BRIEF OXYGENATION EVENT DURING EXTENSIVE ANOXIA ACROSS THE PERMIAN-TRIASSIC TRANSITION AS REVEALED BY THALLIUM ISOTOPES

The Permian-Triassic (P-Tr) transition (~252 Ma) represents a significant period of atmosphere-ocean disturbance in Earth’s history associated with a major extinction event, making this a crucial window to study patterns and effects of climatic variability related to biologic change. The main driver of this extinction event was the eruption of the Siberian Traps, which caused a cascade of events linked to massive release of CO₂. Although latest Permian to early Triassic expansion of ocean anoxia, a direct potential cause for the mass extinction, is well-established, the timing, intensity, and geographic extent of this expansion needs better constraints. Determining these relationships is key to understanding the potential effects of human-induced CO₂ increases and expanding modern ocean anoxia.

Recently, thallium (Tl) isotopes have been used to constrain global-ocean anoxia during several major Mesozoic oceanic anoxic events (OAEs). Thallium isotopes are a useful and unique tool for understanding globally integrated variations in seawater oxygen content because changes in the Tl isotope system are primarily from its adsorption onto manganese oxides, a process which requires the presence of oxygen. In addition, Tl has a relatively short seawater residence time, 18,500 years, allowing it to track detailed changes in oceanic oxygen content.

Here, we use sedimentary records from several Permian-Triassic sections to better constrain the global extent and timing of ocean anoxia in relation to the mass extinction event. The Tl isotopic values document significant changes in the global-ocean redox state. They indicate seafloor anoxia developed prior to the extinction event. Similar to the observed record for Mesozoic OAEs, which expands at the start of the event and remains anoxic long after the end of the event. However, unlike other OAEs and previous research on the P-Tr extinction, a sudden and short-lived return of Tl isotopes to more oxic ocean values indicates a possible oxygenation event at the onset of the extinction event. These data provide a unique window into understanding the development of oceanic anoxia during the P-Tr transition, a likely result of increased volcanic CO₂ emissions, and the short-lived but potentially important undiscovered oxygenation event.