THALLIUM ISOTOPES ACROSS THE SMITHIAN-SPATHIAN BOUNDARY: EVIDENCE FOR PERSISTENT ANOXIA THROUGH THE EARLY TRIASSIC IN THE WAKE OF THE END-PERMIAN MASS EXTINCTION

Following the largest known extinction – the end-Permian mass extinction – the Early Triassic Epoch is often considered an interval of slow biotic recovery. The mechanistic underpinning of this event has been hypothesized to be linked to high temperatures and widespread marine deoxygenation. Additionally, the minor extinction between the Smithian and Spathian substages occurred a couple of million years after the end-Permian during this epoch. This has been linked to a specific interval of rapid change in global climate and expansion of anoxia, further compounding the biotic recovery of this interval. There are still limitations and uncertainties to our understanding of the persistence and extent of anoxia across this time compared to the vast amount of research done on the end-Permian. Therefore, refining the specific changes in anoxic conditions will further improve our understanding of its effect on life in the wake of the largest mass extinction.

Here, we utilized iron speciation and trace metal concentration analysis to determine local to regional redox conditions in concert with thallium (Tl) isotopes to track the earliest changes in global (de-)oxygenation across much of the Early Triassic at the Wallenbergfjellet section of Spitsbergen, part of the long-lasting Sverdrup Basin. Local paleoredox proxies suggest continual anoxia with short-term euxinia within the Sverdrup Basin. Importantly, Tl isotopes suggest widespread persistent global anoxia during the Early Triassic– aligning with previous research – although less severe in the Smithian compared to the start of the Triassic. A sudden perturbation at the Smithian-Spathian boundary suggests a global expansion of anoxia beyond the already prevalent reducing conditions, coinciding with the small biotic crisis that affected the limited fauna that survived the end-Permian. Following this minor extinction, Tl isotopes indicate generally more globally widespread anoxic conditions occurred during the multi-million-year duration of the Spathian, but Tl isotopes additionally suggests these conditions waxed and waned throughout this substage. Both the generally more reducing global conditions and these oscillations in redox likely contribute to poor adaptation and through the remainder of the Early Triassic.