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Sequential thermal analysis allows for deconvoluting the refractory nature and complexity of carbon mixtures embedded in mineral matrices for subsequent offline stable carbon and radiocarbon (14C) isotope analyses. Originally developed to separate Holocene from more ancient sedimentary organic matter to improve dating of marine sediments, the Ramped Pyrolysis and Oxidation (RPO) apparatus, or informally, the “dirt burner” is now used to address pressing questions in the broad field of biogeochemistry. The growing interest in the community now necessitates improved handling and procedures for routine analyses of difficult sample types. Here we report on advances in CO2 purification during sample processing, modifications to the instrumentation at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility, and introduce sodium bicarbonate procedural standards with differing natural abundance 14C signatures for blank assessment. Measurements from different environmental samples are used to compare the procedure to the different generations of sequential thermal analyses. With this study, we aim to improve the standardization of the procedures and prepare this instrumentation for innovations in online stable carbon isotopes and direct AMS-interface measurements in the future.
This note describes improvements of UV oxidation method that is used to measure carbon isotopes of dissolved organic carbon (DOC) at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS). The procedural blank is reduced to 2.6 ± 0.6 μg C, with Fm of 0.42 ± 0.10 and δ13C of –28.43 ± 1.19‰. The throughput is improved from one sample per day to two samples per day.
Radiocarbon (14C) is an isotopic tracer used to address a wide range of scientific research questions. However, contamination by elevated levels of 14C is deleterious to natural-level laboratory workspaces and accelerator mass spectrometer facilities designed to precisely measure small amounts of 14C. The risk of contaminating materials and facilities intended for natural-level 14C with elevated-level 14C-labeled materials has dictated near complete separation of research groups practicing profoundly different measurements. Such separation can hinder transdisciplinary research initiatives, especially in remote and isolated field locations where both natural-level and elevated-level radiocarbon applications may be useful. This paper outlines the successful collaboration between researchers making natural-level 14C measurements and researchers using 14C-labeled materials during a subglacial drilling project in West Antarctica (SALSA 2018–2019). Our strict operating protocol allowed us to successfully carry out 14C labeling experiments within close quarters at our remote field camp without contaminating samples of sediment and water intended for natural level 14C measurements. Here we present our collaborative protocol for maintaining natural level 14C cleanliness as a framework for future transdisciplinary radiocarbon collaborations.
The new species Psoroma nivale is described from an area of late snow-lie in the Keglo Bay area on the eastern side of Ungava Bay, northern Québec, Canada. It is superficially similar to P. hypnorum but has a dark, brownish black thallus colour without reddish hues, much-branched, proliferating squamules, thick paraphyses, distinct but inconspicuous IKI+ ascus tube structures, and characteristic elongate, bacilliform, often asymmetrical ascospores. The new species is compared with possible related taxa and its systematic position discussed. A key to the species of pannarioid lichens reported from arctic areas of North America is also provided.
We estimate the blank carbon mass over the course of a typical Ramped PyrOx (RPO) analysis (150–1000°C; 5°C×min–1) to be (3.7±0.6) μg C with an Fm value of 0.555±0.042 and a δ13C value of (–29.0±0.1) ‰ VPDB. Additionally, we provide equations for RPO Fm and δ13C blank corrections, including associated error propagation. By comparing RPO mass-weighted mean and independently measured bulk δ13C values for a compilation of environmental samples and standard reference materials (SRMs), we observe a small yet consistent 13C depletion within the RPO instrument (mean–bulk: μ=–0.8‰; ±1σ=0.9‰; n=66). In contrast, because they are fractionation-corrected by definition, mass-weighted mean Fm values accurately match bulk measurements (mean–bulk: μ=0.005; ±1σ=0.014; n=36). Lastly, we show there exists no significant intra-sample δ13C variability across carbonate SRM peaks, indicating minimal mass-dependent kinetic isotope fractionation during RPO analysis. These data are best explained by a difference in activation energy between 13C- and 12C-containing compounds (13–12∆E) of 0.3–1.8 J×mol–1, indicating that blank and mass-balance corrected RPO δ13C values accurately retain carbon source isotope signals to within 1–2‰.
In response to the increasing demand for 14C analysis of samples containing less than 25 μg C, ultra-small graphitization reactors with an internal volume of ∼0.8 mL were developed at NOSAMS. For samples containing 6 to 25 μg C, these reactors convert CO2 to graphitic carbon in approximately 30 min. Although we continue to refine reaction conditions to improve yield, the reactors produce graphite targets that are successfully measured by AMS. Graphite targets produced with the ultra-small reactors are measured by using the Cs sputter source on the CFAMS instrument at NOSAMS where beam current was proportional to sample mass. We investigated the contribution of blank carbon from the ultra-small reactors and estimate it to be 0.3 ± 0.1 μg C with an Fm value of 0.43 ± 0.3. We also describe equations for blank correction and propagation of error associated with this correction. With a few exceptions for samples in the range of 6 to 7 μg C, we show that corrected Fm values agree with expected Fm values within uncertainty for samples containing 6–100 μg C.
The majority of samples processed at the National Ocean Sciences AMS Facility (NOSAMS) thus far were collected as part of the World Ocean Circulation Experiment (WOCE). Due to the long storage time (2–3 yr) required to analyze thousands of samples on the accelerator mass spectrometer (AMS), a test was designed and implemented to determine the effects, if any, of storage time on 14C concentration. We find no systematic offsets in AMS measurements made over a 5-yr period between a total of 16 replicate sets from surface and deep water collected at the same locality. Furthermore, the average δ14C value from the deepwater AMS replicates (-213.1%, std. dev. 7.3) agrees very closely with the conventional 14C results published for GEOSECS (-212.7%) from station 320 taken 20 yr earlier.
A total of 73 WOCE shipboard replicate sets (162 AMS measurements) were analyzed with a mean precision of 4.3%. AMS results from 20 more shipboard replicate sets (44 AMS measurements) submitted as CO2 from the Stable Isotope Laboratory (SIL) at the University of Washington were analyzed with a mean precision of 3.4%. These results suggest no significant difference between water stripping methods used in each preparation lab.
To assess reproducibility, we calculate a pooled estimate of σ for replicates called s, which we use as an approximation of σTOT for a given sample type. The s for WOCE seawater replicates is 4.9% and 5.8% for SIL gas replicates. These numbers demonstrate an overall reproducibility of seawater AMS results at NOSAMS that is in line with reported errors. We take the difference between total error s and machine error as the overall standard deviation of combined uncertainties associated with preparation of samples and with AMS. For seawater samples processed at NOSAMS, σSPL is calculated to be 2.4%, and for the SIL gas replicates it is 4.8%.
Reproducibility of samples prepared with an acid hydrolysis technique is demonstrated using 24 coral samples submitted in triplicate by Dr. R. G. Fairbanks of Lamont Doherty Earth Observatory. Seventy-two replicates were prepared and analyzed at NOSAMS with a mean reported precision of 1.2%. The pooled estimate s for the Fairbanks triplicates is 2.6%. We calculate a laboratory reproducibility uncertainty for coral hydrolysis samples of 2.2%.
In 1993, NOSAMS participated in the Third International Radiocarbon Intercomparison (TIRI) Study. We report here 60 AMS analyses of the six TIRI test materials, five of which are organic carbon samples, to validate sample-processing methods for organic carbon sample AMS analyses at NOSAMS.
In July 1986, an AMS radiocarbon target preparation laboratory was established at the Woods Hole Oceanographic Institution to produce graphite to be analyzed at the NSF-Accelerator Facility for Radioisotope Analysis at the University of Arizona (Tucson). By June 1991, 923 graphite targets had been prepared and 847 analyzed. Our lab procedures during this time included the careful documentation of weights of all starting samples, catalysts and final graphite yields, as well as the volume of CO2 gas evolved during CaCO3 hydrolysis or closed-tube organic carbon combustions. From these data, we evaluate the methods used in general and in our lab.
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