MASS EXTINCTION ON A SLOW-BURN: THE LATE TRIASSIC PALEOENVIRONMENTAL RECORD OF WILLISTON LAKE, BRITISH COLUMBIA

The Norian - Rhaetian stage boundary is globally associated with significant evolutionary turnover, leading to the rise of a distinct and comparatively short-lived Rhaetian fauna that is then severely affected by the end-Triassic extinction. The Norian - Rhaetian stage boundary is associated with “chaotic carbon episodes” in which the δ¹³C record shifts between negative and positive excursions, inferred to represent prolonged ecosystem instability. At the Rhaetian - Hettangian boundary, carbon cycle volatility culminated with a negative δ¹³C excursion, potentially driven by Central Atlantic Magmatic Province volcanism leading to collapse of marine productivity. Viewed together, the paleoenvironmental and faunal changes of the Norian - Rhaetian boundary mark the beginning of accelerated extinction rates, possibly cascading into the end-Triassic extinction. Previous work, however, has largely considered these events separately, and the Norian - Rhaetian boundary remains understudied in North America.

Late Triassic strata exposed along the shores of Williston Lake in northern British Columbia preserve a record of marine deposition off the western North American continental margin. The Ne-Parle-Pas Point, Pardonet Creek, and Black Bear Ridge sections sampled for this study all span the Norian through Hettangian stages. The Black Bear Ridge section also extends lower, incorporating the Carnian - Norian boundary, for which it is a Global Boundary Stratotype Section and Point candidate. The location of the Norian - Rhaetian boundary within these strata and the trends of paleoenvironmental change associated with the boundary are interpreted through the integration of carbonate δ¹³C chemostratigraphy, conodont biostratigraphy, and the abrupt disappearance of bivalve packstone beds coinciding with the extinction of the bivalve Monotis. This combination of chemostratigraphy and biostratigraphy is also used to investigate down through the Carnian - Norian boundary, as well as up through the Rhaetian - Hettangian boundary, placing the end-Triassic event in the broader context of Late Triassic paleoenvironmental change. In doing so, this study explores the idea of a drawn-out mass extinction, wherein the end-Triassic event is fundamentally linked to periodic extinctions throughout the Late Triassic.