Planktic foraminifera record the succession of anaerobic metabolisms in particle microenvironments across a pelagic oxygen gradient

Abstract

Microenvironments inside marine organic particles can host anaerobic microbial respiration outside of ocean anoxic zones, but these environments are challenging to directly observe. We present evidence that the planktic foraminifer Globorotaloides hexagonus inhabits a particle microenvironment and suggest that their shell chemistry records anaerobic microbial metabolisms inside particles. We propose a novel interpretation of intrashell trends in Ba/Ca, Mn/Ca, and Zn/Ca ratios as signals of denitrification, Mn respiration, and sulfate reduction. We measure these trace elements in G. hexagonus collected from discrete depth horizons across a pelagic oxygen gradient in the Eastern Tropical North Pacific. Using this intrashell trace element framework, we find that denitrification may have occurred inside particles throughout the water column, and that Mn respiration and sulfate reduction may have occurred inside particles throughout the oxygen minimum zone. Our results have implications for budgets of nitrogen, sulfur, manganese, and other trace elements in regions with expanding oxygen minimum zones and suggest a new method of interpreting intrashell trends in trace element-to-calcium ratios in planktic foraminifera.