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.

Radiocarbon calibration data derived from German oak chronologies, ranging back to 7200 BC, have been published in the previous Calibration Issue (Stuiver & Kra 1986). In recent years, the German oak chronology has been extended to 7938 BC (Becker, this issue). For earlier intervals, tree-ring chronologies must be based on pine, because oak re-emigrated to central Europe at the Preboreal/Boreal transition, at about 8000 BC. We have established a 1784-yr pine chronology centered in the Preboreal, and have linked it tentatively to the absolutely dated oak master. We present here calibration data based on this link, for the age range, 7145–9439 BC.

14C measured in trace gases in clean air helps to determine the sources of such gases, their long-range transport in the atmosphere, and their exchange with other carbon cycle reservoirs. In order to separate sources, transport and exchange, it is necessary to interpret measurements using models of these processes. We present atmospheric 14CO2 measurements made in New Zealand since 1954 and at various Pacific Ocean sites for shorter periods. We analyze these for latitudinal and seasonal variation, the latter being consistent with a seasonally varying exchange rate between the stratosphere and troposphere. The observed seasonal cycle does not agree with that predicted by a zonally averaged global circulation model. We discuss recent accelerator mass spectrometry measurements of atmospheric 14CH4 and the problems involved in determining the fossil fuel methane source. Current data imply a fossil carbon contribution of ca 25%, and the major sources of uncertainty in this number are the uncertainty in the nuclear power source of 14CH4, and in the measured value for δ 14C in atmospheric methane.

14C measurements provide a useful test for determining the degree to which chemical and physical fractionation of soil organic matter (SOM) are successful in separating labile and refractory organic matter components. Results from AMS measurements of fractionated SOM made as part of several projects are summarized here, together with suggestions for standardization of fractionation procedures. Although no single fractionation method will unequivocally separate SOM into components cycling on annual, decadal and millennial time scales, a combination of physical (density separation or sieving) and chemical separation methods (combined acid and base hydrolysis) provides useful constraints for models of soil carbon dynamics in several soil types.

When dating unburnt bone, bone collagen, the organic fraction of the bone, is used. Collagen does not survive the heat of the cremation pyre, so dating of cremated bone has been considered impossible. Structural carbonate in the mineral fraction of the bone, however, survives the cremation process. We developed a method of dating cremated bone by accelerator mass spectrometry (AMS), using this carbonate fraction. Here we present results for a variety of prehistoric sites and ages, showing a remarkable success rate for this method.

By correlating the climate records and radiocarbon ages of the planktonic foraminifera N. pachyderma(s) of deep-sea core PS2644 from the Iceland Sea with the annual-layer chronology of the GISP2 ice core, we obtained 80 marine 14C calibration points for the interval 11.4-53.3 ka cal BP. Between 27 and 54 ka cal BP the continuous record of 14C/cal age differences reveals three intervals of highly increased 14C concentrations coincident with low values of paleomagnetic field intensity, two of which are attributed to the geomagnetic Mono Lake and Laschamp excursions (33.5-34.5 ka cal BP with maximum 550 marine δ14C, and 40.3-41.7 ka cal BP with maximum 1215 marine δ14C, respectively). A third maximum (marine δ14C: 755) is observed around 38 ka cal BP and attributed to the geomagnetic intensity minimum following the Laschamp excursion. During all three events the A14C values increase rapidly with maximum values occurring at the end of the respective geomagnetic intensity minimum. During the Mono Lake Event, however, our A14C values seem to underestimate the atmospheric level, if compared to the 36Cl flux measured in the GRIP ice core (Wagner et al. 2000) and other records. As this excursion coincides with a meltwater event in core PS2644, the underestimation is probably caused by an increased planktonic reservoir age. The same effect also occurs from 38.5 to 40 ka cal BP when the meltwater lid of Heinrich Event 4 affected the planktonic record.

We made a comparative study of AMS 14C ages of organic deposits (minerotrophic peats and gyttjas) and macrofossils in order to evaluate the magnitude of a number of sources of error that may be present in bulk sediment samples. The consistency of 14C ages found for coexisting macrofossils suggests that they are unlikely to record disturbances. Some of our gyttja samples yielded an age 0.2–0.6 ka 14C years too old due to hardwater effect. We also found an aging effect in several bulk samples with a high admixture of siliciclastic material; this is attributed to fluvial input of reworked, older organic debris. Rejuvenation of bulk material as a result of root contamination occurs mainly in samples overlain by slowly accumulated deposits, and particularly in samples affected by (sub)recent roots.

Techniques for making precise and accurate radiocarbon accelerator mass spectrometry (AMS) measurements on samples containing less than a few hundred micrograms of carbon are being developed at the NOSAMS facility. A detailed examination of all aspects of the sample preparation and data analysis process shows encouraging results. Small quantities of CO2 are reduced to graphite over cobalt catalyst at an optimal temperature of 605°. Measured 14C/12C ratios of the resulting targets are affected by machine-induced isotopic fractionation, which appears directly related to the decrease in ion current generated by the smaller sample sizes. It is possible to compensate effectively for this fractionation by measuring samples relative to small standards of identical size. Examination of the various potential sources of background 14C contamination indicates that the sample combustion process is the largest contributor, adding ca. 1 μg of carbon with a less-than-modern 14C concentration.

The program called Geochemical Ocean Section Study (GEOSECS) was initiated in 1971 and designed to make an oceanic inventory of chemical constituents in the world oceans, partly as a baseline study for future chemical changes, and partly to investigate large-scale oceanic transport and mixing processes. Measured were hydrographic parameters (salinity, temperature, and oxygen vs depth), the carbonate system, the nutrients, and assorted other constituents with heavy emphasis laid on the radioactive substances, radiocarbon, tritium, and radon.

Although the reliability of 14C dates of bone has increased greatly since AMS methods permitted better pretreatment on smaller samples, most old, badly contaminated or severely weathered bone still give serious problems. Several groups have recently proposed improvements to sample purification methods, often supported by a number of 14C measurements. We present here an overview of these improvements. The issue is complicated by the following:

1. 1. Different problems are presented depending on age, preservation and degree of contamination of bone.

2. 2. Methods may or may not be developed with routine application in mind.

3. 3. Determining the conditions for which any method can be regarded as reliable is not at all straightforward.

High-quality data from appropriate archives are needed for the continuing improvement of radiocarbon calibration curves. We discuss here the basic assumptions behind 14C dating that necessitate calibration and the relative strengths and weaknesses of archives from which calibration data are obtained. We also highlight the procedures, problems, and uncertainties involved in determining atmospheric and surface ocean 14C/12C in these archives, including a discussion of the various methods used to derive an independent absolute timescale and uncertainty. The types of data required for the current IntCal database and calibration curve model are tabulated with examples.

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.

The result of a radiocarbon determination is commonly expressed as an age given in radiocarbon years. An error is usually assigned to each value as a measure of the statistical uncertainty of the measurement. Date lists published in this journal use a standard form of reporting dates and their errors (see Editorial Statements in Radiocarbon, v. 3 and v. 4). The conversion of a radiocarbon age, given in radiocarbon years B.P. (i.e., radiocarbon years elapsed since the origin of the sample) to a true calendar year makes necessary certain assumptions with respect to: (1) the half-life of C14, (2) the production rate of C14 by cosmic rays, (3) the size of reservoirs into which C14 is distributed and the exchange rate of this distribution. Libby (1955, p. 10) has shown that as an approximation one may assume that reservoir size and production and distribution rates, and therefore the C14 activity in atmospheric CO2 have been constant. However, the more accurate measurements of recent years have shown that at least one of these quantities must have varied with time. This means that a more complicated relationship exists between radiocarbon age and exact calendar age of a sample than had been assumed by Libby. This relationship cannot be determined theoretically, but can be derived empirically by determination of the radiocarbon contents of samples of known age. The following summarizes our present knowledge regarding differences between radiocarbon ages and true ages and the present status of the empirical calibration of the radiocarbon time scale.

14C measurements on continuous weekly samples of atmospheric CO2 and hydrocarbons, collected in a rather densely populated area are presented. The deviation of the measured 14C data from the clean air level is primarily due to CO2 from the combustion of fossil fuels. This is confirmed by fossil fuel admixture estimates individually calculated with an atmospheric dispersion model. Up to 10 percent admixture is predicted by this model and observed from the 14C shift for weekly averages, particularly during the winter season. Natural CO2 admixture due to soil respiration, however, even in winter, is of the same order of magnitude, but much larger in the warm season: the considerable variations in CO2 concentration in summer are almost exclusively controlled by natural sources. Using tree leaf samples, we have been able to identify boiling water reactors (BWR) as weak sources of 14CO2. Atmospheric samples taken in the environment of the pressurized water reactors (PWR) Biblis show that the 14C release of these reactors is primarily in the form of hydrocarbon 14C. The source strength of the various power plants, calculated on the basis of our observations in their environment, ranges from 0.5 to 7Ci per year.

The radiocarbon ages of dendrochronologically dated wood samples, each covering 10 years, are reported back to 2500 yr BC. The decadal calibration curve constructed from these data is an extension of the curve previously given for the AD interval (Stuiver, 1982). A major difference with the previous work, however, is the assessment of the error in the radiocarbon age determination. Whereas previously this error was only based on the Poisson counting statistics of the accumulated number of counts for the sample and standards, the current calibration error is based on an estimate of the reproducibility in the radiocarbon activity determination. As a consequence, the uncertainty in the current calibration curve is, on average, 1.6 times that of the AD curve previously given.

Current conventions for reporting radiocarbon determinations do not cover the reporting of calibrated dates. This article proposes revised conventions that have been endorsed by many 14C scientists. For every determination included in a scientific paper, the following should apply: (1) the laboratory measurement should be reported as a conventional radiocarbon age in 14C yr BP or a fractionation-corrected fraction modern (F14C) value; (2) the laboratory code for the determination should be included; and (3) the sample material dated, the pretreatment method applied, and quality control measurements should be reported. In addition, for every calibrated determination or modeled date, the following should be reported: (4) the calibration curve and any reservoir offset used; (5) the software used for calibration, including version number, the options and/or models used, and wherever possible a citation of a published description of the software; and (6) the calibrated date given as a range (or ranges) with an associated probability on a clearly identifiable calendar timescale.

Apparent marine radiocarbon ages are reported for the northern Indian Ocean region for the pre-nuclear period, based on measurements made in seven mollusk shells collected between 1930 and 1954. The conventional 14C ages of these shells range from 693 ± 44 to 434 ± 51 BP in the Arabian Sea and 511 ± 34 to 408 ± 51 BP in the Bay of Bengal. These ages correspond to mean ΔR correction values of 163 ± 30 yr for the northern Arabian Sea, 11 ± 35 yr for the eastern Bay of Bengal (Andaman Sea) and 32 ± 20 yr for the southern Bay of Bengal. Contrasting reservoir ages for these two basins are most likely due to differences in their thermocline ventilation rates.

The chronology of the Early Bronze Age (EBA) in the southern Levant and the synchronization between the sites, considering seriation and radiocarbon dates, have shown large inconsistencies and disagreement. We have assembled 420 14C dates, most of them previously published and a few provided directly by the excavators. The dates have been re-evaluated on the basis of their archaeological context and using analytical criteria. Bayesian modeling has been applied to the selected dates in relation to the given seriation of the EBA subperiods (EB I, II III, IV). Sites with 2 or more sequential sub-phases were individually modeled in order to define the transitions between the subperiods. The new chronology indicates that the EB I–II transition occurred site-dependently between 3200–2900 BC, with EB II–III around 2900 BC, and EB III–IV ∼2500 BC.

We present a brief discussion of sample preparation procedures at the Keck Carbon Cycle Accelerator Mass Spectrometer (KCCAMS), University of California, Irvine, and a systematic investigation of the use of Mg(ClO4)2 as an absorptive water trap, replacing the standard dry ice/ethanol cold finger in graphite sample preparation. We compare high-precision AMS measurement results from oxalic acid I and USGS coal samples using Mg(ClO4)2 under different conditions. The results obtained were also compared with those achieved using the conventional water removal technique. Final results demonstrate that the use of Mg(ClO4)2 as an alternative water trap seems very convenient and reliable, provided the Mg(ClO4)2 is replaced frequently.

The high-precision radiocarbon calibration curve for short-lived samples (1–4 yr) of the early historical period (3rd millennium BC) presented previously (Vogel et al. 1986) has been further substantiated and extended to link with a similar curve produced by de Jong for part of the 4th millennium BC (de Jong & Mook 1980). The precise dendrochronological age of the sample set measured by de Jong has finally been fixed (de Jong, Mook & Becker 1989), so that the two sets now cover the period 1930–3900 BC, i.e., the Early Bronze Age and Late Chalcolithic periods of the Middle East. The standard calibration curve for the two sets is presented by Vogel and van der Plicht (1993).