HIGH RESOLUTION PALEOECOLOGY AND GEOCHEMISTRY ACROSS THE TRIASSIC/JURASSIC MASS EXTINCTION INTERVAL AND THE IMPORTANCE OF BIOSILICEOUS “GLASS RAMPS”

Mass extinction events are unique intervals to study environmental change paired with rapid ecological reorganization. Fossil data show the Triassic/Jurassic extinction was global, severe, and selective; it produced one of the greatest drops in Modern Faunal diversity of the Phanerozoic. Environmental proxy data show concomitant changes in sea level, climate, and global geochemical cycles associated with phases of Central Atlantic Magmatic Province eruption. A global shift in marine sedimentary facies across the Triassic/Jurassic (T/J) boundary makes conformable shallow fossiliferous records both rare and scientifically valuable.

Field investigations in Nevada and the central Peruvian Andes on conformable T/J strata in thick 100-200 m sedimentary successions with a wide variety of represented depositional environments allow high-resolution geochemical analysis of this interval. Stable isotope proxies (specifically of N and C) are employed to examine trends in marine conditions that would influence productivity, ocean circulation, and oxygen concentrations. Paleontological analyses focus on the metazoan contribution to sedimentation. Available evidence suggests that massive injection of volcanic CO2 just prior to the boundary perturbed the global marine carbonate system, destabilizing carbonate biocalcifiers. In the aftermath, siliceous sponges dominated mid- to inner-ramp settings for ~ 2 Myr, producing an enduring low diversity level-bottom regime. The resulting T/J strata necessarily defy purely-actualistic biofacies interpretations.

Appreciating the role of siliceous sponges in the T/J extinction aftermath is aided by consideration of sponge biogeography, particularly comparison to other shallow siliceous spiculites of the Phanerozoic. Bio-siliceous “glass ramps” represent a 3rd sedimentary regime distinct from both siliciclastic and carbonate systems. Glass ramps are an alternative ecosystem state expressed at the local and continental scale, and in deep time they appear to respond to both long-term geochemical revolution and brief environmental upheaval. High-resolution geochemically-linked mass extinction interval studies help to identify the metazoan influence on these ecosystem shifts.