Biogeochemical Dichotomy and Intra-Order Variability in Miliolid and Rotaliid Foraminifera

Abstract. Foraminiferal geochemical records reflect both environmental and biological influences. Disentangling these factors is essential for improving their application in marine monitoring and contributing valuable insights into the evolution across major foraminiferal lineages. Calcifying foraminifera evolved independently, with miliolids and rotaliids represent the most widespread and ecologically dominant calcifying foraminiferal groups. Most geochemical studies to date have focused on rotaliids, despite the importance of miliolids in ecological and environmental roles as prolific calcifiers. This study leverages the unique southeastern Mediterranean Israeli coastal waters, where dominant representatives of both groups co-occur in the same habitats, allowing for a direct comparison of bioincorporation differences, known as the vital effect. This setting also allowed for within-group variability and the identification of biological and environmental elemental signatures characteristic of specific taxa. Elemental incorporations in tests of six co-occurring taxa were analyzed: three rotaliids and three miliolids, from an oligotrophic Mediterranean marine reserve using whole-test ICP-MS analyses. Results reveal a clear geochemical dichotomy, with miliolids exhibiting consistently higher element/Ca ratios than rotaliids for nearly all measured elements, except Li, which shows the opposite trend. The contrast is strongest for rare earth elements (REEs) with order of magnitude differences (up to 45 times), and moderate but systematic for other elements (e.g., Zn, Cd, Fe). This dichotomy likely reflects fundamental differences in biomineralization pathways between the two orders. Within each order, element/Ca ratios show distinct patterns: in some taxa, variability appears to be biologically controlled through biomineralization processes, while in others it seems environmentally driven, reflecting the chemical composition of the surrounding water.