The following series of dates contains most of the measurements obtained since our last list. Procedures of measurements and calculation are as previously described (R., 1970, v. 12, p. 353–357).

This list contains most of the unpublished radiocarbon dates measured between 1961 and 1968. Analyses of the C14 content of groundwater are excluded and a number of geologic dates will be included in a later list. Certain earlier analyses performed by H. de Vries and H. de Waard are included, as well as a few results obtained since W. G. Mook assumed responsibility for the laboratory in 1969, to complete series. Descriptions and comments are mainly based on information supplied by the submitters or contained in the publications cited. However, our own interpretation is often given in the comments. Sections I A to D2, II B1, and B2 were mainly prepared by the first author and the rest by the second.

The following list and data presented previously (R., 1970, v. 12, p. 494–495) were obtained to investigate the transport of carbon between man and his environment (Harkness, 1970). These data constitute a study of contemporary variations of C14 concentrations viz., Suess effect and bomb effect, as reflected in the tissues and food chain of the population of the United Kingdom (Harkness and Walton, 1969).

The measurements presented in this date list were made in the Radiocarbon Dating Laboratory at Teledyne Isotopes during 1969–70. Samples were analyzed by techniques described in R., 1968, v. 10, p. 246. Methods and references to sample pretreatment are provided in R., 1970, v. 12, p. 87. Errors associated with the age determinations are calculated by combining standard deviations from oxalic acid standard, background, and actual sample data. Samples which approach modern or background are reported with at least 2σ limits. Counting time for calculation of errors of background and standard is the same as that used for the sample. The error associated with the De Vries effect and the uncertainty of the half-life are not included.

This date list was compiled by the Institute of Geological Sciences (U.K.) incorporating data supplied under contract by E. Welin, Radioactive Dating Laboratory, Stockholm. Unless otherwise stated, age figures are in C14 years before A.D. 1950. The half-life of C14 is taken as 5568 and the standard error is given as a standard deviation of 1σ. Correction for C13/C12 has not been made.

This list includes the results of datings made during 1969–70. The method is essentially the same as described in R., 1968, v. 10, p. 29–35; 1971, v. 13, p. 29–31.

The following date list includes all samples processed from December 1969 through November 1970 at the Illinois State Geological Survey Radiocarbon Dating Laboratory. The benzene liquid scintillation technique was used, following the method of Noakes, Kim, and Stipp (1965), and Noakes, Kim, and Akers (1967). Detailed sample preparation procedures used in this laboratory have been reported by Kim and Ruch (1969), and Kim, Ruch, and Kempton (1969).

The following is a list of dates obtained since the compilation of List XIII (R., 1970, v. 12, p. 161–180). The method is essentially the same. Two CO2–CS2 Geiger counter systems were used. Equipment and counting techniques have been described elsewhere (Crane, 1961). Dates and estimates of error in this list follow the practice recommended by the International Radiocarbon Dating Conferences of 1962 and 1965, in that (a) dates are computed on the basis of the Libby half-life, 5570 yr; (b) A.D. 1950 is used as the zero of the age scale; and (c) the errors quoted are the standard deviations obtained from the number of counts only. In Michigan date lists up to and including VII, we quoted errors at least twice as great as the statistical errors of counting in order to take into account other errors in the over-all process.

The following is a list of dates obtained since the compilation of List XIV, and the method is essentially the same. Two CO2–CS2 Geiger counter systems were used. Equipment and counting techniques have been described elsewhere (Crane, 1961). Dates and estimates of error in this list follow the practice recommended by the International Radiocarbon Dating Conference of 1962 and 1965, in that (a) dates are computed on the basis of the Libby half-life, 5570 yr, (b) A.D. 1950 is used as the zero of the age scale, and (c) the errors quoted are the standard deviations obtained from the number of counts only. In Michigan date lists up to and including VII, we quoted errors at least twice as great as the statistical errors of counting, to take account of other errors in the over-all process.

The C14 dates given below are continued from our previous list (R., 1970, v. 12, p. 559–576), and results obtained mainly during 1970 are described. A 2.7 L stainless steel counter and a 3.3 L copper counter are used, yielding background counting rates of 5.5 and 8.5 cpm, respectively, when filled with dead CO2 at ca. 1.8 atm. Dates have been calculated on the basis of the C14 half-life of 5568 yr and 95% of NBS oxalic acid as modern standard.

Measurements reported in this paper were obtained in the Cambridge Radiocarbon Dating Laboratory during 1970–71. For most determinations, pure carbon dioxide at a pressure of 2 atm was used to fill the copper proportional counters, of 0.5 L or 1.5 L volume. The counters were protected by a massive lead shield and a plastic scintillation anticoincidence screen (Switsur, Hall, and West, 1970). Modern sample gas was obtained from the 1845 to 1855 growth rings of a locally grown oak tree which was felled in 1950. Background was prepared from Welsh anthracite. The contemporary standard is compared frequently with the activity of the International NBS oxalic acid standard. Sample ages are calculated using the conventional half-life of 5568 ± 30 years and the uncertainty stated in terms of one standard deviation of the counting statistics.

The following list covers some old measurements not included in previous lists and most of the samples measured at the Uppsala C14 laboratory since the last list (R., 1969, v. 11, p. 515–544); samples utilized for determining the increase of the C14/C12 ratio due to explosion of nuclear devices are omitted.