Original sentence: A variety of factors may have contributed to this drop.Biochemical processes 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  6,500–8,400 gigatons of organic carbon would have to be oxidized and returned to the ocean-atmosphere system within less than a few hundred thousand years to reduce the  by 10 ‰ — this scenario is unrealistic.Gases from volcanic eruptions have a  about -5 to -8 ‰, the amount required to reduce  by 10 ‰ worldwide would require eruptions orders of magnitude larger than those for which occurred.Modelling shows that the dominant factor reducing the  was the release of methane from methane clathrates.Other hypotheses include mass oceanic poisoning releasing vast amounts of  and a long-term reorganisation of the global carbon cycle
Edited sentence: Biochemical processes 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  6,500–8,400 gigatons of organic carbon would have to be oxidized and returned to the ocean-atmosphere system within less than a few hundred thousand years to reduce the  by 10 ‰, which would be an unrealistic timeframe.Gases from volcanic eruptions have a  about 5 to 8 ‰ below normal, but the amount required to produce a reduction of about 10 ‰ worldwide would require eruptions greater by orders of magnitude than any for which evidence has been found.Most proposed causes for such an isotope reduction are insufficient.Only one sufficiently powerful cause has been proposed for the global 10 ‰ reduction in the : the release of methane from methane clathrates