On the tuning of plateaus in atmospheric and oceanic <sup>14</sup>C records to derive calendar chronologies of deep-sea cores and records of <sup>14</sup>C marine reservoir age changes

Abstract

Abstract. We assess the methodology of the so-called 14C plateau tuning (PT) technique used to date marine sediment records and determine 14C marine reservoir ages (MRAs) as recently reviewed by Sarnthein et al. (2020). The main identified problems are linked to the assumption of constant MRA during 14C age plateaus; the lack of consideration of foraminifera abundance changes coupled to bioturbation that can create spurious plateaus in marine sediments; the assumption that plateaus have the same shapes and durations in atmospheric and oceanic records; the implication that atmospheric 14C / 12C peaked instantaneously from one plateau to the next; that the 14C plateaus represent 82 % of the total time spent between 14 000 and 29 000 cal yr BP, whereas during the remaining 18 % of the time, the radiocarbon clock was running almost 5 times faster than the radioactive decay; that the sparsity, combined with the level of analytical uncertainties and additional noise, in both atmospheric and marine data do not currently allow one to reliably or robustly identify plateaus (should they exist) beyond 15 000 cal yr BP; and that the determination and identification of plateaus in the deep-sea cores is reliant upon significant changes in sedimentation rate within those marine sediments which are, a priori, unknown and are not verified with an independent method. The concerns we raise are supported and strengthened with carbon cycle box model experiments and statistical simulations of pseudo-atmospheric and pseudo-marine records, allowing us to question the ability to identify and tune 14C age plateaus in the context of noisy and sparse data.

Estimating Bioturbation From Replicated Small‐Sample Radiocarbon Ages

“…For each sample, the probability density function (PDF) for calendar age was summarized by its median and standard deviation, as none of the PDFs were bi‐ or multi‐modal. Multi‐modality may be more of a concern when using the Intcal20 calibration curve for terrestrial samples, as smoothing of the cosmogenic radiocarbon signal in the atmosphere is much less than that in the ocean (Bard & Heaton, 2021).…”

Radiocarbon as a Dating Tool and Tracer in Paleoceanography

“…However, it is particularly prone to the nexus of foraminifer abundance variability and bioturbation (Costa et al., 2017), due to its dependence on fluctuations in the observed radiocarbon age‐depth relationship (e.g., Sarnthein et al., 2013). Furthermore, its accuracy and universal applicability have been questioned on theoretical grounds (Bard & Heaton, 2021). For example, the inference of surface reservoir age offsets >2 kyr (Sarnthein et al., 2020) is inconsistent with plateau tuning's premise of perfect equilibrium and synchrony between the atmosphere and the surface ocean habitat of planktonic foraminifera, while the sparsity and scatter in atmospheric and marine data can be shown to lead to ambiguous matching of plateaus, and therefore equivocal marine chronologies and reservoir age offsets (Bard & Heaton, 2021).…”

“…Furthermore, its accuracy and universal applicability have been questioned on theoretical grounds (Bard & Heaton, 2021). For example, the inference of surface reservoir age offsets >2 kyr (Sarnthein et al., 2020) is inconsistent with plateau tuning's premise of perfect equilibrium and synchrony between the atmosphere and the surface ocean habitat of planktonic foraminifera, while the sparsity and scatter in atmospheric and marine data can be shown to lead to ambiguous matching of plateaus, and therefore equivocal marine chronologies and reservoir age offsets (Bard & Heaton, 2021). Regardless of the proposed (de)merits of the plateau tuning approach, the conflicting results of Ausín et al.…”

A RESPONSE TO COMMUNITY QUESTIONS ON THE MARINE20 RADIOCARBON AGE CALIBRATION CURVE: MARINE RESERVOIR AGES AND THE CALIBRATION OF 14C SAMPLES FROM THE OCEANS

“…The ocean and the atmosphere respond differently to variation in 14 C production rates. Changes in atmospheric 14 C levels typically take time to be transferred to the surface ocean and are also significantly damped (Druffel and Suess 1983;Grottoli and Eakin 2007;Komugabe-Dixson et al 2016;Bard and Heaton 2021). This is a consequence of mixing between the surface layer of the ocean and the extremely large carbon reservoir in deeper ocean layers (in total, during late-Holocene/pre-industrial times, the ocean contained ∼ 60 times more carbon than the atmosphere), and the slow rate of air-sea CO 2 equilibration (Grottoli and Eakin 2007;Skinner and Bard 2022).…”