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Numerical evaluation of mechanisms driving Early Jurassic changes in global carbon cycling

¹ Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK

The Early Jurassic (early Toarcian, ca. 183 Ma) carbon cycle perturbation is characterized by an -5 ¹³C excursion in the exogenic carbon reservoirs, a 1000 ppm rise in atmospheric CO₂, and a 6-7 °C warming. Two proposed explanations for this presumed global carbon cycle perturbation are the liberation of massive amounts of isotopically light CH₄ from (1) Gondwanan coals by heating during the intrusive eruption of the Karoo-Ferrar large igneous province (LIP) or (2) the thermal dissociation of gas hydrates. Carbon cycle modeling indicates that the release of CH₄ from Gondwanan coals synchronous with the eruption of the Karoo-Ferrar LIP fails to reproduce the magnitude or timing of the CO₂ and ¹³C excursions. However, sensitivity analyses constrained by a marine cyclostratigraphically dated ¹³C record indicate that both features of geologic record can be explained with the huge input of 15,340-24,750 Gt C over 220 k.y., a result possibly pointing to the involvement of hydrothermal vent complexes in the Karoo Basin. The simulated release of >6000 Gt C from gas hydrates also reproduces aspects of the early Toarcian rock record, but the large mass involved raises fundamental questions about its formation, storage, and release.

Key Words: atmospheric CO₂ • carbon cycle • ¹³C • gas hydrates • Gondwanan coals

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