The Eocene-Oligocene Extinction
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About 34 million years ago Earth got hit by at least three large objects. The impactors apparently came from a small asteroid that broke up, much as Comet Shoemaker-Levy did in 1992 before falling into Jupiter in 1994. The first piece hit Siberia about 880 kilometers northeast of Norilsk, creating the 100-kilometer wide Popigai crater at 71 39' N, 111 11' E. Later, after Earth had moved a little farther on its orbit, the other two objects came down, one creating the 22-kilometer wide crater in Tomís Canyon (39 08' N, 73 13' W), 160 kilometers east of the location of Atlantic City, New Jersey, and the other creating an 85-kilometer wide crater at the mouth of Chesapeake Bay (37 17' N, 76 01' W). Shortly, either before or after those impacts, another piece of the comet train hit the farside of the moon, raising a cloud of dust that settled into orbit around Earth as a ring.
The impact craters have been dated at roughly 35 million years ago, but no increase in the extinction rate of life occurs at that time, as we should expect. However, recent (as of June 2014) dating of rocks from Popigai crater redate that crater to 33.7 million years ago and tektites from the North American strewn field, which came from the Chesapeake impact, have been dated at around 34 million years ago. Those dates fall close to an extinction event 33.9 million years ago, marking the transition from the Eocene Period to the Oligocene Period. Further, the effects of the presumed dust ring, normal summers and bitterly cold winters, have been dated to around 34 million years ago.
Not as intense as the mass extinction of the Cretaceous-Paleogene boundary (which extinguished the dinosaurs) or the end-Permian event (the Great Dying), the Eocene-Oligocene event had more in common with the Jurassic-Cretaceous extinction, 145 million years ago. It was relatively minor and most of the extinguished organisms were marine or aquatic in nature. So what does the evidence tell us about what happened and what can we infer about the event?
The event began with a comet or asteroid that was broken up by passing within the Roche limit of one of the planets (or of the sun). The fragments from that breakup drifted apart and formed a cloud or a chain of pieces. Soon thereafter that array of pieces moved to cross Earthís orbit at a place that Earth was occupying at the time and thus precipitated the Popigai-Chesapeake Event.
Coming to ground at a minimum speed of 11 kilometers per second (and likely faster), each kilogram of the impactors carried a minimum of 60.5 million joules of energy (TNT contains 4.184 million joules per kilogram), enough energy to raise one kilogram of stone (such as basalt) to a temperature of over 70,000 degrees Kelvin (roughly 130,000 degrees Fahrenheit and higher). As each impactor punched into Earthís crust, it vaporized itself and the surrounding rock, producing a highly pressurized, super-hot gas that threw material upward and outward in a giant explosion. That material, some of it rising into space before re-entering the atmosphere as meteors, fell as a fiery rain. In the case of the Chesapeake impact and possibly the Tomís Canyon impact, some of the material came down as tektites in an area that geologists call the North American strewn field. Isotope dating of those tektites put the Chesapeake impact at about 34 million years ago.
Immediately following the Chesapeake and Tomís Canyon impacts the Atlantic basin was swept by tsunamis, which likely traveled around the world. We have an example of what happened then in the events surrounding the formation of Burckle Crater, on the floor of the Indian Ocean, in part because people recorded some of those effects. In that impact, on or around BCE 2807 May 10 (the dating provided by an associated solar eclipse that distant observers noted), a comet three miles wide punched into 3800 meters of water some 1500 kilometers southeast of Madagascar (at 30.865 S, 61.365 E) and blasted out a crater 29 kilometers wide. The result was a tsunami that rose as high as 180 meters and massive rain clouds from the condensing steam. The water surging up the Tigris-Euphrates Valley and inundating Mesopotamia while days of heavy rain added to the misery is recorded in the Epic of Gilgamesh and the story of Noahís ark. The Biblical reference to the fountains of the deep opening up describes the tsunami. Thatís the kind of disaster that scoured the east and west coasts of the Atlantic Ocean consequent to the Chesapeake and Tomís Canyon impacts.
Another major effect came from the shock waves propagating through Earthís interior. Spreading out from the impact, the shock waves refocused at the antipode and cracked Earthís crust, thereby producing flood vulcanism, just as the Chicxulub impact produced the flood basalts in India that make up the Deccan Plateau. The antipode of the Popigai impact lies under the Southern Ocean west of the Antarctic Peninsula and the antipodes of the Chesapeake and Tomís Canyon impacts lie under the Indian Ocean west of what is now New Guinea. The fact that the end-Eocene flood vulcanism occurred under deep water explains, in large part, why the Eocene-Oligocene extinction was a minor one and most of the extinguished species lived in the sea.
When lava heaves up out of the ground and surges across the landscape it releases gases, mainly carbon dioxide and sulphur oxides. When such lava flows occur on land, the gases rise into the atmosphere and change the climate in ways that stress the populations of many species to the point of extinction. When the lava flows across the ocean floor, especially under deep water, the gases get dissolved in the water: little of the gases reaches the atmosphere, but the ocean gets acidified and that acidity stresses marine species to extinction.
One last feature of the Popigai-Chesapeake Event added its misery to life on Earth. Paleontologists have inferred from fossil evidence that Earth went through a cooling spell 34 million years ago. Summers were relatively normal, but winters became bitterly cold, with temperatures going as much as twenty Centigrade degrees below normal winter temperatures. This is when the ice cap began to grow across Antarctica, putting that continent into a permanent deep freeze. The pattern suggests that at that time a ring of dust revolved around Earth, its particles following orbits with radii less than 16,000 kilometers. When the North Pole tilted toward the sun, sunlight would pass unimpeded over the ring and the Northern Hemisphere would have a normal summer, but when the North Pole bowed to the stars, sunlight headed toward the Northern Hemisphere would pass through the ring and by partly absorbed, resulting in a colder than normal winter. But whence came the ring?
Not all of the object that caused the Popigai-Chesapeake Event hit Earth. Some of the fragments soared back into interplanetary space, never to return, and at least one piece hit the farside of the moon. We know that it must have hit the farside because there is no major crater on the nearside thatís 34 million years old: the youngest big crater on the nearside is Tycho, dated at 108 million years ago. On the lunar farside the craters Cockroft and Doppler look large enough and new enough to make them candidates for the source of the dust, but we wonít know until expeditions explore them and determine the dates of their formation.
Thrown into space by the explosion, the dust, sharing the moonís orbital motion, would obey Keplerís second law of planetary motion (equal areas swept out in equal times; basically the law of conservation of angular momentum). As it moved away from Earth, the dustís orbital motion slowed, allowing the moon to move further ahead in its orbit, so that when the dust fell back it missed the moon and fell toward Earth. Further interaction with the moonís gravity altered the orbits of the dust cloudís particles and formed the dust into a ring that slowly shrank, altering the climate for centuries before all of the dust spiraled down into the atmosphere.
Thus we see how a single interplanetary object could cause a cluster of events that altered Earth in a way that extinguished many species of life and opened new niches for life to evolve into.
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