During the UppermostP. gigas conodont zone (P. linguiformis zone) the stepwise Frasnian extinction culminates in the global transgressive-regressive Frasnian-Famennian »Kellwasser event«. Most pelagic organisms of the tropical marine environment are more or less affected by this mass extinction. During the early Upper Devonian extinction rates are clearly related to the deposition of black shales and the storage of a great amount of organic carbon in the (shelf) sediments. Contemporaneous positive excursions of the δ13C recorded from pelagic limestones of Europe and Australia prove a worldwide change in the CO2 content of the oceans surface water which is in balance with the atmosphere. If the climate is controlled by the CO2 content of the atmosphere, a cyclic operation process might have developed with the succession: transgression — high organic production — deposition of organic carbon — lowering of CO2 in the atmosphere — icehouse effect — polar glaciation — regression — erosion of organic carbon and re-supply of CO2 to the ocean and atmosphere — greenhouse effect — deglaciation — transgression. During the change from a greenhouse to an icehouse effect an overturn of the anoxic oceans probably was one of the causes for the mass killing of low latitude marine organisms.