SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

ASTEROIDS, CLIMATE CHANGE, AND MASS EXTINCTIONS

Geologists and other scientists have gathered evidence from the fossil record of five major mass extinctions that occurred over the past 550 million years (Table 1). The elimination of up to 50% or more of all plant and animal species then in existence during any one of these episodes points to drastic changes in the Earth's environment that exceeded the tolerance limits of vast numbers of organisms. What caused these mass extinctions?

Prior to 1980, the most popular explanation for mass extinctions was a gradual decrease in species number (perhaps over millions of years) due to long-term climate change coupled with ecological forces. In 1980, however, another much more dramatic explanation took center stage. The father-son team of scientists Luis (1911-1988) and Walter (1940- ) Alvarez of the University of California, Berkeley, proposed that an asteroid impact on Earth was responsible for the mass extinction that took place 65 million years ago. This event was known as the K-T mass extinction, named for the boundary between the Cretaceous and Tertiary periods of geological time. The Alvarez team presented convincing evidence of an asteroid impact including the presence of iridium (Ir) in sedimentary layers from around the world and all dating from 65 million years ago. Iridium is a silver-gray metallic element that is extremely rare in Earth's crust. Asteroids, however, contain a much higher concentration of iridium than does the Earth's crust. The Alvarez hypothesis was bolstered by features found within and near the impact site.

The K-T asteroid produced a 180-km (112-mi) wide crater on the floor of the ancient Caribbean Sea. Marine sediments gradually filled the crater and geological forces later elevated a portion of the crater above sea level. Today, what remains of the Chicxulub crater forms part of Mexico's Yucatán Peninsula. Radar images obtained by the Space Shuttle Endeavour in 2000 revealed a 5-m (16-ft) deep, 5-km (3-mi) wide trough on the Yucatán Peninsula that may mark the outer rim of the crater. Drilling through the layers of sediment on the floor of the nearby Gulf of Mexico recovered cores of fractured and melted rock from the impact zone.

Other evidence of the impact consists of bits of tiny bead-like spherules of glassy rock, which originated as droplets of molten rock blasted into the atmosphere by the impact. These droplets cooled as they fell through the atmosphere onto the land or into the ocean. They were recovered from nearby deep-ocean sediments. Many rocks on land contain mineral grains deformed by the extreme heat and pressure produced by the impact (e.g., shocked quartz). Unusual sediment deposits were produced by enormous waves (tsunamis) generated when the asteroid (about 10 km or 6 mi in diameter) struck the ocean surface. In addition, a layer of soot indicates considerable burning vegetation on land.

The K-T asteroid impact had a catastrophic effect on life. Best known is the extinction of the dinosaurs, which had dominated life on Earth for more than 250 million years. Dinosaurs were not the only victims, however. The asteroid impact destroyed more than 50% of the other life forms then existing on the planet. The impact also caused major extinctions among many groups of marine organisms including plankton.

What precisely caused this ecological disaster? One widely accepted theory is that the asteroid impact vaporized large amounts of sulfur-containing deep-sea sediments. This sulfur was blown into the atmosphere where it generated enormous clouds of tiny sulfate particles likely augmented by meteoric and Earth materials also thrown into the atmosphere by the impact. These clouds greatly reduced the sunlight reaching Earth's surface for a long period (perhaps 8 to 13 years) and most plants died because they could not photosynthesize. Furthermore, precipitation decreased by up to 90%. In this dark, cold and dry environment, dinosaurs and other animals who depended on plants for food starved and the carnivores that fed on them soon met the same fate. Only small animals (like mammals) could survive by eating the dead plants and animals until conditions improved and new food sources became available. Eventually, aerosols settled out of the atmosphere, photosynthesis resumed, and new plants replaced those lost following the impact. Small mammals evolved rapidly to take the place of the dinosaurs. Another possibility is that red-hot, impact-generated particles rained down through the atmosphere making it so hot that most plants and animals were killed directly.

In the 1980s and 1990s, the Alvarez theory of asteroid impact was widely accepted as the cause of all but one of the five major mass extinctions (Table 1). However, a vocal minority of scientists took exception to the preeminent role of asteroid impact, arguing that many of the major mass extinctions were linked to volcanic activity and increased atmospheric CO₂. The largest eruptions of flood basalts closely correspond in age to the times of most major mass extinctions. Flood basalts consist of successive lava flows erupted from fissures in Earth's crust, accompanied by gases released to the atmosphere, including hydrogen sulfide (H₂S), and the greenhouse gases carbon dioxide (CO₂) and methane (CH₄). Flood basalt eruptions can be enormous. The world's largest flood basalt eruptions (that produced the Siberian Traps) delivered about 4.2 million cu km (1 million cu mi) of lava over an area of almost 7.8 sq. km (3 million sq. mi) about 252 to 248 million years ago. This eruption was very near the time of the great Permian mass extinction (about 251 million years ago) when 90% of marine species and 70% of terrestrial species on Earth were wiped out. No evidence of an asteroid impact has been found to explain the Permian extinction. In addition, most mass extinctions took place during times when the concentration of atmospheric CO₂ was relatively high or rapidly rising.

By 2005, a new hypothesis was in place that attributed most major mass extinctions to a combination of chemical and circulation changes in the ocean coupled with global warming due to an enhanced greenhouse effect. In arriving at this alternate explanation for mass extinctions, scientists relied on analysis of biomarkers where fossils were absent. Biomarkers are the organic chemical residue of organisms extracted from ancient strata.

According to research conducted by Lee Kump and his colleagues at Pennsylvania State University, the late Permian ocean was stratified. The bottom water had little or no dissolved oxygen while the shallow surface layer was oxygenated. (Today's ocean is oxygenated from top to bottom.) With the release of greenhouse gases to the atmosphere during eruptions that produced the Siberian Traps, the global temperature rose dramatically. Warming of the surface ocean waters reduced the amount of oxygen absorbed from the atmosphere. A reduction in the equator to pole temperature gradient caused a weakening of the wind and the wind-driven surface ocean currents. Consequently, the ocean circulation changed so that great volumes of warm, nearly oxygen-free water filled the ocean bottom. In this environment, microbes were dominated by anaerobic bacteria that consume sulfur and produce hydrogen sulfide. Biomarkers of green sulfur bacteria and photosynthetic purple sulfur bacteria were extracted from strata of this age. In time the layer of oxygen-poor, H₂S-rich water became thicker and eventually reached the ocean surface. Hydrogen sulfide escaped to the atmosphere where it killed plants and animals directly and indirectly. H₂S is highly toxic especially at high temperatures and it reacts with and destroys the stratospheric ozone. With no ozone shield, lethal levels of solar ultraviolet radiation reached Earth's surface.

For much more on this topic, see Ward, Peter D., 2007. Under A Green Sky. Washington, DC: Smithsonian Books, 242 p.

TABLE 1

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Prepared by Joseph M. Moran, Ph.D., and E.J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.