The calibration curves can vary significantly from a straight line, so comparison of uncalibrated radiocarbon dates (e.g., plotting them on a graph or subtracting dates to give elapsed time) is likely to give misleading results. Marine reservoir variations are partly handled by a special marine calibration curve.
When an organism dies, it contains the standard ratio of 14C to 12C, but as the 14C decays with no possibility of replenishment, the proportion of carbon 14 decreases at a known constant rate.
The time taken for it to reduce by half is known as the half-life of 14C.
The carbon is of geological origin and has no detectable 14C, so the 14C/12C ratio in the vicinity of the volcano is depressed relative to surrounding areas. Hemisphere Effect The northern and southern hemispheres have atmospheric circulation systems that are sufficiently independent of each other that there is a noticeable time lag in mixing between the two. Island Effect It has been suggested that an “island effect” might exist, by analogy with the mechanism thought to explain the hemisphere effect – since islands are surrounded by water, the carbon exchange between the water and atmosphere might reduce the 14C/12C ratio on an island. Contamination Any addition of carbon to a sample of a different age will cause the measured date to be inaccurate.
Contamination with modern carbon causes a sample to appear to be younger than it really is: the effect is greater for older samples. Sample Size How much sample material is needed to perform testing depends on what is being tested, and also which of the two testing technologies is being used: detectors that record radioactivity, known as beta counters, or atomic mass spectrometers (AMS). Measurement Errors For some time, beta counting methods were more accurate than AMS, but there is now little to choose between them, though AMS still cannot compete with the very highest-precision beta counting laboratories, which can provide results with a standard error of ± 20 years. Conversion Errors The calculations to convert measured data to an estimate of the age of the sample require the use of several standards.
These include the main stable isotope (12C) and an unstable isotope (14C).
Through photosynthesis, plants absorb both forms from carbon dioxide in the atmosphere.
These records allow fine-tuning, or “calibration”, of the raw radiocarbon age, to give a more accurate estimate of the calendar date of the material.
One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. Carbon has two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C), and a radioactive isotope, carbon-14 (14C), also known as radiocarbon.
Calibration curve for the radiocarbon dating scale. The calibration method also assumes that the temporal variation in 14C level is global, such that a small number of samples from a specific year are sufficient for calibration. Significantly, in the ill-behaved regions of the calibration curve, increasing the precision of the measurements does not have a significant effect on increasing the accuracy of the dates.
Marine Effect This effect is not uniform – the average effect is about 440 years, but there are local deviations of several hundred years for areas that are geographically close to each other.
Similarly, the assumption that at the time of death an organism always “contains the standard ratio of 14C to 12C” is a statement of faith [not science].