Abstract

The Early Jurassic (early Toarcian, ca. 183 Ma) carbon cycle perturbation is characterized by an ∼-5‰ δ13C 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 δ13C excursions. However, sensitivity analyses constrained by a marine cyclostratigraphically dated δ13C 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.

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