Asteroid Impact Lecture Notes

ASTEROID IMPACT By the end of the Cretaceous, dinosaurs along with 70 percent of all known species were extinct. Because the division between the Cretaceous and Tertiary, called the K-T boundary, is not a sharp breakthrough but may represent up to 1 million years or more, these extinctions are not always sudden. It can happen over a long period of time. Many dinosaurs along with other species were already in decline several million years before the end of the Cretaceous.Triceratops (Figure 171), whose large herds covered the entire world and may have contributed to the decline of other dinosaur species, were among the last dinosaurs to die. One theory attempts to explain the extinction of dinosaurs and more than 70 percent of other species at the end of the Cretaceous.One or so large asteroids or comets hit Earth with an explosive force equivalent to 100 trillion tons of dynamite, which equals 1 million eruptions of Mount St. Helens. The 3-foot-thick Sphereule layer found in the Gulf of Mexico is linked to a 65-million-year-old Chicxulub impact structure off Mexico's Yucatan Peninsula.Spherules resemble glass chondrules (spherical grains) in carbonaceous chondrites, which are carbon-rich meteorites, and in lunar soil. The impact sent 500 billion tons of debris into the atmosphere. Hot chunks of crater debris flying past and back through the atmosphere burned wildfires around the world as they landed. The raging fires burned perhaps a quarter of all vegetation on the continent, turning much of the Earth into smoldering ash. A thick blanket of dust and soot surrounds the entire globe and persists for months, cooling the planet and stopping photosynthesis. The planet is plunged into an environmental catastrophe. A catastrophe on this scale would destroy most terrestrial habitats and cause extinction of tragic proportions. Generally, no animal heavier than 50 pounds has survived extinction. The large body size seems to have been a severe disadvantage among land animals. Species living in the tropics that rely on warmth and steady sunlight, such as coral reef communities, have been particularly hard hit. For example, rudists, who build coral-like structures, completely die out along with half of all bivalve genera. The massive bombardment of meteorites may also have stripped the upper atmosphere of the ozone layer, bathing Earth in the Sun's deadly ultraviolet rays. Increased radiation will kill land plants and animals as well as major producers in ocean surface waters. Mammals, which were no larger than rodents, coexisted with dinosaurs for more than 100 million years. However, because they are mostly nocturnal and remain in their underground loans during the day, going out only at night to feed, mammals will be spared the onslaught of ultraviolet radiation during the day. After the bombing, Earth will succumb to a year of darkness under a thick brown fog of nitrogen oxides. Surface water, poisoned by traces of metal leached from soil and rock, and corrosive global rain such as battery acid will deal a deadly blow to terrestrial life forms. Plants that survive as seeds and roots will be relatively unharmed. High acidity levels will dissolve the calcium carbonate shells of marine organisms, while those with silica shells will survive as they have done during other crises. Land animals living in burrows will be well protected. Creatures living in lakes propped up against acid would have survived meteorite impacts quite well. The impact could also lead to widespread extinction of microscopic marine plants called calcareous nannoplankton, which produce sulfur compounds that aid cloud formation. With the death of these creatures, cloud cover would decrease dramatically, triggering a global heatwave extreme enough to kill dinosaurs and most marine species. This opinion is supported by the fossil record. This suggests that ocean temperatures rose by 5 to 10 degrees Celsius for tens of thousands of years after the end of the Cretaceous. During this time, over nearly half a million years, more than 90 percent of calcareous nannoplankton disappeared along with most of the marine life in the upper oceans. Sixty-five million-year-old sediments found at the boundary between the Cretaceous and Tertiary periods around the world (Fig.172) contain surprising quartz grains with characteristic lamellae, common soot from global wildfires, a rare amino acid known to exist only in meteorites, the mineral stishovite—a form of solid silica found nowhere but at known impact sites, and a unique concentration of iridium—a rare isotope of platinum that is relatively abundant in meteorites and comets but practically absent in the Earth's crust. The geological record holds clues to the impact of other giant meteorites with anomalous iridium concentrations that coincided with episodes of extinction. However, they were not nearly as strong as the iridium concentrations in the beds marking the end of the Cretaceous, which was as high as 1,000 times the background level. This suggests that the late Cretaceous extinction may have been a unique event in the history of life on Earth. After discussing the life that lived in the warm Cretaceous, the next chapter will examine the evolution of mammals and the geology of the Tertiary period.