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

From: Michael Roberts <michael.andrea.r@ukonline.co.uk>
Date: Thu Oct 18 2007 - 06:35:38 EDT

Steve

You have not actually said it, but are you saying that B either doesn't know what he is talking about or his whole approach is full of what Winston Churchill called terminological inexactitudes.

This type of thing is what I found in my studies on YEC presentations of radiometric age dating on pre-RATE stuff going back the work of SF entitled The Genesis Flood.

I find I have both moral and scientific concerns about this type of thing - probably more the former.

Michael
  ----- Original Message -----
  From: Steven M Smith
  To: asa@calvin.edu ; Kirk Bertsche
  Sent: Wednesday, October 17, 2007 5:12 PM
  Subject: Re: [asa] Denver RATE Conference (Thousands...Not Billions)_Part 2

  Kirk Bertsche wrote on 10/15/2007 11:32:28 PM:
> ...
> I recently posted some general comments about radiocarbon and
> contamination. Since then I have gone through the written RATE
> radiocarbon claims in detail, read most of the references, and have
> spoken with the expert who measured the radiocarbon samples for
> them.
> ...

  This information is excellent. Thank you, Kirk!

  I also have obtained Baumgardner's list of 90 "AMS measurements on samples conventionally deemed older than 100,000 years" (Table 1) and have been checking the cited references. I have made it through about half of the articles. Unlike Kirk, I do not have a background in radiocarbon research and am on a steep learning curve. But I thought I would share a few things I have learned so far with the list.

  1) Early expectations of the Accelerator Mass Spectrometry (AMS) technology were that it might be able to measure Carbon-14 (C-14) dates up to 100,000 years old and should allow measurements of much tinier samples than the conventional beta counting method. The reality is best summed up by the title of a journal article in _Nuclear Instruments and Methods in Physics Research B_ (NIM-B), "Early expectations of AMS: Greater ages and tiny fractions. One failure? - One success" (1987, v.29, p. 97-99). The AMS may obtain marginally older dates than the conventional beta counting method but its biggest success is in the ability to process samples much smaller than 500 mg. During the conference, Baumgardner made a big deal out of the fact that AMS technology could potentially obtain C-14 dates back to 100,000 years (100 ka) and suggested that this "failure" was due to intrinsic C-14 in every bit of fossil carbon. The article cited above states, "The age limit depends on true or apparent ion counts, which may originate in sample preparation, in the accelerating system itself, or from ions other than 14C which are not adequately suppressed."

  2) A large number of the reports of C-14 in Carbon "dead" samples that Baumgardner cites come from articles that describe the methodology & instrumental set-up of new AMS facilities. Note some of the following titles: "Accelerator mass spectrometry of 14C at Nagoya University", "Accelerator radiocarbon dating at SFU", "AMS 14C measurements and preparative techniques at NIES-TERRA", "AMS at the TTT-3 tandem accelerator in Naples", "AMS sample handling in Groningen", "An assessment of laboratory contamination at the IsoTrace radiocarbon facility", etc. (That is just the articles with titles that start with the letter A.) As I read these articles, my weird sense of humor kicks in and I am reminded of a bunch of kids at school comparing their cars or stereos. "My tandem AMS can get a date as old as 85 ka on a blank aluminum sample holder with the ion gate closed." "Yeah, well my Tandetron AMS get >90 ka!" These are articles about experiments that are testing the absolute limits of individual laboratory facilities, not experiments designed to measure "intrinsic" or "in-situ" C-14. For some reason, Baumgardner did not list C-14 dates of 85,000 years for blank aluminum sample holders in his Table 1.

  3) As Kirk points out, many of Baumgardner's cited samples also come from studies of sources of laboratory C-14 contamination in the sample preparation or instrumental analyses. The two samples in Baumgardner's list that Kirk noted are prime examples that Baumgardner is ignoring this fact. Each sample was listed twice: First with a C-14 measurement on a "untouched" fraction, and a second measurement made after "graphitization" to measure the component of introduced contamination. The article I mentioned above under note 1 lists "Geological graphite -- powdered and encapsulated under argon: 69,030 +/- 1,700. Same sample -- except prepared in air: 63,800 +/- 1,100. An article by D. L. Kirner, R. E. Taylor and J. R. Southon (Reduction in backgrounds of microsamples for AMS 14C dating, Radiocarbon, 37, 697-704, 1995) summarizes a lot of work on sources and levels of C-14 contamination in AMS work. I reproduce their Table 1 below that gives 4 different general sources of background C-14 -- each with 2 or 3 types of possible contamination (9 types of contamination total). Baumgardner ignored 8 of these sources of C-14 contamination, implying that his measurements were all due to 1 category in this table -- "Non-14C-"dead" sample: sample erroneously assumed to contain no 14C."

  -----
  TABLE 1. Potential Sources of Background in AMS 14C
                   Measurements Using Catalytically Condensed
                   Graphitic Carbon Targets

  A. Machine background: 14C detected when graphite in the
       sample holder does not contain 14C.
      1. Detector anomaly: 14C pulse registered in detector
           circuitry when no 14C ion is present.
      2. Ion identification anomaly: particle of same 14C
           mass/energy ratio as 14C but which is not 14C reaches
           detector.
      3. Beam-line contamination: 14C internally derived from
           an accelerator beam line component reaches detector.

  B. Combustion/acidification background: 14C introduced
       during production of CO2 from sample.
      1. Materials contamination: 14C introduced from materials
           in combustion/acidification tube.
      2. Tube contamination: 14C introduced from combustion
           tube.

  C. Graphitization background: 14C introduced during the
       graphitization process.
      1. Materials contamination: 14C introduced from materials
            in reaction tube.
      2. Tube contamination: 14C introduced from reaction tube.

  D. Pseudo 14C-"dead" sample background: 14C is present
       in a sample material that should not contain 14C because
       of its geologic age.
      1. Non-14C-"dead" sample: sample erroneously assumed
            to contain no 14C.
      2. Contaminated 14C-"dead" sample: 14C introduced into
            material that contains no 14C.
  -----

  4. Radiocarbon measurements are different than other radioisotopic dating methods. Most methods (K-Ar for example) measure the amount of radioactive parent material (K-40) and the amount of resultant daughter isotope (Ar-40) left in the sample. The ratio of K-40/Ar-40 is used to determine the age of the sample. Hence a little bit of Ar-40 contamination (the most common contamination problem in the method) primarily affects the ages of very young samples. Thus K-Ar dating is seldom used, or used with greatest caution, on samples less than 1 million years old (i.e. 5,000 years +/- 1 million years is a problem; 246 million years +/- 1 million years is not). However, radiocarbon instruments do not measure the parent (C-14) to daughter (N-14) ratio, they measure total C-14 present and compare it to modern carbon (either C-14/C or % modern carbon: pMC); therefore a small amount of C-14 contamination becomes critical only for the oldest samples (i.e. 49.25 +/- 0.01 pMC [~5,000 years] is not much of a problem; 0.01 +/- 0.01 pMC [~70,000 years] is a problem).

  It is interesting to me that the more I read the actual literature Baumgardner cites, the less concern I have with his small amounts of C-14 in carbon "dead" materials issue.

  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 Thu Oct 18 07:44:01 2007

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