From: higham@waikato.ac.nz
Newsgroups: sci.archaeology
Subject: Re: Carbon 14
Date: 15 Apr 92 13:05:37 +1200
Organization: University of Waikato, Hamilton, New Zealand

In article <22672@dog.ee.lbl.gov>, mike@fionn.lbl.gov (Michael Helm) writes:
> I've been meaning to post this for a while.  Hope you all 
> still find this of interest, even tho the thread's a bit stale.
> 
> Woody Baker writes:
>>It all boils down to faith.  Everything has to be taken by faith,
>>even the uniformatarian principle that underlies the theory of ...
> 
>>Tom Owens writes (...we have no clear examples of carbon 14 data being
>>incorrect...).  
> 
>>That of course, calls most c14 dates into question, as we can't
>>REALLY know what the c13:c14 ratio was. We have to take it on FAITH.
>>It would be most intersting to analyze the air trapped in ICE cores
>>to see if the RATIO of C..:C14 has ever changed.  It seems that one
> 
> [I've just got to say that I don't have a lot of faith 
> that faith is involved in nuclear physics.  Thank you.]
> 
> Ok.  Here's an extract from a book about Irish archaeology.
> (Michael O'Kelly, "Early Ireland", Cambr 1989)  From App A,
> "Radiocarbon Dating".
> 
> ".... 14C which is constantly produced in the atmosphere out of
> Ni, by nuclaer reactions with neutrons in [by] the so-called
> cosmic radiation...."
> 
> "Calibration
> 
> "In the early days of the radiocarbon 'revolution' it was believed
> that dates expressed in radiocarbon years were the same as solar
> or calendar ones. In the 1960s a series of wood samples of which
> the solar or calendar date had been established by tree-ring
> analysis (see p 343 [another appendix discusses this technique
> & what has been done in Ireland & other places, with references])
> were submitted for radiometric determination & it was found that
> as a rule there was little or no parity between them, particularly
> for dates earlier than the 2nd millenium BC.  The tendency was for
> the 14C dates to be too young (recent).  A calibration table
> based on tree-ring (dendrochronologic) sequence is therefore necessary
> in order to obtain the approximate equivalent in calendar years.
> 
> "As a very ruf-and-ready rule of thumb it may be said that uncalibrated
> dates in the 1st half of the 3rd millenium BP are too recent by 100-200
> years;  in the 4th millenium BP by 200-500+ years;  & in the 5th
> millenium BP by 500-800 years (Burgess 1980,18). "
> 
> Burgess, CB 1980 _The Age of Stonehenge_ London
> Bailie, MGL 1985 "Irish dendrochronology & radiocarbon calibration", UJA 48,
>    11-23
> Pearson &al 1986 "High Precision 14C measurement of Irish Oak to show
>    the natural 14C variations from AD 1840 - 5210 BC" Radiocarbon 28, 911-34
> 
> Harbison in "Pre Christian Ireland" (p 10) says that the Belfast laboratory
> (see Baillie) has shown that 14C dates falling between 800-400bc can
> no longer be trusted, bedause they can't be distinguished from each
> other.  I have seen a picture of the calibration curve of 14C &
> it happens that it is not monotonic during that period!  I don't
> have a ref for it but I would guess looking for Baillie citations
> mite fetch it up for you.  Now I knew that the modern era could
> not be 14C dated -- side effect of nuclear testing -- but this is
> the 1st I'd heard that bumps like this existed in the past.
> 

When radiocarbon dating was first developed, Libby and co suggested that the
method rested upon a number of assumptions that would need to be tested and 
investigated. One was the constancy of the decay process. It was assumed that
the relationship between C14 creation in the cosmic ray flux and the subsequent
decay was equilibrius. Using dendrochronology, this was found to be an
assumption that was incorrect. The calibration curves given in Radiocarbon 28
serve to remind us that the relationship between calendar and conventional
ages is not linear, there are "wiggles" caused by variation in sun activity
which affect the amount of C14 created in the upper atmosphere. This means that
one conventional age may correspond with not one but 2, 3 or even 5 calibrated
ages. This is a natural phenomenon. I don't think its fair to say that
radiocarbon dates can 'no longer be trusted', all it means is that there are
more permutations in calibrating at certain conventional ages. 
	You say that at 800-400 BP the curve is very wiggly, I assume you are
looking at the decaadal curve? Its not actually that bad compared to some of
the others-2000-2500 BC for example. It can sometimes be frustrating to
calibrate and find your conventional age bisects a curve in 4 or 5 places, but
as I say, its just one of those things-it just means you have to accept that
your date, calibrated could be either cal date 1, cal date 2 or cal date 3,
statistically they are all possiblle 'true' ages, 
then there are the errors to consider! 
	Beyond the dendro chronology (around 8000 bp are our oldest trees)
there is no calibration for radiocarbon, however, some  work is being done
to rectify this, and other dating methods which overlap C14 to a certain extent
(uranium series disequilibrium for example) are being used to compare with 
radiocarbon. From what I know, this has been very favourably compared as
precision increases. Also there are varves which have been dated in
Scandinavia but I don't know how far these go back, possibly 10000-12000bp.
Maybe someone knows more about varve dating.


Tom.