Estimating Bioturbation From Replicated Small‐Sample Radiocarbon Ages

Abstract

Age-heterogeneity within sediment layers adds hidden uncertainty to radiocarbonbased age estimates.• The amount of age-heterogeneity depends on the sedimentation rate and bioturbation mixing depth.• We present a method to estimate 14 C age-heterogeneity and lookup figure to estimate age uncertainty.

Radiocarbon as a Dating Tool and Tracer in Paleoceanography

“…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. 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).…”

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

“…Nevertheless, caution should be taken with the work by Ausin et al (2021), who obtain low MRA and benthic 14 C values in this Iberian Margin core, notably during Heinrich stadial 1 (HS1) and the Last Glacial Maximum (LGM), in stark contrast with records obtained on nearby cores (Skinner et al, 2014(Skinner et al, , 2021. The MRA drop (down to 300 years) during HS1 based on PT is also in conflict with modeling results (Delaygue et al, 2003;Ritz et al, 2008;Franke et al, 2008;Butzin et al, 2017). Although Sarnthein and Grootes (2021) present the new paper by Ausin et al (2021) as a "nice test case" of PT, the strong disagreement with the literature is not reassuring.…”

“…4b by Goslar et al, 1995, or Fig. 5 by Hughen et al, 1998; see also simulations performed with more complex models by Marchal et al, 2001;Delaygue et al, 2003;Ritz et al, 2008;Singarayer et al, 2008). Consequently, the assumption which is required to underpin PT, i.e., that the radiocarbon calibration curve has the shape of a staircase, is in conflict with our basic understanding of 14 C as a tracer.…”

“…The study of atmosphere and surface ocean 14 C wiggles linked to spatially variable ocean-atmosphere exchange is inherently more complex as it requires spatial resolution with 2D and 3D models and consideration of regional 14 C data to identify active and passive regions. However, regional gradient changes of surface 14 C simulated by models are generally on the order of a few centuries (Stocker and Wright, 1998;Delaygue et al, 2003;Butzin et al, 2005;Franke et al, 2008;Singarayer et al, 2008;Ritz et al, 2008), rather than the millennia advocated by Sarnthein et al (2020).…”

“…Indeed, this shard profile is the impulse response function of the bioturbation filter and hence provides information that can be used to correct 14 C ages used for the MRA calculation (Bard et al, 1994). Another route to detect and constrain bioturbation is to study the total scatter of 14 C ages measured on single specimens of foraminifera and to compare it with the smaller dispersion linked to analytical errors only (Fagault et al, 2019;Dolman et al, 2021).…”