GEOLOGY 1H - Explosions and Extinctions (PDF)

Explosion and Extinction GEOLOGY 1H Review: What is the Geologic Time Scale? It divides up the history of the earth based on life-forms that have existed during specific times since the creation of the planet. These divisions are called geochronologic units (geo: rock, chronology...

Explosion and Extinction GEOLOGY 1H Review: What is the Geologic Time Scale? It divides up the history of the earth based on life-forms that have existed during specific times since the creation of the planet. These divisions are called geochronologic units (geo: rock, chronology: time). Most of these life-forms are found as fossils, which are the remains or traces of an organism from the geologic past that has been preserved in sediment or rock. Without fossils, scientists may not have concluded that the earth has a history that long precedes mankind. How Old is Old? From the time of Hutton, scientists were convinced that the earth was much older than the 6000 years predicted by the religious scholars. Charles Lyell tried to estimate the age of the earth through the amount of evolution exhibited by marine mollusks in a specific time system. Another method was to estimate the rate of deposition for sedimentary rocks. Another scholar proposed to estimate the age of the earth using salt content of the oceans, assuming that the oceans were once non-saline and that salt addition to the oceans corresponded in some linear fashion with time. Lord Kelvin estimated the age of the Earth at 24-40 million years. He proposed that the Earth has been cooling since it formed, and he calculated the rate of cooling using principles of heat conduction. It wasn’t until Henri Becquerel discovered radioactivity in 1896 and Madame Curie isolated radium 2 years later that people realized that the Earth had it’s own source of heat. Thus it became one of the most useful tools for future scientists. How Old is Old? The oldest rocks found so far on Earth (based on zircon grains from Australia) have been dated currently at about 4.4 billion years. Meteorites have also been dated at 4.6 billion years. Meteorites are considered to be remnants of a planet or asteroid that originally formed at the same time as the Earth, so that the Earth’s age is currently estimated to be 4.6 billion years. The oldest fossils are preserved remains of stromatolites, which are layers of lithified blue-green algae, dating to approximately 3.5 billion years before present. How Old is Old? Other than the origin of life itself, the most significant biologic event of the Archean was the development of the autotrophic process of photosynthesis as much as 3.5 billion years ago. The cells were still prokaryotic and anaerobic, but as autotrophs, they were no longer dependent on preformed organic molecules as a source of nutrients. The Archean microfossils are anaerobic, autotrophic prokaryotes. Numerical Dating Methods Radioactive decay is the process whereby an unstable atomic nucleus is spontaneously transformed into an atomic nucleus of a different element. The half-life of a radioactive element is the time it takes for one-half of the atoms of the original unstable parent element to decay to atoms of a new, more stable daughter element. The half-life of a given radioactive element is constant and can be precisely measured. Half-lives of various radioactive elements range from less than a billionth of a second to 106 billion years. Five of the Principal Long-Lived Radioactive Isotope Pairs Used in Radiometric Dating Explosions What Was the Cambrian Explosion? At the beginning of the Paleozoic Era, animals with skeletons appeared rather abruptly in the fossil record. Their appearance is described as an explosive development of new types of animals, and is referred to as the Cambrian explosion by most scientists. This sudden appearance of new animals in the fossil record is rapid, however, only in the context of geologic time, having taken place over millions of years during the Early Cambrian Period. Explosions What Was the Cambrian Explosion? Up until the evolution of the Ediacaran fauna, Earth was populated primarily by single-celled organisms. The Ediacaran fauna, which is found on all continents except Antarctica, consists primarily of multi-celled soft-bodied organisms. Microscopic calcareous tubes, presumably housing wormlike suspension-feeding organisms, have also been found at some localities. In addition, trails and burrows, which represent the activities of worms and other sluglike animals, are also found associated with Ediacaran faunas throughout the world. The trails and burrows are similar to those made by present-day soft-bodied organisms. Explosions What Was the Cambrian Explosion? The Cambrian explosion probably had its roots firmly planted in the Proterozoic. The mechanism or mechanisms that triggered this event, however, are still being investigated. Although some would argue for a single causal event, the Cambrian explosion was more likely a combination of factors, both biologic and geologic. Explosions What Was the Cambrian Explosion? Another hypothesis is that the rapid evolution of a skeletonized fauna was a response to the evolution of predators. A shell or mineralized covering would have provided protection against the various predators evolving during this time. The conflict between predators, whose goal is to consume prey, and prey, whose goal is to avoid becoming a meal for a predator, is one of the strongest factors in natural selection. Explosions What Was the Cambrian Explosion? Whatever the ultimate cause of the Cambrian explosion, the appearance of a skeletonized fauna and the rapid diversification of that fauna during the Early Cambrian were major events in life history. [emergence of a shelly fauna leading to your modern-day marine ecosystem diversity] Extinctions Mass extinction – are greatly accelerated extinction rates resulting in marked decrease in biodiversity. Judging from the fossil record, most organisms that existed are now extinct—perhaps as many as 99% of all species. Natural selection does not yield some kind of perfect organism, but rather those adapted to a specific set of circumstances at a particular time. Thus, a clam or lizard existing now is not somehow superior to a clam or lizard that lived millions of years ago. Extinctions The continual extinction of species is referred to as background extinction, to clearly differentiate it from a mass extinction, during which accelerated extinction rates sharply reduce Earth’s biotic diversity. When the dinosaurs and their relatives died out, the mammals began a remarkable diversification as they started occupying the niches left temporarily vacant. Extinctions Everyone has heard of the mass extinction of dinosaurs and some of their relatives at the end of the Mesozoic Era. The greatest mass extinction, though, was at the end of the Paleozoic Era, when perhaps more than 96% of all marine invertebrate species and approximately 70% of all terrestrial vertebrate species died out. END of Part 1 What are mass extinctions, and what causes them? In the last 500 million years, life has had to recover from five catastrophic events. In this period, 75 to more than 90 % of all species on Earth have disappeared in a geological blink of an eye in catastrophes we call mass extinctions. Though mass extinctions are deadly events, they open up the planet for new forms of life to emerge. The most studied mass extinction, which marked the boundary between the Cretaceous and Paleogene periods about 66 million years ago, killed off the non-avian dinosaurs and made room for mammals and birds to rapidly diversify and evolve. What are mass extinctions, and what causes them? The single biggest driver of mass extinctions appears to be major changes in Earth’s carbon cycle such as large igneous province eruptions, huge volcanoes that flooded hundreds of thousands of square kilometers with lava. These eruptions ejected massive amounts of heat-trapping gases such as carbon dioxide into the atmosphere, enabling runaway global warming and related effects such as ocean acidification and anoxia, a loss of dissolved oxygen in water. Ordovician-Silurian extinction - 444 Mya the first known mass extinction struck as the Ordovician period ended massive glaciation locked up huge amounts of water in an ice cap that covered parts of a large south polar landmass The icy onslaught may have been triggered by the rise of North America’s Appalachian Mountains. The large-scale weathering of these freshly uplifted rocks sucked carbon dioxide out of the atmosphere and drastically cooled the planet. Ordovician-Silurian extinction - 444 Mya As a result, sea levels plummeted by hundreds of feet. Creatures living in shallow waters would have seen their habitats cool and shrink dramatically, dealing a major blow. Whatever life remained recovered haltingly in chemically hostile waters. Once sea levels started to rise again, marine oxygen levels fell, which in turn caused ocean waters to more readily hold onto dissolved toxic metals. The second worst mass extinction known to science, this event killed an estimated 85 percent of all species. The event took its hardest toll on marine organisms such as corals, shelled brachiopods, eel-like creatures called conodonts, and the trilobites. Late Devonian extinction - 383-359 Mya Starting 383 million years ago, this extinction event eliminated about 75 percent of all species on Earth over a span of roughly 20 million years. ocean oxygen levels dropped, which dealt serious blows to conodonts and ancient shelled relatives of squid and octopuses called goniatites. The worst of these pulses, called the Kellwasser event, came about 372 million years ago. Rocks from the period, in what’s now Germany, show that as oxygen levels plummeted, many reef-building creatures died out, including a major group of sea sponges called the stromatoporoids. Late Devonian extinction - 383-359 Mya It’s been hard to nail down the cause for the late Devonian extinction pulses, but volcanism is a possible trigger. Within a couple million years of the Kellwasser event, a large igneous province called the Viluy Traps erupted 240,000 cubic miles of lava in what is now Siberia. The eruption would have spewed greenhouse gases and sulfur dioxide, which can cause acid rain. Asteroids may also have contributed. Sweden’s 32-mile-wide Siljan crater, one of Earth’s biggest surviving impact craters, formed about 377 million years ago. Permian-Triassic extinction - 252 Mya life on Earth faced the “Great Dying”: the Permian-Triassic extinction the single worst event life on Earth has ever experienced Over about 60,000 years, 96 percent of all marine species and about three of every four species on land died out. The world’s forests were wiped out and didn’t come back in force until about 10 million years later. Of the five mass extinctions, the Permian-Triassic is the only one that wiped out large numbers of insect species. Marine ecosystems took four to eight million years to recover. Permian-Triassic extinction - 252 Mya The extinction’s single biggest cause is the Siberian Traps, an immense volcanic complex that erupted more than 720,000 cubic miles of lava across what is now Siberia. The eruption triggered the release of at least 14.5 trillion tons of carbon, more than 2.5 times what’d be unleashed if every last ounce of fossil fuel on Earth were dug up and burned. Adding insult to injury, magma from the Siberian Traps infiltrated coal basins on its way toward the surface, probably releasing even more greenhouse gases such as methane. Permian-Triassic extinction - 252 Mya The resulting global warming was downright hellish. By 250.5 million years ago, sea surface temperatures at the Equator got as high as 104 degrees Fahrenheit (40ºC), a hot tub's standard maximum temperature. At the time, almost no fish lived near the Equator. Permian-Triassic extinction - 252 Mya As temperatures rose, rocks on land weathered more rapidly, hastened by acid rain that formed from volcanic sulfur. Just as in the late Devonian, increased weathering would have brought on anoxia that suffocated the oceans. Climate models suggest that at the time, the oceans lost an estimated 76 percent of their oxygen inventory. These models also suggest that the warming and oxygen loss account for most of the extinction’s species losses. Triassic-Jurassic extinction - 201 Mya Life took a long time to recover from the Great Dying, but once it did, it diversified rapidly. Different reef-building creatures began to take hold, and lush vegetation covered the land, setting the stage for a group of reptiles called the archosaurs: the forerunners of birds, crocodilians, pterosaurs, and the non-avian dinosaurs. But about 201 million years ago, life endured another major blow: the sudden loss of up to 80 percent of all land and marine species. Triassic-Jurassic extinction - 201 Mya At the end of the Triassic, Earth warmed an average of between 5 and 11 degrees Fahrenheit, driven by a quadrupling of atmospheric CO2 levels. This was probably triggered by huge amounts of greenhouse gases from the Central Atlantic Magmatic Province, a large igneous province in central Pangaea, the supercontinent at the time. Remnants of those ancient lava flows are now split across eastern South America, eastern North America, and West Africa. Triassic-Jurassic extinction - 201 Mya The uptick in CO2 acidified the Triassic oceans, making it more difficult for marine creatures to build their shells from calcium carbonate. On land, the dominant vertebrates had been the crocodilians, which were bigger and far more diverse than they are today. Many of them died out. In their wake, the earliest dinosaurs—small, nimble creatures on the ecological periphery—rapidly diversified. Cretaceous-Paleogene extinction - 66 Mya The Cretaceous-Paleogene extinction event is the most recent mass extinction and the only one definitively connected to a major asteroid impact. Some 76 percent of all species on the planet, including all non-avian dinosaurs, went extinct. One day about 66 million years ago, an asteroid roughly 7.5 miles across slammed into the waters off of what is now Mexico’s Yucatán Peninsula at 45,000 miles an hour. Cretaceous-Paleogene extinction - 66 Mya The massive impact—which left a crater more than 120 miles wide—flung huge volumes of dust, debris, and sulfur into the atmosphere, bringing on severe global cooling. Wildfires ignited any land within 900 miles of the impact, and a huge tsunami rippled outward from the impact. Overnight, the ecosystems that supported nonavian dinosaurs began to collapse. Global warming fueled by volcanic eruptions at the Deccan Flats in India may have aggravated the event. Some scientists even argue that some of the Deccan Flats eruptions could have been triggered by the impact. Extinction today Earth is currently experiencing a biodiversity crisis. Recent estimates suggest that extinction threatens up to a million species of plants and animals, in large part because of human activities such as deforestation, hunting, and overfishing. Other serious threats include the spread of invasive species and diseases from human trade, as well as pollution and human-caused climate change. Extinction today Climate change presents a long-term threat. Humans’ burning of fossil fuels has let us chemically imitate large igneous provinces, through the injection of billions of tons of carbon dioxide and other gases into Earth’s atmosphere each year. By total volume, these past volcanoes emitted far more than humans do today; the Siberian Traps released more than 1,400 times the CO2 than humans did in 2018 from burning fossil fuels for energy. However, humans are emitting greenhouse gases as fast as—or even faster than—the Siberian Traps, and Earth’s climate is rapidly changing as a result. Extinction today As mass extinctions show us, sudden climate change can be profoundly disruptive. And while we haven’t yet crossed the 75-percent threshold of a mass extinction, that doesn’t mean things are fine. Well before hitting that grim marker, the damage would throw the ecosystems we call home into chaos, jeopardizing species around the world—including us. END