Direct AMS ¹⁴C Analysis of Carbonate

Abstract

We have investigated the possibility of direct accelerator mass spectrometry radiocarbon (AMS 14C) measurement of carbonate samples at ANSTO using the STAR 2 MV tandem accelerator. Each carbonate sample was powdered, mixed with metal powder and pressed into an aluminum cathode for direct carbonate measurement by AMS 14C. Of the three high-purity metal powders (Fe, Nb, and Ag) used in our investigation, Nb was found to be the best metal, which delivered higher carbon beam currents and lower background. Beam currents for targets containing the optimal carbonate mass of 1.5–2.0 mg were ∼8% of those obtained from graphite targets of standard size (>0.5 mg C). Typical measured blank for Carrara marble (IAEA-C1) of ∼40 ka was obtained. Background-corrected 14C values of carbonate targets agreed well with their associated values obtained from high-precision analysis of graphite targets within 2σ uncertainties. Typical precision of this rapid AMS analysis was ∼1% for samples <8 ka. Despite lower precision for carbonate target ages (compared to standard graphite target ages), these ages are useful for palaeobiological applications where a large number of dates are required, such as time-averaging studies.

COMPARING DIRECT CARBONATE AND STANDARD GRAPHITE 14C DETERMINATIONS OF BIOGENIC CARBONATES

“…Direct carbonate 14 C analysis of NAU's C1 blank yields 2.10 ± 0.33 pMC (n = 21) using Fe and 1.49 ± 0.66 pMC (n = 115) using Nb powder. Our direct carbonate blank results are similar to Hua et al (2019) who demonstrated that Nb powder yields lower blanks than either Fe or Ag powders. The source of the direct carbonate 14 C contamination in the NAU blank is unclear but likely stems from a variety of sources including, but not limited to, contamination during processing, carbon contamination in both the metal powders and the C1 powder itself, and uptake from atmospheric sources (Longworth et al 2013).…”

“…We compiled 60 additional paired determinations generated at UCI or the Australian Nuclear Science and Technology Organisation (ANSTO) from Bush et al (2011), New et al (2019, Parker et al (2019), andHua et al (2019). The respective publications provide the lab procedures and methods used for the additional paired determinations.…”

“…The direct carbonate AMS 14 C technique uses a cesium sputter source and a metal powder as a binder without the need to convert the carbonate sample to graphite but yields beam currents about an order of magnitude lower than the standard graphite method (Bush et al 2013;Hua et al 2019) which leads to the lower precision. Longworth et al (2013) analyzed several materials with percent Modern Carbon (pMC) between 0.25 and 94.21.…”

“…Using iron powder, the direct carbonate method produced 1σ errors that ranged from 0.31 and 0.62 pMC, whereas 1σ errors on the same materials ranged from 0.03 and 0.11 pMC using graphite. Subsequent study by Hua et al (2019) further established the utility of the direct carbonate technique, testing iron (Fe), niobium (Nb), and silver (Ag) powders before concluding that Nb powder was superior because it produced the highest beam current and lowest background.…”

“…Several studies have compared small numbers of paired direct carbonate and graphite 14 C results, showing that the two methods are comparable (Bush et al 2013;Longworth et al 2013;Kosnik et al 2017;Kowalewski et al 2018;Hua et al 2019;New et al 2019;Albano et al 2020). In this paper, we have compiled a comprehensive dataset (n = 153) of published and unpublished direct carbonate and graphite 14 C determinations from biogenic carbonates belonging to several taxonomic groups (mollusks, corals, echinoderms, brachiopods) to further quantify any bias in the results based on the direct carbonate method.…”