Morphological insights from benthic foraminifera for environmental conditions in the Baltic Sea during the Last Interglacial

Marine environments worldwide are increasingly threatened by warming, deoxygenation, and ocean acidification. These stressors can be recorded as alterations in foraminiferal geochemistry and morphology. We integrated morphological features with assemblage and geochemical records of benthic foraminifera from the Danish Straits at the entrance to the Baltic Sea, covering the Last Interglacial period (LIG, MIS5e), to assess potential indications of environmental changes. Using synchrotron radiation X‐ray microcomputed tomography (SRμCT), we measured Elphidium clavatum in terms of diameter, height, surface area, volume, thickness, and number of pores. Pore patterns and wall thickness were evaluated simultaneously to assess both metabolic and mechanical constraints under environmental changes. The proloculus (initial
chamber) size may indicate the reproduction mode of foraminifera in response to environmental stress, including salinity and oxygen variations. We show that during the early‐mid LIG, the physical resistance of E. clavatum against mechanical constraints remained relatively strong, characterized by high wall thickness and low porosity, coinciding with higher bottom water salinity and oxygen content. In the mid‐late LIG, pore density and wall thickness decreased, while porosity and proloculus size increased, fueling metabolism and increasing survival rates. These traits reflect adaptation to an increasingly stressful environment with lower salinity and oxygen levels, as indicated by declining faunal diversity and increasing E. clavatum abundance. Our study demonstrates that benthic foraminiferal morphological features, including proloculus size, wall thickness, and pore patterns, serve as indicators for assessing stress levels and reconstructing bottom water conditions in brackish and potentially hypoxic environments during the LIG.