BIOGEOCHEMICAL PROCESSES AT THE DEVONIAN/CARBONIFEROUS BOUNDARY IN THE WESTERN CANADA SEDIMENTARY BASIN

Changing pathways of biogeochemical cycling in ancient oceans are tied to evolution, extinction, and biotic response to environmental change. Chemical signatures of these changing pathways in sedimentary rocks, however, are affected by differing depositional environments between geographic locations and over time. To examine this variability in chemical signatures, we analyzed sedimentary rocks from 20 locations in the Western Canada Sedimentary Basin (WCSB) deposited during Devonian–Carboniferous transition. This interval includes a major mass extinction, the Hangenberg Event, which coincides with widespread global black shale deposition around the D/C boundary. Fourteen locations were on the ancient continental shelf and six were in the deeper water of the Peace River Embayment and Prophet Trough. All cores included the 3-8 m thick black shale unit of the Exshaw Formation that has been associated with oceanic anoxia and includes the extinction interval.

A positive carbon isotope excursion (CIE) that has been associated with the Hangenberg Event elsewhere is not evident at all locations in the WCSB. Based on n-alkane distributions (very low values for terrigenous/aquatic ratio and odd-to-even preference), organic matter at all locations was derived from marine algae/bacteria so terrigenous carbon did not overprint the CIE. Sterane distributions (C₂₇: 0-60%; C₂₈: 0-8%; C₂₉: 40-100%) suggest dominant green algae with temporally and geographically variable inputs of red algal biomass. Locations with a greater proportion of C₂₇ steranes also had higher organic carbon content, low bulk d¹⁵N values between 0-2‰, and elevated sterane/hopane ratios that collectively indicate a greater input of biomass from eukaryotic algae assimilating regenerated nitrate or ammonium. An exception to this relationship was found for a deeper water location in the Peace River Embayment, where low sterane/hopane ratios and low d¹⁵N values suggest biomass inputs from nitrogen-fixing cyanobacteria. Upward decreasing stratigraphic trends in the C₂₇/C₂₉ sterane and sterane/hopane ratios correspond with decreasing organic carbon content and increasing d¹⁵N values. These trends suggest a temporal decrease in red algae relative to green algae and increasing bacterial input/reworking for these locations. Ultimately, our data indicate that the global biogeochemical signature of the Hangenberg Event is muted by regional processes that can be characterized by analyzing core from multiple locations in the depositional basin.