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.

We provide detailed contextual information on 25 14C dates for unusually well-preserved archaeological and paleontological remains from Daisy Cave. Paleontological materials, including faunal and floral remains, have been recovered from deposits spanning roughly the past 16,000 yr, while archaeological materials date back to ca. 10,500 BP. Multidisciplinary investigations at the site provide a detailed record of environmental and cultural changes on San Miguel Island during this time period. This record includes evidence for the local or regional extinction of a number of animal species, as well as some of the earliest evidence for the human use of boats and other maritime activities in the Americas. Data from Daisy Cave contribute to a growing body of evidence that Paleoindians had adapted to a wide variety of New World environments prior to 10,000 PB. Analysis of shell-charcoal pairs, along with isotopic analysis of associated marine shells, supports the general validity of marine shell dating, but also provides evidence for temporal fluctuations in the reservoir effect within the Santa Barbara Channel region.

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).

The radiocarbon calibration curve IntCal04 extends back to 26 cal kyr B P. While several high-resolution records exist beyond this limit, these data sets exhibit discrepancies of up to several millennia. As a result, no calibration curve for the time range 26–50 cal kyr BP can be recommended as yet, but in this paper the IntCal04 working group compares the available data sets and offers a discussion of the information that they hold.

An attempt has been made to assemble the large number of C14 dates measured in Groningen since the last date list was published in 1958. We have not succeeded in preparing all the measurements done in this time; the present list contains a more or less random selection. It is hoped the rest will be included in next year's list.

14C measurements are reported for fractionated soil organic matter from a genetic soil sequence which was sampled several times during the period of atmospheric nuclear weapons testing. Fractionation of the soils by density followed by acid hydrolysis was successful in separating the organic matter into components with mean residence times for carbon ranging from 5 to 20 years (low density fraction) to several thousand years (residue after acid hydrolysis). Comparison of the infiltration of bomb 14C into the vertical soil profile with the distribution of 137Cs, gives clues as to the mechanism (most probably dissolved transport) for importing carbon into deeper soil layers.

Acid hydrolysis is used to fractionate the soil organic carbon pool into relatively slow- and fast-cycling compartments on soils from Arizona, the Great Plains states and Michigan collected for carbon isotope tracer studies related to soil carbon sequestration, for studies of shifts in C3/C4 vegetation, and for “pre-bomb” soil-carbon inventories. Prior to hydrolysis, soil samples are first treated with cold 0.5–1N HCl to remove soil carbonates if necessary. Samples are then dispersed in a concentrated NaCl solution (ρ≍1.2 g cm-3) and floated plant fragments are skimmed off the surface. After rinsing and drying, all remaining recognizable plant fragments are picked from the soil under 20x magnification. Plant-free soils, and hot, 6N HCl acid-hydrolysis residue and hydrolyzate fractions are analyzed for carbon content, δ13C and 14C age, and the carbon distribution is verified within 1–2% by stable-carbon isotope mass balance. On average, the recalcitrant residue fraction is 1800 yr older and 2.6% more 13C-depleted than total soil organic carbon. A test of hydrolysis with fresh plant fragments produced as much as 71–76% in the acid-hydrolysis residue pool. Thus, if plant fragments are not largely removed prior to hydrolysis, the residue fraction may date much younger than it actually is.

Bone would seem to be an ideal material for 14C dating because this calcified tissue contains 20 weight per cent protein. Fossil bone, however, can lose most of its original organic matter and frequently contains contaminants having different 14C ages. Numerous 14C dates on bone have been available to archaeologists and geologists but many age determinations have been inaccurate despite over 30 years of research in the field following the first 14C age determinations on bone (Arnold & Libby, 1951). This situation remained unchanged until simple pretreatments were abandoned and more bone-specific fractions were isolated. The ideal solution is to use accelerator mass spectrometer 14C dating, which facilitates the use of milligram-sized amounts of highly purified compounds—an approach impossible to pursue using conventional 14C decay-counting methods.

This paper provides a summary overview of the current state-of-art in the radiocarbon dating of groundwater. While the use of natural 14C measurements in applied hydrogeology still presents a difficult challenge, meaningful dates can be achieved if they are determined and interpreted in conjunction with the analyses of other isotopic species that occur in the natural environment. Although 14C dating of groundwater can be, and often is, carried out as a matter of routine, any specific case study requires its own scientific design and effort. As is widely recognized, and discussed in considerable detail throughout the scientific literature, there are many hydrogeochemical reactions and/or physical processes that can alter the natural 14C enrichment measured in environmental materials. Fortunately, for fresh groundwater resources such effects are in general well defined and therefore of limited significance. The primary challenge in applied groundwater dating is with the development of the appropriate theoretical background against which 14C dates can be used to calibrate numerical analogues of the groundwater system. The hydraulic properties of each of the widely used finite-element models can be well estimated from numerous piezometric data and extrapolations. In contrast, only a few groundwater ages can be provided for the calibration of those models that are complex functions of aging mixture and sometimes also hydrochemical reactions.

An on-line measurement system was installed at the MICADAS in Zurich, using an elemental analyzer (EA) as a combustion unit to enable direct radiocarbon measurement of samples containing carbon in the range of 5–100 μg possible with minimum effort. The samples are combusted in small capsules and the gaseous combustion products are separated by the EA. The carbon dioxide leaving the EA in a high helium flow is concentrated on a small external trap containing X13 zeolite adsorber material. This new concept, avoiding a cryogenic trapping for the enrichment step, allows the construction of a very compact system able to work even with the smallest samples. Concentrated on the external trap, the carbon dioxide is flushed into the gas-tight syringe of our gas inlet system using a low helium stream. The gas mixture is measured with the MICADAS gas ion source. Several different sample capsules were analyzed to minimize the major blank contribution coming from the sample vessel. The best results were achieved with 25-μL tin capsules, which contained only 0.34 ± 0.13 μg carbon at 65 pMC. This work describes the development of the on-line system and the protocol for measurement runs. Results are presented for on-line measurements of reference materials and a comparison is performed with typical dating samples measured previously as graphite targets. Finally, relevance and limitations of on-line measurements are discussed.

Information on the history of the atmospheric CO2 content and the 13C/12 and 14C/C ratios is recorded in natural ice. Measurements on samples from very cold accumulation regions show that CO2 is occluded not only in air bubbles, but also in the ice lattice. The two CO2 components are of similar size. It is very difficult to measure CO2 in the bubbles and CO2 in the ice lattice separately. By melting the samples and extracting the evolving gases in two fractions, it is possible to estimate CO2 concentration in the bubbles and the ice lattice. Enrichment or depletion of CO2 in the bubbles by exchange with the ice is difficult to estimate. Information about this effect is expected from 13C/12C analysis on the extracted CO2 fractions.

To investigate whether atmospheric CO2 content was different during the last glaciation than during the present one, sets of 16 and 20 samples distributed over the last 40,000 years from the two deep ice cores from Camp Century (North Greenland) and Byrd Station (West Antarctica) were measured. The time scales for the two cores are based on a rheological model. Results and conclusions are:

1. — The data series from both cores show similar trends correlated to a certain degree to the δ18O profiles.

2. — For both cores, the values for the CO2 concentration of the first fraction, considered to best represent the atmospheric composition, show lower values during glaciation than in the Holocene, with a minimum before the end of glaciation.

3. — Low CO2 concentrations in the first fractions (200ppm) of certain samples are a strong indication that the atmospheric CO2 concentration during last glaciation was lower than during the postglacial. These low concentrations indicate that, at that time, CO2 concentration in the atmosphere could have been lower than today by a factor of 1.5. Possible explanations for such a change in atmospheric CO2 content as well as its influence on atmospheric 14C/C ratio and on the radiation balance is discussed.

Claims that forest cutting during the last few decades has contributed significantly to the buildup in atmospheric CO2 have cast doubt on the validity of models used to estimate CO2 uptake by the ocean. In this paper we review the existing models and conclude that the box-diffusion model of Oeschger and his co-workers provides an excellent fit to the average distributions of natural and bomb-produced radiocarbon. We also take the first steps toward a more detailed ocean model which takes into account upwelling in the equatorial zone and deep water formation in the polar zone. The model is calibrated using the distribution of bomb-produced and cosmic ray-produced radiocarbon in the ocean. Preliminary calculations indicate that the fossil fuel CO2 uptake by this model will be greater than that by the box-diffusion model of Oeschger and others (1975) but not great enough to accommodate a significant decline in the mass of the terrestrial biosphere over the past two decades.

13C and 14C analyses were performed on a series of modern Jamaican land snails in order to quantitatively determine the sources of shell carbon. A model of these carbon sources, the pathways by which carbon reaches the shell, and the fractionation processes involved are presented. The contribution of limestone to shell carbonate is variable but may comprise up to 33% of the shell. About 25–40% of shell carbonate is derived from plants and about 30–60% from atmospheric CO2. Variation among populations and species with respect to 13C and 14C is attributed to the effects of limestone incorporation, snail size (as it affects CO2 exchange rate), physiological characteristics (presence of urease, respiration rate), and activity patterns of the snails. A formula for correction for isotopic fractionation of 14C of shell carbonate, based on 13C measurements, is derived. Bicarbonate-aragonite fractionation is apparently very minimal. Shell organic carbon appears to be derived largely from plants but also to a lesser extent from inorganic hemolymph carbon. This introduces the possibility of a small age anomaly of shell organic 14C due to limestone incorporation.

With the recent establishment of an unbroken West European tree-ring sequence spanning the past 7272 years (Pilcher et al, 1984) the calibration of the 14C time scale was advanced considerably. It is now possible to use this chronology as an independent cross-check on the 8681-year US Bristlecone Pine series (Ferguson & Graybill, 1983). There also are opportunities for 14C matching (wiggle matching) between the older portion of the Bristlecone Pine series and floating (not tied to the present) parts of the South German Oak sequence. Linick, Suess and Becker (1985) used this approach in matching the earliest part of the Bristlecone Pine series with the Donau 6 Main 4/11 (Becker, 1983) series and thus established a D6M4/11 “zero” point (tree-ring no. 1) of 7215 bc.

The University of Bern has set up the new Laboratory for the Analysis of Radiocarbon with AMS (LARA) equipped with an accelerator mass spectrometer (AMS) MICADAS (MIni CArbon Dating System) to continue its long history of 14C analysis based on conventional counting. The new laboratory is designated to provide routine 14C dating for archaeology, climate research, and other disciplines at the University of Bern and to develop new analytical systems coupled to the gas ion source for 14C analysis of specific compounds or compound classes with specific physical properties. Measurements of reference standards and wood samples dated by dendrochronology demonstrate the quality of the 14C analyses performed at the new laboratory.

The international radiocarbon community met on June 25, 1982. Two reports were presented, and several measures related to radiocarbon dating discussed and adopted.

A sequence of annually laminated sediments is a potential tool for calibrating the radiocarbon time scale beyond the range of the absolute tree-ring calibration (11 ka). We performed accelerator mass spectrometric (AMS) 14C measurements on >250 terrestrial macrofossil samples from a 40,000-yr varve sequence from Lake Suigetsu, Japan. The results yield the first calibration curve for the total range of the 14C dating method.

Layers of volcanic ash, or tephra form widespread chronostratigraphic marker horizons which are important because of their distinctive characteristics and rapid deposition over large areas. Absolute dating of prehistoric layers effectively depends upon 14C analysis. We focus here on Icelandic tephra layers at both proximal and distal sites and consider three strategies to obtain age estimates: 1) the conventional dating of individual profiles; 2) high-precision multisample techniques or “wiggle-matching” using stratigraphic sequences of peat; and 3) a combination of routine analyses from multiple sites. The first approach is illustrated by the dating of a peat profile in Scotland containing tephra from the ad 1510 eruption of Hekla. This produced a 14C age compatible with ad 1510, independently derived by geochemical correlation with historically dated Icelandic deposits. In addition, the ca. 2100 bp date for the Glen Garry tephra in Scotland, determined by a series of dates on a peat profile in Caithness, is supported by its stratigraphic position within 14C dated profiles in Sutherland, and may be applied over a very large area of Scotland. More precise dates for individual tephras may be produced by “wiggle-matching”, although this approach could be biased by changes in peat-bog stratigraphy close to the position of the tephra fall. As appropriate sites for “wiggle-match” exercises may be found only for a few Icelandic tephras, we also consider the results of a spatial approach to 14C dating tephra layers. We combined dates on peat underlying the same layer at several sites to estimate the age of the tephra: 3826 ± 12 bp for the Hekla-4 tephra and 2879 ± 34 bp for the Hekla-3 tephra. This approach is effective in terms of cost, the need for widespread applicability to Icelandic tephra stratigraphy and the production of ages of a useful resolution. We stress the need for accurate identification of tephra deposits without which the conclusions drawn from subsequent 14C dating will be fundamentally flawed.

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.