REE + Y signatures and enamel microstructure constraining early burial diagenesis in Pleistocene Bison from northeastern Mexico: Palaeoenvironmental implications

Dental enamel is regarded as a robust archive for geochemical and palaeoecological studies; however, evaluation of diagenetic alteration is essential prior to palaeoenvironmental interpretation. This study assesses the microstructural and geochemical preservation of dental enamel from four fragmentary Pleistocene molariform teeth attributed to Bison sp., recovered from Quaternary deposits at Ejido San Lázaro, Villagrán, Tamaulipas, north-eastern Mexico. An analytical approach combining optical microscopy, SEM-EDS and LA-ICP-MS was applied to evaluate enamel integrity, major-element stoichiometry and rare earth element plus yttrium (REE+Y) systematics. Microscopic observations reveal a well-preserved hydroxylapatite microstructure, with densely packed crystallites and no evidence of significant recrystallisation. Major-element compositions (Ca, P and F) display near-stoichiometric Ca/P ratios (~1.67) and low F/P values (<0.1), indicating limited fluorapatite formation and mineralogical alteration. LA-ICP-MS analyses yielded low total REE concentrations (ΣREE = 50–137 ppm; ΣREE+Y up to 168 ppm) and flat PAAS-normalised patterns, consistent with restricted post-mortem incorporation of trace elements. Systematic negative Ce anomalies, accompanied by positive La/La* anomalies, indicate early burial under oxidising conditions and interaction with LREE-enriched pore fluids or sediments. Given the regional geological framework, contribution from the surrounding and underlying Cretaceous carbonate and calcareous–argillaceous succession is possible, although Y/Ho ratios are complementary rather than diagnostic evidence for marine influence. The La enrichment is interpreted cautiously as a local early diagenetic signal related to sediment–fluid interaction, pH-dependent REE mobility and sorption/desorption processes, rather than as evidence of a uniquely marine-derived fluid source. Overall, the results indicate a high degree of preservation in the analysed enamel specimens and show that their palaeoenvironmental value depends on recognising limited diagenetic overprinting. By combining microstructural, major-element and REE+Y evidence, this study provides a framework for distinguishing preserved early burial geochemical signals from later alteration, supporting the cautious use of Bison enamel specimens as reliable archives for palaeoenvironmental studies in north-eastern Mexico.