REDOX CONDITIONS AND COMMUNITY EVENNESS FOLLOWING THE END-PERMIAN MASS EXTINCTION

Repeated environmental perturbations and fluctuating oceanic conditions after the end-Permian mass extinction led to highly uneven and depauperate seafloor invertebrate communities in the Early Triassic. Encroachment of suboxic and euxinic conditions on shallow shelf settings and global temperature spikes suppressed evenness, diversity, tiering, and trophic relationships in the Early Triassic marine benthic community. Some aspects of benthic community complexity did not recover to pre-extinction conditions until the Middle Triassic. We correlated paleo-redox proxies with ecological analyses of benthic invertebrate assemblages from a shallow shelf Lower Triassic Tethyan section in order to investigate the biological response to local and global changes in ocean redox state. A high resolution facies and geochemical analysis of the Lower Triassic (Induan to Olenekian stages) Werfen Formation from the U’omo section in Northern Italy was used to determine change in seafloor oxygenation conditions, with lithologic changes and shifts in δ¹³C_carb used to pinpoint the onset of changes in benthic oxygenation. Oxygen availability was compared to ecological metrics of community complexity obtained from bulk samples, including the abundance, diversity, evenness, body size, and degree of dominance of disaster taxa. δ¹³C_carb averaged near 0 ‰, for the first 50 meters (Induan). In this part of the section microbialites and thrombolitic structures co-occur with high abundances of the low-oxygen facies disaster taxon Claraia. In the next 100 meters (Induan to Olenekian), δ¹³C_carb values become more positive (1.7 ‰ on average), indicating an increase in oxygen availability in a shallower, high energy section which allowed the benthic fauna to colonize deeper into the sediment. At the top of the section (Olenekian), δ¹³C_carb values decrease to -2 ‰ on average. Overall community diversity increased throughout the section, but disaster taxa continued to dominate fossil assemblages and community evenness does not appear to track δ¹³C_carb changes. This decoupling of geochemical signals and biotic metrics suggests a more complex control of extinction recovery than oxygen availability alone.