A Lipid Biomarker Stratigraphic Record through the Late Ordovician Mass Extinction

The Late Ordovician (~450-440 Ma) was a period of major environmental change, as indicated by evidence for short-duration (<1 myr) glaciation, with concurrent sea level fall and rise, despite greenhouse atmospheric conditions. These environmental changes are accompanied by at least one positive carbon isotope excursion (Hirnantian Isotopic Carbon Excursion, HICE) and mass extinction event(s). Anticosti Island, Quebec, Canada provides an exceptional opportunity to assemble nearly continuous chemostratigraphic records of the Late Ordovician. In this study we use lipid biomarkers extracted from carbonate shelf sedimentary rocks exposed in outcrop on Anticosti Island to gain insight into the major marine primary producers and microbial community structure in an epeiric sea setting during the Hirnantian mass extinction and HICE. Anticosti biomarkers have low maturities consistent with the thermal burial history of strata on the island, lack signs of petroleum-derived contamination (e.g. zero oleanane signal from angiosperms and other self-checks), and yield the C29 sterane predominance and low C28/C29 sterane ratios typical of the Early Paleozoic. These sediments, which bear marine fossils, lack the marine marker 24-n-propylcholestane, and have high 3β-methylhopane (4-11% of C30 αβ-hopane) and moderate 2α-methylhopane (2-4% C30 αβ-hopane) indices, most commonly associated with methanotrophs and cyanobacteria, respectively. Gammacerane is present only in trace amounts. Hopane/sterane ratios range from 1.8 to 11.2 (average = 4.7), with most values significantly above the Phanerozoic marine average values of 0.5-2.0, indicating a high contribution of bacterial input to sedimentary organic matter. Lower hopane/sterane values (average = 2.2) are generally found coincident with the carbon isotope excursion. Taken together, the lipid biomarker data suggest a stressed oligotrophic marine ecosystem in which N2-fixing bacterial communities dominate over eukaryotic algae, possibly as a response to nitrate-limitation in the low-sulfate Paleozoic ocean.