Paleogeographic Implications of Open-Marine Anoxia in the Permian–Triassic Slide Mountain Ocean

Abstract

The end-Permian mass extinction was associated with the onset of anoxia in widespread marine environments; however, the extent of this anoxia remains controversial. Proposed models range from near-universal “superanoxia” in the Panthalassic Basin to a more limited expansion of anoxia in the upper water column in response to enhanced primary productivity. The Peck Creek and Ursula Creek sections of northern British Columbia were deposited at ~200 m water depth in the Ishbel Trough, on the margin of cratonic North America. This trough was generally contiguous with the Slide Mountain Ocean, and thus with the broader Panthalassic Ocean, though it may have been partially separated by structural highs at various times during the Permian. Both sections include continuous Wordian to Changhsingian sedimentary successions, which span the end-Permian mass extinction boundary and continue into the earliest Triassic. The extinction is recognizable as the disappearance of biogenic silica from the environment, which defines the contact between the Fantasque Formation and overlying Grayling Formation. This surface also corresponds with the onset of anoxia, and the accumulation of redox-sensitive trace metals. The covariation trends in these metals, and in other isotopic proxies, can be used as tools to trace the degree of communication between the Ishbel Trough–Slide Mountain Ocean and the broader Panthalassic Basin. Molybdenum-uranium covariation trends indicate that the northern Slide Mountain Ocean and Ishbel Trough remained in communication with the larger global ocean throughout this interval, suggesting inversion of the Wordian structural high to form a depositional subbasin by the Changhsingian. This is in contrast to the Opal Creek section of southern Alberta, which shows evidence for some degree of restriction, suggesting that the Slide Mountain Ocean may have maintained a north–south gradient in water chemistry. Several lines of evidence suggest that this onset of anoxia was not related to expansion of an upwelling-driven oxygen minimum zone. No clear changes in primary productivity, as recorded by organic carbon or authigenic phosphorus and barium, are observed across the extinction horizon. Changhsingian nitrogen isotope values are generally in the 2 to 3‰ range, suggesting minimal denitrification at thermocline water depths, and these values decreased in the earliest Triassic, likely in response to enhanced nitrogen fixation. This suggests that anoxia was driven by shoaling of a chemocline that developed due to stratification of the Slide Mountain Ocean, rather than western-boundary upwelling effects.