PINPOINTING THE ECOLOGICAL EFFECTS OF DISCRETE HIGH TEMPERATURE AND LOW OXYGEN EVENTS FOR THE BENTHIC RECOVERY FOLLOWING THE END-PERMIAN MASS EXTINCTION

For five million years following the end-Permian mass extinction additional global warming events and oxygen minimum zones had deleterious effects on benthic and pelagic diversity. We studied the benthic fauna to determine how these two types of environmental perturbations impacted the ecological recovery. We also compared shallow and deep water sections to test whether different shoreface environments reinforced or confounded high temperature and low oxygen conditions. In the Early Triassic, extreme global warming limited the ecological recovery of the benthic fauna while repeated low oxygen events restricted community diversity.

An extreme global warming event has been established during the late Smithian Stage. This event occurred simultaneously with the most eastward expansion of an extensive epicontinental sea in the Southwest United States. The Sinbad Limestone in eastern Utah represents deposition in this extremely shallow environment. The benthic community contains an abundant microgastropod fauna (<10mm length) and lacks epifunal tiering, i.e., organisms living above the sediment water interface (SWI). In contrast, deeper water deposits from the Thaynes Formation in western Utah contain larger gastropods (>10mm length) and epifauna, specifically crinoids. We interpret the abundant Lilliputian gastropod fauna and lack of crinoids in the shallow water Sinbad Limestone to be the result of exposure to high temperatures which were not as severe for fauna living in deeper environments.

Extensive oxygen minimum zones have been inferred for equatorial Panthalassa throughout the Early Triassic. Deposits from the offshore transition of the Virgin Limestone show facies evidence for low oxygen conditions and contain a low diversity benthic fauna of molluscs and echinoderms. Despite the low oxygen setting, crinoids were abundant and represent tiering above the SWI and utilization of ecospace not expressed by the high temperature Sinbad Limestone community. The shallow section of the Virgin Limestone formation contains well oxygenated shoal and storm facies. Here, large bivalves (up to 5cm) dominate storm facies and a wider variety of echinoderm groups are present. We attribute the ecological differences to both the degree of oxygenation and the different facies present at the two sites.