We describe here the general operation of the Oxford Radiocarbon Accelerator Laboratory which has now been dating routinely for 18 months.

With Fe powder as a catalyst, CO2 is completely converted to graphite within 90 minutes. The reaction proceeds at 650°C with an excess of H2. The reaction rate is enlarged by forced circulation and by keeping the water vapor pressure very low. The graphite samples obtained, consisting of 5mg of carbon, almost immediately produce stable 12C− currents of the order of 20–30μA in the sputter source of a Van de Graaff accelerator. The currents can be maintained for at least 10 hours and are comparable to those from commercial graphite. No memory effects in the preparation system have been observed.

A method has been developed at the Arizona AMS facility for production of graphite directly from CO2 by catalytic reduction of CO/H2 mixtures. The method is a modification of procedures discussed by Vogel et al (1984). Currents of 12C− from these targets vary considerably, but have a median value which is 70% of that achieved from spectroscopic graphite. Precisions of ±60 to 100 yr are routinely achieved for 14C ages of 5000-yr-old samples containing one milligram of carbon. The yield of 14C from “blank” targets is 0.6 ± 0.1% of that from modern material.

A description is given of the use of a Tandetron accelerator mass spectrometer system for measuring the isotopes 10Be and 26Al.

A description of accelerator mass spectrometry (AMS) measurements with the long-lived radioisotope 36Cl is given. All measurements were made at the Munich tandem accelerator laboratory. Results are presented for 36Cl measurements in ground waters, in the meteorite Bjurböle, in ice-core samples of the Vernagtferner, Austria, and in granite samples from Hiroshima, Japan, irradiated by the atomic bomb explosion in 1945.

Tests have been carried out on the measurement of 14C directly from CO2, using a source designed by Middleton, and a Tandetron accelerator mass spectrometer system. These tests were very promising from the point of view of minimum quantity of carbon necessary (<50μg), but suffered from a background (apparently not memory effect) giving an equivalent age of ∼25,000 years. As a demonstration of the technique, a single thread from the mummy wrapping of the Egyptian king Ramses II was dated.

The Simon Fraser University accelerator mass spectrometry (AMS) facility has now been in semi-routine use since the beginning of 1985. The advantages and limitations for 14C dating are described.

A new laser-based analytical technique is described for isotope selective noble gas atom counting. The method has been used to detect 81Kr atoms in a groundwater sample.

Since August 1984 over 100 samples have been analyzed. 12C, 13C, and 14C isotopes were measured after acceleration and all dates were corrected for natural and preparation-induced fractionation. A precision of better than 1.0% was obtained for samples 10,000 yr BP or less and these results were shown to be reproducible at precision levels as low as 0.3%. An accuracy of better than 1.0% was demonstrated for the same age range by analyzing samples which were previously dated with the conventional technique. The machine background, measured on blank sample holders in a clean ion source, yielded a count rate equivalent of 0.015 ± 0.007% modern.

The University of Washington FN tandem accelerator mass spectrometry (AMS) system has been used in a series of 14C studies. 1) The 14C concentrations in annual growth rings for 1962, 1963, and 1964 of a Sitka spruce, each divided into ten sequential segments, were measured; a full and rapid response of tree-ring cellulose to atmospheric changes in 14CO2 is indicated, with a delay, if any, of not more than three weeks. 2) The C concentrations in two chemical fractions of dissolved organic carbon and in two fractions (by size) of particulate organic carbon were measured for Amazon River samples from several locations. All contain bomb carbon, but the amounts differ significantly. 3) Algae samples from lakes in the dry valleys of Antarctica were dated in order to assist in the reconstruction of the climatic history of Antarctica. 4) Background studies indicate that the contribution of the AMS system itself to the observed 14C concentrations is equivalent to an age of ca 60,000 14C yr BP; for a prepared sample of 5mg of carbon the background corresponds to ca 50,000 years.

The ETH/SIN AMS dating facility is routinely used for 10Be, 14C, 26Al, and 36Cl measurements. The present status and performance of this facility are reviewed and some of its major applications summarized.

Measurement of 14C in samples of wood of precisely known age have shown that the cosmogenic 14C content of the CO2 in the atmosphere has not remained constant during the period of time covered by radiocarbon dating. As the terrestrial atmosphere mixes with a time constant of less than 3 years, these variations must be essentially independent of geographic location. The 14C in atmospheric CO2 must be a quantity that, at a given time, pertains to the terrestrial atmosphere as a whole. Not only is its knowledge necessary for deriving accurate radiocarbon dates, but it is also valuable in connection with many geophysical and astrophysical problems. Unfortunately, progress in our knowledge of 14C variations in the terrestrial atmosphere has been delayed by hidden experimental errors in results obtained by many laboratories. By rigorous statistical analysis of the La Jolla results, it is now possible to show that the 14C variations are not simple random fluctuations but show distinct regularities. Similar patterns of variations have been found in the growth rate of trees during the last 5000 years. Measurements of radiogenic 10Be currently being done by European workers promise to conclusively elucidate the 14C findings.

New Arizona high precision Δ14C data back to 6500 BC plot close to an 11,300-yr period sinusoid extrapolated from the post 5300 BC data (offset = +32‰, half amplitude = 51‰ and phase lag = 2.29 radians). The trend curve is modulated by high latitude components of the non-dipole field with a fundamental period of 2400 yr. Based upon a model of Lund and Banerjee (1985), the non-dipole field rotates and every 1200 yr the high latitude maxima pass over the north magnetic pole and near the south magnetic pole in reversed polarity. This modulates cosmic ray production producing extended maxima ca AD 1700, 700 bc, 3100 bc, and 5500 bc. The 2400 period appears to be stationary. The magnetic field also modulates the amplitude of the solar activity induced cycles of periods 200, 80, and 11 yr as can be seen in the Zürich-Bern Camp Century ice core data as well as in the Δ14 C fluctuation data. Reinterpretation of the Camp Century 10Be data indicates that it is in agreement with magnetic field as well as solar activity modulation of terrestrial radioisotope production.

Since the 11th International Radiocarbon Conference considerable advances have been made in European dendrochronology giving several long continuous absolute chronologies. Recent collaboration between European laboratories provides confirmation of the accuracy of these chronologies and, thus, of the standards used for radiocarbon calibration.

Seasonal variations in 18O content, in acidity, and in dust content have been used to count annual layers in the Dye 3 deep ice core back to the Late Glacial. In this way the Pleistocene/Holocene boundary has been absolutely dated to 8770 BC with an estimated error limit of ± 150 years. If compared to the conventional 14C age of the same boundary a value of Δ14C = 53 ± 13‰ is obtained. This Δ14C value suggests that 14C levels during the Late Glacial were not substantially higher than during the Postglacial.

Absolute high-precision radiocarbon time-scale calibration is now available and is shown by Stuiver and Pearson (1986) and Pearson and Stuiver (1986) to be internationally valid. The technique of curve fitting presented in this paper shows how certain samples may be dated calendrically to within a very narrow band width ca ± 20 years, at time periods when single sample analysis would give ambiguous calendrical conversion. Samples of known deposition rate covering a time span of >50 years are subdivided to produce a ‘floating curve’ and this curve is then ‘wiggle matched’ with the high-precision calibration to give a precise calendrical age.

We measured the Δ14C values in 57 rings (from AD 1824 to 1880) of a white spruce grown in Mackenzie Delta (68°N, 130°W), as part of our continuing study of the Δ14C variation related to solar activities. The values exhibit a 10‰ fluctuation with an 11-year periodicity anti-correlated with the solar activity cycle. We also measured the Δ14C values in 6 rings (from AD 1940 to 1945). The abnormally high value in the 1943 ring may be due to two large solar flares occurring in 1942.

The hottest topic for those interested in the earth's carbon cycles is the change in atmospheric CO2 content between glacial and interglacial time. What caused it? What is its role in glacial cycles? We evaluate here the hypotheses that have been put forward to explain the CO2 change with evidence from deep sea sediments. We conclude that all the hypotheses have serious drawbacks and that much effort will have to be expended in gathering more data from ice cores and ocean sediments before we will be pointed toward the correct scenario. Also, thoughtful modeling aimed at depicting the ties between pCO2, O2,13C/12C, 14C/12C, and nutrient constituents in the sea for various modes of circulation will have to be done before the evidence from ocean cores can be properly interpreted.