Investigating crystal orientation patterns of foraminiferal tests by electron backscatter diffraction analysis

Abstract

Abstract. We studied the crystallographic orientation of calcite crystals in benthic foraminifers by electron backscatter diffraction (EBSD). Individuals of two species, Gyroidinoides soldanii and Cibicidoides grimsdalei, featuring different test structures, were investigated for a time span covering 43 Myr. The aims of this study are to visualize test structure differences in foraminifers and to reveal potential changes in crystal orientation and grain size over time caused by diagenetic reactions such as recrystallization. Such recrystallization effects over time may aid in the interpretation of time-resolved geochemical data obtained on foraminiferal samples for paleo-environmental reconstructions. The EBSD patterns clearly resolve the different test structures of the two species. Cibicidoides grimsdalei has the c axes perpendicular to the test surface. An apparent shift in the preferred crystal orientation can most likely be attributed to a mismatch between the equatorial plane and cutting plane of the foraminiferal test, highlighting the importance of reproducible preparation techniques. In Gyroidinoides soldanii, the c axes of the calcite crystals show a broader distribution of the crystals with no preferred orientation. The specimens show no change in crystal sizes over time, with a frequency maximum corresponding to the spot size of the electron beam. Overall, the differences between the two species demonstrate that EBSD is a powerful tool to visualize and differentiate between foraminiferal test structures.