National Ocean Sciences AMS Research Articles

SEQUENTIAL THERMAL ANALYSIS OF COMPLEX ORGANIC MIXTURES: PROCEDURAL STANDARDS AND IMPROVED CO2 PURIFICATION CAPACITY

ABSTRACTSequential 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.

COMPARABILITY OF RADIOCARBON MEASUREMENTS IN DISSOLVED INORGANIC CARBON OF SEAWATER PRODUCED AT ETH-ZURICH

ABSTRACT Radiocarbon observations (Δ14C) in dissolved inorganic carbon (DIC) of seawater provide useful information about ocean carbon cycling and ocean circulation. To deliver high-quality observations, the Laboratory of Ion Beam Physics (LIP) at ETH-Zurich developed a new simplified method allowing the rapid analysis of radiocarbon in DIC of small seawater samples, which is continually assessed by following internal quality controls. However, a comparison with externally produced 14C measurements to better establish an equivalency between methods was still missing. Here, we make the first intercomparison with the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility based on 14 duplicate seawater samples collected in 2020. We also compare with prior deep-water observations from the 1970s to 1990s. The results show a very good agreement in both comparisons. The mean Δ14C of 12 duplicate samples measured by LIP and NOSAMS were statistically identical within one sigma uncertainty while two other duplicate samples agreed within two sigma. Based on this small number of duplicate samples, LIP values appear to be slightly lower than the NOSAMS values, but more measurements will be needed for confirmation. We also comment on storage and preservation techniques used in this study, including the freezing of samples collected in foil bags.

A high output, large acceptance injector for the NOSAMS Tandetron AMS system

We have completed a major upgrade of the National Ocean Sciences AMS Facility (NOSAMS) Tandetron AMS system in two stages. First, the simultaneous (recombinator) injector was replaced with a fast-cycling sequential injector and changes to the low-energy acceleration section. Data after the injector commissioning show an improvement in background, with mean machine background (commercial graphite) of Fm 0.0004 (62ka). Second, we replaced the original ion source with a high-output 40 sample MCSNICS source. This improved beam currents and raw ratio fractionation, and increased sample to detection efficiency fivefold.

Palaeoecology and geoarchaeology of the Varna Lake, northern Bulgarian Black Sea coast

The coastal lakes are rich sources of biostratigraphic information that is very useful in palaeoecological reconstructions of climate changes and human impact on the natural vegetation. This information is of great importance for the archaeological descriptions of submerged praehistorical settlements found in the northern Bulgarian Black sea area. There are 4 archaeological sites in this area that have been palynologically studied for the last 30 years: the Durankulak Lake, the Shabla-Ezeretz Lake system, the Lake Bolata, as well as the Varna-Beloslav Lake system. Because of the lack of AMS radiocarbon dates for these sites, it was not possible to correlate adequately all palaeoenvironmental results with the available archaeological chronology. Aimed to receive additional information on the Holocene vegetation dynamics and lake level changes, as well as on the anthropogenic impact during the Late Eneolithic and Early Bronze Age, the high-resolution spore-pollen analysis of AMS dated laminated sediments from a new Core 3 – Varna Lake was combined with analyses of dinoflagellate cysts, acritarchs, and other non-pollen palynomorphs. The location of the core is close to several sites of submerged praehistorical settlements and the Varna Late Eneolithic (Chalcolithic) Necropolis, which is famous with the oldest hand-made gold treasure in the Worlds, and permits the palaeoenvironmental correlations of obtained results with available archaeological and geochronological data. The core is 995 cm long, but its palynologically investigated length is 870 cm. It contains dark grey clay and laminated sediments (varves). Seven samples of sediments were submitted for radiocarbon dating to the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility of Woods Hole Oceanographic Institution (WHOI). The dates have been calibrated using the program CALIB version 6.1.0 of using the IntCal09 curve. An Age Model for the sedimentation rate was created by the newest version 1.17.16. of the TILIA software.

A High-Throughput, Low-Cost Method for Analysis of Carbonate Samples for 14C

The National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility at the Woods Hole Oceanographic Institution has developed an automated system for high-throughput, low-cost analysis of radiocarbon in carbonate samples (e.g. corals, carbonaceous sediments, speleothems, etc.). The method bypasses graphitization and pretreatment, and reduces costs to about 1/5th the price of a graphite-based 14C carbonate analysis, with a throughput of 60 unknowns per day and an analytical precision of better than 2%. Additionally, a simple mixing experiment indicated that extensive cleaning of carbonate samples to remove organic material is not necessary.

A gas-accepting ion source for Accelerator Mass Spectrometry: Progress and applications

The National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility at the Woods Hole Oceanographic Institution has developed an Accelerator Mass Spectrometry (AMS) system designed specifically for the analysis of 14C in a continuously flowing stream of carrier gas. A key part of the system is a gas-accepting microwave ion source. Recently, substantial progress has been made in the development of this source, having achieved ion currents rivaling that of a traditional graphite source (albeit at relatively low efficiency). Details and present performance of the gas source are given. Additionally, representative results obtained from coupling the source to both a gas chromatograph and gas bench are presented.

Progress with a gas-accepting ion source for Accelerator Mass Spectrometry

The National Ocean Sciences AMS (NOSAMS) facility at Woods Hole Oceanographic Institution has developed a novel, gas-accepting microwave-plasma ion-source. The source is a key component of a compact Accelerator Mass Spectrometry (AMS) system built for the analysis of 14C in a continuously flowing gas stream. The gas source produces carbon currents from a stream of CO 2 with currents typical of a traditional graphite source. Details of the gas source, including ion current achieved, optimal flow rate, efficiency, and memory are presented. Additionally, data obtained from coupling a gas chromatograph to the source shown. Details about ion optics are presented separately [1].

Design and reality: Continuous-flow accelerator mass spectrometry (CFAMS)

In 2007 we published [1] the design of a novel accelerator mass spectrometry (AMS) system capable of analyzing gaseous samples injected continuously into a microwave plasma gas ion source. Obvious advantages of such a system are drastically reduced processing times and avoidance of potentially contaminating chemical preparation steps. Another paper in these proceedings will present the progress with the development of the microwave gas ion source that has since been built and tested at the National Ocean Sciences AMS Facility in Woods Hole [2]. In this paper we will review the original design and present updates, reflecting our recent encouraging experience with the system. A simple summary: large acceptance ion beam optics design is beneficial to accelerator mass spectrometry in general, but essential to AMS with plasma gas ion sources.

Software development for continuous-gas-flow AMS

The National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility at Woods Hole Oceanographic Institution is presently completing installation of a novel continuous-flow AMS system. A multi-year development of an AMS microwave gas ion source in collaboration with Atomic Energy Canada Limited (AECL), Chalk River, has preceded this final step of an implementation that is expected to add a new dimension to 14C AMS. National Instruments, NIM, and CAMAC modules have been programmed with LabVIEW on a Windows XP platform to form the basis for data acquisition. In this paper we discuss possible applications and include simulations of expected data acquisition scenarios like real-time AMS analysis of chromatograms. Particular attention will have to be given to issues of synchronization between rapidly changing input amplitudes and signal processing cycles in hardware and software.

Radiocarbon Dating of Alkenones from Marine Sediments: II. Assessment of Carbon Process Blanks

We evaluate potential process blanks associated with radiocarbon measurement of microgram to milligram quantities of alkenones at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility. Two strategies to constrain the contribution of blanks to alkenone 14C dates were followed: 1) dating of samples of known age and 2) multiple measurements of identical samples. We show that the potential contamination associated with the procedure does not lead to a systematic bias of the results of alkenone dating to either younger or older ages. Our results indicate that alkenones record Δ14C of ambient DIC with an accuracy of approximately 10‰. A conservative estimate of measurement precision is 17‰ for modern samples. Alkenone 14C ages are expected to be reliable within 500 yr for samples younger than 10,500 14C yr.

Installation of a 134-sample MC-SNICS ion source at NOSAMS and first results

In April 1999, the National Ocean Sciences AMS (NOSAMS) facility received the shipment of a 134-sample MC-SNICS ion source from the National Electrostatics Corporation (NEC). It replaced one of the two 59-sample spherical ionizer sources (US-AMS Model 3090A, HVEE Model 846). In this paper, we will describe the adaptation of the NEC ion source to the US-AMS/HVEE recombinator injector at NOSAMS. This is the first time that the two leading sputter ion source designs are directly comparable on the same system. We will present ion beam optics calculations for both in comparison with measured results. Once fully operational, the new ion source will assume all of the sputter sample measurements at NOSAMS, while the remaining 59-sample source will be replaced by the newly developed microwave gas ion source. One of the most desirable features of the MC-SNICS design is the smaller cathode geometry. At comparable extracted currents, the surface diameter of the pressed samples is reduced from 1.5 to 1 mm. A new, automatic sample pressing procedure is being developed for this design. This will improve the capability for preparing and analyzing smaller sputter samples (carbon weight <100 μg). Our US-AMS cathodes showed considerable reduction in the extracted currents when the sample diameter was reduced to 1 mm, even with samples of normal size (>200 μg C).

The NOSAMS sample preparation laboratory in the next millenium: Progress after the WOCE program

Since 1991, the primary charge of the National Ocean Sciences AMS (NOSAMS) facility at the Woods Hole Oceanographic Institution has been to supply high throughput, high precision AMS 14C analyses for seawater samples collected as part of the World Ocean Circulation Experiment (WOCE). Approximately 13,000 samples taken as part of WOCE should be fully analyzed by the end of Y2K. Additional sample sources and techniques must be identified and incorporated if NOSAMS is to continue in its present operation mode. A trend in AMS today is the ability to routinely process and analyze radiocarbon samples that contain tiny amounts (<100 μg) of carbon. The capability to mass-produce small samples for 14C analysis has been recognized as a major facility goal. The installation of a new 134-position MC-SNICS ion source, which utilizes a smaller graphite target cartridge than presently used, is one step towards realizing this goal. New preparation systems constructed in the sample preparation laboratory (SPL) include an automated bank of 10 small-volume graphite reactors, an automated system to process organic carbon samples, and a multi-dimensional preparative capillary gas chromatograph (PCGC).

Ten years after – The WOCE AMS radiocarbon program

The National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility is measuring all of the samples collected as part of the US WOCE Program – over 13,000 samples. We designed our extraction lines so that we also measure precise, oceanographically useful δ 13C- ΣCO 2 values. We have completed the analysis of samples from the Pacific and Southern Oceans and are processing those from the Indian Ocean now. At present, this constitutes the world’s largest AMS data set. Reviews of the Pacific radiocarbon data are available and demonstrate the increased penetration of the “bomb signal” into the water column since the 1970s. Stable isotope data are being combined with those collected as part of NOAA’s Ocean-Atmosphere Carbon Exchange Study to study the ocean’s role in the anthropogenic CO 2 cycle. The relationship of δ 13C to other chemical tracers, e.g., PO 4, O 2 and chlorofluorocarbons, will further our understanding of basic oceanographic processes. We present preliminary results from these studies as well as investigate the relationship of 14C to 13C in the ocean.

Performance characteristics of the 3 MV Tandetron AMS system at the National Ocean Sciences AMS facility

Operational and machine performance parameters are discussed for the National Ocean Sciences AMS system. The system now routinely measures between 50 and 100 carbon samples per week in largely unattended mode using one of two functional high-current ion sources. System development and procedures are described that enable us to reach and maintain the high precision level required for the measurement of deep sea water dissolved inorganic carbon samples.

High-precision AMS radiocarbon measurements of central Arctic Ocean sea waters

We report on the first sub-5%. precision radiocarbon dataset measured on single targets using accelerator mass spectrometry (AMS). A 13 sample water column profile collected in the Canada Basin (74°N, 150°W, 3850 m water depth) of the central Arctic Ocean in September 1992 has been analyzed in duplicate and the average total precision achieved for the 26 targets is ± 3.2%.. The reproducibility of the 13 paired analyses averages +- 4.8%. as determined by a chi-square fit minimization for a quality factor of unity, and ± 7.8%. using ANOVA. Eliminating two of the 13 paired analyses because of apparent outlier behavior in one of the two analyses comprising the pair results in a total precison of ± 3.2%., a chi-square fit of ± 3.5%., and ANOVA precision of ± 3.5%.. Comparison with a recently published AMS 14C profile from the same basin suggests these data are accurate as well. Results show that the deep waters of the Canada Basin have a renewal rate of 430 years, in comparison with 250 years estimated for the deep waters of the Eurasian Basin. Although the major requirement of the World Ocean Circulation Experiment (WOCE) for a radiocarbon analysis precision of ± 3 to 4%. for deep water samples can now be met with the AMS technology available at the National Ocean Sciences AMS Facility at the Woods Hole Oceanographic Institution for single-target analyses, a careful program of duplicate analyses should be included to insure the highest quality in the WOCE Δ 14C dataset.

Optimized data analysis for AMS radiocarbon dating

Because efficiencies of detection of 14C and 12C vary with time during AMS data acquisition, it is desirable to have a data analysis technique which recognizes time variations and uses all available data to extract the maximum possible information content from a data set while providing meaningful statistical information about measurement errors. Toward this end, a new method of data reduction and error analysis is being developed for the determination of 14C to 12C ratios for radiocarbon dating from data sets taken at the National Ocean Sciences AMS facility.

Methods for data screening, flagging and error analysis at the National Ocean Sciences AMS Facility

All data collection, from sample submittal through processing into targets and AMS analysis, is integrated within a large relational database (Sybase). Over 50 tables are linked through key fields. Through structured queries, the information is analysed and presented for a wide variety of applications. Benefits include enhanced quality control, more complete reports to users and more accurate transfer of data among the several laboratories on the network.

Automated sample processing at the National Ocean Sciences AMS facility

The high throughput and high precision requirements for the NOSAMS facility have made it essential to automate many of the stages in sample processing. These automated procedures increase the sample capacity for the lab while reducing errors in sample preparation. Automation has also allowed sample histories to be recorded and saved in Sybase, a relational data base.

The Rapid Preparation of Seawater ΣCO2 for Radiocarbon Analysis at the National Ocean Sciences Ams Facility

We have established a laboratory for extracting ΣCO2 from seawater samples for AMS analysis of the radiocarbon content. The seawater samples are collected at sea, poisoned and stored until analysis in the laboratory. Each sample is acidified; the inorganic carbon is stripped out as CO2 with an inert carrier gas and then converted to graphite. We present results for Buzzards Bay surface H2O and Na2CO3 standards that demonstrate we strip &gt;98% of inorganic carbon from seawater. Stable isotope analyses are performed to better than 0.2‰, and the reproducibility of 14C measurements on Buzzards Bay seawater is better than 13‰. Finally, we compare data from samples collected in 1991 to those collected in the 1970s and to large volume samples.

A High Throughput 14C Accelerator Mass Spectrometer

I present design details of a tandem accelerator mass spectrometer, which has been installed at the National Ocean Sciences AMS Facility at Woods Hole, Massachusetts, to provide precision 14C/13C/12C isotopic ratios for sub-milligram-size samples of graphite with throughputs of &gt;4000 samples per year. A unique feature is the capability for simultaneous measurement of all three isotopes after acceleration, to avoid differential transmission effects and to allow on-line fractionation corrections and diagnosis of instrument health. Using filamentous graphite fabricated from a recent sample, we have established the counting rate of 14C ions at between 60–120 s–1.