The number of paleoecological records for the Sierra Nevada of California has increased substantially since the compilation of Adam (1985). We examine here the geographical and temporal distribution of records within the range in order to identify areas for which “gaps” exist in our paleoecological knowledge. Seventy-two sites with paleoecological information are identified; these sites are dated with 234 radiocarbon dates. Sites occur primarily between ca. 36°N and 38°30'N latitudes, and from ca. 1000 m to over 3000 m elevation on both sides of the Sierran crest, although more sites have been analyzed on the west side of the crest than the east side. In general, packrat (Neotoma) midden series are located at the lowest elevations, meadow and marsh cores originate from mid-elevations, and lake sediments have been analyzed from the highest elevations. Significant gaps in our knowledge occur for much of the east side of the crest, for both sides of the range above modern treeline, and for time periods older than the latest Pleistocene.

Many mammoth remains have been radiocarbon-dated. We present here more than 360 14C dates on bones, tusks, molars and soft tissues of mammoths and discuss some issues connected with the evolution of mammoths and their environment: the problem of the last mammoth; mammoth taphonomy; the plant remains and stable isotope records accompanying mammoth fossils; paleoclimate during the time of the mammoths and dating of host sediments. The temporal distribution of the 14C dates of fossils from the northern Eurasian territory is even for the entire period from 40 to 10 ka bp.

Systematic treatment of the data recorded by our guard counters and corrections introduced for meteorological factors has allowed observations on solar events clearly manifested in the readings. Examples are the solar flares of March 1989 and especially of June 1991, which caused a ca. 10% decrease in the cosmic radiation flux reaching the counters. A sinusoidal variation in the cosmic-ray flux with a period of one year is also clearly manifested in the data. The observation that the background in the 14C measurements depends on the intensity of the cosmic radiation has led to the use of monthly correlations for the determination of the best background value to be used in the age calculations. This reduces the error significantly. However, various factors such as random statistical fluctuations of the background measurements may affect the slope of the correlations and consequently the calculated age of the samples. Long-term observations of the relation between background values and coincidence counts have led to constraints in the slope of the correlation. A simple extension of the fitting procedure is explored, which maintains the physically meaningful range of the slopes, but is flexible to adjust for the seasonally varying contributions to the variations of the cosmic-ray flux.

We measured apparent radiocarbon ages of live-collected, pre-bomb mollusk shells from the northern and central Gulf of California to determine the source of the reservoir ages and the reservoir age correction offsets for calibrating 14C dates of fossil samples. Reservoir ages average 860 yr in the northern Gulf and 725 yr in the central Gulf. The corresponding ∆R values (the deviation from typical worldwide values) are 540 yr and 395 yr, respectively, with variabilities (SD) of 90 and 110 yr. This variability significantly limits the precision of calibrated 14C ages. The apparent 14C age of Colorado River water (as measured in a freshwater mussel, collected in the 1890s, before diversion of river flow) is not sufficiently high (1420 yr) to account for the high reservoir ages in the Gulf. The lack of a relation between the stable isotope composition of Gulf mollusks and their reservoir ages is further evidence that the Colorado River does not make a significant contribution to Gulf reservoir ages. Upwelling of old, deep Pacific-derived water appears to be the cause of the large reservoir ages.

Dating pollen concentrated from eolian sediments provides a new way to establish a chronological framework on the Loess Plateau of China. We show that pollen deposited simultaneously with sediment in a stable environment can provide reliable ages. We suggest that the reliability of pollen dating can be evaluated by comparison with wood cellulose or charcoal ages from the same stratigraphic level. Dating pollen concentrates from the various profiles indicates paleomonsoon precipitation variability at the loess/desert transitional belt from the late Pleistocene to the early Holocene.

Twenty paired 14C and U/Th dates covering most of the past 50,000 yr have been obtained on a stalagmite from the Cango Caves in South Africa as well as some additional age-pairs on two stalagmites from Tasmania that partially fill a gap between 7 ka and 17 ka ago. After allowance is made for the initial apparent 14C ages, the age-pairs between 7 ka and 20 ka show satisfactory agreement with the coral data of Bard et al. (1990, 1993). The results for the Cango stalagmite between 25 ka and 50 ka show the 14C dates to be substantially younger than the U/Th dates except at 49 ka and 29 ka, where near correspondence occurs. The discrepancies may be explained by variations in 14C production caused by changes in the magnetic dipole field of the Earth. A tentative calibration curve for this period is offered.

A new analysis of previous results on conflicts between shell and charcoal dates and on burnt human bones, with new data presented here, suggests that alternative interpretations are possible for the archaeology and environmental history of the Willandra Lakes region. Black sediment samples from archaeological sites at Lake Outer Arumpo exhibit wide variation in burnt and unburnt carbon content; high humic acid concentrations in midden layers and in one group of hearth/ovens are absent in another, older, group of hearth/ovens. There are no acceptable results on charcoal from hearth/ovens older than ca. 31 ka bp, and no evidence that these samples are associated with numerous midden shell dates at 34–37 ka bp. Similar logic applied to humic-free residue dates on burnt human bones places five gracile skeletons (including Mungo 1) as post–Last Glacial Maximum.

We present radiocarbon determinations for 271 New Zealand archaeological samples measured at the University of Waikato Radiocarbon Dating Laboratory between 1975 and 1995. A discard protocol is applied to the series and the list culled to winnow the acceptable dates from those that may incorporate error. None of the 221 acceptable 14C determinations older than 600 bp (in the case of terrestrial samples) or 930 bp (in the case of marine and estuarine shell) extends beyond cal ad 1250. This conclusion supports the short chronology model of New Zealand prehistory presented by Anderson (1991).

We performed radiocarbon measurements using accelerator mass spectrometry (AMS) on 6 stalagmites 3 stalactites and 7 seepage waters from four different caves in Southwest France and Belgium in order to calculate the dead carbon proportion (dcp). All the speleothems studied are modern and annually laminated, which offers the advantage of an accurate chronology, with better than one-year resolution. Coupled with the fact that very little calcite is necessary for an AMS measurement (between 1.5 and 7 yr of calcite deposit), we obtained dead carbon values within an uncertainty limit of ± 1.5%. Results show that the dead carbon proportion varies from 9.2% to 21.9% for calcite deposits and from 3.6% to 21.9% for water. In each sampling site, the dcp is homogeneous. Although the inter-site dcp varies by >11%, its average value of 15.5% ± 4.4 still lies within the uncertainty range of the accepted value of 15% ± 5 (dilution factor of 0.85 ± 0.5). We compare the average dcp of each site with the local geology, vegetation and climate. Given similar geology and temperature the highest dcp values are found under forest cover; dcp difference is up to 9%. However, the Belgian site, which is also under a forest shows a dcp very close to the dcp found under grassland sites of Southwest France, which proves that other unknown factors may play an important role in dissolution processes. Secondary calcite deposition and redissolution in the soil zone or more likely in the fracture system before reaching the cave itself could also explain the inter-site differences. The IAEA isotopic model (Pearson model adapted for open systems) is in good agreement with the measured activities.

Rivers and shallow groundwater are deep groundwater precursors. Their dissolved inorganic carbon content (DIC) and its isotopic composition are end members in the evolution of these properties in confined situations, and are therefore essential information when applying carbon isotopes as tracers of groundwater processes and determining aquifer residence times using 14C.

During studies of regional aquifer systems in New Zealand, a simple model has been developed to explain the isotopic compositions of DIC encountered in rivers and shallow groundwater. The model format incorporates a diagrammatic approach, providing a framework for tracing the subsequent evolution of DIC in both precipitation- and river-recharged aquifers under closed conditions.

DIC concentration of rivers continuously adjusts toward chemical and isotopic equilibrium between direct addition of CO2 to the water (via plant respiration and decay of dead organic material) and exchange of CO2 across the river-air interface. In the shallow groundwater situation, the gaseous reservoir is soil CO2, generally at significantly higher partial pressure. In both cases, calcite dissolution or other processes may be an additional source of DIC directly added to the bicarbonate and dissolved CO2 components; while these may add or remove DIC, steady-state isotopic concentrations are considered to be determined only by the dynamic balance between directly added CO2 and gas exchange. This model allows the calculation of steady states, using selectable parameters in river or groundwater situations. These appear as straight lines in 13C or 14C vs. 1/DIC, or total 14C vs. DIC plots, into which the experimental data can be inserted for interpretation. In the case of 14C, the steady-state balance is very often complicated by the presence of an old component in the directly added DIC; the understanding achieved via the 13C patterns is helpful in recognizing this.

Data from four contrasting aquifer systems in New Zealand. The success of the approach has depended crucially on DIC concentrations measured very accurately on the isotope samples, rather than separate chemical analyses.

We Fourier transformed and filtered calibration curve data to compensate for the averaging effect of radiocarbon-dating sets of adjacent tree rings. A Wiener Filter was also applied to minimize the effects of the counting errors of the dates on the resulting calibration curve and to produce a least-squares curve through the data. The method is illustrated using a short 14C-dated tree-ring sequence from New Zealand to produce a calibration curve at yearly intervals for New Zealand matai (Prumnopitys taxifolia). The resulting curve has a nominal standard error of 10 ± 3 yr, which is ca. half the average standard error of the original raw data.

We present and discuss quasi-continuous long-term 14CO2 observations from the continental background station Schauinsland (48°N, 8°E, 1205 m asl, Black Forest, southern Germany). The observed steady decline of atmospheric 14CO2 from 1977 to 1996 can be described by a single exponential function with an e-folding time of (16.3 ± 0.2) yr. Summer means (May to August) in atmospheric 14CO2 at Schauinsland compare within Δ14C = ±4‰ with measurements made on individual rings from a tree grown in the near vicinity of the Schauinsland site. Both data sets are slightly depleted by up to 5‰ if compared to maritime background measurements of atmospheric 14CO2 made at Izaña, Tenerife. This is due to the influence of fossil fuel CO2 emissions over the European continent as well as generally in mid latitudes of the Northern Hemisphere. δ13C analyses from the Schauinsland samples show mean seasonal variations with an amplitude of ±0.4‰, caused by atmosphere-biosphere exchange, and a mean decrease from 1977 to 1996 of δ13C = −0.017‰ yr−1. This trend is mainly due to an increasing quantity of fossil fuel CO2 in the atmosphere, depleted in 13C/12C ratio, and compares well to trends measured at other stations in mid-to-high northern latitudes.

The Cariri region is the largest sedimentary basin in the state of Ceará, Brazil. Located in the southern portion of the state, it comprises the Araripe Plateau and the Cariri Valley on its northern foot. The region's groundwaters are being heavily exploited. Using electric conductivity (EC) and 18O, 14C and 3H data, we differentiate groundwaters from various origins. We identified three horizons of springs on the slope of the Plateau through their geologic environment and the EC of their waters. Groundwaters from wells in the Cariri Valley are classified according to the aquifers exploited as indicated by the drilling profiles. However, strong tectonic features and intense fracturing in the Valley produce a great many horizontal discontinuities, which result in a mixing of groundwaters from different aquifers. Mixing systems are described in terms of δ18O–14C and EC–14C linear trends.

We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10–100 μg carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures.

Chemical and isotopic (13C, 14C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 μg (central mean) with a dispersion of 0.50 μg (standard deviation). Reductions of 10 μg carbon onto targets were complete in 3–6 h with all targets, after correction for the blank, reflecting the 13C signature of the starting material. The 100 μg carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the 13C data suggested the presence of kinetic isotope effects during the reduction. The observed CO2-graphite 13C fractionation factor was 3–4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%.

Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the 12C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to 14C uncertainties of 5–1% for 10–100 μg carbon samples, respectively. A bias of 5–7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.

Penguin is a Macintosh computer application that facilitates the use of CALIB 3.03, the 14C age calibration program by Stuiver and Reimer (1993). Penguin offers an easy user interface based on the well-known Macintosh standard multiwindow environment to create and edit the CALIB 3.03 calibration files and to export data in text format. Penguin and CALIB interact at the file level, i.e., Penguin is capable of reading and writing files in CALIB formats. Files containing the data are created in the Penguin environment and then saved on disk in the Penguin format. Penguin allows multiple editing of the calibration parameters and recalibration of the list of samples without the need to insert any modifications manually throughout the list. Penguin can also be used to read already calibrated files in order to extract the “cal” ages and display them in a spreadsheet-like window.

Since 1991, radiocarbon analyses of exhaust air have been part of the regular radioactivity monitoring program at the Krško Nuclear Power Plant (NPP), a Westinghouse 632 MWe pressurized water reactor (PWR). Activity of CO2 and hydrocarbons has been identified; the former contributes ca. 43%. A normalized release of total 14C of 0.219 TBqGWe−1a−1 was obtained. Indoor air 14C concentrations in selected rooms inside the plant have generally been <5 Bq m−3, although rare peaks of >1000 Bq m−3 may be reached. Tree rings have shown slight enhanced 14C activity due to the operation of the plant.