Lipid biomarker chemostratigraphy in Arctic Canada: Evaluating microbiology ecology and carbon cycling during Hirnantian cooling

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The Late Ordovician Mass Extinction (LOME) event is one of the most well-known climatic and environmental transition events in the Phanerozoic Eon. The Hirnantian glaciation and associated cooling during the latest Ordovician is widely considered to be the key driver for the major mass extinction event as well as changes in the climate and oceanographic systems1. Evidence from sedimentological, faunal, and geochemical data from around the globe has demonstrated that the transition from pre-glacial, to glacial, and post-glacial times was associated changes in carbon cycling, a large drop in eustatic sea level, and a series of extinction pulses2. The extinction patterns of marine fauna and perturbations to the carbon cycle have been well documented. However, the effects on the microbial communities that underpin marine food webs and mediate essential biogeochemical cycles are poorly constrained. New pristine outcrop samples have provided an opportunity for a detailed microbial lipid biomarker and stable isotope investigation on a succession that spans the Late Ordovician (Katian Stage) to Early Silurian (Rhudanian stage) time interval3. Here, we investigate how the significant environmental changes associated with the LOME and HICE affected the microbial communities.Lipid biomarker and stable isotope (δ13Corganic, δ13Ccarbonate, δ15Ntotal) stratigraphic records were acquired from a 10-m interval of outcrop section from Cornwallis Island, Nunavut, Canada. Rock extracts were analyzed for a suite of branched and polycyclic hydrocarbon biomarkers utilizing the sensitivity and selectivity of Metastable Reaction Monitoring-Gas Chromatography-Mass Spectrometry (MRM-GC-MS). Baseline conditions, prior to the HICE, exhibit typical Ordovician marine biomarker characteristics which have been observed from various lithologies and different Ordovician marine depositional settings. These characteristics include low hopane/sterane (H/St) ratios, high relative abundance of C29 steranes from green algae, and high abundances of 3-methylhopanes (many times the Phanerozoic average), likely sourced from methanotrophic bacteria4. The rising and falling limbs of the HICE locally at our site are associated with a significant increase in total organic carbon (TOC) content (

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