Carbon-cycle changes during the Toarcian (Early Jurassic) and implications for regional versus global drivers of the Toarcian oceanic anoxic event

Abstract

The Early Jurassic Toarcian oceanic anoxic event (T-OAE) is characterized by an extinction event, a major sea-level rise, enhanced marine primary productivity, elevated seawater temperatures, widespread deposition of organic-rich sediments, and a negative carbon-isotope excursion (CIE). However, the T-OAE exhibits significant interregional variation in its expression, with strong spatial variation in lithology, total organic carbon (TOC) content, and the magnitude of the negative CIE in both carbonate and organic carbon profiles. It is better developed as a distinct geological event on the Northwest European Shelf (NWES), exhibiting both a large organic CIE (to –5 to –7 ‰) and high TOC content (to ~10 %), than in other regions globally. The reason for the regionally variable expression of the T-OAE has been a matter of debate, with models based on both regional and global factors proposed. We review these models in the context of the global sedimentary record of the T-OAE. The T-OAE records a global carbon-cycle perturbation that has been linked to Karoo-Ferrar Large Igneous Province magmatism, suggesting that volcanic emissions of greenhouse gases were the main cause of contemporaneous paleoenvironmental changes. Increases in seawater temperature have been documented only on the NWES to date, although lithologic and geochemical evidence of enhanced chemical weathering intensities is transregional. Mercury (Hg) enrichments are found mainly in shallow-marine settings, and their provenance in volcanic emissions remains uncertain. The exceptional expression of the T-OAE on the NWES points to regional oceanographic factors in the development of watermass stratification, deepwater anoxia, and enhanced organic matter accumulation. At a global scale, shifts toward more reducing oceanic redox conditions were spatially variable, and the T-OAE may therefore be more widely recognizable by its negative CIE than by paleo-environmental redox changes.