Extension of the C record beyond the 0 to 11,900 year long tree ring record is well underway, being measured in many different archives, and undoubtedly an enormous amount of scientific knowledge will stem from these studies.
In our laboratory, we have overlapped and extended the tree-ring radiocarbon calibration from 3,000 to 55,000 yrs.
A line or curve is fit to the data and the resulting equation is used to convert readings of the unknown samples into concentration.
An advantage of this method is that the random errors in preparing and reading the standard solutions are averaged over several standards.
You can add and delete calibration points at will, to correct errors or to remove outliers; the sheet re-plots and recalculates automatically.
In analytical chemistry, the accurate quantitative measurement of the composition of samples, for example by various types of spectroscopy, usually requires that the method be calibrated using standard samples of known composition.
Terrestrial (Int Cal04) and Marine (Marine04) radiocarbon calibration curves for the past 26,000 cal yr BP. Calibrated ages are shown for 1σ and 2σ (68.2% and 95.4% confidence levels, respectively).
The spreadsheet automatically plots and fits the data to a straight line, quadratic or cubic curve, then uses the equation of that curve to convert the readings of the unknown samples into concentration.
Additional complexities, such as variations in marine reservoir effects, marine carbon in the diet, and regional offsets, require consideration.
The potential for future extensions to the calibration curve is discussed.
To produce a curve that can be used to relate calendar years to radiocarbon years, a sequence of securely dated samples is needed which can be tested to determine their radiocarbon age.
The study of tree rings led to the first such sequence: tree rings from individual pieces of wood show characteristic sequences of rings that vary in thickness because of environmental factors such as the amount of rainfall in a given year.