BP stands for “Before Present” or “Before Physics” as some would refer to it.
It should be noted that a BP notation is also used in other dating techniques but is defined differently, as in the case of thermoluminescence dating wherein BP is defined as AD 1980.
Suess’s curve, based on the bristlecone pine, used tree rings for its calendar axis.
There have been many calibration curves published since Suess’s curve, but their proliferation brought more problems than solutions.
The tree rings were dated through dendrochronology.
At present, tree rings are still used to calibrate radiocarbon determinations.
The trees often used as references are the bristlecone pine (Pinus aristata) found in the USA and waterlogged Oak (Quercus sp.) in Ireland and Germany.Calibration is not only done before an analysis but also on analytical results as in the case of radiocarbon dating—an analytical method that identifies the age of a material that once formed part of the biosphere by determining its carbon-14 content and tracing its age by its radioactive decay.Carbon-14 is a naturally occurring isotope of the element carbon.It is also called “radiocarbon” because it is unstable and radioactive relative to carbon-12 and carbon-13.
Carbon consists of 99% carbon-12, 1% carbon-13, and about one part per million carbon-14.The science of dendrochronology is based on the phenomenon that trees usually grow by the addition of rings, hence the name tree-ring dating.