Fieldwork for the World Ocean Circulation Experiment (WOCE) radiocarbon program was recently completed. Ca. 9000 samples were collected for analysis using both conventional β-counting techniques and the newer AMS technique. The mean uncertainty for the β analyses is 3‰; for AMS analyses, ca. 4.5‰.

We compared four adjacent soil plots in an effort to determine the effect of land use on soil carbon storage. The plots were located at the High Plains Agricultural Research Laboratory near Sidney, Nebraska. We measured 14C, total carbon, total nitrogen and 137Ce to determine the size and turnover times of rapid and stable soil organic matter (SOM) pools, and their relation to land-use practices. Results were consistent with the model produced by Harrison, Broecker and Bonani (1993a) in that the 14C surface soil data fell on the time trend plots of world 14C surface soil data, indicating that the natural sod and non-tilled plots had a rapidly turning over SOM pool, comprising ca. 75% of surface soil carbon, and the tilled plots had a rapidly turning over SOM pool, comprising only 50% of surface soil carbon.

AMS radiocarbon results from the World Ocean Circulation Experiment in the Pacific Ocean show dramatic changes in the inventory and distribution of bomb-produced 14C since the time of the GEOSECS survey (8/73–6/74). Near-surface Δ14C values for the eastern portion of both the northern and southern subtropical gyres decreased by 25–50‰, with the change being greater in the north. Equatorial near-surface values have increased by ca. 25‰. Changes in the 250–750-m depth range are dramatically different between the northern and southern basins. The intermediate and mode waters of the southern basin have increased by as much as 75‰ since GEOSECS. Waters of similar density in the northern hemisphere are not exposed to the Southern Ocean circulation regime and are significantly less ventilated, showing maximum changes of ca. 50‰.

At the University of Miami Tritium Laboratory and the University of Washington Quaternary Isotope Laboratory, more than 1000 large-volume Pacific Ocean radiocarbon samples were measured for the WOCE program. Here we present a comprehensive data set, and a brief discussion of our findings.

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.

Δ14C records are reported for post-bomb corals from three sites in the tropical Atlantic Ocean. In corals from 18°S in the Brazil Current, Δ14C values increased from ca. −58% in the early 1950s to +138% by 1974, then decreased to 110‰ by 1982. Shorter records from 8ºS off Brazil and from the Cape Verde Islands (17°N) showed initially higher Δ14C values before 1965 than those at 18ºS, but showed lower rates of increase of Δ14C during the early 1960s. There is general agreement between the coral results and Δ14C of dissolved inorganic carbon (DIC) measured in seawater previously for locations in the tropical Atlantic Ocean. Δ14C values at our tropical ocean sites increased at a slower rate than those observed previously in the temperate North Atlantic (Florida and Bermuda), owing to the latter's proximity to the bomb 14C input source in the northern, hemisphere. Model results show that from 1960–1980 the Cape Verde coral and selected DIG Δ14C values from the North Equatorial Current agree with that calculated for the North Atlantic based on an isopycnal mixing model with a constant water mass renewal rate between surface and subsurface waters. This is in contrast to Δ14C values in Bermuda corals that showed higher post-bomb values than those predicted using a constant water mass renewal rate, hence indicating that ventilation in the western north Atlantic Ocean had decreased by a factor of 3 during the 1960s and 1970s (Druffel 1989).

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.

The carbon content, pH and 14C concentration of humic acids were determined for three soil series of Arctic and Subarctic ecosystems. The measured 14C ages were interpreted in the light of an equilibrium model of humus formation and of mineralization processes in recent soils, and the coefficient of renovation, Kr, was calculated for humic acids. The comparison of Kr for series formed under different climatic conditions suggested that global warming could accelerate decomposition of soil organic matter and possibly increase productivity of ecosystems of the Arctic region.

We have refined marine reservoir age estimates for eastern Pacific coastal waters with radiocarbon measurements of mollusk shells collected prior to 1950. We have also investigated interspecific variability in 14C ages for historic and ancient shells from San Francisco Bay.

In order to verify sediment trap samples as indicators of upper ocean 14C concentrations, particulate inorganic radiocarbon (PICΔ14C) collected by time-series sediment traps in the Sea of Okhotsk and the Bering Sea was measured by accelerator mass spectrometry (AMS). All of the PICΔ14C measurements were < 0‰, in contrast to GEOSECS 14C data in the upper ocean from the northwestern North Pacific. This difference is attributed to the upwelling of deepwater that contains low Δ14C of dissolved inorganic carbon (DICΔ14C) and to the decrease over time of surface DICΔ14C owing to the decrease of atmospheric Δ14C values. In addition, PICΔ14C values showed significant seasonal variability: PICΔ14C collected in the fall was the greatest (-22‰ on average), whereas PICΔ14C collected in winter showed an average minimum of −48‰. It is likely that this difference was caused by changes in mixed layer thickness. Although some uncertainties remain, further study on PICΔ14C will enable us to estimate seasonal variability in DICΔ14C and air-sea CO2 exchange rate.

For several undisturbed sites in Germany, 14C data are reported for soil organic matter (SOM) (4 sites), soil CO2 (10 sites) and dissolved organic carbon (DOC) (1 site). With the assumption of a fast degradable component (lifetime ca. 1 yr) and a slow degradable component (lifetime ca. 100 yr), a range between 0.6 and 1.6 mm yr-1 has been determined for the downward migration rates of soil organic carbon at the sampling sites from the soil 14C data. The soil CO2 measurements show that in deciduous forests the fast degradable component is ca. 60% and the slow degradable component is ca. 40% of the SOM. In coniferous forests this ratio is reversed. The 14C results for DOC could not be explained with the assumption of a first-order decay process. The removal of soil organic carbon by DOC is of minor importance for the estimation of carbon budgets for the investigated site.

Extensive measurements of radiocarbon have been used in New Zealand since the mid-1960s to follow carbon (C) movement and turnover in soils. We present here unpublished radiocarbon (14C) measurements on a range of eight New Zealand soils with details of the sites, ecosystems, climates, soil descriptions and associated analytical data. An overview is also given of published 14C measurements on soils, and the use of these measurements to model soil C turnover.

Accelerator mass spectrometry (AMS) measurement of 25 single-year tree rings from AD 1861–1885 at ca. ±3.5‰ precision shows no evidence of an anomalous 11-yr cycle of 14C near the Arctic Circle in the Mackenzie River area. However, the Δ14C measurements are lower on average by 2.7 ± 0.9 (ō)‰ relative to 14C measurements on tree rings from the Pacific Northwest (Stuiver and Braziunas 1993). We attribute this depression of Δ14C to thawing of the ice and snow cover followed by melting of frozen earth that releases trapped 14C-depleted CO2 to the atmosphere during the short growing season from May through August. Correlation of Δ14C with May–August estimated temperatures yields a correlation index of r = 0.60. The reduction in Δ14C is dominated by seven years of anomalous depletion. These years are 1861, 1867–1869, 1879–1880 and 1883. The years 1867–1869 are coincident with a very strong ENSO event.

Radiocarbon measurements on organic carbon from the varved cores of Elk Lake, Minnesota suggest that the varve count may underestimate calendar years by 18% for the most recent 3800 varve years.

The Geological Survey of Canada (GSC) has developed, over the past decade, a user-oriented database, Date Locator File, of Canadian samples dated by the 14C technique. This database presently contains >3500 soil and soil-related dates. The primary category in this suite of dates is peat, as a large portion of the Canadian landscape is covered with this type of organic soil. The data is available gratis to all researchers in a large variety of formats from simple lists to complex tables for inclusion in publications. The site localities can also be plotted on base maps suitable for publication. The database is actively augmented on an ongoing basis, but to continue to be relevant, it depends largely on the altruism of the scientific community.

Recently, in an intercomparison of the Hohenheim German oak chronology (Becker 1993) and the Göttingen chronology (Leuschner and Delorme 1988), an error was discovered in the former (Leuschner, in preparation). Due to an error in adding sections at 5241 bc, 41 yr are missing in the published Hohenheim chronology. After correction of the error, the two chronologies synchronize over their entire common length, back to 7200 bc.

We present a compilation of 14C soil dates measured at the University of Hamburg through 1984 (HAM-1597).

We report here the remainder of the Hamburg University dates on thin soil layers (HAM 1652–3129).

We have studied DNA adduction with 14C-labeled nicotine and nicotine-derived nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), by accelerator mass spectrometry (AMS) in mouse liver at doses equivalent to low-level exposure of humans. The dose ranges of nicotine and NNK administered were from 0.4 μg to 4.0×102 μg kg b.w.-1, and from 0.1 μg to 2.0×104 μg kg b.w.-1, respectively. In the exposure of mice to either nicotine or NNK, the number of DNA adducts increased linearly with increasing dose. The detection limit of DNA adducts was 1 adduct per 1011 nucleotide molecules. This limit is 1–4 orders of magnitude lower than that of other techniques used for quantification of DNA adducts. The results of our animal experiments enabled us to speculate that nicotine is a potential carcinogen. According to the procedure for 14C-labeled-NNK synthesis, we discuss the ultimate chemical speciation of NNK bound to DNA. From the animal tests we derived a directly perceivable relation between tobacco consumption and DNA adduction as the carcinogenic risk assessment.