Terrestrial record of methane hydrate dissociation in the Early Cretaceous

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 (∆ ≈ -5‰) in atmospheric δ¹³C, followed by a rapid positive compensation (∆ ≈ +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.