Reconstructing the OMZ in the east Pacific during the Pliocene using G. hexagonus

Ocean oxygen concentration has been reported to decline over the past few decades due to the ongoing climate crisis. The development of the Oxygen Minimum Zone (OMZ) during the Pliocene (5.3–2.6 Ma) offers a valuable analog for understanding ocean oxygenation under modern climate change, as the Pliocene shares similar climatic conditions with those in the present. However, the mechanisms controlling OMZ development during this period are not fully understood, highlighting the need for suitable proxies to provide further insights. Recent studies have proposed the planktic foraminifer Globorotaloides hexagonus as a potential direct indicator of OMZ variability, assisting investigations into oxygen-depleted environments. While previous research suggests that modern G. hexagonus responds to change in water oxygen concentration through variations in shell porosity, its application in paleo-reconstructions remains unexplored. Thus, the biogeochemical relationship between G. hexagonus and OMZs requires additional investigation. In this study, we quantified the abundance of G. hexagonus at Ocean Drilling Program (ODP) Site 1241 in the East Equatorial Pacific to investigate its relationship with oxygen concentrations during glacial-interglacial cycles in the Pliocene. Our results indicate significant variability in G. hexagonus abundance, suggesting changes in OMZ. The results initially focus on Marine Isotope Stages (MIS) 96-100 (~2.55–2.4 Ma), where a correlation appears between the trend in G. hexagonus abundance and precession variability cycles (insolation). In contrast, no significant variability patterns are observed around MIS M2 (~3.3 Ma). We then selected four samples characterized by high and low G. hexagonus abundance and captured their images using scanning electron microscopy (SEM). A deep learning algorithm, initially trained for pore morphometry in benthic foraminifera, was retrained specifically to analyze G. hexagonus SEM images, enabling efficient obtaining of pore parameters (porosity, pore size, and pore density). Significant differences in porosity between high- and low-abundance groups suggest that increased porosity is associated with stronger OMZ, consistent with the abundance counts. Statistical analysis indicates that porosity variations are primarily driven by changes in pore size rather than pore density. Although the current findings cover only the ~2.55–2.4 Ma interval, they provide robust evidence for oxygenation-related adaptations in G. hexagonus abundance and pore morphology. Furthermore, extending the G. hexagonus abundance study from 3.3-2.4 Ma and including Mn/Ca analyses on the tests of G. hexagonus will provide more robust evidence on the controlling forces behind the variability in OMZ intensity during the Pliocene.