Comparative Diagenesis and Rare Earth Element Variation in Miocene Invertebrate and Vertebrate Fossils from Panama

摘要

The incorporation of rare earth elements (REEs) into the mineral lattices of skeletonized fossils has been used, particularly in vertebrates, to understand diagenesis and other postmortem paleoenvironmental parameters. Little is known about similar processes in invertebrates. Invertebrate and vertebrate fossils were analyzed for REE concentrations and patterns from five Miocene formations in central Panama (similar to 20-6 m.yr. ago). These formations (Culebra, Cucaracha, Gatun, Chucunaque, and Chagres) contain sediments interpreted to range from freshwater to shallow marine environments (depth of <300 m). We also compared different compositional types of fossils, including hydroxylapatite (bone) and aragonite (otoliths) in vertebrates and calcite and aragonite in invertebrates. REEs from modern invertebrates indicate that, as previously reported for vertebrates, invertebrates do not incorporate significant amounts of REEs into their mineral lattices during life; therefore, uptake must primarily occur during diagenesis. Comparisons of REEN concentrations (REE concentrations normalized to Post-Archean Australian Shale) of the fossils from Panama indicate differences in diagenesis. Within our largest sample from the Gatun Formation, there are significance differences in REEN concentrations and patterns within different compositional types. For example, shark teeth have the highest REEN concentrations and otoliths the lowest, with these differences likely a result of physical, chemical, and mineralogical factors. Calcitic and aragonitic fossils from the Gatun Formation show REEN patterns indicating differential uptake of individual elements during diagenesis. The cerium anomaly, previously used to indicate relative paleobathymetry, did not discriminate water depth in the marine formations. Comparative analysis of REEs in vertebrates and invertebrates provides a powerful tool to understand diagenetic processes, as demonstrated in the five formations and different compositional types studied from the Miocene of Panama.