Cessation of a productive coastal upwelling system in the Panthalassic Ocean at the Permian–Triassic Boundary

Abstract

While the end-Permian extinction in the marine realm is well known from the Tethys Ocean, it remains little studied in the vast Panthalassic Ocean. Opal Creek, Alberta, Canada is a biostratigraphically continuous Permian–Triassic Boundary (PTB) section that is interpreted to have been deposited in a deep outer shelf setting along the Panthalassic western margin of Pangaea. Significant organic carbon and nitrogen isotope excursions precede the extinction of the dominant Late Permian benthic organisms (siliceous sponges). Geochemical comparison with underlying Guadalupian age rocks and analysis of the conodont fauna suggest that the latest Permian at Opal Creek records the shutdown of a productive cold-water upwelling ecosystem with a mid-water column oxygen minimum followed by the transition to a warm and less vigorously circulating system with a bottom-water oxygen minimum. The transition from a low-diversity, cold-water conodont fauna in the Middle Permian to a more diverse latest Permian fauna containing equatorial species coincides with a transition from elevated δ 15N values characteristic of coastal zones with denitrification in a mid-water oxygen minimum to more normal marine values. A spike in the δ 13C of organic carbon following a latest Permian marine transgression along western Pangaea is here attributed to transient increased productivity in the photic zone preceding the main extinction, likely related to the synergistic effects of warming, increased nutrient runoff, and residual upwelling. This is followed by an equilibration to lower δ 13C org values characteristic of a lower productivity regime in the Early Triassic.