Thallium Isotopes Across the Smithian-Spathian Boundary: Evidence for Anoxia Throughout Most of the Early Triassic.

Following the largest known Phanerozoic extinction, the end-Permian mass extinction, the Early Triassic is often considered an interval of slow biotic recovery. During this time, the minor Smithian-Spathian extinction occurred, further delaying the biotic recovery of this epoch. One mechanism linked to the slow recovery is widespread marine anoxic conditions, but the spatiotemporal changes to anoxia following the mass extinction remain poorly constrained. Here, we utilized local and global redox-sensitive metal proxies to track variations in (de-)oxygenation at Wallenbergfjellet, a section in Spitsbergen, Arctic Norway. Local proxies (e.g., Fe speciation and trace metal concentrations) indicate basinal anoxia and possibly euxinia prevalent in the Boreal Ocean during this interval. Importantly, thallium isotopes, a global paleoredox proxy sensitive to Mn oxide burial, have not previously been applied to the Early Triassic recovery interval that followed the end-Permian mass extinction, though basinal restriction may limit geographic extent of interpretations. Our results show a shift in ε205Tl values from ~ -4.0 in the Smithian to ~ -2.0 at the Smithian-Spathian boundary (SSB), consistent with a rapid expansion of global anoxia coincident with the boundary extinction. A rapid perturbation in thallium isotopes at the SSB (from -4.0 to -2.0 ε-units) indicates this interval experienced an expansion of anoxic conditions that can be linked to the minor extinction event during the recovery interval following the end-Permian mass extinction. This suggests that global anoxia persisted throughout the Early Triassic and contributed to delayed biotic recovery, with the SSB representing a major intensification of reducing conditions.