Unmixing of stable isotope signals using single specimen δ¹⁸O analyses

Abstract

[1] The resolution at which foraminiferal stable isotopes are applied in paleo-environmental studies is ever increasing, resulting in continuous sampling of sediment cores. The resolution of such continuously sampled records depends on the rate of sedimentation of foraminiferal shells in its relation to the intensity of bioturbation. Bioturbation essentially mixes sediment layers of different age, altering the primary climate signal, thereby impacting the accuracy of both the timing and magnitude of reconstructed climate changes. A new approach to assess and correct the impact of bioturbation is investigated here, based on the d 18 O of individual specimens of planktonic foraminifera Globorotalia inflata from a series of boxcore samples in the Eastern North Atlantic. Average d 18 O values decrease southward from 1.62 to 1.07% with the exception of site T86-11 (1.35%). The d 18 O distribution of each station can be fitted with a uni-to polymodal distribution. A nonunimodal distribution strongly suggests admixing of bioturbated individuals. Quantification of these distributions allows deconvolving the original and bioturbated signals and subsequently provides a correction for bioturbation.

Paleoclimate and paleoceanography over the past 20,000 yr in the Mediterranean Sea Basins as indicated by sediment elemental proxies

“…The resolutions provided are at the millennial to centennial time scale, or even decadal in certain regions of high-sedimentation rates. In order to unravel the highly diverse and interactive properties recorded in marine sediments, multiple geochemical methods have been used to develop proxies for paleoenvironmental studies, i.e., molecular biomarkers (e.g., Eglinton and Eglinton, 2008;Kim et al, 2010;Rampen et al, 2012Rampen et al, , 2014Rontani et al, 2013;Schouten et al, 2013;Weijers et al, 2014), stable isotopes (e.g., Pearson, 2012;Wit et al, 2013;Henkes et al, 2014;Levin et al, 2014) radiogenic isotopes (e.g., Frank, 2002;Bayon et al, 2009;Garçon et al, 2014), microfossil assemblages (e.g., Cl eroux et al, 2013;Evans et al, 2013 and references therein), sediment grain size (e.g., McCave and Hall, 2006;Garzanti et al, 2009;deGelleke et al, 2013), and elemental ratios (e.g., Tribovillard et al, 2006;Calvert and Pedersen, 2007).…”

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.…”

“…It is worth noting that the spread in foraminiferal values is generally considered to be an underrepresentation of the total seasonal signal, either because foraminifera have a monthly life cycle (Hemleben et al, 1989;Spindler et al, 1979); the flux of all or a particular species of foraminifera is limited to a particular season (Mix, 1987;Roche et al, 2018); or the sedimentology of the core is unrepresentative (i.e., a slow sedimentation accumulation rate). One benefit of such analysis is that the individual specimen's δ 18 O can be considered by the researcher prior to the calculation of a mean δ 18 O; anomalous values can be screened via outlier analysis (Ganssen et al, 2011) or through "unmixing" of the sample into discrete subsamples representing specimens from unique populations (Wit et al, 2013) so that a more robust downcore species-specific δ 18 O can be produced. In combination with other analytical techniques, such as photographing and weighing of shells (e.g., Altuna et al, 2018;Pracht et al, 2018) prior to analysis or removing a portion of the shell for radiocarbon analysis, taxonomic issues (Pracht et al, 2018) and benthic processes (Lougheed et al, 2018) can be nullified or subsequently corrected.…”

SEAMUS (v1.20): a Δ<sup>14</sup>C-enabled, single-specimen sediment accumulation simulator

“…Moreover, machine analysis of multi-specimen samples will only report the mean value and machine error, thus hiding the true distribution of values within the sample. Advances in mass spectrometry eventually allowed the analysis of single specimens (Killingley et al, 1981) and, since single specimens capture a single year or season of the climate signal, researchers can in principle study the full distribution of isotope or trace element values obtained within various discrete depths of sediment cores, thereby making inferences regarding variability in climate, habitat or specimen morphology for various specific time periods during the Earth's history (Spero and Williams, 1990;Tang and Stott, 1993;Billups and Spero, 1996;Ganssen et al, 2011;Wit et al, 2013;Ford et al, 2015;Metcalfe et al, 2015;Ford and Ravelo, 2019;Metcalfe et al, 2019b). However, the accuracy with which the aforementioned studies can quantify time-specific variation for a particular climate period, habitat or morphological variable is strongly dependent upon the constraint of the age range of the specimens contained within a given discretedepth interval.…”

“…This bias is an interesting finding, seeing as it has long been assumed that pooled specimen samples used for dating (e.g. 14 C dating) should be retrieved from abundance peaks (Keigwin and Lehman, 1994;Waelbroeck et al, 2001;Galbraith et al, 2015). This assumption is largely based on the fact that 14 C dates sampled from abundance peaks are younger than the immediately surrounding sediment (Rafter et al, 2018).…”