Recommended dating age
If the reservoir corrected conventional radiocarbon age calculated is within the past 200 years, it should by convention be termed 'Modern' (Stuiver and Polach, 192).If a sample age falls after 1950, it is termed greater than Modern, or Where Aabs is the absolute international standard activity, 1/8267 is the lifetime based on the new half life (5730 yr), Y = the year of measurement of the appropriate standard.
Another standard, Oxalic Acid II was prepared when stocks of HOx 1 began to dwindle. The ratio of the activity of Oxalic acid II to 1 is 1.29330.001 (the weighted mean) (Mann, 1983). There are other secondary radiocarbon standards, the most common is ANU (Australian National University) sucrose.Obviously, this activity is additional and must be removed from calculations.In order to make allowances for background counts and to evaluate the limits of detection, materials which radiocarbon specialists can be fairly sure contain no activity are measured under identical counting conditions as normal samples.In an earlier section we mentioned that the limit of the technique is about 55-60 000 years.Obviously, the limit of the method differs between laboratories dependent upon the extent to which background levels of radioactivity can be reduced.D14C is calculated using: Figure 1: Decay curve for C14 showing the activity at one half-life (t/2).
The terms "%Modern", or "pm C" and D14C are shown related in this diagram along with the Radiocarbon age in years BP (Before 1950 AD).
A copy of this paper may be found in the Radiocarbon Home Page The radiocarbon age of a sample is obtained by measurement of the residual radioactivity. T (National Institute of Standards and Technology; Gaithersburg, Maryland, USA) Oxalic Acid I (C). The activity of 1890 wood is corrected for radioactive decay to 1950.
This is calculated through careful measurement of the residual activity (per gram C) remaining in a sample whose age is Unknown, compared with the activity present in Modern and Background samples. Thus 1950, is year 0 BP by convention in radiocarbon dating and is deemed to be the 'present'.
This is the International Radiocarbon Dating Standard.
The Oxalic acid standard was made from a crop of 1955 sugar beet. The isotopic ratio of HOx I is -19.3 per mille with respect to (wrt) the PBD standard belemnite (Mann, 1983). T designation SRM 4990 C) was made from a crop of 1977 French beet molasses.
According to Stuiver and Polach (1977), all laboratories should report their results either directly related to NBS Oxalic acid or indirectly using a sub-standard which is related to it.