Encyclopedia of scientific dating methods
An initial reading dates the specimen which is then calibrated by considering this date and its correspondence with the measurable level of carbon 14 stored over time in the growth rings of certain tree species, including redwood and pine bristol.
The results of radiocarbon dating are expressed in years and include a time range (eg, 630± 60 BP).
The bones were buried under (and are therefore older) a layer of ash that resulted from a volcanic eruption dating back to 7000 years BP (Before Present; "present" indicates c. Subsequently, radiocarbon dating, an absolute dating technique, was used to date the bones directly and provided a date of 8250 BP, showing how useful the combined used of relative and absolute dating can be.
Radiocarbon dating, however, can only be used for dating objects that are less than 50 000 years.
However, this method is sometimes limited because the reoccupation of an area may require excavation to establish the foundation of a building, for instance, that goes through older layers.
In this case, even if the foundation of the building is found in the same stratigraphic level as the previous occupation, the two events are not contemporary.
The amount of carbon 14 remaining in the material to date is compared to a reference standard (ratio 14C/total carbon, 12C and 13C) to calculate the time elapsed since its occurrence.
A sample requires 10 to 20 grams of matter and usually consists of charred organic material, mainly charcoal, but bones (zooarchaeology) and shells can also be dated using this technique.
Stratigraphic dating remains very reliable when it comes to dating objects or events in undisturbed stratigraphic levels.