HR: 10:50h
AN: PP42A-03    [Abstracts]
TI: Thermogenic or biogenic methane? Interpreting negative isotopic excursion during Oceanic Anoxic Events using atmospheric pCO₂ records from fossil plant stomata.
AU: * McElwain, J C
EM: jmcelwain@fieldmuseum.org
AF: The Field Museum, Department of Geology 1400 S. Lake Shore Drive, Chicago, IL 60605 United States
AU: Hesselbo, S P
EM: stephess@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX13P3 United Kingdom
AU: Murphy, J W
EM: wJ.Wade-Murphy@students.uu.nl
AF: Utrecht University, Laboratory of Paleobotany and Palynology, Utrecht, 3584 CD Netherlands
AB: 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 ñ 100 ppmV and 2.5 ^oC global cooling followed by an abrupt 1200 ñ 400 ppmV pCO₂ increase and 6.5 ñ 1^oC 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.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0439 Ecosystems, structure and dynamics (4815)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005