TRACE METAL DATA FROM LOWER AND LOWERMOST MIDDLE TRIASSIC ROCKS FROM URSULA CREEK, BRITISH COLUMBIA REVEALS PERSISTENT ANOXIA AND VARIABLE PALEOPRODUCTIVITY HISTORY IN THE AFTERMATH OF THE PERMIAN-TRIASSIC MASS EXTINCTION

Recovery from the Permian-Triassic mass extinction was complex and strongly controlled by the distribution of environmental stress. The 105.5 m-thick Ursula Creek section of western British Columbia preserves a complete Lower-lowermost Middle Triassic sedimentary record of deep-water facies and provides a means to examine how deep, offshore oceanic conditions varied along the western continental shelf of Pangea during the Permian-Triassic recovery interval. A total of 204 samples were collected from the Griesbachian – Dienerian Grayling Formation and the Smithian – Anisian Toad Formation and analyzed for major, minor, and trace elements in addition to %TOC contents. Anoxic to euxinic conditions were persistent during deposition of the entire study interval based on elevated V and Mo enrichments factors (EFs) within both formations. There is no evidence of an oxygenation event near the Permian-Triassic boundary that has been documented elsewhere. Ba, Cu, Ni and Zn enrichment factors shift between low productivity (EFs < 1) and elevated productivity (EFs > 1) within the lower 10 m of the 36.5 m-thick Grayling Formation, indicating that productivity was variable following the mass extinction, and did not return in a robust manner until deposition of the upper part of the Grayling Formation. Fluctuations in productivity in the period immediately following the extinction are indicative of initial post-extinction environmental turmoil prior to the establishment of stable conditions later in the Early Triassic. A steady increase in Ba, Cu, Ni and Zn EFs across the Smithian-Anisian Toad Formation is diagnostic of further strengthening of primary productivity, while a concomitant increase in V and Mo EFs suggest a heightened degree of anoxia and euxinia that was driven by higher levels of primary productivity. The results of this study document the persistence of anoxic to euxinic conditions in deep ocean environments during the post-extinction recovery interval, but also demonstrates the complex interaction between deep ocean chemistry and the export of organic matter from surface waters as productivity was reestablished.