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

The Permian-Triassic (P-Tr) transition (~252 Ma) was a significant period of environmental and climatic disturbance in Earth’s history associated with the largest mass extinction of the Phanerozoic. Thus, it is a vital window to study patterns and effects of climatic variability related to biologic and chemical evolution. The initial driver of this climatic perturbation was the eruption of the Siberian Traps, with its massive release of CO₂ causing a cascade of environmental events. One of these potential effects, expansion of ocean anoxia, is well-established for the P-Tr transition. However, the timing, magnitude, and geographic extent of this process need better constraints. Determining these relationships is key to understanding the interplay between climate and ocean oxygenation.

Recently, thallium (Tl) isotopes have been used to constrain global-ocean anoxia during several major Mesozoic oceanic anoxic events (OAEs). This isotopic system is a useful tool for constraining global variations in seawater oxygenation. Tl isotopes are primarily affected by Mn-oxides during burial, which Tl adsorbs onto, one of the first processes affected by changes in ocean oxygenation. In addition, Tl’s relatively short seawater residence time, 18,500 years, allows for tracking detailed changes in oceanic oxygen content during major events.

Here, we use sedimentary records from three P-Tr sections to better constrain the global extent and timing of ocean anoxia in relation to the mass extinction event. Their Tl isotopic profiles document a significant perturbation in the global-ocean redox state during the P-Tr transition. This isotopic shift suggests that widespread seafloor anoxia developed prior to the extinction event, expanded at the start of the event, and remained anoxic long after the end of the event, similar to the observed record for two Mesozoic OAEs. The event’s onset is marked by. These sections also document a punctuated return to more negative Tl values at the event’s onset, which suggests widespread Mn-oxide burial, possibly due to a better oxygenated ocean. 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 a brief, but potentially important, oxygenation event at its onset.