Standard modern radiocarbon

In this way, it is possible to reach an extremely high selective sensitivity down to 1 part in 1015, which in 14C dating corresponds to being able to date samples about 50 000 years old. Moreover, modern systems can measure isotopic ratios in modern carbon, both C/ C and C/ C, with an ultimate precision as good as 2%o and l%o, respectively. The former value corresponds to determining the conventional radiocarbon age with an absolute error, smaller than in the past, better than 20 years, while the l%o precision for the 13C/12C allows an adequate correction for isotopic fractionation effects. Even in routine measurements, at least in the case of historical samples, a precision of 5%o in the 14C/12C measured value is standard, corresponding to an uncertainty in the radiocarbon age of 40 years.[27]... 

Figure 6. Radiocarbon content of modern pre-bomh marine shell from the west coast of North and South America, Data from Refs. 73 and 74. Values have been expressed with respect to 0.95 NBS oxalic acid standard and corrected in light of their values.

The significance of these eflFects to archaeologists are limited. Except when combined with variations in the production rate, they make it difficult to distinguish radiocarbon concentrations in organic samples over the past several hundred years and force laboratories to use 100 or 150 years as the minimum age which can be cited. This aflFects the accuracy of values for late prehistoric or protohistoric archaeological contexts for several areas of the world. It also explains why radiocarbon laboratories cannot use modern wood as a contemporary reference standard. Rather, artifical standards must be used to provide what the value of the activity in the atmosphere would have been without human intervention. 

Thirteen additional radiocarbon analyses are now available to supplement the eight results reported by Herrdez et al. (1979). In the present discussion we will use the corrected ages appearing in the last column of Table VI which were computed based on the conventional radiocarbon dates (Table VI, column 11. The calculation of conventional dates assumes (a) the initial radiocarbon content of the water was lOOpmC. (percent modern carbon), i.e. an activity equal to the U.S. National Bureau of Standards Activity and (b) the half-life or radiocarbon equals the Libby half-life of 5568 yr. Since the actual initial radiocarbon activity of the infiltrating water was not 100 pmC, these conventional dates must be corrected. This initial... 

The presentation of radiocarbon measurements has been rigidly formalized. All measurements are made relative to a modern standard of oxalic acid, the activity of which has been related to the theoretical... 

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