Reconstruction of changing C isotopic composition of Early Cretaceous atmospheric CO2 from fossilized C3 vascular land-plant tissue revealed a brief and striking negative excursion (Δ ≈ −5‰) in atmospheric δ13C, followed by a rapid positive compensation (Δ ≈ +5‰) during the Aptian (ca. 117 Ma). Mass-balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion; this would also result in an increased CO2:O2 mixing ratio as O2 is consumed during CH4 oxidation to CO2, spurring the exponential phase of angiosperm biogeographic expansion.

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