The Evolution of Bioturbating Animals and the Development of the Sedimentary Mixed Layer
In order to resolve the ecological-environmental feedbacks that accompanied the emergence of animal-dominated seafloor ecosystems, I use the sedimentary and trace fossil records to investigate the evolution of infaunal (sediment-dwelling) animals and bioturbating (burrowing and sediment-mixing) behaviors. The radiation of bioturbating animals is frequently invoked as the trigger for a myriad of geochemical, ecological and preservational phenomena associated with the Cambrian Explosion (such as oxygenation of the seafloor and deep ocean and the closing of several key windows of exceptional fossilization). However, the evolution of the mixed layer—the zone of sediments physically and chemically homogenized by burrowing animals and the zone which is mechanistically most responsible for the ecological, biogeochemical and preservational impact of bioturbation—has, historically, been poorly constrained. My recent field-based sedimentological and paleontological work, coupled with geochemical modeling, indicates that the development of the sedimentary mixed layer was a protracted process. The implementation of efficient sediment-mixing behaviors lagged behind both the Cambrian Explosion and the Great Ordovician Biodiversification Event, and the delayed development of bioturbation directly impacted marine sulfate concentrations and the rheology of the early Paleozoic seafloor (Tarhan et al., 2015, Nature Geoscience; Tarhan, 2018, Geology; Tarhan, 2018, Earth-Science Reviews; Tarhan and Droser, 2014, Palaeo-3). I am currently, with colleagues, developing a coupled sediment diagenesis and ocean chemistry model to test the impact of Paleozoic bioturbation upon marine phosphorus burial, productivity and ocean-atmosphere oxygenation.