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Hereafter these isotopes will be referred to as 12C, 13C, and 14C.
14C is radioactive and has a half-life of 5730 years.
When a plant or animal dies it no longer exchanges CO with the atmosphere (ceases to take 14C into its being). 14C decays by emitting an electron, which converts a neutron to a proton, converting it back to its original 14N form.
The current internationally-ratified calibration curve for terrestrial samples (e.g., woods, charcoals and macro-fossils) from the Northern Hemisphere is Int Cal04, which covers the past 26,000 calendar years (cal yr) (Fig. This curve is based on dendrochronologically-dated tree rings for the period 0-12,400 cal yr before present (BP, with 0 BP being AD 1950).There is a small difference in the natural atmospheric C levels in the southern troposphere are therefore usually lower than those in the northern troposphere, and the radiocarbon ages of terrestrial materials in the Southern Hemisphere for a particular period of time are usually older than those in the Northern Hemisphere.The current internationally-ratified radiocarbon calibration curve for terrestrial samples from the Southern Hemisphere is SHCal04.For the remaining period 12,400-26,000 cal yr BP, the curve is derived from independently dated marine samples such as foraminifera and corals.A new internationally-ratified calibration curve (Int Cal09) covering the whole radiocarbon timescale (~50,000 cal yr) is being prepared by the Int Cal Working Group.
For a radiocarbon value measured in a sample S (Fs), bomb radiocarbon delivers two possible calendar dates (T1 and T2), indicated by the grey boxes (Hua, 2009).