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Current models of the global carbon cycle lack natural mechanisms to explain known large, transient shifts in past records of the stable carbon-isotope ratio (δ¹³C) of carbon reservoirs^,. The injection into the atmosphere of ∼1,200–2,000 gigatons of carbon, as methane from the decomposition of sedimentary methane hydrates, has been proposed to explain a δ¹³C anomaly^, associated with high-latitude warming and changes in marine^,, and terrestrial biota near the Palaeocene–Eocene boundary, about 55 million years ago. These events may thus be considered as a natural ‘experiment’ on the effects of transient greenhouse warming. Here we use physical, chemical and spectral analyses of a sediment core from the western North Atlantic Ocean to show that two-thirds of the carbon-isotope anomaly occurred within no more than a few thousand years, indicating that carbon was catastrophically released into the …