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Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date of the age scale, reflecting the fact that radiocarbon dating became practical in the 1950s.

The abbreviation "BP", with the same meaning, has also been interpreted both of which requested that publications should use the unit "a" for year and reserve the term "BP" for radiocarbon estimations. A large quantity of contemporary oxalic acid dihydrate was prepared as NBS Standard Reference Material (SRM) 4990B. This value is defined as "modern carbon" referenced to AD 1950.

The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.

Other corrections must be made to account for the proportion of throughout the biosphere (reservoir effects).

The ages are expressed in years before present (BP) where "present" is defined as AD 1950.

Uncalibrated radiocarbon dates should be clearly noted as such by "uncalibrated years BP", because they are not identical to calendar dates.

The idea behind radiocarbon dating is straightforward, but years of work were required to develop the technique to the point where accurate dates could be obtained.

Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years.

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Uncalibrated radiocarbon ages can be converted to calendar dates by means of calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (dating on the basis of tree growth-rings) and stratigraphy (dating on the basis of sediment layers in mud or sedimentary rock).

The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died.

The older a sample is, the less (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.

Stratigraphy uses the assumption that higher layers or strata were laid down after lower layers.

Ice core sampling normally uses the assumption that the ring bands observed represents years.

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