Traps and thus may have been a significant additional factor explaining the severity of the end-Permian extinction. Methane clathrate gasification Methane clathrates, also known as methane hydrates, consist of methane molecules trapped in cages of water molecules. The methane, produced by methanogens (microscopic single-celled organisms), has a about 6.0% below normal ( −6.0%). At the right combination of pressure and temperature, the methane is trapped in clathrates fairly close to the surface of permafrost and, in much larger quantities, on continental shelves and the deeper seabed close to them. Oceanic methane hydrates are usually found buried in sediments where the seawater is at least deep. They can be found up to about below the sea floor, but usually only about below the sea floor. The release of methane from the clathrates has been considered as a cause because scientists have found worldwide evidence of a swift decrease of about 1% in the in carbonate rocks from the end-Permian. This is the first, largest, and most rapid of a series of negative and positive excursions (decreases and increases in ) that continues until the isotope ratio abruptly stabilised in the middle Triassic, followed soon afterwards by the recovery of calcifying life forms (organisms that use calcium carbonate to build hard parts such as shells). While a variety of factors may have contributed to this drop in the a 2002 review found most of them to be insufficient to account fully for the observed amount: Gases from volcanic eruptions have a about 0.5 to 0.8% below standard ( about −0.5 to −0.8%), but an assessment made in 1995 concluded that the amount required to produce a reduction of about 1.0% worldwide requires eruptions greater by orders of magnitude than any for which evidence has been found. (However, this analysis addressed only CO2 produced by the magma itself, not from interactions with carbon bearing sediments, as later proposed.) A reduction in organic activity would extract C more slowly from the environment and leave more of it to be incorporated into sediments, thus reducing the Biochemical processes preferentially use the lighter isotopes since chemical reactions are ultimately driven by electromagnetic forces between atoms and lighter isotopes respond more quickly to these forces, but a study of a smaller drop of 0.3 to 0.4% in ( −3 to −4 ‰) at the Paleocene-Eocene Thermal Maximum (PETM) concluded that even transferring all the organic carbon (in