Abstract Fossil foraminifera store a wealth of information about past climates and ecosystems in their shell chemistry. However, these element and isotope signals may be affected by post‐mortem diagenesis (i.e., alteration). Here, we use Electron Backscatter Diffraction (EBSD; crystal orientation mapping) to evaluate the structural integrity of sub‐fossil (core‐top) Globorotalia inflata and truncatulinoides shells versus live‐caught (net‐tow) specimens from the Subantarctic. All shells have smaller, less crystalline grains in the inner wall compared to the outer wall, with more extreme inner‐ versus outer‐wall differences in crystallinity for core‐tops. In tow‐shells, larger outer‐wall grains are elongated, whereas in core‐tops, they vary from elongated to polygonal. Sectioning geometry contributes to more polygonal shapes in some maps, and the addition of gametogenic calcite (not yet present in tow‐shells) explains the thicker, double‐layer structure in core‐tops. Weaker diffraction from the inner/ontogenetic layer likely implicates partial dissolution. Crystal orientations in tow‐caught shells are strictly radial, with the c ‐axis normal to and following the curvature of the shell wall. The dominant misorientation (i.e., difference in orientation between neighboring grains) is 60° driven by extensive c ‐axis twinning. In core‐tops, crystal orientations are less strictly radial and sometimes exhibit fanning, which are possible indicators of recrystallization. In G. inflata , particularly those without a veneer, c ‐axis twinning is weak. We propose a combination of originally less‐twinned gametogenic calcite and greater diagenetic loss of twin boundaries than G. truncatulinoides . Variable preservation of inner versus outer layers and potentially different shells/chambers from the same sediments highlights the challenge of selecting appropriate targets for paleo‐reconstructions.
Smart et al. (2026) studied this question.