QUANTIFYING FOSSIL ABUNDANCE IN UPPER ORDOVICIAN STRATA OF THE ELLIS BAY FORMATION FROM EASTERN ANTICOSTI ISLAND, QUEBEC, CANADA

The Late Ordovician Mass Extinction (LOME) was the first of the Big Five mass extinctions of the Phanerozoic. Strata exposed across Anticosti Island preserve an excellent sedimentary and paleontological archive of the LOME. The LOME may have been a two-pulse event, possibly caused by global glaciation and/or anoxia; there is also evidence that supports a single-pulse event, possibly associated with massive volcanism. Here, we attempt to quantify the abundance of marine taxa in the Upper Ordovician Ellis Bay Formation in eastern Anticosti Island before and during the LOME. We document fossil composition and abundance in samples from shallow water mixed carbonate-siliciclastic stratigraphic sections at Anse Mauvaise and Rivière aux Saumons. First, thin sections were made from these samples and were studied under a microscope to make petrographic characterizations. High resolution digital scans of the thin sections were then analyzed to calculate the percentage of skeletal fragments, micrite, cement, and other allochems. Second, millimeter-size chips cut from the same hand samples were partially dissolved in weak acetic acid, and the microscopic fossil residues were examined and identified under a binocular microscope and scanning electron microscope. Calculating and comparing fossil abundance in association with published stable isotope proxy data for environmental change and recently developed sequence stratigraphic correlations at both localities on eastern Anticosti Island provides greater context about the tempo of the extinction event and its effects on animal abundance. The application of these results to single-pulse and two-pulse models helps us better understand how macrofossil and microfossil records can further be used to investigate possible drivers of mass extinctions.