Radiocarbon Dating of Foraminiferal Shells

The majority of the organic matter contained in the initial sediment was extracted with organic solvents following Ohkouchi et al. (2005) to use the organic fraction in a follow‐up investigation. To assess the possible influence of this procedure on the foraminifera contained in the solvent‐extracted residue, we also analyzed five samples of G. bulloides tests selected from nonextracted sediments. Between 15 and 30 g of dry sediment were diluted in MiliQ® water and sonicated for only 15 s for disaggregation while avoiding shell fragmentation. The solution was then wet sieved through 300‐ and 250‐μm mesh sieves and thoroughly washed using a high‐pressure stream of MiliQ® water. The resulting 250‐ to 300‐μm size fraction was immediately dried at 60 °C overnight, prior to collecting 45–100 well‐preserved shells of G. bulloides or G. ruber from each sample. In some intervals, only 7–20 specimens of G. ruber were available, limiting the amount of measured carbon (Tables S1 and S2 in the supporting information). Radiocarbon determinations (¹⁴C/¹²C) were performed with a gas ion source in a Mini Carbon Dating System at the Laboratory of Ion Beam Physics, ETH Zürich, with an automated method for acid digestion of carbonates whose sensitivity allows for less than10 μg of total carbon to be measured (Wacker et al., 2013). The method is outlined as follows: vials (septa sealed 4.5‐ml exetainers vials from Labco Limited, UK) containing the samples were purged for 10 min with a flow of 60 ml/min He to remove atmospheric CO₂. Later, samples were briefly leached by adding 100 μl of ultrapure HCl (0.02 M) with an automated syringe to remove possible surface contaminants. The CO₂ released from the leachate, referred to as leachate was transported by helium to a zeolite trap and automatically injected into the ion source to be measured for radiocarbon. The remaining sample, containing 12 μg C and referred to as leached sample, was subsequently acidified by adding 100 μl of ultrapure H₃PO₄ (85%) that was heated to 60 °C for at least 1 hr. The released CO₂ was loaded in a second trap and injected into the ion source to be analyzed for radiocarbon (Wacker et al., 2014). Bard et al. (2015) showed that the F¹⁴C (fraction modern according to Reimer et al. (2004)) of leachates from sequential leaching of discrete samples converge toward a comparable value to that of the F¹⁴C of the leached sample (Bard et al., 2015). Thus, we propose differences <5% between the two values as an indication of near‐complete removal of surface contaminants. Five replicates of G. bulloides samples, referred to as untreated, were directly measured without leaching the outer shell to assess the necessity of this method. This gas ion source Accelerator Mass Spectrometry (AMS) system has a background ¹⁴C/¹²C value of F¹⁴C 0.0020 + −0.0010 (50000 BP), determined on marble (IAEA‐C1). Radiocarbon determinations were corrected for isotopic fractionation via ¹³C/¹²C isotopic ratios and are given in conventional radiocarbon ages. Radiocarbon ages and errors were not rounded to avoid artificial increments of age offsets and propagated errors.