NEW RECORD OF ATMOSPHERIC CARBON DIOXIDE INDICATES THAT THE TOARCIAN OCEANIC ANOXIC EVENT (~183 MA) WAS TRIGGERED BY INTRUSION OF GONDWANAN COALS BY KAROO AND FERRAR DOLERITES

The marine sedimentary record is punctuated by brief episodes (<1 myr) of dramatically enhanced primary productivity and/or organic carbon preservation known as “oceanic anoxic events” (OAE's). They are characterized by carbon-isotope excursions in marine and terrestrial reservoirs and mass extinction of marine faunas. Despite the importance of OAE's to our understanding of the carbon cycle and biotic evolution, causal mechanism(s) remain to be determined. We have tested two leading hypotheses for an abrupt negative carbon isotopic excursion marking initiation of the Toarcian OAE (~183 myr ago) using a high-resolution atmospheric carbon dioxide (pCO₂) record obtained from changes in stomatal frequency on fossil leaves preserved within nearshore sediments of the Sorthat (formerly Bagå) Formation at Korsodde, Bornholm, in the eastern Danish Basin. The data reveal that coincident with the isotope excursion was a pCO₂ drawdown of 350 ± 100 ppmV and 2.5 ^oC global cooling followed by an abrupt 1200 ± 400 ppmV pCO₂ increase and 6.5 ± 1^oC greenhouse warming. The detected CO₂ draw-down and inferred global cooling occur coincidently with a second order mass extinction of marine organisms, suggesting a possible additional extinction mechanism to marine anoxia. The pattern and magnitude of CO₂ change are not consistent with catastrophic input of isotopically light methane from methane hydrates as the cause of the negative isotopic signal. Our pCO₂ record better supports a magma-intrusion hypothesis and suggest that a massive injection of isotopically light carbon from release of thermogenic methane (CH₄) occurred due to the intrusion of Gondwanan coals by Toarcian aged Karoo-Ferrar dolerites. We propose that CH₄/CO₂ generation associated with Karoo-Ferrar magmatism was an important triggering mechanism for the Toarcian OAE. Furthermore our results indicate that organic carbon burial/preservation at the onset of the OAE resulted in atmospheric conditions (CO₂ < 400 ppmV) that associate with cooling, possibly of a sufficient magnitude even to enable transient ice cap growth (DeConto, pers. comm.) on higher elevations of northern and southern high latitudes of Pangea.