Evaluation of shark tooth diagenesis-screening methods and the application of their stable oxygen isotope data for palaeoenvironmental reconstructions

Abstract

The δ18O(PO4) of shark teeth can be used as a geochemical proxy for the palaeoenvironment during tooth mineralization. A well-preserved state of the teeth is necessary for this approach. We used polarization, cathodoluminescence (CL) and scanning electron microscopy to investigate two fossil shark teeth (Sphenodus nitidus) from the Late Jurassic Nusplingen Plattenkalk. The data are compared with those for two recent shark teeth (Hemipristis elongata, Lamiopsis temminckii). δ18O(PO4) of six further Jurassic shark teeth was measured and is compared with δ18O data from co-occurring belemnite rostra and carbonate sediments. The enameloid of both the fossil and the recent shark teeth shows orange–red CL, which is due to incorporation of Mn during biomineralization. The δ18O(PO4) values of the fossil enameloid are constant around 21.50‰ V-SMOW whereas those of the fossil dentine are more negative with values around 20‰ V-SMOW, which indicates that the enameloid is well preserved. Palaeotemperatures calculated from the enameloid are like those derived from belemnites. We demonstrate that CL microscopy should only be used cautiously as a diagenesis-screening method for fossil biophosphates. Geochemical and microstructural analyses of tissue types with different susceptibility to diagenetic alteration on the other hand provide an effective way to identify well-preserved tissues. Supplementary material: Polarization microscopy images, energy-dispersive X-ray data and cathodoluminescence spectra are available at https://doi.org/10.6084/m9.figshare.c.4373720