Deciphering paleoceanographic signals: Mg/Ca variability, calcite preservation, and morphometric trends in benthic foraminifera

The use of foraminiferal proxies in paleoceanographic research is critical for reconstructing past ocean conditions and understanding Earth's climate history. However, the reliability of these proxies is influenced by several factors, including complexities in geochemical signals, diagenetic alteration, and environmental conditions. This study addresses these complexities through the investigation of Mg/Ca ratios in benthic foraminifera, the development of a novel method for assessing calcite preservation, and the examination of morphological responses in benthic foraminifera to the Paleocene-Eocene Thermal Maximum (PETM). Laser Ablation Mass-Spectroscopy (LA-ICP-MS) based Mg/Ca measurements in Cibicidoides wuellerstorfi and Uvigerina peregrina reveal significant intra- and inter-test variability, challenging the assumption of proxy consistency across spatial and temporal scales. This variability underscores the difficulty in disentangling the influence of complex environmental factors on Mg/Ca ratios, indicating that temperature is not the sole determinant of Mg/Ca variability. To address the potential effects of diagenetic alteration on foraminiferal based geochemical proxies, a novel method using micro-CT is introduced to assess calcite preservation in foraminiferal tests. By quantifying diagenetic alteration and sediment infill at the individual test level, this technique enhances the reliability of geochemical datasets. The study further utilizes micro-CT to examine benthic foraminiferal responses to the PETM, analyzing changes in reproductive strategies and test morphology. Results indicate that shifts in reproductive modes and proloculus size correlate with environmental stressors such as oxygen depletion and reduced food availability. Unexpected increases in test volumes in some of the studied genera points to the complex interplay of conditions and stressors during the sudden changes of the PETM. Internal dissolution patterns observed in the Micro-CT scans also reflect the dynamics of the carbonate compensation depth (CCD) during the period. This work highlights the importance of accounting for heterogeneity, diagenetic alteration, and environmental influences in paleoceanographic reconstructions.