Radiocarbon Dating of Small-sized Foraminifer Samples: Insights into Marine sediment Mixing

Abstract

Radiocarbon ( 14 C) can be used to build absolute chronologies and reconstruct ocean ventilation over the last 40 ka. Sample size requirements have restricted 14 C measurements in marine cores with low foraminifer content, impeding 14 C-based studies focused on abrupt climate events. Recent developments have demonstrated that small-sized foraminifer samples can now be dated using a gas introduction system at the cost of a small decrease in precision. We explore the potential of gas measurements on benthic and planktonic foraminifers from core SU90-08 (43°03 0 1 00 N, 30°02 0 5 00 W, 3080 m). Gas measurements are accurate, reproducible within 2σ uncertainty and comparable to graphite measurements. Both techniques yield negative 14 C benthic-planktonic (B-P) age-offsets after Heinrich event 1. We argue that negative B-P ages result from bioturbation and changes in foraminifer abundances, with the chance of negative B-P especially increased when the 14 C age gradient between the deep and surface waters is decreased. Small-sized 14 C measurements seem to capture the variance of the foraminifera age distribution, revealing the active mixing in those archives. Sediment deposition and mixing effects possibly pose a greater obstacle for past 14 C-based dating and ocean ventilation reconstructions than the measurement precision itself, particularly in relatively low sedimentation rate settings.

Radiocarbon as a Dating Tool and Tracer in Paleoceanography

“…This serves to underline the potential difficulties that arise at low sedimentation rate sites. Setting aside poorly understood species‐specific effects (Magana et al., 2010), it is likely that where moderate to high sedimentation rates prevail, bioturbation of foraminifera (acting in conjunction with changes in their abundance) will exert the most significant impact on measured radiocarbon ages (Bard et al., 1987; Broecker et al., 1984; Missiaen et al., 2020). Furthermore, bioturbation will affect planktonic as well as benthic substrates, and will interact with particle abundance variations and particle preservation biases (Barker et al., 2007; Broecker, Andree, Bonani, Wolfli, Klas, et al., 1988) to create offsets between radiocarbon ages and sediment deposition age (and between radiocarbon ages for entities with different abundance histories and/or preservation potential; Bard et al., 1987; Broecker, Andree, Bonani, Wolfli, Klas, et al., 1988).…”

“…Auxiliary assessments of diagenetic overprinting or recrystallization may also be obtained from scanning electron microscope (SEM) imaging, for example to provide evidence of secondary phases/coatings or altered surface structures (Stott et al., 2009; Wycech et al., 2016). In this context, quantifying the effects of diagenesis or alteration can be a major challenge, even when age differences between visually distinct fossils or species (Ausín et al., 2019; Barker et al., 2007; Broecker, Andree, Bonani, Wolfli, Klas, et al., 1988; Wycech et al., 2016), or between leachates and bulk samples (Ausín et al., 2019; Missiaen et al., 2020), can be identified. Furthermore, any such impacts are very likely to be specific to a given site, sediment core, or even a given set of samples, depending on the conditions of sedimentary deposition, sediment core recovery, and sample treatment.…”

“…In general, it is fair to say that the effects of syndepositional contamination, habitat biases, bioturbation, and post‐depositional overprinting/preservation have been relatively under‐studied, largely due to the relatively large sample sizes needed for radiocarbon analyses, even with accelerator mass spectrometry methods. However, this is set to change with the advent of accurate gas‐source AMS methods capable of routinely measuring very small aliquots of carbon (e.g., single foraminifera; Fagault et al., 2019; Wacker et al., 2013), potentially in sequential or continuous fashion (e.g., on carbonate leachates, or by laser‐ablation; Ausín et al., 2019; Missiaen et al., 2020; Welte et al., 2016). Thus, age distributions for fossil entities (Dolman et al., 2021; Fagault et al., 2019), or age biases in sample leachates (Ausín et al., 2019; Missiaen et al., 2020), might be used to inform on the effects of bioturbation, population sampling, preservation or diagenesis.…”

Using sedimentological priors to improve 14C calibration of bioturbated sediment archives.

“…This age distribution is mainly governed by the sediment accumulation rate (SAR) and bioturbation depth (BD), the latter of which is typically around 10 cm (Trauth et al 1997;Boudreau 1998). The presence of the aforementioned age distribution is supported by studies of, e.g., particle mixing, stable isotopes, 14 C, species abundance and tephras (Bramlette and Bradley 1942;Nayudu 1964;Ruddiman and Glover 1972;Peng et al 1979;Hutson 1980;Pisias 1983;Schiffelbein 1984;Andree 1987;Bard et al 1987;Wheatcroft 1992;Trauth et al 1997;Henderiks et al 2002;Löwemark and Grootes 2004;Sepulcre et al 2017;Lougheed et al 2018;Abbott et al 2018;Missiaen et al 2020;Dolman et al 2020).…”

USING SEDIMENTOLOGICAL PRIORS TO IMPROVE 14C CALIBRATION OF BIOTURBATED SEDIMENT ARCHIVES

“…This age distribution is mainly governed by the sediment accumulation rate (SAR) and bioturbation depth (BD), the latter of which is typically around 10 cm (Trauth et al 1997;Boudreau 1998). The presence of the aforementioned age distribution is supported by studies of, e.g., particle mixing, stable isotopes, 14 C, species abundance and tephras (Bramlette and Bradley 1942;Nayudu 1964;Ruddiman and Glover 1972;Peng et al 1979;Hutson 1980;Pisias 1983;Schiffelbein 1984;Andree 1987;Bard et al 1987;Wheatcroft 1992;Trauth et al 1997;Henderiks et al 2002;Löwemark and Grootes 2004;Sepulcre et al 2017;Lougheed et al 2018;Abbott et al 2018;Missiaen et al 2020;Dolman et al 2021).…”