Biogeochemical dichotomy and intra-order variability in Miliolid and Rotaliid foraminifera
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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 representing the most widespread and ecologically dominant calcifying foraminiferal groups today. 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 element incorporation differences, known as the vital effect. This setting additionally enables assessment of within-group variability and the identification of biological and environmental elemental signatures characteristic of specific taxa. Elemental incorporation in tests of six co-occurring taxa was 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 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 differences for other elements (e.g., Zn, Cd, Fe). This dichotomy likely reflects fundamental differences in biomineralization pathways between the two orders, potentially including differences in calcifying-fluid regulation, Rayleigh-type enrichment, Mg exclusion, and element-specific transport processes. Within each order, element / Ca show distinct taxon-specific patterns: in some taxa, variability appears to be biologically controlled through biomineralization processes, while in others it appears to be partly environmentally driven, reflecting the chemical composition of the surrounding microhabitat.
