REDOX DYNAMICS AND REDUCED BENTHIC FAUNAL ABUNDANCE ACROSS THE TRIASSIC-JURASSIC MASS EXTINCTION

The biodiversity crisis at the end of the Triassic Period was one of the most severe mass extinctions in the history of animal life, substantially reducing the taxonomic diversity and ecological complexity of marine ecosystems. However, the impact of the extinction on animal abundance, a critical component of energy flow in ecosystems, remains poorly constrained. The goal of this study was to quantify the abundance of skeletal animals before and after the mass extinction and the relationship of any observed changes to variations in redox conditions. We quantified the abundance of skeletal grains in shallow-marine limestones by point-counting 293 thin sections from four stratigraphic sections, three representing a shallow-marine carbonate ramp in the Lombardy Basin and one representing a peritidal setting in the southern Apennines. To constrain shallow-marine redox conditions, we measured I/Ca ratios from two of the Lombardy Basin sections. Skeletal abundance decreased significantly across the Triassic/Jurassic boundary in all four studied sections and I/Ca decreased significantly in both sections analyzed. The limestones from the upper Hettangian to lower Sinemurian increased significantly in skeletal abundance and I/Ca relative to the lower Hettangian. A statistical test across all 293 samples indicates that the time period of deposition (p<0.001, η²=0.30) has more significant control on skeletal abundance than the location of the stratigraphic section (p<0.001, η² =0.15) itself. It also indicates that the observed variations in the skeletal abundance are indicative of a control acting across all sampled locations irrespective of depth of sediment deposition. Furthermore, the correspondence between changes in skeletal abundance and I/Ca, as well as other paleoredox proxies such as δ²³⁸U, is consistent with either a direct redox control on animal abundance in shallow-marine ecosystems or a common control on both ecosystem structure and shallow-marine redox conditions, such as paleoclimate. The redox proxies indicate a sharp reduction in dissolved oxygen in the water column across the Triassic-Jurassic transition which could have greatly reduced the aerobic habitat volume of shallow marine fauna, thus explaining the observed reduction in animal abundance.