Eight atmospheric carbon dioxide samples (as calcium carbonate—CaCO3—precipitates) from Lindesnes site (58ºN, 7ºE), belonging to 1963 and 1980 (four samples from each year) and stored at the National Laboratory for Age Determination (NTNU), have been reevaluated through radiocarbon (14C) analysis. Previous 14C results indicated the presence of a contaminant, which was not removed through different chemical cleansing procedures (e.g., hydrochloric acid—HCl and/or hydrogen peroxide—H2O2). Here, we present a follow up investigation using 14C step-combustion and Fourier-transform infrared spectroscopy (FTIR) analysis. Results from 14C data indicate unsuccessful removal of the contaminant, while further FTIR analysis displayed the presence of moisture. This finding alludes to the possibility that the contaminant is of ambient air-CO2 deeply embedded in CaCO3 powders (within clogged CaCO3 pores and/or bonded to the lattice). Samples were found exposed to air-CO2 and humidity. These conditions may have lasted for years, possibly even decades, leading to the 14C offsets detected here.

This paper presents a compilation of atmospheric radiocarbon for the period 1950–2019, derived from atmospheric CO2 sampling and tree rings from clean-air sites. Following the approach taken by Hua et al. (2013), our revised and extended compilation consists of zonal, hemispheric and global radiocarbon (14C) data sets, with monthly data sets for 5 zones (Northern Hemisphere zones 1, 2, and 3, and Southern Hemisphere zones 3 and 1–2). Our new compilation includes smooth curves for zonal data sets that are more suitable for dating applications than the previous approach based on simple averaging. Our new radiocarbon dataset is intended to help facilitate the use of atmospheric bomb 14C in carbon cycle studies and to accommodate increasing demand for accurate dating of recent (post-1950) terrestrial samples.

We consider one misconception of those who currently reject the general validity of radiocarbon (14C) age determinations older than, at most, 10,000 BP. There is an allegation that the presence of 14C reported by accelerator mass spectrometry (AMS) laboratories in their measurements of 14C infinite age (>100,000 years) organics used to define background levels, support their point of view. This article has been written for a general audience, primarily for those who have questions about the validity of these arguments. However, they may not be familiar with the literature relevant to providing a clear response to the claims of these individuals. We conclude that, in our view, of all of the possible explanations for the reports of the presence of 14C in these background samples, the least probable explanation has been advanced by those rejecting the validity of the 14C time scale in excess of, at most, 10,000 BP.

Over the last few decades, radiocarbon laboratories have used different procedures for measuring a broad range of carbonaceous materials. To produce reliable results, the processes employed for sample processing, graphite target production, and spectrometer measurement must be rigorous, well tested, and reproducible. Most of the procedures have been developed, improved, and published as part of the laboratories’ quality control and research programs, and can be frequently found in the literature. Nevertheless, there are suites of laboratory techniques (or “small useful skills”), products, and other resources that either have never been described in publications, or have been somewhat hidden in much larger scientific articles and reports. We feel that with the rapid rise of newer laboratories and facilities, a set of resourceful suggestions might come in handy. Here we gathered these skill sets that can be used in all aspects of 14C sample processing, with the intention to simplify and expedite procedures, from glass-tube making to graphitization and measurements. We also included some miscellaneous items to help in laboratory setup.

This study used high-precision radiocarbon bomb-pulse dating of selected wood rings to provide an independent validation of the tree growth periodicity of Pseudolmedia rigida (Klotzsch & H. Karst) Cuatrec. from the Moraceae family, collected in the Madidi National Park in Bolivia. 14C content was measured by accelerator mass spectrometry (AMS) in 10 samples from a single tree covering over 70 yr from 1939 to 2011. These preliminary calendar dates were determined by dendrochronological techniques and were also used to select the samples for 14C AMS. In order to validate these preliminary dates using the established Southern Hemisphere (SH) 14C atmospheric concentration data set, the targeted rings were selected to be formed during periods before and after the 14C bomb spike nuclear tests (i.e. 1950s–1960s). The excellent agreement of the dendrochronological dates and the 14C signatures in tree rings associated with the same dates provided by the bomb-pulse 14C atmospheric values for the SH (SHCal zone 1–2) confirms the annual periodicity of the observed growth layers, and thus the high potential of this species for tree-ring analysis. The lack of discrepancies between both data sets also suggests that there are no significant latitudinal differences between the 14C SHCal zone 1–2 curve and the 14C values obtained from the selected tree rings at this geographic location (14°33′S, 68°49′W) in South America. The annual resolution of P. rigida tree rings opens the possibility of broader applications of dendrochronological analysis for ecological and paleoclimatic studies in the Bolivian tropics, as well as the possibility of using wood samples from some tree species from this region to improve the quality of the bomb-pulse 14C SHCal curve at this latitude.

In forensic sciences, radiocarbon found in modern human nails and hair is evaluated to determine the year of death. However, 14C analyses presented herein of fingernails and hair from the same infant demonstrated 14C values of hair that were lower than would be expected (e.g. depleted relative to the fingernails by at least 10‰). These results prompted a series of 14C measurements on infant hair strands, fingernails, and infant shampoo, which suggested the presence of C contamination due to cosmetic products. To further evaluate these discrepancies, several hair strands and fingernail samples from multiple donors were collected, pretreated by several approaches, and measured using isotopic analysis (δ13C, δ15N, and C/N as well as 14C accelerator mass spectrometry). SEM images of the surface of chemically pretreated hair strands were also taken to inspect the performance of the chemical pretreatments applied. Our 14C and stable isotope results show that modern human hair is likely contaminated with fossil-fuel-derived carbon, which is found in most hair care products. Currently, the various chemical pretreatments available in the literature and presented herein show that it is not possible to completely remove foreign carbon contaminates. Thus, the human 14C and δ13C values between keratinous tissues (fingernails and hair) arc not in agreement. From these observations, it becomes apparent that isotopic interpretations using human hair should be used with extreme caution and additional work is needed for its use in forensic and dietary research.

We have developed a simple, rapid method to screen carbonates for survey applications, which provides radiocarbon dates with decreased precision at lower cost. The method is based on previous work by Longworth et al. (2011) and involves mixing pulverized CaCO3 with Fe powder, followed by pressing into aluminum target holders for direct 14C accelerator mass spectrometry (AMS) measurements. An optimum beam current averaging ∼10% of those produced by >0.7 mg C graphite targets was obtained for carbonate samples of 0.3–0.5 mg (0.04–0.06 mg C). The precision of the method was evaluated by measuring triplicates of 14C reference materials, as well as by comparing results from this rapid method with results from high-precision AMS measurements on graphite (typically 0.2–0.3%). Measurement reproducibility was ∼1.8% (1σ) for samples <10 ka BP, and it increased drastically for older samples. However, t tests on paired samples resulted in p values greater than 0.05, indicating a good correlation between this survey method and the conventional one. An average blank (calcite) of 0.0075 Fm (∼39 ka BP) was achieved. The simplicity of the technique allowed us to process and measure 72 deep-sea coral samples in less than 25 hr.

The acid-base-acid (ABA) procedure is a common chemical pretreatment used on most organic matter samples. Although this pretreatment is straightforward, there is no consensus among labs about the optimum strength of the chemicals, or even the temperature, to be used for digestions. Comparisons between 14C results obtained by samples undergoing ABA against other wet oxidations, such as acid-base oxidation followed by stepped-combustion (ABOX-SC) or α-cellulose on wood, have sometimes suggested that ABA does not always remove all contaminating carbon. In addition, if not all ABA protocols are the same, could it be said that one procedure is better than others or comparable to wet-oxidation pretreatments, if rigorously applied? To determine whether 14C-free samples showed any fraction modern carbon (FmC) deviations due to pretreatment strategies, 3 experiments were carried out. The first compared 14C results for wood samples at or near the limit of 14C dating, pretreated with a standard ABA protocol used at UC Irvine's Keck Carbon Cycle Accelerator Mass Spectrometry (KCCAMS) facility, with those obtained from ABA- and ABOX-SC-prepared aliquots of the same samples performed at Australian National University (ANU) in 2001. The second experiment subjected wood samples, ranging from ∼12 ka BP to 14C-free, to 5 selected published ABA pretreatments. Third, we evaluated whether the ABA protocol needs a higher-strength final acid step (particularly important for removing CO2 absorption when samples are submerged in alkaline solutions). We are able to show that rigorous ABA treatments exist that can provide robust, reproducible results for many wood samples that are known to be >>50 ka BP, implying that a lack of control over the procedure's outcome might be sample-related, rather than due simply to inadequacies in the chemical pretreatment.

Marine radiocarbon (14C) is a widely used tracer of past ocean circulation, but very few high-resolution records have been obtained. Here, we report a time series of carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from the Newport Beach pier in Orange County, within the Southern California Bight, from 2005 to 2010. Surface seawater was collected bimonthly and analyzed for Δ14C, δ13C, and salinity. Results from May 2005 to November 2010 show no long-term changes in δ13C DIC values and no consistent variability that can be attributed to upwelling. Δ14C DIC values have lowered from ∼34‰ to about ∼16‰, an 18‰ decrease from the beginning of this project in 2005, and is consistent with the overall 14C depletion from the atmospheric thermonuclear bomb pulse at the end of the 1950s. Δ14C DIC values, paired with salinity, do appear to be suitable indicators of upwelling strength with periods of upwelling characterized by more saline and lower DIC Δ14C values. However, a similar signal was not observed during the strong upwelling event of 2010. These results were obtained in the Southern California Bight where upwelling is fairly weak and there is a complex occanographic circulation in comparison with the remaining western USA coastline. It is therefore likely that the link between DIC Δ14C, salinity, and upwelling would be even stronger at other sites. These data represent the longest time series of Δ14C data from a coastal Southern California site performed to date.

We report carbon isotope abundances of dissolved inorganic carbon (DIC) in surface seawater collected from a time-series site off the Newport Beach Pier in Orange County, California. These data represent the first time series of Δ14C data for a coastal southern California site. From a suite of samples collected daily from 16 October to 11 November 2004, Δ14C values averaged 32.1 ± 4.4‰. Freshwater input from the Santa Ana River to our site caused Δ14C and δ13C values to decrease. Since this initial set of measurements, a time-series site has been maintained from November 2004 to the present. Surface seawater has been collected bimonthly and analyzed for Δ14C, δ13C, salinity, and ΣCO2 concentrations. Water samples from the Santa Ana River were collected during the wet season. California sea mussels and barnacle shells, ranging from 4 to 6 months old, were also collected and analyzed. Results from May 2005 to January 2008 show no long-term changes in δ13C DIC values. Δ14C DIC values over the 2005–2006 period averaged 33.7‰; high Δ14C values were observed sporadically (every 6–7 months), suggesting the presence of open water eddies at our site. Finally, in 2007, a stronger upwelling signal was apparent as indicated by correlations between Δ14C, salinity, and the Bakun index, suggesting that the Δ14C record is an indicator of upwelling in the Southern California Bight.

We report compound-specific radiocarbon analyses of organic matter in ocean sediments from the northeast Pacific Ocean. Chemical extractions and a preparative capillary gas chromatograph (PCGC) were used to isolate phospholipid fatty acids (PLFA) and n-alkanes from 3 cores collected off the coast of California, USA. Mass of samples for accelerator mass spectrometry (AMS) 14C analysis ranged from 13–100 μg C. PLFA extracted from anaerobic sediments in the Santa Barbara Basin (595 m depth) had modern Δ14C values (–20 to +54‰), indicating bacterial utilization of surface-produced, post-bomb organic matter. Lower Δ14C values were obtained for n-alkanes and PLFA from coast (92 m depth) and continental slope (1866 m) sediments, which reflect sources of old organic matter and bioturbation. We present a brief analysis of the blank carbon introduced to samples during chemical processing and PCGC isolation.

We present a status report of the accelerator mass spectrometry (AMS) facility at the University of California, Irvine, USA. Recent spectrometer upgrades and repairs are discussed. Modifications to preparation laboratory procedures designed to improve sample throughput efficiency while maintaining precision of 2–3‰ for 1-mg samples (Santos et al. 2007c) are presented.

The Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS/UCI) has developed protocols for analyzing radiocarbon in samples as small as ∼0.001 mg of carbon (C). Mass-balance background corrections for modern and 14C-dead carbon contamination (MC and DC, respectively) can be assessed by measuring 14C-free and modern standards, respectively, using the same sample processing techniques that are applied to unknown samples. This approach can be validated by measuring secondary standards of similar size and 14C composition to the unknown samples. Ordinary sample processing (such as ABA or leaching pretreatment, combustion/graphitization, and handling) introduces MC contamination of ∼0.6 ± 0.3 μg C, while DC is ∼0.3 ± 0.15 μg C. Today, the laboratory routinely analyzes graphite samples as small as 0.015 mg C for external submissions and ≅0.001 mg C for internal research activities with a precision of ∼1% for ∼0.010 mg C. However, when analyzing ultra-small samples isolated by a series of complex chemical and chromatographic methods (such as individual compounds), integrated procedural blanks may be far larger and more variable than those associated with combustion/graphitization alone. In some instances, the mass ratio of these blanks to the compounds of interest may be so high that the reported 14C results are meaningless. Thus, the abundance and variability of both MC and DC contamination encountered during ultra-small sample analysis must be carefully and thoroughly evaluated. Four case studies are presented to illustrate how extraction chemistry blanks are determined.

Over the past decades, analysis of occluded carbon in phytoliths (opaline silica mineral bodies that form in and between plant cells) has become a workhorse of paleoclimate and archaeological studies. Since different plant types exhibit distinctive phytolith morphologies, their assemblages are used in identifying vegetation histories or food culture adaptations. A few direct radiocarbon AMS measurements of phytoliths have been carried out, but these measurements are difficult due to the low concentrations of phytoliths in some plant species, and the small amount of C per phytolith (<2%). In addition, no phytoliths samples of a known 14C age are available to verify measurement accuracy and precision, and to check sample preparation protocols. Background corrections are also difficult to address due to the lack of suitable material. In this work, we designed a procedure to quantify a suitable blank using SiO2 powder samples (close to the opal structure, and free of 14C). The full phytolith extraction showed high carbon contamination components: a) ∼3 μg of modern C and ∼2 μg of dead C. We also performed accuracy tests on large phytolith-occluded carbon samples extracted from soils and harvested plants. The unexpected 14C ages in some of the results triggered further investigations of possible sources of carbon contamination.

Radiocarbon (Δ14C) measurements of monthly samples from a Galapagos surface coral are among the first data sets from the new Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine. An average Δ14C value of −62 is obtained for 144 measurements of samples from monthly coral bands that lived from about AD 1760–1771 (±6 yr). High Δ14C values were found during January through March, when upwelling was weak or absent at the Galapagos Islands. Low Δ14C values were obtained mid-year during strong upwelling. The average seasonal variability of Δ14C was 15–25, which is greater than that at other tropical and subtropical locations in the Pacific Ocean because of intense seasonal upwelling at this site. Periods of sustained high Δ14C values were found during 1762–1763 and 1766. A spectral analysis revealed that the spectral density for the Δ14C data displays most of its variance at the 5-yr cycle, which is reflective of El Niño periodicity during the 20th century.