Calcareous Benthic Foraminifera Under Ocean Alkalinity Enhancement: Synthesis, Knowledge Gaps, and Future Research Agenda

Abstract

Limiting global warming to 1.5°C will require carbon dioxide removal, and Ocean Alkalinity Enhancement (OAE) is increasingly being explored as a marine carbon dioxide removal pathway. However, benthic biological responses remain much less constrained than water-column processes and planktonic responses, even though calcareous benthic foraminifera play important roles in sediment–water interface biogeochemistry and coastal carbonate budgets. This review synthesizes current understanding of benthic foraminiferal biomineralization, experimental evidence relevant to elevated alkalinity, and analytical approaches for detecting OAE-driven changes in test structure and chemistry. Available evidence indicates that calcification in benthic foraminifera is a tightly regulated, energy-dependent process controlled by the interaction between inorganic-carbon supply, biological pH regulation, and external carbonate chemistry. While inverse analogs such as ocean acidification studies provide mechanistic clues regarding species-specific sensitivities, direct experimental evidence of benthic foraminiferal responses to true OAE conditions remains a critical knowledge gap. We propose an analytical framework that integrates morphometric measurements, X-ray diffraction, Raman spectroscopy, SEM/micro-CT imaging, and calibrated geochemical proxies such as δ¹¹B, U/Ca, and selected element ratios to distinguish primary biomineralization responses from redox, nutrient, and diagenetic overprints. We propose a four-tier framework for geochemical proxies that links primary carbonate-system indicators with feedstock-related tracers and quality-control metrics for interpreting OAE-driven perturbations. The framework proposed here enables cross-calibrated workflows for laboratory, mesocosm, and field settings, supporting evaluation of benthic responses, refining environmental assessment, and identifying where foraminiferal observations may inform OAE efficacy and benthic feedbacks on alkalinity retention.