An MP tandem Van de Graaff accelerator at the University of Rochester has been employed since May 1977 to detect 14C in various terrestrial samples. The carbon sample sizes required are 1mg or less. Dating accuracies based on reproducibility now approach (± 80 years). Measurements have been made on 1850 wood, Australian sucrose, a carbon sample from Mt Shasta, a baby woolly mammoth, and an Egyptian bull mummy wrapping.

A description is given of an accelerator-based radioisotope measuring system, being supplied to the Universities of Arizona and Toronto and in part to the University of Oxford. This system will be capable of measuring 14C/12C and 13C/12C ratios in milligram samples of cracked acetylene. At present 200µg of carbon obtained from cracked acetylene appears to be adequate for an isotopic ratio determination. Previous performance and new calculations indicate that a precision better than 1 percent will be achieved in a half-hour period only limited by counting statistics. A precision of 0.2 percent will be obtained in a ten-hour period. Using a carbon sample with an age greater than 60,000 years, the measured background 14C/12C ratio will be less than 0.07 percent of modern.

A standard heavy ion nuclear structure facility has been used to detect several long-lived radioisotopes by counting the accelerated ions. The problem of eliminating strong isobaric background beams has been solved by combining the energy loss dispersion through a uniform Al foil stack with the high momentum resolution of an Enge split-pole magnetic spectrograph. Radioisotope concentrations in the following ranges have been measured: 14C/12C = 10–12 to 10–13, 26Al/27Al = 10–10 to 10–12, 32Si/Si = 10–8 to 10–14, 36Cl/Cl = 10–8 to 10–11.

In the two years since the first successful measurements of 14C by high energy mass spectrometry, the advantages and the basic techniques of the method have been sufficiently reported. They will not be repeated here. This paper describes the work being done at Oxford to fill the gap between demonstrating the effectiveness of the approach and the creation of a facility dedicated to the carbon dating of milligram samples.

The evolution of a tandem accelerator 14C dating system at Chalk River is recounted. Background problems and sources of instability are discussed and solutions are described. Details of sample chemistry and source preparation are presented.

Direct detection of 14C using the Harwell 6MV Tandem accelerator has been achieved using a gaseous ion source. The implications of our results for the use of such sources in this role and for the machine, in general, are discussed.

The University of Washington Model FN tandem Van de Graaff accelerator is being used for the measurement of extremely small isotopic abundance ratios, notably 14C/12C and 10Be/9Be, in a joint project of the Nuclear Physics Laboratory (NPL) and the Quaternary Isotope Laboratory (QL). The experimental arrangements and technical developments are described, and some preliminary results on isotopic ratios in carbon and beryllium are presented.

The addition of organic materials derived from the upper soil layer yields, for samples at greater depth, younger 14C dates than the date of deposition. To find a criterion for the contamination with younger carbon in a soil sample, we examined the radiocarbon concentrations in two humic acid fractions and humin taken from the same sample. The humic acid extracted from a soil sample was divided into two fractions HA1 and HA2. HA1 is the first fraction extracted by 30 minutes' heating with 2 percent NaOH solution, and HA2 is the second fraction extracted by 2 hours' heating with 2 percent NaOH solution after the extraction of HA1. The residue was assumed as the humin (HM).

Many of the peat or soil samples taken from the layer just above the nonpermeable layer contain appreciable amounts of organic materials transported from the upper layer after the sedimentation of the deposits. For the limited number of cases tested here, there is a trend in which the contaminants are selectively extracted by HA1 or HA2. When the soil samples are contaminated the ages of the HA1 and HA2 fractions appeared to differ widely in most cases. Agreement between the HA1, HA2, and HM ages may be used as a criterion for the reliability of the soil dating.

Systematic dating of tufa samples has been undertaken to establish a chronology of tufa deposits in Plitvice National Park, Yugoslavia. We conclude that tufa samples give reproducible data within the time span of ≈ 40,000 years up to recent. The 14C/12C ratio of carbon in the ambient biosphere and hydrosphere gives a detailed picture of the distribution of carbon isotopes in the investigated system. Despite the susceptibility of calcareous material to ambient conditions in the hydrosphere, the original 14C composition of tufa has not been substantially changed. A vertical profile which was cut in tufa during pathway construction showed that the tufa isochrones run vertically in the investigated area. The vertical isochrones and the 14C gradient were horizontal due to tufa build-up in Plitvice National Park which occurs on vertical escarpments that have been flooded. The relatively uniform radiocarbon content of most tufa deposits indicates redistribution of radiocarbon during the formation period. While detailed stratigraphy is often obscured, a general chronology of tufa deposits in the investigated area has been established.

Cosmic-ray produced atmospheric 39Ar activity (T1/2 = 269 yr) has been determined at 0.11 ±0.012 dpm/lt argon. Ice samples from two profiles in Greenland bore holes showed conclusively that 39Ar dating leads to correct ages. Corrections can be made for possible contamination of the samples with ambient air during field extraction and during laboratory processing by measuring 85Kr in the same samples.

The following isotopes: 14C, 39Ar, 85Kr, 3H, partly 32Si, 13C, and 18O were investigated in 20 groundwater samples. Unexpectedly large discrepancies between “14C ages” and “39Ar ages” were observed for many of these samples. For example, a horizontal profile of a confined sandstone aquifer in the Franconian Albvorland showed decreasing 39Ar and 14C activities from respectively, 100 percent to 17 percent and 80 percent to 0.3 percent of modern activity, corresponding to elapsed time periods of 700 and >20,000 years, respectively.

It seems unlikely that gas exchange through the aquiclude is the cause of this discrepancy. It can neither be explained by only assuming that the water represents a mixing of components with different ages. We detected the possibility of underground production of 39Ar in thermal spring water from Zurzach, Switzerland. Its argon showed higher specific activity than atmospheric argon. Elsewhere, however, samples were found containing no detectable 39Ar activity: two wells of a confined carstic aquifer in Ingolstadt, Germany, show less than 7 percent atmospheric activity.

In our opinion, the assumptions on which the 14C method in hydrology are based need to be critically re-examined. It is possible that for some aquifers carbonate exchange between solid and liquid phases in the aquifer changes 14C results to a larger degree than generally assumed.

Frequency distributions of more than 300 14C groundwater ages from various regions in northern and southern Sahara reflect the alternating sequence of humid and arid periods in the Sahara during the Holocene and late Pleistocene. A broad frequency maximum between 20,000 and 50,000 years bp indicates a long humid period. During this time span, the northern Sahara received rain from the Western Drift, which is concluded from a west-east decrease of deuterium and oxygen 18 of these groundwaters (continental effect). In the time-slice between 14,000 and 20,000 years bp, groundwater formation was significantly lower due to a cool and (semi-)arid period. In the Holocene, the Saharian climate is characterized by a sequence of dry and wet periods.

Radiocarbon dates, obtained from paleosols, sediments, fossils, and groundwater samples of North Africa and especially, Tunisia, were investigated for information on phases of pedogenesis throughout the younger Pleistocene and Holocene in north and central Tunisia. This paper evaluates available data, while a larger set of new samples is under study, which, hopefully will exhaust the problem and will reveal whether extrapolations such as those made in this paper, eg, phases of pedogenesis from groundwater data, are correct.

Frequency distribution of the dates from groundwaters taken by systematic sampling, as well as from random soil samples from open pits that yielded access to buried paleosols, indicate that organic matter was being produced for 7 or 8 periods. The evidence suggests major pedogenic activity at about 2000 BP, 4000 to 6000 BP, 8000 to 12,000 BP, and perhaps 21,000 to 25,000 BP.

14C and 13C enrichment values are reported for a series of surface soil profiles which represent the progressive transition from mor to mull humus induced by birch (Betula pendula) colonization.

Variations in Δ and δ13C, which range between 85 to 154‰ modern and −28.1 to −25.3‰ (PDB), respectively, reflect changes in the rate and mode of organic decomposition. The most marked alterations in soil character occur over the first few decades following the introduction of birch, with clear isotopic evidence for the deeper penetration and accelerated mineralization of organic material.

Δ14C and δ13C values for organic matter in forest and grassland soils, in the presence or absence of earthworms, indicate that it should be possible to quantify the effects of earthworms on soil organic matter by this means. Without earthworms, both in forest and grassland soils, plant debris tends to accumulate on the surface of the mineral soil and little organic matter is incorporated into or is translocated down the soil profile. Where earthworms are present, there is much more marked incorporation of fresh plant debris in the mineral soil. This is shown especially by the pulse of ‘bomb’ carbon and also by the δ13C values.

Tufa is a superficial, secondary deposit of calcium carbonate which accumulates on precipitation from emergent spring waters. It occurs as discrete, localized masses in regions of calcareous country rock.

In the United Kingdom, deposits vary widely in structure and thickness but little is known of the rate of deposition. Some deposits contain laminae assumed to represent annual growth increments and which should contain a 14C and stable isotope record related to the original water from which it was precipitated.

Investigations are reported on tufa from, three areas of different limestones in the United Kingdom (Northwest Yorkshire, South Derbyshire and North Oxfordshire).

Hydrologically, the dating of tufa by 14C involves the same problems as the dating of groundwater. In the case of actively forming tufa, however, it is possible to derive a sequence of measurements, beginning with present day deposition, which clearly demonstrates the applications of age corrections.

At Gordale Scar, Northwest Yorkshire, a profiling study of laminated tufa appears to show bomb trial 14C to a depth of 18mm below the surface, with almost constant values around 50 percent modern (raw data) from 18 to 48mm below the surface. The 14C content of surface tufa lies within the seasonal range of 14C measurements from the parent stream waters.

Results of 50 percent modern in the sequence are consistent with the simplest correction procedures based on δ13C balance and the observed δ13C change on tufa precipitation is a practical demonstration of the fractionation factor ∊13. However, the application of corrections to active, surface tufa and parent waters collected monthly over a period of study (14 months) from all three sites, produce results higher than would be expected from published world 14C levels.

Carbon isotope measurements in soil CO2 are presented and discussed. Soil CO2 concentration and 13C profiles were measured using a new technique. A simple model describing the CO2 transport from the soil to the atmosphere is derived. The finding that CO2 in the soil is richer in 13C than the CO2 leaving the soil is attributed to isotopic fractionation in molecular diffusion.

Contamination by recent carbon and the turnover of organic matter make dating of ancient soils difficult. In order to isolate the oldest organic fraction of sediments, two main extraction methods were previously proposed: 1) alkaline solubilization of humus that separates humins, humic acids, and fulvic acids, and 2) successive hydrolyses that solubilize increasingly resisting products. Both preparation methods were tested on the same actual or fossil soils of different pedologic types from five geologic profiles on which other chronologic data are available.

Analytic results show that 14C ages obtained from alkaline extraction products differ according to the duration of treatments and characteristics of soils: while hydrolysis should yield more homogeneous results and isolate oldest fractions. It seems likely that true ages of geologic formations were never obtained from their organic matter and that the oldest organic fraction, contemporaneous with the sediment formation, completely disappears. Thus, most ages from 14C dating of organic matter of soils must be too recent.

A preliminary review is given of the evaluation of the International Cross-Calibration Radiocarbon Data. Consensus values and standard errors are D14C = 508.1 ± 2.0% (ANU Sucrose) and Δ = −2.5 ± 1.5% (1850 Wood). Reporting format follows recommendations of Stuiver and Polach (1977).

An interlaboratory comparison experiment for 14C measurements has been organized jointly by the Harwell and British Museum laboratories to include the working radiocarbon laboratories of the United Kingdom. The experiment has been run along the lines of that organized by the IAEA for tritium and has explored the problems of sample preparation, verification of equivalent levels, and presentation of results. Samples of benzene representing 5 age equivalent levels between twice modern and 20,000 years old have been prepared and distributed for measurement either by direct counting (liquid scintillation) or full process tests. Preliminary results received show excellent agreement both between laboratories and in comparison with the known relative activities of the prepared solutions. The possibility of extending the experiment to cover different sample types and a wider distribution of testing laboratories is briefly discussed.