ELECTRONIC COPY OF THE ORIGINAL
Dr. Francis P. McManamon
October 17, 1999
Dear Dr. McManamon:
Please find enclosed the radiocarbon dating result for one bone sample "CENWW.97.R.24(MTa)/DOI1a" which was received on September 10. It was very small, requiring us to convert the sample carbon to graphite and then to count the radiocarbon atomically using an accelerator mass spectrometer (AMS). It provided plenty of carbon for reliable measurements and all analytical steps went normally. The quoted errors represent 1 sigma statistics. Since these errors cannot include uncertainties outside of those which can be quantified during measurement, it is best to consider them as minimum quotes.
Note that we notified your office upon beginning the analysis with an observation that the "R" in the submitter number on the sample package was not listed on the sample datasheet. Since it was listed on the sample package, we have used it in the reported sample designation number.
The bone sample was highly encrusted and in-filled with non-calcareous minerals. These minerals were physically eliminated with grinding, prior to demineralization of the apatite fraction with hydrochloric acid. The resultant protein extracted was subjected to alkali in high enough concentration to eliminate any secondary organic acid contamination. SEM analysis (photo-micrographs enclosed) were examined prior to pretreatment and after pretreatment (but prior to AMS analysis) to establish the integrity of the sample material.
The report sheet contains calibration results which enhance the accuracy of the radiocarbon dating. A hard-copy is enclosed showing the radiocarbon year/calendar year correlation curve segment associated with the radiocarbon date, along with explanation sheets. You will notice the X axis (cal BC age) that multiple two sigma ranges are possible for the radiocarbon date. This is discussed on the report sheet.
The results are reported in three formats; the Conventional
Radiocarbon Age (BP) which is systematic with radiocarbon dates quoted
without calendar calibration, calibrated calendar age (cal BC) which
is corrected for true half life and atmospheric fluctuations and reported
in calendar years, and calibrated Conventional Radiocarbon Age (cal
BP), where the same half life and atmospheric fluctuation corrections
are applied to provide a corrected BP format result (BP = before present,
present being AD 1950). The cal BC and cal BP results are reported using
the two sigma, 95% probability limitation. As noted on the report sheet,
if other lines of evidence give you confidence to use the one sigma
range on the calibrated results, you may use that range instead (which
is listed on the hard-copy calibration print-out). In summary, the results
Also enclosed is a Quality Assurance report showing the expected and measured ages for standards and a blind measured in the AMS. As I previously mentioned, we only rely on the AMS for the measurement. The machine is provided with our own standards, blanks, and blinds, already loaded in the target holder. The machine simply makes a measurement for us, which we verify. The QA report shows the measurement of two secondary standards (TIRI wood and TIRI turbidite). These two targets are international standards, with known consensus values. The "expected values" listed on the report are those consensus values. The "blind" listed on the QA report is a sample which had been previously analyzed by us. The AMS facility did not know the previous result for this blind.
A photo-documentary of the analysis is enclosed. Given the sensitivity of this analysis, each step of the analysis was carefully documented. Notes were taken by each individual involved in the analysis which consisted of myself Mr. Darden Hood, Director (20 years experience), Mr. Ronald Hatfield, Laboratory Manager (18 years experience), Mr. Christopher Patrick, Associate Manager (15 years experience), Ms. Teresa Zilko-Miller (12 years experience), Ms. Lethia Cerda, Office Coordinator (8 years experience), and Mr. David Miller, Staff (6 years experience). The sample graphite along with the necessary standards, already pressed into the target holder under our control, was sent to the AMS facility at Lawrence Livermore National Laboratory for measurement, and the result verified through our QA program.
One comment on the results is the 13C/12C ratio result. The value is elevated, indicating the individual had a C4 plant, or marine diet. Corn is the staple diet of most individuals with an elevated 13C/12C ratio. Since corn was not present 9000 years ago (to our knowledge), it suggests the likelihood of a marine diet. If this is the case, the presence of a "reservoir effect" in the diet may need to be considered. This effect may make the radiocarbon dating "too old" by some amount, perhaps by several hundred years.
The cost of the analysis was charged to your MASTERCARD. A receipt is enclosed. Also enclosed is excess poor quality bone which was not used in the analysis and the remaining protein extracted from the sample. As always, if you have any questions or would like to discuss the results, don't hesitate to contact me.
SAMPLE #: CENWW.97.R.24(MTa)/DOI1a
The equivalent calibrated calendar age (using the two sigma, 95% probability) in years BC is;
"cal BC 7560 to 7455 and cal BC 7395 to 7370"
The equivalent calibrated radiocarbon age (using the two sigma, 95% probability) in years BP is;
"cal BP 9510 to 9405 and cal BP 9345 to 9320"
Two ranges are possible due to "wiggles" in the calibration curve in this time region. A graphical representation of this calibration is enclosed. The two sigma range is quoted to encompass the delineation between separate radiocarbon events. One sigma ranges may be more appropriate for your research if other lines of evidence allow the use of higher precision. The one sigma ranges are "cal BC 7535 to 7480 and cal BP 9485 to 9430".
These calibration results are unique to the single Conventional Radiocarbon Age. Multiple measurements of the sample would provide statistically indistinguishable radiocarbon ages, each with its own unique calibrated range. For this reason, it is recommended that the calibration results be used in general terms.
When comparing the statistical agreement between radiocarbon dates, it is best to compare Conventional Radiocarbon Ages, as the calibration results may vary depending on the calculation format and time of calibration (ie calibration tables have changed through the years). The best average for muliple dates is to calculate a weighted average for Conventional Radiocarbon Ages and then do the calibration.
Reference standard results for Beta-133993