Moving beyond the age–depth model paradigm in deep-sea palaeoclimate archives: dual radiocarbon and stable isotope analysis on single foraminifera

Abstract

Abstract. Late-glacial palaeoclimate reconstructions from deep-sea sediment archives provide valuable insight into past rapid changes in ocean chemistry. Unfortunately, only a small proportion of the ocean floor with sufficiently high sediment accumulation rate (SAR) is suitable for such reconstructions using the long-standing age–depth model approach. We employ ultra-small radiocarbon (14C) dating on single microscopic foraminifera to demonstrate that the long-standing age–depth model method conceals large age uncertainties caused by post-depositional sediment mixing, meaning that existing studies may underestimate total geochronological error. We find that the age–depth distribution of our 14C-dated single foraminifera is in good agreement with existing bioturbation models only after one takes the possibility of Zoophycos burrowing into account. To overcome the problems associated with the age–depth paradigm, we use the first ever dual 14C and stable isotope (δ18O and δ13C) analysis on single microscopic foraminifera to produce a palaeoclimate time series independent of the age–depth paradigm. This new state of the art essentially decouples single foraminifera from the age–depth paradigm to provide multiple floating, temporal snapshots of ocean chemistry, thus allowing for the successful extraction of temporally accurate palaeoclimate data from low-SAR deep-sea archives. This new method can address large geographical gaps in late-glacial benthic palaeoceanographic reconstructions by opening up vast areas of previously disregarded, low-SAR deep-sea archives to research, which will lead to an improved understanding of the global interaction between oceans and climate.

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

“…This can be mostly attributed to the fact that Holocene stable isotopes have similar values throughout the Holocene period, minimizing the effect of post depositional mixing (bioturbation). Additionally, as proposed by Lougheed et al (2018), this can be attributed to the nonlinear relationship between 14 C activity and 14 C age. In fact, the addition of a relatively small amount of 14 C young material significantly shifts the final 14 C age toward a younger value compared to the actual average 14 C age of the sample.…”

“…Interestingly, graphite pairs of 14 C B-P ages correspond to the average of the replicated gas 14 C B-P ages except for one depth (35 cm, 10.1 ka) where the graphite pair 14 C B-P age is completely offset compared to the gas measurements. This can be related to the sample size requirement of each measurement: because the graphite analysis requires more material, the sampled individuals (n >100 shells) are more likely to statistically represent the average of the analyzed level than the gas samples (n < 50 shells) (Lougheed et al 2018). We observe that the screened 14 C B-P ages record obtained by discarding potentially altered measurements and averaging the replicated 14 C B-P ages for each level, contain less negative 14 C B-P ages ( Figure 5B).…”

“…The possibility of analyzing ultra-small foraminiferal samples (<0.5 mg CaCO 3 ) (Lougheed et al 2012;Wacker et al 2013a) and even single foraminifer (<0.1 mg CaCO 3 ) (Wacker et al 2013b;Lougheed et al 2018) has been demonstrated but for age determination sample mass should ideally be larger than 40 μg C (~0.3 mg CaCO 3 ) and should contain a sufficient number of individuals to reliably represent the sediment level analyzed. Gottschalk et al (2018) showed that gas 14 C analysis of small-sized (~0.3-0.6 mg CaCO 3 ) foraminifera samples give accurate results with only a slight precision decrease (a few tens of years on 14 C scale) compared to graphite target AMS measurements.…”

Sedproxy: a forward model for sediment-archived climate proxies

“…While the assumption of complete mixing with a sharp cutoff is unlikely to be true, the general effects of bioturbation should also apply under conditions of incomplete mixing and the code could be modified to use a more complex bioturbation model (e.g., Guinasso and Schink, 1975;Steiner et al, 2016). However, when sedimentation rates are low relative to mixing rates, more complex mixing models converge to the simple box-type model that is employed here (Matisoff, 1982). Sedproxy further assumes a constant bioturbation depth over time, as the bioturbation depth is generally not known for each setting and cannot easily be reconstructed down-core.…”

Oxygen Isotope Variability of Planktonic Foraminifera Provide Clues to Past Upper Ocean Seasonal Variability

“…Furthermore, potential disequilibrium processes during calcification, commonly referred to as the vital effect and postmortem effects (i.e., dissolution or bioturbation), serve to further alter the signal premortem or postmortem. In section 4.3, we elaborate upon the potential influence of bioturbation, although it is worth noting here that previous work does suggest that the influence of bioturbation upon this section of T90-9P is not as extreme as in other cases (Lougheed et al, 2018;Wit et al, 2013), although appearances can be deceiving. This leaves either vital effect or the influence of salinity.…”