Forty-five tree rings (1881–1925) were taken from a white spruce grown near Campbell River in Mackenzie Delta, Canada, for the measurement of 14C activity variation. Because of the narrowness of the rings, 2 and sometimes 3 rings were combined to yield a total of 21 specimens. The 14C content in these specimens was measured with a liquid scintillation-PM tube counter system of the History Department of Peking University. The data points exhibit a 10‰ variation, anti-correlated with sunspot numbers. The physical implication is discussed.

Parallel determinations of 14C and ionium dates on a stalagmite from the Cango Caves provide evidence of variations in the 14C content of the atmosphere beyond the range of the California tree-ring sequence. During the Holocene growth period the 230Th dates are compatible with the tree-ring calibrated 14C dates. At 18,000 BP and between 30,000 and 40,000 BP the 14C ages are markedly younger than the 230Th ages, suggesting that the 14C level of the atmosphere was considerably higher at these times. Between the 230Th ages of 35,000 and 29,000 BP the 14C ages remain nearly constant at 29,500 BP, indicating that 14C production must have been drastically reduced during this period. The 14C fluctuation is greater than that predicted by Barbetti (1980) but it may be explained by postulating a substantial increase in the geomagnetic dipole field, for which there is mounting evidence.

Procedures and equipment have been described in previous date lists. Except as otherwise indicated, wood, charcoal, and peat samples are pretreated with dilute NaOH–Na4P2O7 and dilute H3PO4 before conversion to the counting gas methane; marls and lake cores are treated with acid only. Very calcareous materials are treated with HCl instead of H3PO4. Pretreatment of bone varies with the condition of the bone sample; solid bone with little deterioration is first cleaned manually and ultrasonically. The bone is treated with 8% HCl for 15 minutes, then dilute NaOH–Na4P2O7 for 3 hours at room temperature, washed until neutral, and the collagen extracted according to Longin (1971). Charred bone is treated with dilute HCl, NaOH–Na4P2O7, and then dilute HCl again.

The following list summarizes the post-1970 laboratory results of 14C dating of ancient sea-level indicators from the coasts of the Soviet Union. One of the aims of the International Geologic Correlation Programme Project No. 61 “Sea level movements during the last deglacial hemicycle” is the global cataloguing and mapping of ancient sea levels. The laboratory, which acts as the USSR National curator for these age measurements obtains dates sampled from its own expeditions and from other institutions of the country.

This list reports measurements made on archaeologic samples by our laboratory from September 1980 to May 1982. Results of measurements made during that period which lack review by submitters will be reported in a subsequent date list. Dates were obtained by liquid scintillation counting of benzene using the laboratory procedure outlined previously (R, 1982, v 24, p 344–351). All dates are expressed in 14C years relative to ad 1950 based on the Libby half-life for 14C of 5568 yr. The laboratory standard continues to be ANU sucrose. Data analysis is now performed with the aid of an RT-11 microcomputer interfaced with our Packard model 3255 LS counter. Dates have been corrected for isotopic fractionation only when the δ13C value is given. No corrections have been made for natural 14C variations. The following descriptions of samples are based on information provided by the submitters.

The AD time series of Δ14C, δ13C, and cosmic ray fluxes, Q, were analyzed for similarities. Two cases of significant correlation between Q and tree-ring δ13C were encountered, for which up to 25% of the variance can be attributed to changes in the tree's isotope fractionation that related to solar induced climatic changes. However, it is possible that the demonstrated correlation is fortuitous because actual climate proxy records generally do not correlate significantly with the Q record.

This list comprises age measurements carried out at the Institute from 1979 to 1981. Anticoincidence variant was applied to attain higher counting efficiency of a 2-channel scintillation device. The introduction of a plastic scintillator as an active guard around the detector in the 2π geometry decreases the influence of high energy cosmic radiation and reduces the background ca 35 to 75% (Rajamäe & Punning, 1980). The application of an active guard with 4π geometry has proved most effective; as a result, the background decreased 85% to 0.65cpm with the volume of scintillation cocktail 5ml. We have improved our methods for decreasing the actual error of the background. The application of a control channel and consideration of barometric effect results in the increase of dating limits of up to 5000–6000 years (Rajamäe, 1981). Samples are calculated using a half-life of 5568 ± 30 years for 14C. Results are reported in years before 1950. δ13C measurements were performed on the mass-spectrometer MI-1201 and are reported with PDB as a reference standard. The reproducibility of results is ±0.2‰.

The history of the earth's magnetic field is preserved in the fossil magnetism of archaeologic specimens, natural rocks and sediments. Samples such as lava flows and baked sherds that acquired a thermoremanent magnetization on cooling can be used to estimate ancient geomagnetic field intensities and directions. Paleofield directions can also be obtained from fine-grained sediments that acquired detrital magnetic remanence when deposited. Study of the earth's magnetic field over the last few tens of thousands of years yields information on geomagnetic dynamo theories, causes of fluctuations in cosmic-ray activity, and the formulation of a new regional chronologic tool.

The geomagnetic field is one of the major physical fields of the earth. Because its source is fluid motion in the outer core, it exhibits temporal changes, called secular variation, which are quite rapid compared to most geologic phenomena. The prehistoric secular variation is usually inferred from paleomagnetic data. We will discuss here how changes in the atmospheric 14C content can be used to gain additional insight into the behavior of the dipole moment over the past 8500 years. By rewriting the differential equations representing the 14C geochemical cycle in finite-difference form, we are able to convert the atmospheric 14C activity record into an equivalent radiocarbon dipole moment.

Radiocarbon dates reported in this date list are totally derived from a cooperative effort between the Romanian Institute of Geology and Geophysics and the Center for Applied Isotope Studies (CAIS). The project was carried out between 1972 through 1978. The purpose of the study was to add to the knowledge of the various stages of the Holocene development of the Danube and Black Sea (Ghenea et al, 1971; Panin, 1976).

Relatively precise quantitative observations of geophysical parameters are available to evaluate the fluctuations of atmospheric 14C activity during the past three centuries. As reviewed by Damon, Lerman, and Long (1978), these fluctuations seem to result from three factors: 1) changes in the earth's dipole magnetic field intensity, which has been decreasing since the first measurements by Gauss (McDonald and Gunst, 1968); 2) solar modulation of the cosmic-ray production, which has been correlated with the sunspot record of Waldmeier (1961), and more recently, to the Aa geomagnetic index by Stuiver and Quay (1980); and 3) the combustion of fossil fuels (Suess, 1955). A relationship between the climatic time series and the 14C-derived record of solar change has not yet been demonstrated (Stuiver, 1980).

The following radiocarbon dates are a partial list of samples measured for a variety of projects and materials since May 1982. Chemical and counting procedures remain the same as indicated in R, v 20, p 274–282.

Calculations are based on the 5568-year Libby 14C half-life. Precision is reported as one-standard deviation based only on statistical counting uncertainties in the measurement of the background, NBS modern standard, and sample activities. δ13C values are measured relative to PDB and reported ages are corrected for isotopic fractionation by normalizing to −25‰.

Measurements have continued with the same proportional counter system, pretreatment procedure, methane preparation and measurement, and calculation, as described previously (R, 1970, v 12, p 298–318). Uncertainties quoted are single standard deviations originating from standard, sample, and background counting rates. No 13C/12C ratios were measured. Sample descriptions have been prepared in cooperation with submitters.

Previous investigations on short-term 14C variations in tree rings are compared with 14C measurements in wine samples. The comparison is made for 4 solar cycles (1903–1944) with the same method of statistical evaluation of measured results. The average amplitude of Δ14C variations as observed by various authors in tree-ring samples is ca 2 ± 1 ‰; however, wine samples show an average amplitude of 4.3 ± 1.6 ‰. The anticorrelation dependence of Δ14C on Wolf sunspot numbers was observed with a time shift between W maxima and Δ14C minima of 3–5 yr for different solar cycles.

When used with 14C records, an accurate reconstruction of the 13C/12C changes of atmospheric CO2 may be a key constraint to determining the historic activity of the biosphere as CO2 source or sink. We recently detailed the derivation of an atmospheric δ13C record for 1930–1979 from Arizona juniper tree rings (Leavitt and Long, 1983). Each tree was close to a weather station, allowing us to account for the effects of climate on the tree-ring δ13C records. The resulting atmospheric δ13C trend shows an overall decrease of ca 1.3‰, but instead of decreasing exponentially over the whole 50-year period, the best-fit curve decreases exponentially to ca 1960 and then flattens out. The drop from 1956 to 1978 was ca 0.55‰, still very similar to the measured atmospheric drop over that period of 0.65 ± 0.13‰ (Keeling, Mook, and Tans, 1979).