Increase the magnification, they are shrinking! Morphometric variations of the planktic foraminifera Globigerina bulloides in response to environmental changes over the last 27.4 ka in the Ligurian Sea

This study investigates the test size variability of the planktic foraminifera Globigerina bulloides over the past 27.4 ka from sediment core NDT_22_2016, collected in the Ligurian Sea. This climatically sensitive region provides an ideal setting to explore how large-scale glacial-interglacial oscillations influenced the phenotypic plasticity of the non-symbiotic planktic foraminifer G. bulloides through changes in surface-water conditions. We explore the relationships between test size and sea surface temperature (SST), relative salinity (δ 18 Osw-ivc), and atmospheric CO₂. To test this, over 3400 specimens from 110 samples were analysed. Morphometric data were obtained through high-resolution imaging and automated area measurements, while environmental variables were derived from Mg/Ca-based SST reconstructions and stable isotope analyses, both performed on G. bulloides tests, and from published ice core CO₂ records. Descriptive statistics reveal a significant reduction in test size from the Pleistocene to the Holocene, with the largest specimens recorded during the Younger Dryas. Generalized linear mixed models (GLMMs) indicate that G. bulloides tests size responds significantly to environmental forcing: size decreases with increasing SST, consistent with Atkinson's temperature-size rule; increases (although less significantly) with increasing δ 18 Osw-ivc; and exhibits a non-linear (quadratic) relationship with CO₂. The occurrence of larger tests at lower temperatures and higher salinities could also be related to higher productivity in a paleoceanographic context that favoured water-column mixing and surface fertilization. To test the possible productivity-size relationship, we used the abundance of G. bulloides as a proxy for productivity, but no significant relationship was revealed by the GLMM. • Variation of Globigerina bulloides test size over the last 27.4 ka cal BP • Decreasing of the test size with the increase of sea surface temperature • Decrease in mean test size from the Pleistocene to the Holocene • Optimal shell size at intermediate CO₂ concentrations