Stable Oxygen Isotope Composition Is Biased by Shell Calcification Intensity in Planktonic Foraminifera

Abstract

Planktonic Foraminifera are widely used for environmental reconstructions through measurements of their shell's geochemical characteristics, including its stable oxygen and carbon isotope composition. Using these parameters as unbiased proxies requires a firm knowledge of all potential confounding factors influencing foraminiferal shell geochemistry. One such parameter is the shell calcification intensity (shell weight normalized for shell size) that may influence the shell δ 18 O value either bioenergetically (by reducing energy available and required for equilibrium isotope fractionation during faster calcification) or kinetically (by influencing calcification depth through the shell's density contrast with seawater). Specimens from the Globigerinoides ruber/elongatus compound from a sediment trap in the North Atlantic have been used to quantify the influence of shell calcification intensity on shell δ 18 O values. Shell calcification intensity was found to have a significant effect on the shell stable oxygen isotope composition in all species. Through model fitting, it is suggested that the effect size may be in a range of 1 to 2 ‰ (depending on species, depth migration, and local oceanographic conditions). We show that the confounding effect of shell calcification intensity on stable oxygen isotope composition can be of importance, depending on the anticipated precision of the derived reconstructions. A framework is provided to quantify this effect in future studies.

Searching for Function: Reconstructing Adaptive Niche Changes Using Geochemical and Morphological Data in Planktonic Foraminifera

“…Furthermore, geochemical methods are advancing at an astounding rate making ecological inferences such as life history, metabolic rate, gene flow and geolocations accessible in deep time (Trueman et al, 2016). In planktonic foraminifera these advances have allowed geochemical measurements to be taken at the level of the individual, enhancing our knowledge of paleoclimate (Thirumalai et al, 2013;Schmitt et al, 2019;Glaubke et al, 2021) and the impact of individual planktonic foraminifera ecology on geochemical signatures (Eggins et al, 2003(Eggins et al, , 2004Friedrich et al, 2012;Fehrenbacher et al, 2018;Groeneveld et al, 2019;Weinkauf et al, 2020). We leverage these advances in analytical techniques alongside the exemplary evolutionary record of planktonic foraminifera to investigate trait, organismal and community responses to climatic change on geological timescales.…”