Triassic-Jurassic and Cretaceous-Tertiary Extinctions

Triassic-Jurassic Extinction

          The fifth great extinction happened about 200 mya in the late Triassic-early Jurassic and killed more than half the species on Earth; mammal-like reptiles and true mammals, which evolved during the Triassic Period, were hit particularly hard. Those effects were especially severe in the marine realm with massive destruction of bivalve and gastropod species and the loss of 20 percent of the non-dinosaurian archosaurs and therapsids and the extinct of the last large amphibians. With the sudden abandonment of so many ecological niches dinosaurs assumed dominant roles in the Jurassic. Surprisingly, dinosaurs did not show  lineage losses at that transition, which some paleontologists have recently inferred was a direct consequence of their ability to survive in an oxygen-depleted atmosphere.

          Recently, Peter Ward published research that demonstrated that low oxygen levels could have triggered two giant extinctions, the first 250 mya at the Permian-Triassic boundary between periods, and the second about 200 mya, at the Triassic-Jurassic boundary. According to Ward, since dinosaurs first appeared during a long period of low oxygen they developed highly efficient breathing mechanisms that allowed them to thrive while many other species became extinct because oxygen levels were so low, comparable to levels found now at altitudes of 12,600 feet and higher. Those low oxygen levels may have resulted either from massive outpouring of lava in flood basalts or from the impact of a large bolide. Ward and many other scientists believe that the huge amounts of carbon dioxide and sulfur aerosols (especially hydrogen sulfide) that were released into the air from flood basalts could have been responsible for triggering a devastating atmospheric effect that warmed the Earth and depleted oxygen from the atmosphere, causing environmental deterioration, and finally global species collapse. Other studies have found that the warming also interrupted and even stopped the ocean’s global circulation, making a major part of the water column above the sea-floor devoid of oxygen, crippling the oceans’ ability to replenish their oxygen supply, destroying marine life on a global scale and allowing anaerobic bacteria (which do not require oxygen) to release poisonous hydrogen sulfide gas into the air.

          Although the several explanations for that event have been suggested, all have difficulties that have not been resolved. Gradual climate change or sea-level change (regressions and transgressions) during the late Triassic cannot explain the suddenness of the marine extinctions. Bolide impact is possible but no impact structures have been found that coincide with the Triassic-Jurassic boundary. Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, certainly released enormous quantities of carbon dioxide and sulfur dioxide, which would have caused either intense global warming (from the former) or cooling (from the latter). However, isotopic analyses of fossil soils of late Triassic and early Jurassic have demonstrated no evidence of changes of the atmosphere’s CO₂ composition. More recently however, some evidence has been collected from near the Triassic-Jurassic boundary suggesting a rise in atmospheric CO₂. Several researchers have suggested that the cause of that rise, and of the mass extinction itself, could have been a combination of volcanic CO₂ outgassing and catastrophic dissociation of gas hydrates (clathrates).

          The most plausible source of the vulcanic CO₂ occurred at the boundary of the Triassic/Jurassic as a major rifting event unfolded, certainly one of the Earth’s largest, which began in Pangaea as the massive continent began breaking apart. At the same time, the 3,600-mile diameter Central Atlantic Magmatic Province was erupting huge amounts of basaltic lava (volumes of at least 480,000 cubic miles) that stretched from Pangaea’s east to west coasts through what is now known as northwest Africa, eastern America (both North and South), and parts of Europe. The vulcanism would have erupted massive floods of basaltic lavas and caused the release of large amounts of gas. It is now believed that the warming effects of the CO₂ release may have triggered the dissociation of massive amounts of methane hydrates and thereby greatly exacerbated the warming trend. In 2004, a large consortium of Italian, French, U.S., Moroccan, and Swiss scientists addressed the sedimentary and igneous record around the Triassic-Jurassic boundary in Morocco’s Atlas Mountains. They investigated one of the few uneroded continental flood basalt sequences of the Central Atlantic Magmatic Province (most preserved CAMP magmas are sills and dikes in eroded offshore basins) whose base deformed the underlying sediments, suggesting that eruption materials collected on top of unlithified sediments shortly after deposition. Both age and geochemistry of the flows released by massive vulcanism are remarkably similar to those of flood basalts from the other side of the Atlantic. Magmatic duration, like that in other large igneous provinces, was short, no more than several million years. Their conclusion was that the mass extinction event may have been related to flood basalt vulcanism and the gases released in the CAMP eruptions.

          In 2010, two separate studies (Whiteside, et al., 2010. Compound-specific carbon isotopes from Earth's largest flood basalt eruptions directly linked to the end-Triassic mass extinction. PNAS 107(15): 6721-6725 and Deenen et al., 2010. A new chronology for the end-Triassic mass extinction. Earth and Planetary Science Letters, 291(1-4), 113-125) independently reached the conclusion that contemporaneous CAMP eruptions, mass extinction, and the carbon isotopic excursions are found in the same locations, strengthening the case for a volcanic cause of mass extinction.

          Thus, as of 2011, additional solid evidence is needed to support or reject any specific theory of extinction causation. We wait for science to do its thing.

 Cretaceous-Tertiary (KT) Extinction

          The last and best known extinction, in terms of its widespread popularity with us common folk, that sent the dinosaurs into the deep dirt nap hit around 65 mya at the Cretaceous-Tertiary boundary [Author’s Note: Tertiary is a historical term for the period of time now covered by the Paleogene and Neogene periods). This mass extinction resulted in the loss of about 85 percent of all life on Earth (you should be thinking dinosaurs, pterosaurs, belemnoids, many species of plants except the ferns and seed-producing plants, ammonoids, marine reptiles, and rudist bivalves as well as 93 percent of nannoplankton in the oceans went extinct) died out during this event, though perhaps it would be better to call it a period since it certainly extended for more than a couple thousand years. The really good news is that our ancestors, including most mammals, birds, turtles, crocodiles, lizards, snakes, and amphibians were primarily unaffected and we are around today as a direct result of their persistence.

          In 1978, while studying sedimentation rates in rocks around Gubbio, Italy, geologist Walter L. Alvarez discovered a centimeter-thick clay layer between limestones deposited between two geologic time intervals, the Cretaceous and Tertiary. Analysis of that clay layer revealed large concentrations of iridium, a dense and rare metal that can be found in the Earth’s core but is rare on the surface, usually occurring in concentrations of 0.3 parts per billion. However, the clay layer at Gubbio had concentrations 30 times higher. Iridium is also found, in much higher levels, in asteroids. A team of scientists including Walter Alvarez, Luis Alvarez (Walter’s father and a Nobel Laureate experimental physicist), and several colleagues at the University of California — Berkeley proposed that the clay was the altered remains of the dust cloud that spread around the world when a huge bolide struck the Earth. They theorized the tremendous impact and the resulting cloud of debris, ash, dust, and acid rain caused the mass extinctions of over 80 percent of all life on Earth. This theory was a bomb shell of its own in the geological community, generating no end of controversy, especially since many geoscientists have no truck with catastrophism and its unsavory historical associations.

          The 1991 discovery of the Chicxulub Crater on the Yucatán peninsula in Mexico lent support to the Alvarez theory. Since then, similar clay KT boundary deposits (in the literature, this mass extinction is usually abbreviated KT) have been found to contain not only iridium but also small spherules of molten glass deposited as close to Yucatán and the Chicxulub Crater, Haiti, and as far away as Colorado and Canada.

A competing school of thought  disputed the Alvarez theory and pointed to large-scale volcanic activity in flood basalts that had continued for millions of years (most particularly the Deccan Traps of India), which spewed seemingly endless flows of lava and filled the atmosphere with carbon dioxide and sulfur aerosols, killing most of the land animals, leaving behind a small number of survivor species, such as cockroaches, crocodiles, and turtles. The gigantic volumes of smoke, ash, and sulfuric gases released could easily have resulted in a type of volcanic-nuclear winter similar to the vulcanic event in Laki, Iceland, in 1783 that killed 75 percent of the livestock and 25 percent of the human population that had inhabited the area. Other scientists have combined both theories to argue for an even greater worldwide catastrophe that spelled the end of most life on Earth. But belief in the meteorite theory kept geoscientists around the globe searching for craters buried beneath centuries of sediments. All told, about 150 have been catalogued since the Chicxulub Crater was discovered in 1991.

          Before that major breakthrough, in 1988 Asish Basu, a geologist-geochemist at the University of Rochester, found deposits of shocked quartz (special crystals split along planes that indicated a powerful impact — see shock metamorphism) immediately beneath the Deccan Traps, suggesting that a giant impact preceded these enormous lava flows.  Also in the late 1980s, paleontologist Sankar Chatterjee of Texas Tech University and geologist Dhiraj Kumar Rudra of the Indian Statistical Institute identified a 360-mile-long crater, mostly submerged in the Arabian Sea off Bombay that they called the Shiva Crater, for the Hindu god of destruction and renewal. In the early 1990s, based on new geological evidence, Chatterjee argued that the Shiva Crater was actually one-half of a larger crater; the other segment was buried undersea near the Seychelle Islands, 1,700 miles southeast of India.
          When pieced together, the original crater (which had been separated by movement of continental plates) would be 360 miles long, 270 miles wide, and seven miles deep, suggesting a considerably larger meteorite than the one that landed in Chicxulub. Chaterjee and Rudra speculated that both craters may have been caused by chunks of the same meteor striking in different locations 12 hours apart as the Earth rotated. Their evidence (it must be added that the Shiva Crater has yet to be studied comprehensively, a process that requires extensive chemical and physical evidence, normally gathered from offshore oil exploration) and that of Basu persuaded many scientists that the impact theory was right on the money. Recently, geologists in Gujarat detected abnormally high levels of iridium, a white metal commonly found in meteorites and the same mineral that was present in the Italian clays investigated by Walter Alvarez and that led to his original bolide theory. Another piece of the puzzle fell into place. Chaterjee and Rudra and many other geoscientists believe the meteor triggered the larger catastrophe in which the extinction of dinosaurs was a result of a very complicated geological event, a catastrophe of the sort all uniformitarians of Hutton’s and Lyell’s day would have roundly rejected. And that’s how you spell scientific progress.

          Nearly twenty-five years of research since the Alvarez announcement has resulted in many geoscientists accepting their hypothesis, especially if the bolide impact effects occurred in conjunction with massive volcanic activity, such as that responsible for the Deccan Traps, an enormous flood basalt lava outflow and one of the world’s largest vulcanic provinces, which may have covered as many as 800,000 square miles and contributed gigundus amounts of vulcanic gases, especially  CO₂, and heat to the atmosphere.

          In June 2004, three geoscientists published results of their research into the fossil record across the Cretaceous-Tertiary boundary at a well-known site at El Kef, Tunisia. In their investigation, Simone Galeotti, Henk Brinkhuis, and Matthew Huber  demonstrated that a prolonged cooling phase that was consistent with the oceanographic response to an impact winter (from a large bolide, better be thinking Chicxulub Structure or maybe the Shiva Crater) resulted in the sudden appearance of tiny, cold-loving ocean organisms (dinoflagellates and benthic formanifera) in an ancient sea that had previously been very warm, suggesting that an approximately 2,000-year cooling occurred during the earliest Paleocene (Danian). That discovery establishes the first geophysical evidence of global cooling (caused by the injection of huge quantities of sulfate aerosols into the atmosphere that filtered out a significant amount of the sunlight and cooled the Earths surface for perhaps five years after the impact) that most probably followed the vaporization of the bolide and rocks surrounding the impact site. Their research also is important because it confirms what climate modelers have been saying for years about the global effects of a nuclear/volcanic winter. It must be noted that those exact effects, however, can also be caused by massive volcanic eruptions in flood basalts as was demonstrated in 2005 by Anne-Lise Chenet of the Laboratoire de Paleomagnetisme, Institut de Physique du Globe de Paris, in a detailed study of the Deccan Traps.

          However, detractors of the bolide extinction theory point to the fossil record, maintaining that it took a much broader time span for the dinosaurs and other great reptiles to die off. Paleontologist Robert Bakker argued that an enormous bolide impact would have killed frogs as well as dinosaurs; however, evidence demonstrates that frogs survived the extinction event. Gerta Keller of Princeton University argued that recent core samples from Chicxulub show the impact occurred about 300,000 years before the mass extinction, and thus could not have been the causal factor. As of 2011, no final decision has been reached with which most geoscientists would agree, though more research has uncovered (literally) several other large craters that may have resulted from bolides that struck the Earth at the same time as the bolide that caused the Chicxulub Crater. And several researcher teams in France, India, and the U.S. (computer models created by Elizabeth Parfitt at the University of Bufallo) have discovered additional evidence documenting the catastrophic effects of the huge amounts of volcanic materials released in the fissure eruptions of the Deccan Traps. Either way, it’s definitely worth thinking about and investigating.

          Author’s Note: In a March 5, 2010, review article published in the journal Science (Vol. 327. no. 5970, pp. 1214-1218), a team of geoscientists led by Peter Schulte, from the University of Erlangen-Nürnberg in Germany, examined global stratigraphic records across the Cretaceous-Paleogene boundary to assess the proposed causes of the mass extinction. They found that a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Upper Paleogene boundary. According to the research team, the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations — for example, darkness and cooling — was significantly robust for them to conclude that the Chicxulub impact triggered the mass extinction. In another March 2010 publication in the journal Science, a research team led by Michael Prauss, a paleontologist at Freie Universitaet Berlin, posited that long-term climate fluctuations that continued over several million years and peaked at the Cretaceous-Paleogen boundary were probably the main reason for the extinction of the dinosaurs and other creatures 65 million years ago. According to Prauss, the Chicxulub bolide impact was only one in a chain of catastrophic events that caused substantial environmental perturbations, probably largely controlled by the intermittent activity of the Deccan volcanism near the then-Indian continent.

         Until other, more compelling, evidence is presented, the Chicxulub impact appears to be one of the most likely causes of the KT Extinction but perhaps not the only or most critical cause. The debate continues.