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Two different trends can be seen in the tree ring series.First, there is a long-term oscillation with a period of about 9,000 years, which causes radiocarbon dates to be older than true dates for the last 2,000 years and too young before that.Other corrections must be made to account for the proportion of throughout the biosphere (reservoir effects).Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s.
The half-life of a radioactive isotope describes the amount of time that it takes half of the isotope in a sample to decay.
14c dates are frequently reported as cal bp, cal bc, or cal ad, again with bp referring to the year 1950 as the zero date.
the calculations involve several steps and include an intermediate value called the "radiocarbon age", which is the age in "radiocarbon years" of the sample: an age quoted in radiocarbon years means that no calibration curve has been used − the calculations for radiocarbon years assume that the 14c/12c ratio has not changed over time.
these counters record bursts of ionization caused by the beta particles emitted by the decaying 14c atoms; the bursts are proportional to the energy of the particle, so other sources of ionization, such as background radiation, can be identified and ignored.
stands for "before present", referring to a reference date of 1950, so that 500 bp means the year 1450 ad.