Transcript ppt
Class 10: Other mass extinctions & the Nemesis theory Other mass extinctions – impacts? Search for the corresponding craters. Periodic mass extinctions. The Nemesis theory. Jon Burnett, Oct 1/03, Wales I : The Big Five extinctions… There were 5 extinctions that killed 70% or more of species… 65 Myr (Cretaceous-Tertiary) 202 Myr (Triassic-Jurassic) 250 Myr (Permian-Triassic) 367 Myr (Late Devonian) 438 Myr (Ordovician-Silurian) But there were other periods of mass extinction as well… II : Other mass extinctions RaupSepkoski Seems to be a periodicity in the mass extinctions… Appears to be a mass extinction approx. every 26-30 million years. Regularly spaced… like ticks of a clock. Warning: this claim is still very controversial! It can sometimes be very hard to prove or disprove that a particular signal is periodic. III : Are all mass extinctions caused by impacts? This would be a very extreme view… If true, impacts would be one of the most important forces in evolution. Still very very controversial and vigorously debated. Impact theory would be boosted if we could find craters corresponding to the other mass extinctions. But remember how hard (sometimes impossible) it is to find impact craters on Earth… and we must accurately date them. Morasko, Poland In fact, this is a whole group of craters… Late Devonian Siljan, Sweden (368 Myr) 52 km diameter Permian-Triassic Arjguinha, Brazil (245 Myr) 44 km diameter Triassic-Jurassic Manicouagan, Canada (214 Myr) 100 km Jurassic-Cretaceous Morokweng, South Africa (145 Myr) 70 km (not exposed) Cretaceous-Tertiary Chicxulub, Mexico (65 Myr) 170 km Eocene-Oligocene transition (35 Myr ago) Chesapeake Bay crater… 35Myr IV : What could cause periodic impacts and extinctions? Let’s assume that large impacts are periodic (Shiva Hypothesis). Only plausible scenario… One/several of them occur like clockwork every 2630 million years. Oort cloud suffers a regular disturbance every 26-30 million years. Disturbance shifts the orbits of many distant comets, making them plunge into the inner solar system. A small number of those comets hit the Earth, causing the mass extinctions. But what could disturb the Oort cloud like this? Possibility I As the Sun orbits the galaxy, we “bob” up and down within the galactic disk. We probably pass through middle of galactic disk once every ~30 million years. Maybe this, somehow, disturbs the Oort cloud. Comets rain inwards… Possibility II (Nemesis Theory) The Sun has a companion star in orbit. Once every 30 million years, that star comes close to (plunges through?) Oort cloud. Comets rain inwards… We’ve yet to find this star (people have looked!).