Patterns of 14C enrichment in the superficial plant debris and mineral soil horizons of an established woodland have been monitored at regular intervals during the past 15 years. These data are compared with a model evaluation of carbon turnover based on the recorded changes in atmospheric 14C concentration since AD 1900.

Leaf litter and decomposing plant debris are characterized by steady-state turnover values of ca 2 and ca 8 years, respectively. A two-component system of ‘fast’ (≤20 yr) and ‘slow’ (ca 350 yr) cycling carbon is indicated for the surface (0–5cm) soil humus; below 10cm, the “fast’ component is rare (<5%).

Selective microbal humification of leaf litter, branch, and root debris is proposed to explain a delay of several years in the peak transfer of ‘bomb’ 14C to the soil carbon pool.

A 6-year and a 2-year record of 14C measurements of soil CO2 in two soils are presented and discussed. The annual 14C variation of soil CO2 is controlled by the seasonally varying contribution of root respiration and of microbial decomposition of organic matter producing soil CO2. The Δ14C soil CO2 difference between summer and winter is ca 50‰ in a soil where turnover of organic matter is fast (τ = 2.5a) and ca 100‰ in a soil of slow turnover (τ = 60a). A simple model describing the movement and turnover of organic matter is derived, giving the depth distributions of organic carbon and of 14C. The model needs a subdivision of the carbon reservoir into at least two reservoirs with residence times of τ1 = la and τ2 = 100a, respectively, and with a vertical transfer velocity in the order of 0.6mm/a.

The 14C content in 1961–1967 rings of each of three spruce pines grown at (68° N, 130° W), (47° 30' N, 129° 16' E) and (27° 13' N, 100° 20' E) were measured. Δ14C values of the three specimens rise dramatically from a common level (∼250‰) in 1961 to their respective maxima, 964‰, 909‰, and 743‰ in 1964 and then fall to a common level ∼680‰ in 1967. The observed Δ14C increase comes most likely from the nuclear bomb test of the USSR at 75° N in 1961, although there were many other tests since the 1950s. The different effects at different latitudes reflect the atmospheric circulation patterns in the stratosphere and the transport of 14C nuclei from the stratosphere to the troposphere.

Composite curvature averages for 14C age depth profiles of deep ocean sediment, continental sediment, and soil each indicate a global trend for 14C age increment per cm depth to increase with 14C age over the range for which a definitive statistical sample is available. The global trend indicated for peat profiles is constant 14C age increment per cm depth over the past 10,000 14C yr. Correlation coefficients between changes in 14C yr/cm and maximum profile thickness contradict compaction as an adequate explanation for the global trend indicated by sediment and soil profiles. This trend must be explained by additional factors such as progressively decreasing contamination from older carbon, increasing cosmic ray intensity, decreasing geomagnetic intensity, diminishing 12C in the active biosphere during profile accumulation, and climate factors affecting the rate of accumulation. The diverse trend of peat profiles may indicate climatic conditions more favorable to peat growth during the earlier portion of the past 10,000 yr.

Radiocarbon has been measured in two soil profiles, one of which has been covered by a building since 1956. A comparison of the Δ14C values in horizons of each profile gives an estimate of the total input of atom bomb 14C into the soil profile. From the Δ14C and carbon density profile data, the carbon input rates, respiration rates, and diffusivity are calculated. The lack of vegetation on one soil affects the mobility and the respiration rate of the soil carbon in that soil. The data from this soil profile are also used to check the assumption, used in previous analyses, that there is a uniform distribution of ‘old’ carbon down the soil profile. The input rate, turnover time, and diffusivity parameters determined from the Δ14C profiles in these soils are compared with other published data on pasture and forest soils.

The pattern of global water column inventories of bomb-produced 14C suggests that a sizeable portion of bomb 14C that entered the Antarctic, northern Pacific, and tropical oceans has been transported to adjacent temperate regions. Models of lateral transport of surface water in the Atlantic, Indian, and Pacific Oceans are based on this distribution pattern. Upwelling of bomb-14C-free water from below takes place in the Antarctic, northern Pacific, and tropical regions; downwelling of surface water occurs in the temperate oceans and northern Atlantic. Uptake of excess CO2 by these models is calculated using the observed Mauna Loa pCO2 record as an input function. Results indicate that 35% of fossil fuel CO2 is taken up by these model oceans during the period 1958–1980. Considering the observed airborne fraction of 0.55, it appears that ca 10% of the global fossil fuel CO2 is still missing.

An accurate atmospheric 13C/12C chronology can provide important constraints to models of the global carbon cycle. Trees accumulate carbon from atmospheric CO2 into growth rings and offer potential for 13C/12C reconstructions, but results have not been reproducible. This paper presents δ13C curves from 5 sites, representing 20 pinyon (Pinus edulis) trees, where cores of 4 trees from each site have been pooled into a composite sample. Isotopic analysis of cellulose in 5-yr ring groups produces curves with a general trend of decreasing δ13C after 1800, but with pronounced short-term fluctuations superimposed upon the trend. Evidence indicates the fluctuations are strongly related to moisture availability (drought). A mean curve of the 5 δ13C chronologies from which the fossil-fuel component is subtracted suggests a substantial biospheric CO2 contribution to the atmosphere since 1800.

The natural 14C and 13C content of soil organic matter and their dependence on depth for two Alfisols are presented. This soil type which covers a large area of the earth's surface is characterized by clay migration processes (“Lessivé”). The samples were taken as successive horizontal layers of 2cm depth from an area of ca lm2 size as deep as the C content allows 14C analysis. The minima of the D14C distribution decrease with depth, while the maxima increase in the upper, leached horizon (A1) due to bomb 14C and decrease in the lower, clay illuviated (Bt). δ13C indicates proceeding decomposition in A1 and protection of carbon, probably due to the formation of clay humus complexes in Bt. δ13C values were also used for age correction of the 14C data due to isotopic fractionation. The D14C and δ13C depth distributions are characterized by sharp peaks at the boundaries of the horizons, probably caused by the influence of textural changes on the transport of C with percolating water.

The renewal of east Atlantic deep water and its large-scale circulation and mixing have been studied in observed distributions of temperature, silicate, ΣCO2, and 14C. 14C variations in northeast Atlantic deep water below 3500m depth are small. Δ14C values range from − 100‰ to −125‰. 14C bottom water concentrations decrease from Δ14C =−117‰ in the Sierra Leone Basin to Δ14C = − 123‰ in the Iberian Basin and are consistent with a mean northward bottom water flow. The characteristic of the water that flows from the west Atlantic through the Romanche Trench into the east Atlantic was determined by inspection of θ/Δ14C and θ/SiO2 diagrams. A mean potential temperature of θ = 1.50 ± .05°C was found for the inflowing water. A multi-box model including circulation, mixing, and chemical source terms in the deep water has been formulated. Linear programing and least-squares techniques have been used to obtain the transport and source parameters of the model from the observed tracer fields. Model calculations reveal an inflow through the Romanche Trench from the west Atlantic, which predominates over any other inflow, of (5 ± 2) Sv (potential temperature 1.50°C), a convective turnover of (150 ± 50) years and a vertical apparent diffusivity of (4 ± 1) cm2/s. Chemical source terms are in the expected ranges.

Carbon isotope ratios were measured in organic and inorganic carbon of settling particulate matter collected with a sediment trap at Ocean Station “P” in the Gulf of Alaska from March to October, 1983. Dissolved inorganic carbon (DIG) in surface sea water collected during two different seasons in 1984 were analyzed using large gas proportional counters and revealed a minimum seasonal Δ14C variation of 14‰. Results show that the Δ14C of calcium carbonate scdimenting to the deep sea is the same as that measured in surface water DIC. In contrast, particulate organic carbon (POC) had significantly higher Δ14C values (by 25–70‰) than that in surface water DIC. Also, the δ13C of the POC was markedly lower than previously reported values from other trap stations and marine particulate matter in general. Results from this study suggest that a significant amount of the POC settling to the deep sea at this pelagic station is of terrestrial origin, not strictly of marine origin as had previously been believed.

Macrofossils of terrestrial plants have been picked from a sediment core taken in Lake Lobsigen, a small lake on the Western Swiss Plateau. The sediments were previously analyzed for pollen composition, plant and animal macrofossils, and stable isotopes. Plant macrofossils were selected near pollen zone boundaries in Late Glacial and early Postglacial sediment for 14C dating by AMS. In the same lake carbonate and gyttja (aquatic plant) samples were dated by decay counting. The dates on terrestrial material are generally younger than those on carbonate and gyttja, ie, material reflecting the 14C/C ratio of dissolved bicarbonate in lake water. This is probably due to a contribution of dissolved limestone carbonate and thus a somewhat reduced 14C/C ratio in the lake's water (hard water effect).

Results of 14C/12C ratio measurements on CO2 extracted from air bubbles in polar ice are presented. The samples investigated originate from the Dye 3, South Greenland, deep ice core and span approximately the last 10,000 years. The results are calibrated with tree-ring records. The 14C ages are compared with information obtained from seasonal variations of ice-core parameters and rheologic model calculation.

14C ages were determined on samples of foraminifera separated from cores from three areas of the tropical Pacific (East Pacific Rise, Oontong Java Plateau, and South China Sea). Analyses were made on four planktonic species and on mixed benthics. The purpose of the multiple analysis on planktonic species is to assess the importance of artifacts resulting from the bioturbation-abundance change couple, from the bioturbation-partial dissolution couple and from redeposition by bottom currents. The goal is to use the benthic-planktonic age difference as a means of establishing changes in deep sea ventilation rate over the past 25,000 years. Results of a part of this work are presented in this paper.

It is a well-established fact that 14C dates from lake sediments are usually too old because of contamination with allochthonous material and/or due to discrete reservoir effects. The latter can occur in soft water lakes and may be examined by the 14C measurement of aqueous plants or the carbon dissolved and suspended in the water column.

Some plants assimilate CO2 from the sediment. Their 14C activity is then dependent on the sediment accumulation rate and nutrients stored in the root system may also contribute misleading results. If water is filtered through ultra-fine membranes and then treated chemically, several fractions can be isolated for dating. The present study shows that the 14C activity of such fractions varies widely but with the weighted mean indicating an overall deficiency. Even the water from a raised bog evidences a 14C deficiency relative to contemporary atmospheric CO2.

Charcoal from an archaeologic site and peat from corresponding layers in a nearby bog have yielded significantly different ages. The fact that the Cladium peat was from a very calcareous area is significant. The risk of contamination by younger root material is also documented.

The 14C activity of total dissolved inorganic carbon (DIC) as well as tritium activity and stable isotope content (13C, 2H, 18O) of spring water were measured for 3 consecutive years at 3 karst springs that feed Plitvice Lakes, NW Yugoslavia. The constant chemical and stable isotope composition and temperature indicated well mixed aquifers. A very short mean residence time of underground water (2–5 yr) was determined by tritium activity measurements. Measured C activities of 3 karst springs varied from 60.0% to 82.5%. The measured C activities were compared with calculated 14C activities based on various models using measured values of C activity of soil at different depths and 13C content of limestone, plants, and soils from the recharge area. Our measurements indicate that recharge systems in karst aquifers are open to the atmosphere. The atmospheric CO2 contributes to the 13C content and the C activity of ground water to an estimated extent which varies between 10 and 40% of DIC.

In 14C dating of sediment, the date of deposition is associated with its C age. Most sediments are complex mixtures containing little organic material frequently derived from several sources. The most conspicuous sources of error result from 1) the incorporation of “fossil” carbon (eg, graphite, lignite, etc) into a more recent sediment. This is particularly important in low carbon sediments (Olsson, 1972); 2) the incorporation of older, ‘reworked’ sedimentary material, eg, from terrigenous sources into a lacustrine environment (Schoute, Mook & Streuerman, 1983); 3) the dating of mainly autochthonous material which has metabolized carbon from dissolved bicarbonate carbonates originating from dissolution of fossil.

To provide more information for the 14C ages of components of a sediment, we have used the small sample capability (ie, > = 1 mg carbon) of the Oxford Radiocarbon Accelerator to date specific fractions. Within the limitations of the ‘conventional’ method, different fractions in soils have been the subject of two investigations (Scharpenseel, 1979; Sheppard, Syed & Mehringer, 1979). In general the results show that a measurement on undifferentiated sediment may lead to serious errors in the 14C date, that specific fractions do not guarantee a better date for deposition, but nearly always provide valuable information on the particular history of the individual sediment in relation to its specific context. It is usually possible to estimate the occurrence of the first two sources of error listed above, but more difficult to quantify the extent of terrestrial input and “hard water” error.

14C dating of bone has been unreliable in comparison with more stable materials such as wood or charcoal. Attempts have been made to use various components or fractions isolated from the raw bone sample; these include dilute acid soluble, dilute acid insoluble, collagen, and gelatin, as well as alkali soluble and insoluble fractions of burned bone, and carbonate or apatite fractions of organic-poor bone. All of these fractions have yielded useful data in some cases, but no single method has proven suitable in all situations. The work reported here describes the isolation and purification of amino acids from the dilute acid insoluble fraction of bone collagen and parchment, with some preliminary experiments on amino acids from shell conchiolin.

14C dating of shells from terrestrial and freshwater mollusks is prone to distortion by post-depositional diagenesis as well as incorporation of material depleted in 14C while the mollusk was alive. Three types of diagenetic change can result: etching, the development of surface crusts, and replacement of aragonite by calcite. Inspection under the light microscope, x-ray diffraction, and scanning electron microscope make it possible to assess the relative importance of the changes. When they are confined to the surface, mechanical cleaning combined with judicious leaching can reduce them to <1% of the total sample. The corresponding errors, which can now be specified, are often no greater than those associated with the statistics of counting.

This review is based on geologic surveys carried out in Australia and China as well as on more than 300 14C dates published in Radiocarbon, Evaluated are the origins and pathways of carbonate formation, stable isotopic composition, carbonate nodule growth rates and paleo-climatic effects. The three identified δ 13C abundance peaks are unrelated to environment and carbon source whilst 14C ages group themselves into periods corresponding to past humid warm climate. It is concluded that the major error in caliche dating is due to incorporation of old limestone whilst error on nodule dating is related to their slow growth rate, Thus, caliche antedates and nodules postdate the times of their deposition. Delta 13C values cannot be used to correct for limestone or atmospheric contamination effects.

The Lubbock Lake site, on the Southern High Plains of Texas, contains one of the most complete and best-dated late Quaternary records in North America. A total of 117 14C dates arc available from the site, determined by the Smithsonian and SMU Laboratories. Of these dates, 84 have been derived from residues (humin) and humates (humic acids) of organic-rich marsh sediments and A horizons of buried soils. Most of the ages are consistent with dates determined on charcoal and wood, and with the archaeologic and stratigraphic record. The dates on the marsh sediments are approximate points in time. Dates from the top of buried A-horizons are a maximum for burial and in many cases are close to the actual age of burial. Dates from the base of the A-horizons are a minimum for the beginning of soil formation, in some cases as much as several thousand years younger than the initiation of pedogenesis. A few pairs of dates were obtained from humin and humic acid derived from split samples; there are no consistencies in similarities or differences in these age pairs. It also became apparent that dates determined on samples from scraped trench walls or excavations that were left open for several years are younger than dates from samples taken from exactly the same locations when the sampling surfaces were freshly excavated.