These include : large or multiple meteorite impacts , increased volcanism , sudden release of methane hydrates from the sea floor . Gradual changes include sea level change , oceanic anoxic events , increasing aridity , and a shift in ocean circulation driven by climate change . Suggested mechanisms for the latter include large or multiple bolide impact events , increased volcanism , and sudden release of methane clathrate from the sea floor ; gradual changes include sea-level change , anoxia , increasing aridity , and a shift in ocean circulation driven by climate change .