A knowledge of what has happened in the past seems helpful in improving the predictability of the link between global-scale phenomena and the carbon cycle; this paper therefore attempts to reconstruct the end-Cretaceous carbon cycle (65 million years ago) by means of modeling. The performed simulation suggests that a great amount (130 gigatons at least) of biogenic carbon was rapidly injected to the atmosphere. Methane originating from gas hydrate (GH) is the most likely candidate for the input of biogenic carbon at the end of the ereCretaceous period because it is considered that thick GH stability zones were damaged by perturbations associated with the Chicxulub asteroid impact, and the vast amount of methane was released to the atmosphere as a gas blast. Though GH deposits are greater than other major reservoirs of carbon, these deposits are not commonly categorized as typical carbon reservoirs in terms of the global carbon cycle. How to integrate GH-related methane with well-known carbon reservoirs remains for a future study in order to improve the predictability of the future carbon cycle.