Global organic carbon burial during the Toarcian oceanic anoxic event: Patterns and controls

The early Toarcian oceanic anoxic event (T-OAE, -183 Ma), also known as the Jenkyns Event, was one of the most important hyperthermal events of the Phanerozoic, caused by the large-scale release of carbon from massive volcanism and/or surficial carbon reservoirs. Organic-rich facies record the event in basins worldwide, but the precise controls on organic carbon burial, and the relative importance of this burial as a mechanism of sequestering excess carbon from the Toarcian atmosphere, are uncertain. In this study, we have compiled total organic carbon (TOC) data from 67 lower Toarcian sections to reconstruct the pattern of organic enrichment through the T-OAE. Shallow marine sites dominate the compilation, and rates of organic carbon burial during the T-OAE were low relative to shallow water margins at the present day. Redox was a major control on organic enrichment and burial rates, but the very high TOC (>5%) observed in hydrographically restricted anoxiceuxinic basins in northern Europe and elsewhere were largely a consequence of low sedimentation rates. Globally, organic enrichment and redox conditions were highly variable, but a majority of sites show an increase in TOC at the T-OAE relative to pre-event values. TOC increases were generally highest where deoxygenation was most severe. In anoxic-euxinic marine basins in Europe organic carbon burial rates may have increased -500% on average during the T-OAE, potentially sequestering an extra -791 Gt of carbon relative to the same time period immediately prior to the event. Sites outside of Europe and those that remained oxic-suboxic or were dominated by terrestrial organic matter show variable, and often negligible, changes in organic enrichment across the T-OAE. Globally, an extra -9000 Gt of carbon may have been buried in shallow seas during the T-OAE relative to before the event. Nevertheless, a paucity of data and significant uncertainties mean that the precise amount of excess carbon sequestered during the T-OAE remains uncertain.

Automated summary

In this study, total organic carbon (TOC) data from 67 lower Toarcian sections were compiled to reconstruct the pattern of organic enrichment during the early Toarcian oceanic anoxic event (T-OAE). The findings suggest that redox conditions and sedimentation rates were major controls on organic enrichment and burial rates. Globally, there was a significant increase in TOC at most sites during the T-OAE.