Geologic Time: A Story in the Rock PDF

SH1632 Geologic Time gradual extinctions-but they did not find them. Eventually, the evidence for near instantaneous extinctions became compelling. A Story i...

SH1632 Geologic Time gradual extinctions-but they did not find them. Eventually, the evidence for near instantaneous extinctions became compelling. A Story in the Rock From studies of this type, scientists learned that at least six (6) times in Earth history, catastrophic extinctions decimated life on Earth Rocks, Earth History, and Mass Extinction Earth. In many locations, 19th century geologists found thick The most dramatic extinction occurred 248 million years sequences of rock layers with abundant fossils that we know ago, at the end of the Permian Period. At that time, 90% of all represent millions, or hundreds of millions, of years of Earth’s species in the oceans suddenly died out. On land, about two- history. But younger layers above the fossil-rich rocks contained thirds (2/3) of reptile and amphibian species and 30% of insect few fossils or none at all. Looking even higher in the sequence species vanished. of rock layers, they found abundant fossils again, but of A modern scientist exclaimed that this extinction event organisms very different from those in the older rocks. Even was “the closest life has come to complete extermination since more surprising, fossils of many of the most abundant animals its origin.” and plants in lower layers were never seen again in younger The death of the most life-forms at the end of Permian rocks. Those organisms had simply disappeared from the face of time left huge ecological voids in the biosphere. Ocean Earth forever. ecosystems changed as new organisms emerged in an This fossil record suggests that sudden catastrophic environment free of predators and competition. On land, events had abruptly decimated life on Earth, and that new life- dinosaurs and many other terrestrial animals and plants emerged forms emerged following these mass extinctions. However, and proliferated. scientists did not immediately accept the idea that catastrophic About 180 million years later, at the end of Cretaceous events had caused sudden mass extinctions. Instead, they times, another catastrophic extinction wiped out one-fourth (1/4) suggested that the rock record might not be complete. Maybe of all species, including the dinosaurs. This mass extinction some rocks had been destroyed by erosion, or perhaps occurred 65 million years ago. Small mammals survived this sedimentary rocks had not formed for a long period of time. disaster, facing a new world free of the efficient predators that According to this reasoning, the part of the record that was lost had haunted them. The number of new mammal species could have contained evidence for the gradual decline of the increased rapidly after this extinction event, eventually leading species that became extinct, and for the slow emergence of new to the evolution of humans. species. Following conventional wisdom of time, scientists concluded that the extinctions of old-life forms and emergence Extraterrestrial Impacts of new ones occurred slowly, as a result of gradually changing In 1977, the father-and-son team of Walter and Luis conditions. Alvarez were studying rocks that formed at the time the But if pieces of the rock record were missing in some dinosaurs and so many other life-forms became extinct. In one locations, they must exist in other places on Earth. Geologists layer, they found abundant dinosaur fossils. Just above it, they searched for rocks to fill the gaps and provide evidence for 10 Handout 1 *Property of STI Page 1 of 3  SH1632 found very few fossils of any kind and no dinosaur fossils. gas, sulfur dioxide, forms small particle called aerosols that also Between these two (2) rock layers, they found a thin, sooty, clay reflect sunlight and cool Earth. Carbon dioxide is another gas layer. They brought samples of the clay to the laboratory, and emitted by volcanoes. It is a greenhouse gas that can cause found that it contained high concentrations of the element warming of Earth's atmosphere. Thus, volcanoes erupt materials iridium. This discovery was surprising because iridium is rare in capable of causing both atmospheric cooling and warming. In Earth rocks. Where did it come from? many eruptions, the cooling overwhelms the warming effects. Although rare in Earth’s crust, iridium is more abundant This cooling can occur rapidly-within a few weeks or months of in meteorites. Walter and Luis Alvarez suggested that 65 million the eruption. years ago, a meteorite 10 kilometers in diameter hit the Earth Scientists have noted that some mass extinctions with energy equal to 10,000 times that of today’s entire global coincided with unusually high rates of volcanic activity. For nuclear arsenal. The collision vaporized both the meteorite and example, massive flood basalts erupted onto Earth's surface in the Earth’s crust at the point of impact, forming a plume of hot Siberia 248 million years ago-at the same time that the Permian dust and gas that ignited fires around the planet. Soot from the extinction occurred. According to one hypothesis, explosive global wildfires and iridium-rich meteorite dust rose into the volcanic eruptions blasted massive amounts of volcanic ash and upper atmosphere, circling the globe. The thick, dark cloud sulfur aerosols into the air. The ash and aerosols spread like a blocked out the Sun and halted photosynthesis for as much as a pall throughout the atmosphere, blocking out the Sun. Earth's year. Surface waters froze, and many plants and animals died. atmospheric and oceanic temperatures plummeted, plants This event, called the terminal Cretaceous extinction, killed off withered, and animals starved or froze to death. If this the dinosaurs and many other life-forms. Then the sooty, hypothesis is correct, the Permian extinction (and perhaps iridium-rich dust settled to Earth to form the distinctive clay others) resulted from a classic Earth systems interaction, where layer. a volcanic eruption occurring within the geosphere altered the Other scientists have found evidence of huge meteorite composition of the atmosphere, with profound consequences for crater that formed 65 million years ago in the Caribbean Sea atmospheric and marine temperatures, and for life. north of the Yucatan Peninsula. Most scientists now agree with the meteorite impact hypothesis for the terminal Cretaceous Supercontinents and Earth’s Carbon Dioxide Budget extinction. Some scientists have suggested that an In the modern oceans, cold polar seawater sinks to the extraterrestrial impact also caused the Permian extinction, but sea floor and flows as a deep ocean current toward the equator. the evidence is less compelling. This cur- rent transports oxygen to the deep ocean basins and forces the deep water to rise near the equator, where it is Volcanic Eruptions warmed. The current thus mixes both heat and gases throughout A volcanic eruption ejects gas and fine volcanic ash into the oceans and the atmosphere. the atmosphere. The ash acts as an umbrella that reflects sunlight Ocean currents are partially controlled by the positions back into space and thus causes climatic cooling. One volcanic of the continents. Recall that the continents slowly drift across 10 Handout 1 *Property of STI Page 2 of 3  SH1632 the globe. Several times in Earth’s history, all the continents At this point, the cool, dense surface water sank. The sinking have joined together to form one giant supercontinent. One such waters forced the carbon dioxide-rich deep water to rise to the supercontinent assembled during Permian time. Computer sea surface, where it rapidly released massive amounts of carbon calculations indicate that the assembly of all continents into a dioxide into the atmosphere. According to these hypotheses, the single global ocean prevented mixing between the surface water carbon dioxide asphyxiated life both in the seas and on the and the ocean depths. continents, killing most of life on Earth. Later, new organisms Surface marine organisms absorb atmospheric carbon evolved when the carbon dioxide levels fell. dioxide and use it to produce organic tissue. These organisms Each of these three (3) hypotheses involves large-scale then die and settle to the sea floor. This process removes carbon interactions among the atmosphere, the hydrosphere, the dioxide from the atmosphere, converts the carbon to organic biosphere, and the geosphere, and the first also includes an tissue, and transports it to the sea floor. Organisms living near extraterrestrial factor. The third hypothesis, if correct, is a classic the deep sea floor then consume the fallen litter and release example of both Earth systems feedback mechanisms and a carbon dioxide into the deep ocean water. Much of this gas threshold effect. The feedback process occurred as tectonic plate dissolves in seawater and is held there by the pressure of movement altered the shapes of the ocean basins. Ocean currents overlying water. Modern ocean currents mix deep and shallow changed when the basins changed. The alteration in currents, in seawater, returning the carbon dioxide to the atmosphere. turn, changed atmospheric composition and unleashed a However, because there was little vertical mixing in the threshold event that rapidly killed most living organisms on Permian oceans, carbon accumulated in ever-increasing Earth. concentration in the deep oceans. This process continued to remove carbon dioxide from the atmosphere and stored it in the References: Jenkins, J. (2003, December 1). The essential guide to black canyon of the deep oceans during late Permian time. But carbon dioxide is a gunnison national park. Retrieved from greenhouse gas that absorbs heat and warms the atmosphere. As https://books.google.com.ph: carbon dioxide was removed from the air and stored in deep https://books.google.com.ph/books?id=W31yWpLZMxUC&pg=P water, the atmosphere cooled. Some scientists have suggested A46&lpg=PA46&dq=black+canyon+of+gunnison+national+park+ that continental ice sheets formed as a result of the global gneiss+and+dikes&source=bl&ots=xDthw4qD9m&sig=6SsPEa04 XqluSlqzZWlsT5AsYvY&hl=en&sa=X&ved=0ahUKEwiG6Nql3 cooling. Sea level fell as water evaporated from the oceans and JbKAhXBGZQKHceUBA0Q6AEIHzAB#v=onepage&q=bla accumulated on continental glaciers, exposing continental shelves and stressing populations of shallow-dwelling marine Turk, & Thompson. (2012). Introduction to earth science. Pasig organisms. City,Philippines: Cengage Learning Asia Pte Ltd. The cool air cooled the surface of the sea. When water cools, it becomes denser. Eventually, a threshold was reached at which polar surface water became denser than the deep water. 10 Handout 1 *Property of STI Page 3 of 3