NORIAN ARIDIFICATION TRENDS IN THE DIAGENETIC ENVIRONMENT REVEALED THROUGH GEOCHEMICAL ANALYSIS OF CHINLE FORMATION PHYTOSAUR TOOTH DENTIN

The Upper Triassic (Norian; ~227-208 Ma) Chinle Formation of Petrified Forest (PeFo) National Park was deposited during a time of major tectonic breakup and profound changes in atmospheric circulation and faunal compositions. Little has been done however to assess whether surficial paleoenvironmental trends related to deterioration of the Late Triassic Megamonsoon resulted in similar trends in the early diagenetic environment. Here, we use phytosaur tooth dentin as a proxy for diagenetic conditions in the Chinle of PEFO, as its greater susceptibility to chemical alteration relative to enamel makes it an excellent substrate from which changing diagenetic conditions may be studied in western equatorial Pangea, and document trends in the diagenetic environment which may be compared with other Norian environmental findings. Cement stratigraphy and up-section increases in diagenetic calcite in fossil phytosaur teeth suggest progressive porewater alkalinization and improving drainage through time in PEFO, consistent with Norian aridification. Dentin carbonate δ¹³C and δ¹⁸O both trend similarly to contemporaneous paleosol carbonates, with the onset of positive-trending dentin δ¹⁸O values coincident with the Adamanian-Revueltian Faunal Transition in PEFO. Together, these data indicate that the geochemistry of the early diagenetic environment was influenced by surficial Norian aridification processes, and that tooth dentin can be utilized as a proxy for understanding how diagenetic conditions changed through time. However, our findings raise doubts about whether tooth dentin can be used as a direct replacement for paleosol carbonate nodules in the Chinle Formation. While trends in the isotopic compositions are similar, Chinle Formation phytosaur dentin and pedogenic carbonate exhibit distinctly different values where they stratigraphically co-occur. We attribute this to the different local hydrological and sedimentological conditions prevailing at the site of fossilization and soil formation. Further studies should document a fossil’s sedimentological context to assess the relationship between fossil geochemistry and taphonomy, and determine whether and under what conditions fossil remains can be used as proxies for paleosols when temporal gaps in the paleosol record exist.