Thermogenic or biogenic methane? Interpreting negative isotopic excursion during Oceanic Anoxic Events using atmospheric pCO2 records from fossil plant stomata.

Oceanic anoxic events (OAE) are characterized by carbon-isotope excursions in marine and terrestrial reservoirs and mass extinction of marine faunas. Deceiphering between multiple possible causal mechanisms for these negative isotopic excursion may help to determine the triggering mechanism of OAE's - currently a subject of much debate. 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 Formation (formerly Bag) in the eastern Danish Basin. The data reveal that the negative carbon isotopic excursion was coincident with 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 +/- 1oC inferred 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.