Investigating I/Ca ratio in planktic foraminifera: an integrated approach in the Mediterranean Sea and beyond

Global ocean oxygen depletion is a growing concern, with direct observations showing widespread decline that is not fully captured by existing models. The dynamics of oxygenation are critical to marine ecosystem health and are influenced by physical and biogeochemical factors. To reconstruct past variation of dissolved oxygen content, foraminiferal iodine to calcium (I/Ca) ratio has been developed. This proxy is based on the fact that iodine speciation is dependent on oxygen content and that only iodate ions (IO3-) can be incorporated into the calcite lattice by substitution for carbonate ions. However, our knowledge of foraminiferal I/Ca behavior needs improvement because the relationship between planktonic foraminiferal I/Ca and oxygen content in the upper 500m water column appears to be empirical and the influence of physical and biogeochemical parameters has not been fully addressed yet, and thus prohibits the development of a fully quantitative proxy.

In this study, we present the first planktic foraminiferal I/Ca data from core-top material (late Holocene) in the Mediterranean Sea and a compilation of previously published core-top and plankton tow I/Ca data from the global ocean. We use this database to examine the influence of the maximum and minimum oxygen concentration, temperature, salinity, nutrients, pH, alkalinity, chlorophyll, water velocities and mixed layer depth on the I/Ca proxy. Additionally, we assess the effects of water depth and sample age to monitor possible iodine accumulation during the foraminiferal test settling and early diagenetic effects. We also investigate the potential for differential behaviour between foraminiferal species by categorising by the presence/absence of symbionts.

We find unexpectedly low I/Ca as low as 1 µmol/mol in samples from the western basin of the Mediterranean Sea, providing the first low I/Ca from fossil foraminiferal which lived in waters thought to be highly oxygenated. Principal Component Analysis (PCA) confirms the dominance of oxygen content on foraminiferal I/Ca, however, analysis of the foraminiferal I/Ca residuals after subtraction of oxygen dependence suggest a potential role of temperature. No clear trend is not observed between the residuals and sample age, suggesting a negligible influence of burial diagenesis. Iodine accumulation during the settling of foraminiferal tests has been proposed to explain the high I/Ca of fossil samples when compared to plankton tows, but no trends are visible between our residuals and water depth, implying that there is no systematic effect from post-mortem settling. The cause of low foraminiferal I/Ca in oxygenated waters remains unclear and may involve poorly constrained local oxygen variability, the chemical dynamics of iodine, iodine incorporation mechanisms, or other unknown parameters.