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All radiocarbon laboratories either standardize to the US National Bureau of Standards Oxalic Acid I (OX-I) which is derived from Sugar Beets in 1955 or a secondary standard NBS OX-II (grown in 1977) or Australian National University Sucrose (ANU), which is sugar from the 1974 growing season in Australia.Both the OX-II and ANU have been extensively cross-calibrated to OX-I and can be used to normalize a sample for radiocarbon dating.

This is especially true for old samples with low beta activity.This brings us to two reasons why a radiocarbon date is not a true calendar age.The true half-life of 14C is 5730 years and not the originally measured 5568 years used in the radiocarbon age calculation, and the proportion of 14C in the atmosphere is not consistent through time.This means that it can be difficult to effectively clean the samples and remove enough contaminating carbon to obtain an accurate date.In the late 1970s and early 1980s the dating of small samples became possible using Accelerator Mass Spectrometry (AMS; Muller, 1977; Nelson et al., 1977).

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