Carbon and nitrogen isotopic signatures in the Devonian Canol Formation: implications for paleodepositional conditions

In sedimentary successions, the carbon isotopic composition of organic matter and bulk nitrogen isotopic composition are influenced by several factors, including isotopic signatures of carbon and nitrogen sources, carbon and nitrogen assimilation pathways of primary producers, microbial degradation of organic matter, and thermal maturation of organic matter. Here we used nitrogen and carbon isotopic profiles to shed light on the paleoenvironmental conditions and biogeochemical processes that operated during deposition of the Canol Formation, an organic-rich mudstone present in the Central Mackenzie Valley and Mackenzie Mountains of the Northwest Territories. In this study, samples were collected every two meters through the Canol Formation in the ConocoPhilips N-20 core from the Central Mackenzie Valley and analyzed for δ¹⁵N and δ¹³C composition. δ¹⁵N values ranged from –3.7‰ to –0.6‰. These isotopic values are characteristic of N₂fixation, and suggest that diazotrophic organisms (e.g. certain cyanobacteria species) were the dominant primary producers at the time. These results imply that NO₃^- and NH₄⁺were likely limited in surface waters because N₂ fixation consumes more energy than NO₃^- and NH₄⁺assimilation. The δ¹³C values range between –28‰ to –30‰, falling into the δ¹³C range of many types of primary producers that include modern cyanobacteria. Two trends are apparent in δ¹³C: moving stratigraphically upward through the Canol Formation, a decreasing trend in δ¹³C from about –28‰ to –30‰ is observed, followed by an upward increase in δ¹³C from about –30‰ to –26‰ near the top of the Canol Formation and its transition into the overlying Imperial Formation. These shifts do not match trends in vitrinite reflectance, indicating that they were not produced by variations in thermal maturity. Similarly, the observed trends in δ¹³C are not apparent in the δ¹⁵N profile, which implies that they were not likely a result of changes in the proportions of different types of primary producers, because this should also affect δ¹⁵N values. The observed trends are better explained by changes in the carbon source of primary producers or changes in paleoproductivity. Gaining insight into the conditions that led to the accumulation of this succession enhances our understanding of the controls on organic matter accumulation and preservation in organic-rich mudstone units, and specifically, the Canol Formation’s potential as an unconventional resource target.

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