STRONTIUM ISOTOPE EXCURSIONS, SILICEOUS SPONGES, AND SILICIFIED CARBONATE MICROFOSSILS ACROSS THE NORIAN-RHAETIAN BOUNDARY IN SHALLOW MARINE DEPOSITS OF PANTHALASSA (LATE TRIASSIC, NEW YORK CANYON, NV)

Microfossils can yield important ecological information from sedimentary rocks, but different forms of microfossil preservation can artificially inflate or diminish abundance patterns in rock samples. To understand more about these preservational differences, microfossils were extracted from 18 limestone samples from Nevada, which were deposited in shallow marine conditions during the Norian and Rhaetian stages of the Late Triassic (~210-202 Ma). Using a buffered acetic acid extraction process, the most common microfossils in the limestone residues included carbonate microfossils (echinoderm stereom and bivalve shell fragments), and siliceous sponge microfossils fossils (hexactinellids and demosponge desmids). Most echinoderm and molluscan microfossils from these bulk samples were fully or partially composed of silica, in carbonate matrix, suggesting that the original carbonate mineralogy had been replaced soon after deposition. The presence of siliceous sponge microfossils indicate large amounts of dissolved silica in seawater and porewater during this Late Triassic interval, leading to the silicification of biosediments after deposition. The proliferation of siliceous sponge spicules and silicified microfossils coincides with the Norian-Rhaetian stage boundary which is known as a period of environmental perturbation prior to the End-Triassic mass extinction. A large negative strontium isotope excursion is present in carbonates across the NRB boundary, suggesting rapid weathering of basalts or hydrothermal vent activity that may have supplied an influx of dissolved silica to shallow marine environments. Increased concentrations of silica may have persisted in shallow marine environments during this interval, supporting large populations of silica-limited sponges, and in near-surface porewater resulting in abundant silicified stereom. Thus, microfossil populations and preservation modes can provide insights into global weathering patterns and geochemistry.