Identification of diagenetic overprint and recrystallization of foraminiferal shell calcite by Electron Backscattered Diffraction (EBSD)

Abstract. Electron backscatter diffraction (EBSD) measurements and data evaluation render the mode of crystal organization in the scanned sample volume, many crystallographic attributes of the comprising crystals and structural information of the entire hard tissue. Knowledge is gained on crystal microstructure, crystal texture, degree of misorientation and/or co-orientation between crystals, crystal-twin-formation, among others. As foraminiferal Ca-carbonate records chemical and physical parameters of the ambient environment, the shells serve as archives for environmental and climate change. However, the recorded information is often biased due to the alteration of shell Ca-carbonate by dissolution, precipitation, recrystallization, and the transformation of the biocrystals into their inorganic analogues.

We demonstrate that the diagenetic overprint of foraminiferal Ca-carbonate can be identified using structural characteristics measured with EBSD and information obtained from the corresponding data evaluation. We investigated modern/pristine and fossil Trilobatus sacculifer shells and highlighted an undisturbed shell surface for both. Nevertheless, we demonstrate that there is an increase in recrystallized calcite in the shells and a decrease in pristine, twinned calcite with increasing fossilization. We show that crystal microstructure and texture, and the frequency of the 60°-{001}-twin-misorientation and other misorientations are modified with an increasing degree of overprint. We propose the use of structural attributes provided by EBSD to identify recrystallized/overprinted foraminiferal carbonate. Highlighting low degrees of overprint is of main importance as, in contrast to strong overprint, small structural changes of overprinted shells are easily overlooked with SEM-imaging, while even a minute change of shell microstructure and texture is easily identified with EBSD measurements.