PROTRACTED ANOXIA DURING THE PERMIAN-TRIASSIC TRANSITION AS REVEALED BY THALLIUM ISOTOPES

The Permian-Triassic 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 CO_2­­, including climatic hyperwarming, ocean deoxygenation, and ocean acidification. However, an understanding of globally integrated changes in these processes remains elusive. 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 ocean anoxia in the modern ocean.

Recently, thallium (Tl) isotopes have been used to constrain global-ocean anoxia during several major Mesozoic anoxic events. Thallium isotopes are a useful and unique tool for understanding globally integrated variations in seawater oxygen content due to Tl’s large isotopic fractionation during adsorption onto manganese oxides, a process which requires the presence of oxygen. The other major Tl sink, altered oceanic crust, operates only at multimillion-year timescales, while the source fluxes of Tl to the global ocean all have similar isotopic compositions. Thus, changes in neither source fluxes nor other sink fluxes can drive fractionation on shorter timescales, leaving seawater Tl to fractionate primarily through Mn-oxide burial. 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 deposits 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 from two ocean basins document significant changes in the global-ocean redox state. These data provide a unique window into understanding the earliest onset of P-Tr oceanic anoxia, a likely result of increased volcanic CO₂ emissions.