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Re: Radiocarbon Dating (last post)



Haist, Brandon wrote:
> 
> "[carbon 14] Its half-life of 5,730 years made it useful for measureing
> prehistory and events occurrring within the past 35 to 50 thousand years.
> ...  ...  In this way, the age of an animal or plant 50,000 yrears old or
> less can be determinded."
> 

This has little to do with dinosaurs, but it is a radiometric dating
method, so I'll expand a bit on the C-14 method. Although 5730
years is the correct half-life, it is not the one used for most
C-14 dating, simply because the original half-life used to determine
dates back in the 1950s was wrong, and to be consistent we still tend
to use the wrong value (a bit like the direction of current flow
in electronics, which is the opposite of that which the electrons take,
but was the original and incorrect assumption).

The practical range for dating is in the order of a few hundred to
about 40,000 years BP. Any further back than that and your standard
deviations go way up. Also, C-14 years do not correlate with actual
calendar years, since the amount of C-14 isotopes in the atmosphere
has fluctuated in the past, and the dating method assumes it was
constant. Tree ring data (dendrochronology) can be used to even out
this inconsistency, however the oldest trees used for calibration
are in the order to about 6,000 years old, so any further back than
that and you can't correct your dates (although there are reportedly
some preserved huon pines in Tasmania that could take this right back
to 30,000 years or so, if anyone wants to spend half their life time
counting tree rings). Even if dates are corrected with tree ring data
they are still not considered calander years, but rather radiocarbon
years. So a 40,000 year C-14 date and a 60,000 year thermoluminescense
date could easily come from the same strata, right next to each other,
and possibly reflect a date of anything between 30,000 and 70,000
calender years depending on the standard deviations of your dates.
I know of some TL dates that are in the order of 50,000 years +/- 25,000
years, which with a two standard deviation limit puts it anywhere
between yesterday and 100,000 years ago.

Of course C-14 would never be of any use for dating dinosaur bearing
deposits, unless you want everything to date to around 40,000 years.

-- 
____________________________________________________
        Dann Pigdon
        GIS Archaeologist
        Melbourne, Australia

        Australian Dinosaurs:
        http://www.geocities.com/capecanaveral/4459/
        http://www.alphalink.com.au/~dannj
____________________________________________________