The following list sums up the results of radiocarbon age determinations obtained at the Laboratory in 1974-1975. The list is prefaced with a brief description of the measurement procedure and discussion of some methodological aspects of the investigation that result in a higher accuracy of dating.

The effects of nonconservative sources (inputs) and sinks (outputs) of carbon are indicated by the behavior of Δ14C and δ13C of the total dissolved inorganic carbon (ΣCO2) in San Francisco Bay and Chesapeake Bay. Isotopic distributions and model calculations indicate that in North San Francisco Bay the net CO2 flux to the atmosphere and carbon utilization in the water column are balanced by benthic production. Municipal waste appears to be a dominant source in South San Francisco Bav. In Chesapeake Bay, atmospheric exchange has increased the Δ14C and δ13C in the surface water. Decomposition of organic matter in the water column is indicated to be the dominant source of excess ΣCO2 in the deep water.

A box-diffusion model for the carbon cycle is used to estimate the magnitude of 14C variations caused by changes of reservoir sizes and exchange fluxes in the global carbon system. The influence of changes in atmospheric CO2 concentration, biomass, CO2 exchange rate between atmosphere and ocean, and ocean mixing is considered. Steady-state 14C concentrations as well as the transients are calculated. For changing biomass, atmospheric CO2 levels and 13C/12C ratios are also calculated.

Carbon-cycle-induced 14C variations may have been significant in the transition period from Glacial to Postglacial when drastic changes in environmental conditions took place within short time periods, while they were probably less important during the climatically more stable Postglacial.

Changes of the oceanic circulation, as supposedly occurred, are considered the most important factor, besides variations of the production rate, affecting the global distribution of 14C. 14C variations due to changes of the atmospheric CO2 level or the air-sea-exchange probably did not exceed one to a few percent. Fluctuations of the forest biomass, which may have occurred between Glacial and Postglacial, hardly affected the 14C concentration over a long term.

Responses of the atmospheric 14C concentration are also calculated for variations of the 14C production rate by cosmic radiation. The following cases are considered: a step change, square-wave changes producing “wiggles”, and sinusoidal variations.

Most of the 14C measurements reported here were made between October 1978 and October 1979. Equipment, measurement, and treatment of samples are as reported previously (R, 1968, v 10, p 36-37; 1976, v 18, p 290) except for some minor improvements of the electronic equipment. Sediment samples too small to allow pretreatment, are now burned at <650°C in order to avoid pyrolysis of carbonates that may be present in the samples. Same max combustion temperature is used for samples with suspected graphite contamination. A movable thermocouple (Philips TERMOCOAX TCI 30/50/3) in the combustion zone is used for checking the temperature.

The following list includes samples dated at the Institute of Geology, Academy of Sciences of the Estonian SSR in 1978. The measurement of natural 14C activity is performed by 1-channel and 2-channel scintillation devices (Punning & Rajamäe, 1977). Ages are calculated using the half-life of 5568 ± 30 years and 0.95 NBS oxalic acid modern standard with ad 1950 as reference year.

Palaeomagnetic field strength measurements for the last 50,000 years are summarized. The period before ~12,000 yr bp is characterized by low dipole moments, but high values are associated with the Lake Mungo polarity excursion between ~32,000 and ~28,000 yr bp. The variation since 12,000 yr bp, based on new results from Australia and published data from the Northern Hemisphere has a quasi-cyclic appearance with maxima at ~10,000 and ~3500 yr bp. The geomagnetic record is used to predict variations in atmospheric 14C concentration, and the results are compared with independent comparisons between 14C and other dating methods. Long-term variations in the 14C time-scale are readily explained by known geomagnetic changes.

Radiocarbon activity of successive parts of Pleistocene and Holocene ooids and mollusk shells from the Persian Gulf, Kuwait, was measured. The inner part of the ooids showed the lowest activity and the cement between grains the highest. Radiocarbon activities correspond to the general stratigraphy and to the position of the sediments. Radiocarbon ages of Pleistocene sediments seem to be very low due to recrystallization of aragonite. Higher radiocarbon activity of cement indicates that atmospheric carbon dioxide was involved in the subaerial diagenetic process. The radiocarbon age of well-preserved mollusk shells seems to be lower than their allegedly Pleistocene geologic age.

The following list includes some measurements made during 1978 and 1979 in the radiocarbon laboratory of the Ecole Nationale Supérieure de Géologie Appliquée et de Prospection Minière (ENSG) de Nancy. Equipment, measurement and treatment of samples are as reported previously (R, 1979, v 21, p 453-464) where the sample is synthesized to benzene. Radiocarbon ages are calculated using the 14C half-life of 5568 years and 95% activity of NBS oxalic acid is used as modern standard. Anthracite coal and Merck commercial benzene are used for the dead carbon run.

It has become generally accepted during the last year that 14C fluctuations, the so-called “wiggles”, observed in wood, dated by its tree rings, do indeed exist. Furthermore the La Jolla measurements show that apart from experimental noise, they do not represent random red noise, but characteristic, recurring features. In 1971, Houtermans found indications for the existence of cyclic components and recent Fourier analyses of all the available data by Neftel and Hartwig show a 200-year component. Cyclic oscillations with other periods appear to be present during limited time intervals. The character of the oscillations is not harmonic. The time derivative of many fluctuations is remarkably constant and such that the 14C rises by 1 percent in about 20 years and decreases by 1 percent in slightly more than twice that length of time. The properties of the overall radiocarbon record have to be considered in attempts to explain the variations in terms of variations of the cosmic ray-production rate and changes of the geochemical distribution of radiocarbon.

This list contains analyses completed between January, 1976 and April, 1977. Details of laboratory operation are contained in our first list (R, 1975, v 18, p 205). Samples submitted for analysis are reviewed by a committee consisting of W DeBoer, E Hansen, Anthropology; L Marcus, Biology; W S Newman, D L Thurber, Earth and Environmental Sciences; and Richard Pardi, Radiocarbon Laboratory.

The following radiocarbon dates are a partial list of samples measured for a variety of projects and materials since April 1979. Chemical and counting procedures remain the same as indicated in R, v 20, p 274-282.

The reservoir effect of Scandinavian sea water has been determined by dating seals and whales killed well before man's impact on the natural 14C concentration became significant. The samples were collected at different places along the Swedish coasts and in the Gulf of Finland. They derive from ad 1657 or 1658, 1868, 1875, 1894, and 1906. The EDTA treatment of bones was used to obtain collagen free from contaminants.

An elk, originating from ad 1881 was selected for comparison. A seal from ad 1899 from the Caspian Sea was also included in the investigation.

All results have been normalized to δ13C = —25‰ vs PDB. The determinations yield values of the reservoir effect in agreement with earlier results obtained from shells and mammals. The final results are discussed in light of previous variations of 14C content in the atmosphere. Using a smoother curve, the reservoir effect is slightly smaller than was hitherto believed.

An event thought to be of cosmic origin caused the count rate of both proportional and Geiger counters to increase significantly around December 4, 1978. The correction for this has been studied. The statistics for background, oxalic acid, and unknown samples, measured repeatedly after this correction, were as good as usual.

Rates of CO2 exchange across the air-water interface in oceans and lakes measured to date by the L-DGO group are summarized. They range from 3 to 38 moles/m2/yr. The possible causes for this range include the differences in salinity, mean wind speed, and pH. Wind tunnel studies comparing fresh water and sea water are required before a satisfactory explanation can be found.

The present list includes 74 dates of geologic samples determined using the liquid scintillation 14C method at the Geobiochemical sector of the Institute of Zoology and Botany, Academy of Sciences, Estonian SSR.

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, background counting rates and half-life. No 13C/12C ratios were measured.

Ionium dates for the Upper Lisan Formation in the Dead Sea valley average (10 ± 3) percent higher than a set of radiocarbon dates from the same profiles. No analytical explanation can be found so that the discrepancy may be real for the period 15,000 BP to 35,000 BP (conventional radiocarbon years). This would have implications for the chronology of the Upper Pleistocene.

Radiocarbon variations between 3900 and 2800 bc have been established in La Jolla and Groningen using oak tree rings from a 2350-year floating oak series. Comparison of these variations with the bristlecone pine 14C variations provides precise ages for tree-ring dates of Neolithic settlements of Switzerland and Germany over a period of 1400 years. 14C variations measured in Heidelberg in absolutely dated oak-ring series from ad 250 to 720 show trends similar to those of long-term growth variation of oaks during the same period of time. The influence of the climatic regime on oak growth of this period is discussed.

This list reports certain 14C measurements completed by December 1979; other projects completed by this time will be reported later. Age calculations are based on 14C half-life of 5568 yr and modern standard of 95% NBS oxalic acid, supplemented by tree rings of pre-industrial wood from a log cut in the 1850's (Tx-540; R, 1970, v 12, p 249). Deviations reported are based on counting statistics of sample, background and modern, and are ± 1σ, except that when sample count approaches either modern or background, 2σ limits are reported. Unless noted, 12C/13C measurements were not made and results are not corrected for 13C fractionation (assumed ratio = −25‰ WRT PDB). Our laboratory uses liquid scintillation counting of benzene, with Li2C2 and vanadium-activated catalyst in preparation; chemical yields range between 95% and 99%. Three counters are employed; a Packard Tri-Carb Model 3002 and 2 Beckman LS230 spectrometers obtained through a grant from the National Science Foundation.

14C measurements were carried out on sea water samples collected in 1973, in the Indian ocean. The results obtained for 9 vertical profiles between 27° S and 48°S are presented. In surface water, the bomb 14C content is maximum at middle latitudes. A time lag relative to the north hemisphere bomb 14C delivery is apparent. In the more southern latitudes, 14C content remains very low.

Procedures and equipment have been described in previous date lists. Except as otherwise indicated, wood, charcoal and peat samples are pretreated with dilute NaOH 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.