INVESTIGATING THE DEVELOPMENT OF ANOXIA WITHIN THE EUROPEAN EPICONTINENTAL SEAWAY DURING THE TOARCIAN OCEANIC ANOXIC EVENT (T-OAE)

The loss of oxygen from portions of the oceans today due to climatic warming is a growing environmental concern. We can look to the geologic record for past episodes with similar environmental changes to better understand the present and future impact of decline in marine oxygen levels. One such time was a portion of the Early Jurassic Period (a time known as the Toarcian Oceanic Anoxic Event or T-OAE: ~183 million years ago). The sedimentary record shows that there was a large influx of carbon dioxide from volcanic activity that had cascading effects on the environment including a warmer and wetter climate, mass extinctions, and potentially large-scale oxygen loss from the oceans resulting in portions of the oceans becoming anoxic and euxinic (anoxic water columns with free hydrogen sulfide). However, the geographic and temporal extent of deoxygenation during the T-OAE is still not well defined.

In order to better understand the history of deoxygenation during the T-OAE, we investigated the record of local redox conditions across this event as recorded within three basins of the European epicontinental seaway: the Cleveland Basin (United Kingdom), the Paris Basin (Luxembourg), and the Southern German Basin (Germany and Switzerland). Specifically, we utilized the iron speciation proxy and sulfur isotope composition of sedimentary pyrite to reconstruct the water column redox conditions at each site. These data show that the first intervals of local anoxia occurred in the late Pliensbachian and near the Pliensbachian-Toarcian boundary in the Southern German and Cleveland Basin, respectively. Both anoxic and euxinic conditions are more persistent and most common in the interval attributed to T-OAE, but in all three basins evidence for anoxia persists well after this interval. This importantly demonstrates a greater temporal extent of anoxic conditions and suggests a greater sensitivity of marine oxygen levels to climatic change than previously considered for this event. More broadly, these records provide a timeline of marine deoxygenation in response to an episode of rapid climatic warming in Earth’s history.