STABLE ISOTOPE STRATIGRAPHY OF LATE DEVONIAN CARBONATES IN THE ROCKY MOUNTAIN FRONT RANGES: A FIELD TEST OF THE AUTHIGENIC LEVER

Authigenic carbonate can form as isotopically depleted cements within sedimentary pore spaces. Driven primarily through anaerobic oxidation of organic matter, it has been postulated that the widespread precipitation of these cements may have acted as a prominent alkalinity sink during times of low O₂ concentrations, including during transient ocean anoxic events of the Phanerozoic. The Famennian, last age of the Devonian Period, is characterized by expanded anoxia, enhanced siliciclastic vs. carbonate deposition and is also bookended by mass extinctions of marine calcifiers. If appreciable amounts of isotopically depleted authigenic carbonate were being sequestered in organic-rich sediments, then this would have the effect of driving the δ¹³C values of dissolved inorganic carbon (DIC) towards more positive values. Here, we evaluate the influence of authigenic carbonate on the carbon cycle by developing δ¹³C profiles of Famennian carbonates within the Front Ranges of western Canada, including the Sassenach and overlying Palliser formations. The Sassenach formation is a shoaling-upwards, mixed clastic-carbonate unit, and displays little variation in δ¹³C values (between 1-2‰). The Palliser Formation is more carbonate-rich, and displays depositional heterogeneity with facies alternating between intertidal to deep subtidal environments, and δ¹³C values are relatively constant at 0‰. Localized isotopic variability may be attributed to dolomitization or disconformities. The isotopic composition of the Sassenach-Palliser succession falls within normal values for marine carbonates, and we do not observe an overall increase in δ¹³C values. Next research steps include study of coeval siliciclastic formations outboard of the Sassenach and Palliser formations, as well as the overlying Exshaw Formation, which represents the Late Devonian maximum in black shale deposition in North America. Collectively, we will be able to quantify the carbonate content and δ¹³C value of Late Devonian shale units from Western Canada, and the associated response (if any) recorded in shallow marine carbonates.