A DATABASE OF NERC RADIOCARBON MEASUREMENTS DETERMINED BY ACCELERATOR MASS SPECTROMETRY

Abstract

Radiocarbon (¹⁴C) measurements undertaken by the NERC Radiocarbon Laboratory using accelerator mass spectrometry (AMS) are now freely available on a new online database. The data presented covers measurement of the wide range of sample types that are processed for research projects in the fields of Earth and environmental science, supported by the United Kingdom’s Natural Environment Research Council. Sample types within the database include organic remains, soils, sediments, carbonates, dissolved organic and inorganic carbon, and carbon dioxide. Currently, the database contains ¹⁴C data for over 2400 individual samples that were measured and reported between 2005 and 2013, but it is envisaged that this will expand considerably as more data are made available. Contextual information such as sampling location and associated publications are provided where available, and searches can be performed on sample location, sample type, project number, and publication code. This new database compliments an existing, publicly available database of measurements performed using radiometric methods by the laboratory which has recently been expanded to present over 2000 measurements. It is hoped that this archive will prove useful to workers in the community who would benefit from greater availability of measurements for particular locations or sample types, and for the purposes of performing meta-analyses, and/or synthesis of larger datasets.

INTRODUCTION

In 1971 the UK Natural Environment Research Council (NERC) established the NERC Radiocarbon Laboratory at East Kilbride, near Glasgow. The laboratory became a NERC central facility in 1976, enabling UK researchers in the Earth and environmental sciences to apply for radiocarbon (¹⁴C) analyses via application and peer review by the NERC Radiocarbon Laboratory Steering Committee. Since 2019, the laboratory has been a member of the National Environmental Isotope Facility (NEIF) which is “an integrated platform of state-of-the-art isotope and organic geochemistry analytical capabilities and specialisms,” being delivered for eligible researchers by five organizations spread across the UK (https://www.isotopesuk.org/). The laboratory is now known as the NEIF Radiocarbon Laboratory and is hosted by the Scottish Universities Environmental Research Centre (SUERC) at the same location that was established in 1971.

A key principle of NERC’s data policy is that “environmental data produced by the activities funded by NERC are considered a public good and they will be made openly available for others to use” (NERC 2022). In support of this, the NERC Radiocarbon Laboratory published date lists in Radiocarbon during the first decade of its operation (Harkness and Wilson 1973, 1974, 1979; Harkness 1981). Subsequently, ¹⁴C data have been made available by the lab as a CD-ROM in an issue of Quaternary Science Reviews (Harkness et al. 1997), and most recently, on an interactive spreadsheet accessible via the World Wide Web (Garnett et al. 2010). This latter compilation of ¹⁴C results has subsequently been expanded and transferred to a searchable web database (https://www.environmental14c.co.uk/form.php).

The ¹⁴C results in these collections of data previously presented to the community were all determined from radiometric measurements using liquid scintillation counters. However, since the 1990s the laboratory has prepared graphite targets for ¹⁴C analysis using accelerator mass spectrometry (AMS); the SUERC AMS Laboratory, commissioned by the NERC Radiocarbon Laboratory to undertake the AMS analyses considered here, was itself a NERC Recognised Facility before its role was included in NEIF. Here, we describe a new, freely available, web database of these AMS radiocarbon results.

ACCESSING THE WEB DATABASE OF AMS RESULTS

The web address for the new database of AMS ¹⁴C results is https://www.environmental14c.co.uk/form_ams.php and it can also be accessed via the laboratory’s website (https://environmental14c.co.uk/). The database can be searched using four fields which are accessed in dropdown boxes or textboxes: sampling location, sample type, project allocation number, and publication code. Sample locations are mostly classified by country or water body (e.g., sea or ocean for marine samples). The type of sample material analyzed has been classified using the scheme of sample types shown in Table 1. Allocation number is a unique identifier for the project for which the samples were analyzed, with the numbers after the decimal point representing the month and year (last 2 digits) of the original project application. Publication code is the unique identifier for individual radiocarbon measurements provided by the AMS Laboratory. Location and sample type can be specified individually in searches, or the entire database can be interrogated using the “All” option.

Table 1 Types of sample material in the AMS radiocarbon web database.

Performing a search of the database using the project allocation number returns a table of results listing all sample records associated with the specified allocation number. Sample details include publication code, material identifier, radiocarbon concentration (as percent modern; pMC), conventional radiocarbon age (in years BP, where 0 BP = AD 1950; Stuiver and Polach 1977), sample location details (as provided by the submitter) and the date that the age results were formally reported (samples were usually reported in the same year that they were measured). In addition, details of the project (surname and institute of the lead applicant, and project title) are provided as well as publications associated with the project.

A search on location and sample type returns a table of samples that meet the specified location and sample type criteria. This table provides the same output for individual samples as a search on project allocation number, but also includes a web link in the column titled “Allocation.” Clicking on this link performs a search using the project allocation number as described above and can be used to retrieve the additional project information and samples of other types or locations associated with the project. Searching using publication code returns the same data as a search on location or sample type, but for a single sample only.

PROCESSING METHODS AND DEFINITIONS OF RESULTS

Methods for processing the samples described in the new database follow relevant protocols of pretreatment for particular sample types and project aims, conversion to carbon dioxide (CO₂), cryogenic purification of the CO₂, graphitization, pressing of the iron/graphite mixture into an aluminium target, and AMS measurement. Organic samples were combusted either using the sealed quartz tube method (Boutton et al. 1983) or an elemental analyzer (Costech ECS 4010, Italy) with the combusted gases transferred to a vacuum rig for cryogenic purification without passing through a gas chromatography column. Carbonates were hydrolyzed to CO₂ in sealed glass vessels using orthophosphoric acid. The Fe:Zn reduction method (Slota et al. 1987) was used to convert all samples to graphite. Ascough et al. (forthcoming) present an overview of current sample processing methods at the NEIF Radiocarbon Laboratory which are mostly identical to those applied to the samples in the new database. More details of pretreatments and sample processing, including sample- or project-specific information are not included within the new database, but this information was reported to the project researchers and therefore the reader should consult publications that refer to the analytical data to obtain further details.

All ¹⁴C results currently in the database were measured by AMS at SUERC, using either a National Electrostatics Corporation (NEC) 5 MV tandem accelerator mass spectrometer (Freeman et al. 2004) or a NEC 250 kV single-stage accelerator mass spectrometer (Freeman et al. 2010). Prior to the establishment of SUERC AMS in 2003, AMS samples prepared at the NERC Radiocarbon Laboratory were analyzed at the NSF Accelerator Facility at the University of Arizona (Donahue et al. 1990) or Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, University of California (Roberts et al. 1997), and these results will be included in future updates of the database.

Laboratory background contamination was quantified using ¹⁴C-dead standard materials (e.g., coal and calcite) for each processing method and used to correct results that were significantly above background. Conventional ¹⁴C ages are not reported by the database if a background correction was not applied (for samples <100 pMC), or if the samples were modern (>100 pMC). NIST Oxalic Acid II (SRM 4990 C; National Institute of Standards and Technology, USA) was used as the primary reference standard. Following convention (Stuiver and Polach 1977), ¹⁴C results were normalized to a delta ¹³C of –25‰ using isotope ratio mass spectrometry (IRMS) measurements of an aliquot of carbon dioxide from the pretreated sample on a VG Optima (Micromass, UK) or Delta V (Thermo-Fisher, Germany). For a small number of samples, online AMS delta ¹³C measurements were used to normalize the ¹⁴C results, but these are not reported by the database. Analytical confidence of age measurements incorporates uncertainty from the background and delta ¹³C corrections, in addition to that derived from AMS counting statistics and measurement scatter. In-house quality assurance was monitored to verify the reliability of results via processing of internationally-accepted standard materials of known ¹⁴C value alongside the unknown samples. These materials were derived from either international standards agencies (i.e., IAEA or NIST), from the International Radiocarbon Intercomparisons (Gulliksen and Scott 1995), or from in-house materials whose ¹⁴C value and homogeneity had been verified by repeated measurements in comparison to the aforementioned international standards.