Diagenetic processes in quaternary vertebrate fossils from coastal environments

Abstract The fossilization of Quaternary mammals in coastal environments of southern Brazil results from the interaction between taphonomic and diagenetic processes, strongly controlled by hydrological dynamics, interstitial fluid chemistry, and sedimentary reworking. Through the integration of macro- and microscopic analyses with SEM, XRD, and Raman spectroscopy, two main diagenetic pathways — Route A and Route B — were identified. Route A develops in silica-rich environments, characterized by mechanical sediment infiltration and precipitation of siliceous phases. These processes promote increased structural disorder in apatite, reduction in crystallite size, pronounced removal of B-type carbonate, and the development of vibrational bands associated with phosphate–silicate interactions. In parallel, Route B is associated with carbonate-supersaturated solutions. In this pathway, calcite precipitation and the formation of larger crystallites indicate a buffered geochemical system capable of delaying apatite decarbonation. Evidence of this buffering is preserved in Raman data, which reveal a diagenetic gradient marked by narrowing of the ν₁(PO₄³⁻) band (decreased FWHM) together with progressive but partial depletion of B-type carbonate. XRD data confirm the formation of fluorapatite through diagenetic substitution of OH⁻ by F⁻, as well as variations in crystallite size consistent with distinct fluid–mineral interaction histories. Taphonomic evidence indicates variable redox conditions and intense pore-fluid circulation. The distinction between Route A and Route B provides a useful interpretative framework for understanding bone preservation in Quaternary coastal environments.