New screening method for well-preserved brachiopod shells and shell portions for paleoenvironmental analysis: Raman spectroscopy

Abstract Brachiopod shells have been widely used as paleoenvironmental recorders because of their robust preservation in fossil records. However, accurately reconstructing past seawater temperatures from their oxygen (δ18O) and clumped isotope compositions requires identifying shells/shell portions that are minimally affected by kinetic isotope effects (KIE) and diagenetic alteration. This study investigated the potential of Raman spectroscopy as a rapid and non-destructive screening method for shells/shell portions. We conducted Raman spectroscopy on fossil and modern brachiopod calcite shells from Japan and Thailand, focusing on the ν₁ peak center position (peak center) and full width at half maximum (FWHM). The results revealed that intraspecific variations in the peak center and FWHM were relatively small, with each species exhibiting a distinct peak center and FWHM range. The brachiopod shell portions with low Mg and Sr concentrations exhibited peak centers and FWHM values that were closer to those of pure calcite. Depth profile analyses further indicated that shell portions at a 60–90% relative depth from the outer surface were the least affected by KIE and crystal distortion. Shells altered by meteoric diagenesis exhibited significant shifts in the peak center and FWHM, with higher Mn concentrations being more intense, corroborating diagenetic alteration. These findings suggest that Raman spectroscopy is an effective screening tool for selecting well-preserved brachiopod shells/shell portions suitable for paleoenvironmental reconstructions. Integrating Raman spectroscopy with conventional screening methods to identify well-preserved shells/shell portions is expected to raise the reliability of δ18O- and clumped isotope-based paleotemperature reconstructions.