* The results were less clear in those individuals that were older at the time of the 14C peak, presumably due to already decreased turnover rates.

The importance of this paper lies in the fact that it's another tantalising glimpse of the method at work. The conclusion drawn by the authors is that bdf does in fact separate the bone into older-denser and younger-lighter fractions. It offers a hint at the conclusions of the paper above, with regard to the slow turnover rates after cessation of growth and the long residence times of bone. Adequate milling might be something to consider in the case of Wendell.


dystopia said...

adequate milling? is that even possible?

akika said...

It better be. If it isn't, I have a problem. But since this is a routine method in a number of labs, it should be. Admittedly, they're not doing palaeodietary studies, they're mostly medical labs, but that's not the point. This part of the method is the same.

dystopia said...

are they using leaky mills that remove sanity?

akika said...

They aren't, but I am.

dystopia said...

indeed, hence my question. which was not so much, is it possible at all. but rather is it possible with your evil mill

akika said...

Now I understand.
Hopefully, is the answer. I've managed to find a way to stop it leaking, so let's just hope that it continues to work. Either way, we'll find out next week [or possibly even tomorrow]!

dystopia said...

good luck!

maybe i'll come and make a nuisance of myself next week

my word verification word is 'mental', how rare for it to be an actual word (and i nearly misspell it, grrrr)