Re: [asa] Denver RATE Conference (Thousands...Not Billions)_Part 2

Kirk Bertsche <Bertsche@aol.com> wrote on 09/24/2007 11:41:54 PM:
(http://www.calvin.edu/archive/asa/200709/0573.html)

> On Sep 24, 2007, at 4:01 PM, Steven M Smith wrote:
>> Baumgardner continued: [ ... ] Now the laboratories
>> subtract a "standard background from the actual C14
>> measurement that is equal to about 40,000 years." The
>> laboratories do this to "avoid the embarrassing
>> reality that essentially all fossil carbon contains C14.
>
> No, laboratories do this because AMS mass separation is
> not perfect, and there is always a small background that
> must be subtracted.
>
[ ... ]
> It would be interesting to know where they sent these
> coal and diamond samples, how they were prepared, and
> whether or not backgrounds were subtracted. Based on
> what he said earlier, I suspect he is quoting raw
> numbers without background subtraction; in this case
> his dates are just the AMS background.

Kirk,

Thanks for sharing from your perspective and experience with C14. I also
appreciated the link to the LLNL report. I am not a radiocarbon
specialist and probably know just enough to be dangerous. I have been
perusing the Radiocarbon literature to check out some of Baumgardner's
statements and have also obtained a partial xeroxed copy of a few pages
from Baumgardner's RATE vol. II chapter. I don't know where they sent
their samples but here is a quote concerning how the first diamond sample
was prepared, analyzed, and reported.

  "Our sample consisted of about 50 mg of sub-millimeter diamond chips
obtained by shattering a diamond from the Kimberley district in South
Africa in a sapphire mortar and pestle. Because of the laboratory's lack
of experience in oxidizing diamond, it required several attempts before
they were successful. ... The 14C/C value for the diamond, which as in the
case for the coal samples was a composite number based on four separate
AMS runs, was 0.096+/-0.026 pMC, where the precision represents +/-l
[standard deviation], or 68.3% confidence limits. This number, unlike the
coal results presented above, does not have the laboratory's standard
background of 0.077+/-0.005 pMC subtracted from it. The reason for
reporting the uncorrected measurement here is to be able more clearly to
compare it directly with the laboratory's standard background value. From
a statistical standpoint, the result for this diamond overlapped, in terms
of its confidence limits, the value obtained from a much larger number of
runs on the purified natural gas the laboratory uses as its background
standard. The fact that the diamond displayed a comparable 14C/C value as
the natural gas background standard, however, was consistent with our
working hypothesis that all carbon in the earth contains a detectable and
reproducible (using the AMS technique) level of 14C." (p. 609-610)

Baumgardner then sent in 6 more diamond samples (including the original
sample) - 5 from kimberlite pipe mines and 1 from alluvial deposits.

  "The 14C/C values for these six diamonds cluster tightly about the mean
value of 0.12+/-0.01 pMC [background not subtracted]. From a statistical
standpoint, this mean value is consistent with an identical 14C/C ratio in
all six of the diamonds. With the larger number of diamond analyses, there
was now a clear statistical separation between the mean diamond value and
the laboratory's background value of 0.08 pMC obtained by repeated runs on
a sample of purified natural gas. The laboratory concurred with this
conclusion. We note that by using the usual uniformitarian assumptions for
converting a 14C/C value into an age (which obviously do not apply since
these diamonds almost certainly have not experienced any recent exchange
of their carbon atoms with those in the atmosphere), one gets a
uniformitarian age from this mean 14C/C ratio of 55,700 years." (p.
610-611).

Next Baumgardner sent in an additional 6 diamond samples from Namibian
alluvial deposits. The results for all 12 diamonds were reported in table
6 (p. 614) with the laboratory standard background correction applied.

  "Although the 14C/C ratios shown in Table 6 are small when the standard
background is subtracted away, and in some cases are smaller than the
confidence interval of the measurements from which they are derived, they
nevertheless are all positive, that is, greater than the laboratory's
standard background value. If one averages the values in Table 6 of the
five diamonds from kimberlite mines to get better statistics, one obtains
the value 0.04 pMC. Doing the same for the seven alluvial samples, one
obtains 0.12 pMC. These data suggest, at least from a statistical
standpoint, that 14C exists in these diamonds to a high degree of
certainty, as astonishing as that may seem." (p. 613-614)

Although 12 diamonds is not a sufficient number of samples to
statistically determine average C14 concentration ranges, I find it
interesting that their own data suggests a possible source for excess C14.
 The kimberlite mine (deep underground) diamonds have corrected 14C/C
ratios ranging from 0.01-0.07 pMC (average 0.04 pMC) - the highest values
being barely above the confidence interval of measurement. The alluvial
diamonds (mined from surfical stream gravels with presumably 1,000's of
years worth of exposure to modern C14 levels) have corrected 14C/C ratios
ranging from 0.03-0.31 pMC (average 0.12 pMC). This suggests to me that,
despite all the care that may have been taken during sample preparation,
there is probably still a problem with modern C14 contamination.

Combine possible modern atmospheric C14 contamination with other
non-atmospheric sources of C14 (thermal neutrons from U/Th decay
converting trace N14 to C14; cosmogenic C14; ?) and you *may* have an
explanation for the slight enrichment in some diamonds. To be fair,
Baumgardner does address some of these non-atmospheric C14 sources and
concludes that their influence is 4-5 orders of magnitude too small to
explain the RATE results.

Steve
(Disclaimer: Opinions expressed herein are my own and are not to be
attributed to my employer ... or anyone else.)
_____________
 Steven M. Smith, Geologist, U.S. Geological Survey
 Box 25046, M.S. 973, DFC, Denver, CO 80225
 Office: (303)236-1192, Fax: (303)236-3200
 Email: smsmith@usgs.gov
 -USGS Nat'l Geochem. Database NURE HSSR Web Site-
  http://pubs.usgs.gov/of/1997/ofr-97-0492/

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Received on Wed, 26 Sep 2007 09:56:51 -0600