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.
Highlights
Foraminifers belong to the most frequently applied paleoenvironmental archives (e.g. Katz et al, 2010)
We studied the crystallographic orientation of calcite crystals in benthic foraminifers by electron backscatter diffraction (EBSD)
We focused on benthic foraminifers because their relatively slow evolution and comparatively strong resistivity to dissolution and diagenesis make them suited for long-term paleo-environmental reconstructions (e.g. Edgar et al, 2013; Drury et al, 2014; Voigt et al, 2016)
Summary
Foraminifers belong to the most frequently applied paleoenvironmental archives (e.g. Katz et al, 2010). More than 50 000 species have been described in marine sediments, ∼ 40 000 fossil and ∼ 10 000 recent (Adl et al, 2007; Pawlowski et al, 2014). Besides the classification according to the preferred habitat, the taxon foraminifera is divided into three main types based on the material and structure of the test (Armstrong and Brasier, 2005). There are foraminifers with a secreted test wall, composed of an organic chitinous material, second, tests formed from agglutinated and cemented sedimentary particles, and third, tests composed of secreted calcareous or siliceous minerals with the calcareous types (aragonite and calcite) predominating (Armstrong and Brasier, 2005; Rönnfeld, 2008). Aragonitic foraminifers play only a subordinate role for paleo-environmental reconstructions, because aragonite-forming species are less common, and aragonite is less resistant against dissolution and recrystal-
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