and since then much of the evidence that would have pointed to the cause has been destroyed or is concealed deep within the Earth under many layers of rock. The sea floor is completely recycled over around 200 million years by the ongoing process of plate tectonics and seafloor spreading, leaving no useful indications beneath the ocean. Yet, scientists have gathered significant evidence for causes, and several mechanisms have been proposed. The proposals include both catastrophic and gradual processes (similar to those theorized for the Cretaceous–Paleogene extinction event). The catastrophic group includes one or more large bolide impact events, increased volcanism, and sudden release of methane from the seafloor, either due to dissociation of methane hydrate deposits or metabolism of organic carbon deposits by methanogenic microbes. The gradual group includes sea level change, increasing hypoxia, and increasing aridity. Any hypothesis about the cause must explain the selectivity of the event, which affected organisms with calcium carbonate skeletons most severely; the long period (4 to 6 million years) before recovery started, and the minimal extent of biological mineralization (despite inorganic carbonates being deposited) once the recovery began. Volcanism Siberian Traps The flood basalt eruptions that produced the Siberian Traps constituted one of the largest known volcanic events on Earth and covered over with lava. Such a vast aerial extent of the flood basalts may have contributed to their exceptionally catastrophic impact. The date of the Siberian Traps eruptions and the extinction event are in good agreement. The timeline of the extinction event strongly indicates it was caused by events in the large igneous province of the Siberian Traps. A study of the Norilsk and Maymecha-Kotuy regions of the northern Siberian platform indicates that volcanic activity occurred during a small number of high intensity pulses that exuded enormous volumes of magma, as opposed to flows emplaced at regular intervals. The rate of carbon dioxide release from the Siberian Traps represented one of the most rapid rises of carbon dioxide levels in the geologic record, with the rate of carbon dioxide emissions being estimated by one study to be five times faster than the rate during the already catastrophic Capitanian mass extinction event, which occurred as a result of the activity of the Emeishan Traps in southwestern China at the end of the Middle Permian. Carbon dioxide levels prior to and after the eruptions are poorly constrained, but may have jumped from between