Assessing the importance of thermogenic degassing from the Karoo Large Igneous Province (LIP) in driving Toarcian carbon cycle perturbations

The emplacement of the Karoo LIP occurred synchronously with the Toarcian crisis, which is characterized by negative carbon isotope excursions. Here the authors use carbon cycle modelling to show that thermogenic carbon released during LIP emplacement represents a plausible source for the negative excursions. The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. A marked increase in the atmospheric concentration of CO2 (pCO(2)) attests to significant input of carbon, while negative carbon isotope excursions (CIEs) in marine and terrestrial records suggest the involvement of a C-12-enriched source. Here we explore the effects of pulsed carbon release from the Karoo LIP on atmospheric pCO(2) and delta C-13 of marine sediments, using the GEOCLIM carbon cycle model. We show that a total of 20,500 Gt C replicates the Toarcian pCO(2) and delta C-13 proxy data, and that thermogenic carbon (delta C-13 of -36 parts per thousand) represents a plausible source for the observed negative CIEs. Importantly, an extremely isotopically depleted carbon source, such as methane clathrates, is not required in order to replicate the negative CIEs. Although exact values of individual degassing pulses represent estimates, we consider our emission scenario realistic as it incorporates the available geological knowledge of the Karoo LIP and a representative framework for Earth system processes during the Toarcian.

Automated summary

The study explores the role of the Karoo Large Igneous Province in the Toarcian crisis, finding that the thermogenic carbon released during its formation serves as a plausible source for the negative carbon isotope excursions, without the need for an extremely isotopically depleted carbon source such as methane clathrates to explain this phenomenon.