2012年11月3日雅思阅读机经[A类]

Mass Extinctions

Cases in which many species become extinct within a geologically short interval³ of time are called mass extinctions. There was one such event at the end of the Cretaceous period (around 70 million years ago). There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar⁴ species perished at that time.
The fossil record shows at least five mass extinctions in which many families of marine⁵ organisms died out. The rates of extinction happening today are as great as the rates during these mass extinctions. Many scientists have therefore concluded that a sixth great mass extinction is currently in progress.
What could cause such high rates of extinction? There are several hypotheses, including warming or cooling of Earth, changes in seasonal⁶ fluctuations⁷ or ocean currents, and changing positions of the continents. Biological hypotheses include ecological⁸ changes brought about by the evolution of cooperation between insects and flowering plants or of bottom-feeding predators⁹ in the oceans. Some of the proposed mechanisms¹⁰ required a very brief period during which all extinctions suddenly took place; other mechanisms would be more likely to have taken place more gradually, over an extended period, or at different times on different continents. Some hypotheses fail to account for simultaneous extinctions on land and in the seas. Each mass extinction may have had a different cause. Evidence points to hunting by humans and habitat destruction as the likely causes for the current mass extinction.
American paleontologists David Raup and John Sepkoski, who have studied extinction rates in a number of fossil groups, suggest that episodes of increased extinction have recurred¹² periodically, approximately every 26 million years since the mid-Cretaceous period. The late Cretaceous extinction of the dinosaurs and am monoids was just one of the more drastic in a whole series of such recurrent extinction episodes. The possibility that mass extinctions may recur¹¹ periodically has given rise to such hypotheses as that of a companion star with a long-period orbit deflecting¹³ other bodies from their normal orbits, making some of them fall to Earth as meteors and causing widespread devastation¹⁴ upon impact.
Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid¹⁵-impact hypothesis first suggested by Luis and Walter Alvarez. According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids¹⁶, the combined mass of which was comparable. The force of collision spewed large amounts of debris¹⁷ into the atmosphere, darkening the skies for several years before the finer particles settled. The reduced level of photosynthesis¹⁸ led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores. The mass extinction would have occurred very suddenly under this hypothesis.
One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth' s crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric¹⁹ currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly²⁰ and precisely²¹ at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.
An asteroid of this size would be expected to leave an immense crater²², even if the asteroid itself was disintegrated²³ by the impact. The intense heat of the impact would produce heat-shocked quartz²⁴ in many types of rock. Also, large blocks thrown aside by the impact would form secondary craters²⁵ surrounding the main crater. To date, several such secondary craters have been found along Mexico's Yucatan Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti. A location called Chicxulub, along the Yucatan coast, has been suggested as the primary impact site.