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

Abstract

The δ 18 O (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 ). δ 18 O (PO4) of six further Jurassic shark teeth was measured and is compared with δ 18 O 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 δ 18 O (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