Uncertainty of Radiocarbon Date

Poznan Radiocarbon Laboratory worksation

Uncertainty of radiocarbon date quoted by 14C laboratory usually comprises uncertainties connected with 14C counting statistics, and with fluctuating level of contamination introduced during sample preparation, and fluctuating parameters of 14C- measurement.

Typical precision of our 14C dates

Uncertainty (error, precision) of 14C date (1-sigma) quoted by the Poznań Radiocarbon Laboratory is determinted basing on counting statistics or standard deviation of results (5 at least) of partial 14C measurements of  the dated sample, and of the samples of modern standard and background. Practically, significant deviations between these two estimates occur very rarely.

Average precision of 14C date depends on the age of sample. For young samples, the precision is mainly limited by 14C counting statistics, and when very many 14Cs are counted – by reproducibility of sample preparation. For very old samples, the crucial component of date uncertainty is fluctuation of background level, depending on the AMS spectrometer itself, and on contamination introduced during sample preparation.

Uncertainty of individual 14C date may differ from the average. The statistics of uncertainties of 14C dates obtained for samples containing >1 miligram of carbon, is presented in diagrams shown below. Uncertainties of dates of samples smaller than 1 mgC increase with decreasing mass of sample.

Uncertainty of Radiocarbon Date - Poznan Radiocarbon Laboratory

 

Uncertainty of Radiocarbon Date - Poznan Radiocarbon Laboratory
Statistics of uncertainty of 14C dates quoted by the Poznań Radiocarbon Laboratory. Distributions of uncertainty are different for different intervals of sample age. The presented statistics concerns samples containing >1mg of carbon

Small samples

Routinely, for 14C AMS measurement we forward 1 miligram of carbon. To analyse smaller samples, we studied the dependence of background on sample mass (using many small background samples), and studied efectiveness of correction for isotopic fractionation. These studies showed increasing scatter of 14C/12C ratios with decreasing mass of background samples. They also showed decrease of 13C/12C ratio with decreasing sample mass and demonstrated, that fractionation correction normally used in 14C measurements was not always effective for samples smaller than ca. 0.2 mg C.

The error of dates increases with decreasing mass. For young samples, it is mostly due to uncertainty of isotopic fractionation correction. For old samples, uncertainty of background and low number of 14C counts becomes most critical. Average errors of our 14C dates, for different sample ages and sample masses, is shown in table displayed below.

For very small samples, we from time to time fail to make 14C measurement. Frequency of so unlucky measurements (denoted in the table as “failed”) increases with decreasing mass of carbon (in the rage 0.1-0.2 mgC we fail to date ca. 10%  of samples).

Average uncertainties of 14C dates reported by the Poznań Radiocarbon Laboratory, shown for different sample ages and different masses of carbon
Age (kyr BP) failed 0-1 1-3 3-6 6-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50
Mass (mg C) %
± average error (yrs )
>1.0
—
 30 35 40 50 60 100 270 340 520 660 1200 2000
 0.8-1.0
—
 30 35 40 50 60 110 270 360 550 700 1300 3000
 0.6-0.8
—
 35 35 40 60 70 140 290 400 600 800 1500 —-
 0.4-0.6
—
 40 40 50 70 100 140 290 400 600 800 2000 4000
 0.3-0.4
0.5%
 40 50 50 70 100 160 300 400 900 1000 3000 5000
 0.2-0.3
  2%
 50 50 60 80 110 170 300 410 1000 1300 3000 —-
 0.1-0.2
 10%
 60 60 70 100 150 200 370 530 1200 2000 4000 —-
 0.0-0.1
  51%
 100 110 110 160 190 250 360 900 —- —- —- —-