Soil organic matter sequesters close to three times the carbon existing totally in the living biomass and nearly the same for the total carbon in the atmosphere. Models, such as Jenkinson's or Parton's Century model, help to define soil organic matter fractions of different functions, based on residence time/14C age. Rejuvenation of soil carbon was felt to be the principal impediment to absolute soil dating, in addition to the ambiguity of the initiation point of soil formation and soil age. Recent studies, for example, of Becker-Heidmann (1989), indicate that a soil 14C age of >1000 yr cannot have >0.1% rejuvenation in the total soil organic matter compartments/fractions to be possible and sustainable. Always problematic in earlier observations were age vs. depth increases, in 14C profile curves showing an inflection of reduced age in the deepest samples, i.e., from the rim of the organic matter containing epipedon. We attribute this phenomenon, in mollic horizons, to earthworm casts in the terminal part of the escape tube. Becker-Heidmann (1989) has shown, in thin layer soil profile dating, a highly significant correlation between the highest 14C ages and the highest clay content. Thus, optimization of soil dating is, to a lesser degree, related to the applied extracting solvent system than to soil texture fractions. Such observations allow us to mitigate error ranges inherent in dating dynamic soil systems.

We present 14C AMS measurements and discuss the extraction procedure used on pollen extracted from peat samples. Microscopic examination of the extracts shows that the procedure is sufficient to remove most non-pollen materials and results in an extract that is composed predominantly of pollen. The 14C dates that we obtained for pollen extracts from peat samples associated with the Mazama Ash layer are consistent with the range of bulk-sample dates obtained by others in previous studies. The limited measurement time and resulting precision (± 100 yr) of these initial measurements restrict our ability to draw firm conclusions from a comparison of the pollen extract dates with previous bulk-sample dates. We intend to adjust our procedure to improve the rejection of non-pollen materials and to increase the precision of our 14C measurements on pollen extracts from peat samples in future studies.

We have developed and tested a practical device for manually separating pollen from pollen concentrates in sufficient quantity for AMS 14C dating. It is a combination of standard, commercially available equipment handled in a clean room by an individual trained to recognize pollen. A typical example requires about 15–20 h of hand-picking under the microscope. We show the usefulness of this procedure with results on a mid-Holocene segment from a core from Mono Lake. Sediments from this hardwater lake contain pollen and finely disseminated organic matter, but no macrofossils. The pollen dated ca. 1000 yr younger than the bulk sediment. The sediment “date” is most likely affected by incorporation of limestone-derived carbon, and is erroneously old.

We have measured 14C, 210Pb and 137Cs profiles in two representative cores from Manasbal Lake, Kashmir, India. The sedimentation rate derived from 210Pb and 137Cs in the upper part of the core is in the range of 3.4 to 5.5 mm yr−1. In contrast, 14C ages show an inversion at depths >20 cm. These results are attributed to the erosion of the ubiquitous 10–20-m-thick loess mantle, based on the similarity of 14C ages of the inversion layer in the sediments and the paleosols present in the catchment area. Frequency-dependent mineral magnetic susceptibility (χfd), carbon to nitrogen ratios and pigment concentrations in the profile show a significant amount of allochthonous component in the lake deposits and support the conclusion that the 14C dates do not reflect the chronology of the in-situ lake sedimentation but episodic deposition of the surrounding loess. Thus, 14C serves as a useful tracer to understand source components of the sediments.

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.

A 450-year correction is required to make Anodonta14C dates comparable to 14C dates on other materials in the Mojave River basin. The internal stratigraphic consistency of 34 conventional 14C dates on Anodonta in this drainage basin indicates that such dates are usually reliable. The validity of most conventional 14C dates in the Mojave River basin may be a product of the basin's crystalline bedrock in a region usually typified by thick Paleozoic carbonate sections.

Anthropogenic activities that introduce an excess of nutrients and other pollutants into rivers and lakes are causing significant changes in their aquatic environment. Excessive nutrients greatly accelerate eutrophication, and lake marl formed during eutrophication differs from that formed in oligotrophic water.

We analyzed recent sediment cores from Prošće and Kozjak lakes located in Plitvice National Park, central Croatia. Analyses consisted of 14C activity of calcareous lake marl, the ratio of stable isotopes (δ13C, δ18O), organic compounds in the sediment and the distribution of diatoms. Previous 14C activity measurements helped to determine the sedimentation rate and thus the time period of increased input of nutrients into lakes. We determined the increased 14C activity in lake sediments caused by nuclear bomb effect in recent depth profiles. We attributed the sudden increase in diatom species, Cyclotella operculata unipuctata and Achnanthes clevei rostrata, in the uppermost 5-cm layer, to eutrophication of the lake water.

We performed a molecular characterization of hydrocarbons isolated from the sediments, and applied computer-assisted high-resolution gas chromatography/mass spectrometry to estimate contributions of biogenic, fossil and pyrolytic hydrocarbons.

We have reconstructed the initial activity of calcareous sediments by 1) measuring the 14C activity of recent pre-bomb-test calcareous deposits, 2) calculating the ratio of the 14C activity of the calcareous sediment to that of the adjacent coeval wood, and 3) extrapolating 14C activity of the deep lacustrine sediment layers to the surface, assuming a constant sedimentation rate. We show here that the uppermost sediment is affected by the global increase of atmospheric 14C activity due to thermonuclear bomb tests in the early 1960s. 14C activity of submerged aquatic plants showed values similar to 14C activity of DIC. Thus, organic residue of the sediment cannot be used for reliable 14C dating because of the unknown fraction of aquatic plant detritus. The initial activity, A0, obtained in a case study of the geochronology of carbonates in the Plitvice Lakes area, ranged from 70.5 to 72.2 pMC for Lake Prošće sediment, and from 75.9 to 76.7 pMC for Lake Kozjak sediment. These values also agree with calculated values based on the downstream increase of 14C activity of DIC in freshwater open to the atmosphere.

Authigenic micrite from a playa in the northern Sahara has been dated by both the U/Th and 14C methods. The U/Th ages can be a few thousand years greater or less than the 14C ages obtained on the smallest crystals of micrite. The latter are considered to form a reasonable chronology for the Holocene deposits. The deviations of the U/Th ages are explained by quantifiable losses and gains of uranium under changing redox conditions. Under conditions where U is conservative (reduced sediments with low permeability), the U/Th method can provide good chronologies for lacustrine deposits.

We discuss U-Th and 14C measurements in coral. Samples with U-Th dates in excess of 50 ka BP were chosen for study. Some bulk samples from this group have measurable 14C dates, which range from 30 ka to 43 ka bp. These can be explained by 0.5–2.5% contamination by modern carbon. This small amount of contamination can produce significant offsets in 14C dates of coral samples older than ≃10 ka. It may be undetectable in X-ray powder diffraction patterns. We describe a sample pretreatment that removes the modern carbon by selective dissolution and produces accurate 14C dates.

We have studied the application of carbonates and organic matter to the radiocarbon dating of a paleolake. The results show a systematic apparent age shift of these materials with respect to contemporary wood. The apparent age of carbonates is evidently due to the hard-water effect, whereas the apparent age of organic matter, systematically younger than carbonates, is attributed to aquatic plants, which metabolize dissolved CO2. Terrestrial plants that deposit organic matter also cause apparent age discrepancies between carbonates and organic matter.

The 14C content of dissolved organic carbon (DOC) in streams, soil water and groundwaters in the Harp Lake catchment in Ontario, Canada, reflect a mixture of DOC sources, including both contemporary plant material and 14C-depleted soil organic matter. The concentration and isotopic content of DOC in streams is highly variable, reflecting the complex flow path of the source water entering the streams. The characteristics of groundwater DOC are set in the soil column, either through DOC production in the deeper soil layers, or through preferential decomposition and/or sorption of 14C-enriched DOC components from percolating waters. We estimate the relative magnitudes of decomposition, transport and sorption as sinks for DOC produced in forested catchment soils.

Dissolved inorganic carbon (DIC) is the main acid buffer in forested lake watersheds in Canada. We used carbon isotopes (13C, 14C) to evaluate the production and cycling of DIC in an acid-sensitive lake watershed of the Precambrian Shield. Soil CO2, groundwater and stream DIC were characterized chemically and isotopically. Soil CO2 concentration profiles reflect both changes in production and in losses due to diffusion. δ13C soil CO2 profiles (δ13C values of −23‰ in summer, slightly enriched during the fall and −25%‰ during the winter) are a reflection of the isotopic composition of the sources and changes in isotopic fractionation due to diffusion. Carbon isotopic composition (13C, 14C) of the groundwater and stream DIC clearly indicate that weathering of silicates by soil CO2 is the main source of DIC in these watersheds. 14C data show that, in addition to recent groundwater, an older groundwater component with depleted 14C activity is also present in the bedrock. The carbon isotope pattern in the groundwater also implies that, besides the main springtime recharge events, contributions to the groundwater may also occur during late winter/early spring.

Since the pioneer publication of K. O. Münnich (1957), 14C systematics have been used in many hydrogeological studies. Because of the complexity of carbon geochemistry, numerous models have been proposed to correct the dilution effect of “dead” carbon in groundwater. All the 14C correction models for dating groundwater are based on either open- or closed-system conditions. I present here a simple model that accounts for the effects of both open- and closed-system dissolution of carbonate and aluminosilicate minerals. For systems involving precipitation of carbonate minerals and redox reactions of organic matter, reaction-path simulations are essential for reliable 14C dating of groundwater.

The Aquia (Paleocene) and Magothy (Late Cretaceous) Formations of the Atlantic Coastal Plain represent two well-characterized (hydrodynamically and geochemically) aquifers in southern Maryland. 14C measurements of the dissolved organic (DOC) and inorganic carbon (DIC) of Aquia and Magothy groundwaters have been made using accelerator mass spectrometry (AMS). Both DI14C and DO14C concentrations in the initial flow path are unexpectedly low. As the water progresses farther from the recharge area, the DI14C percent modern carbon (pMC) is consistently lower than the DO14C pMC; this difference stays constant for all samples. The 14C-derived ages for an Aquia water sample downgradient at Site 4 are 17 ka and 12 ka for DI14C and DO14C, respectively. Radiocarbon ages have been compared to ages determined by two other independent dating methods: computer-simulated hydrodynamic modeling and age estimates based on changes in Cl−, 18O and 2H distributions, which are interpreted to be influenced by sea level and climate.

Radiocarbon, δ13C and major-element data were used to construct a geochemical framework for interpretation of the hydrological flow system in the lower San Pedro basin, southeastern Arizona, USA. The 14C and major-element data show a regional confined aquifer that extends throughout most of the basin. Groundwater ages, after correcting for chemistry, are greater than 10 ka bp. The groundwater ages do not increase in a downvalley direction, the assumed direction of groundwater movement in most intermontane basins in the region, but along general flow paths normal to the mountains toward the center of the basin. Recharge to the confined aquifer originates from infiltration of precipitation and runoff near the alluvium-mountain contact along the Galiuro Mountains and is discharged by evapotranspiration along the center of the basin. The hydrogeological concept of the 14C model is supported by the water chemistry and by the mass transfer defined by the chemical model. Weathering of primary silicate minerals in the confined aquifer does not occur downvalley, but only along the direction of flow. Hydraulic conductivities calculated for the aquifer from 14C velocities are about an order of magnitude slower than those determined through hydrological methods. The lower hydraulic-conductivity values are attributed to a thick confining layer overlying the discharge area along the San Pedro River.

We have studied the isotopic composition of recently precipitated travertines to determine the initial 14C activity of ancient travertine deposits. We found that the 14C activity of recent travertines of northeastern Spain was a function of the distance from the spring, resulting in variations in the initial 14C activity of the ancient travertine formations. We calculated the ancient travertine radiocarbon ages by using the ratio between the 14C activity of recent travertines and that of present atmospheric CO2 as the initial 14C activity.

Groundwater samples were collected from several different depths in Illinois glacial deposits at a site in east-central Illinois. Dissolved gases were extracted from many of the water samples, measured volumetrically, and analyze by gas chromatography. The DIC was precipitated as barium carbonate and analyzed for both δ13C and 14C. Due to the formation of microbial CH4, some of the DIC had unusually heavy δ13C values of −3 to −1‰. The standard groundwate 14C-age correction models were developed to account for carbonate dissolution as the primary carbon input beneath the soil zone. If the heavy δ13C values observed in this study are used in readily available groundwater dating models without accounting for the effect of microbial methane formation, many of the resultant calculated ages are negaitive (future ages) mathematically unsolvable. Isotopic and analytical result show a positive correlation (r2 = 0.90) between the δ13C of the DIC and the concentration of methane in the groundwater. With this correlation, we were able to correct the δ13C values of the DIC which were altered due to microbial CH4 formation. This adjustment of δ13C values, along with estimation of dead carbon input from the redox processes, allowed us to calculate 14C ages using standard groundwater age correction models.

We have studied plant remains in thick beds of lacustrine sediments in the upper part of the Markarfljót drainage area in southern Iceland. We collected small samples of plant species from the same horizon and 14C dated them at the Aarhus AMS Dating Laboratory. Terrestrial plants yielded an age of 9 ka BP, whereas aquaic moss (Fontinalis antipyretica Hedw.) yielded the surprisingly old 14C age of 16 ka bp. We believe the age of the terrestrial plants reflects the true age of the sediment. The anomalously old 14C age of the aquatic moss may be an effect of geothermal water on the moss, as the area is known to be geothermally active today. Modern aquatic moss growing in geothermal water showed a similar 14C anomaly, with measured ages ranging from 6 to 8 ka BP, which may be explained by the equally old ages measured for the corresponding water samples. The 14C content of geothermal springs and neighboring rivers in the area ranges from 9 to 50 pMC, equivalent to an apparent age of 20–5.5 ka bp.

14C is produced as an activation product in nuclear reactors, and may be discharged to both the atmosphere and the marine environment during nuclear fuel reprocessing. In the UK, 14C is discharged, under license, into the Eastern Irish Sea by the British Nuclear Fuels plc (BNFL) reprocessing plant at Sellafield, Cumbria, northwest England, and is then transported into Scottish coastal waters. We analyzed intertidal biota samples to determine the effect of these discharges. The specific activities of 14C found in these samples indicate that the uptake and bioaccumulation of 14C is dependent on the type of organism and its feeding behavior. Measured 14C concentrations in mussels (Mytilus edulis) were higher than those in winkles (Littorina littorea), which were greater than those found in seaweed (Fucus spp.); maximum observed activities were ca. 7, 5 and 3.5 times the accepted current ambient level of 260 Bq kg−1 C, respectively. Annual Nori (Porphyra umbilicalis) samples were analyzed for their 137Cs, 241Am and 14C contents; both the 137Cs and 241Am results correlated well with published Sellafield discharge data (r = 0.877 and 0.918, respectively), whereas there was no significant correlation between measured 14C activities and the discharge record, indicating increased complexity in the chemical and biological behavior of 14C or some discrepancy in the estimated discharge records.