LIPID BIOMARKERS OF MICROBIAL COMMUNITY STRUCTURE IN THE LEAD-UP TO LATE ORDOVICIAN CLIMATE CHANGE AND MASS EXTINCTION

The Late Ordovician has received a great deal of attention as a mass extinction linked to climate change. Due to the intricate connections between microbial communities and environmental conditions (nutrients, dissolved oxygen), constraints on microbial communities provide insight into the manner in which Latest Ordovician cooling impacted marine geochemical conditions (e.g. changes in dissolved O₂ or upwelling rates). Lipid biomarker records indicate substantial changes in microbial communities during the glacial maximum and mass extinction (Rohrssen et al., in press); to better constrain microbial community structures leading up to the glacial maximum and mass extinction, we have analyzed lipid biomarkers from three cores along a shallow to deep water paleobathymetry gradient preserved in the Maquoketa Formation, IA. We compare these results with previous work to constrain the extent to which changes in microbial communities that took place during the Late Ordovician mass extinction and cooling event are attributable to regional or global drivers such as climate or oceanographic changes as opposed to local facies effects. Maquoketa Formation biomarkers are thermally well-preserved, lack compounds associated with younger contamination (i.e. oleanane from angiosperms), and have characteristics typical of Lower Paleozoic-age bitumens (e.g. low C₂₈/C₂₉ sterane ratios). Maquoketa biomarker results include elevated levels of 3β-methylhopanes, attributable to high relative contributions from Type 1 methanotrophic proteobacteria, suggesting that conditions suitable to proliferation of those organisms preceded the mass extinction by several million years.