Biosediment assemblages reveal disrupted silica cycling and redox conditions throughout the Rhaetian Stage: Evidence for a precursor event to the end-Triassic mass extinction

Abstract

The Norian–Rhaetian boundary (Late Triassic) represents an important precursor extinction event to the end-Triassic mass extinction, but the biotic and geochemical shifts are not well-understood due to poor stratigraphic constraints. Here we examine the microfossil record for metazoans and protists on a Panthalassan carbonate ramp (Gabbs Formation, Nevada, U.S.A.) during the late Norian to mid-Rhaetian, and correlate changes in these assemblages with macrofossil shifts and geochemical data (strontium and carbon isotopes). In the latest Norian, demosponge spicules represent a small proportion of shallow marine biosediments. Demosponges are joined in the earliest Rhaetian by increasingly abundant hypersilicified sponge spicules and silica-limited hexactinellid sponge spicules synchronous with a negative strontium isotope excursion indicating increased hydrothermal or volcanic activity. Common carbonate microfossils such as echinoderm ossicles and ostracods are typically silicified in these deposits as well, suggesting increased silicic porewater. The source for increased dissolved silica in shallow marine systems is suggested to be hydrothermal vent degassing, likely associated with increased tectonic rifting activity. Mid-Rhaetian microfossil assemblages exhibit evidence for intermitted anoxia in reducing conditions, supporting a scenario in which environmental stress was a prolonged feature of much of the Rhaetian Stage, rather than a short-term event in the terminal Rhaetian. While there is no marine sedimentary record of volcanism recognized for this interval, biosedimentary assemblages may serve as proxies for geochemical conditions associated with rifting.