Permian-Triassic Extinction

            This topic has generated intense research since the late 1980s and early 1990s. Competing conclusions have been reached and lines drawn in the sand, so to speak, concerning the primary event cause ranging from gradualistic environmental change to a catastrophic event, either flood basalt vulcanism or bolide impact. But as recently as 2003 and 2004 several independent research teams have collected and analyzed rocks from the period in question and have come to similar conclusions. The following material attempts to organize and present that rapidly evolving situation.

            The Permian-Triassic extinction event occurred approximately 252 mya at the Permian-Triassic boundary. It was the Earth’s most severe extinction event, marking the dying off of more than 90 percent of all marine species and nearly 80 percent of terrestrial vertebrate species (Author’s Note: No wonder that event is widely known as The Great Dying). Until the last two decades, most geoscientists thought the catastrophe was probably caused by active reconstruction of the Earth’s crustal plates and climatic change during formation of the super continent Pangaea and took quite a few million years to develop. In 1991, however, Asish Basu, a geochemist at the University of Rochester in New York, published the results of research that showed a massive and ancient fissure flow in what are now known as the Siberian Traps dated precisely to that greatest of extinctions 250 mya.

            In a more recent investigation of the Siberian Traps, geochemists Basu, Stein Jacobsen (Harvard University) and Robyn Hannigan (then at the University of Rochester) concluded that the Siberian flood basalt was generated from super-heated, buoyant rock that rose in a narrow column (mantle plume?) from a depth of 1,800 miles into a gigantic mushroom-shaped mass of hot material just 40 to 50 miles below present-day Siberia. Then, some 250 mya, 12 to 16 percent of that hot rock suddenly melted (probably a result of depressurization or decompression) and broke through fissures in the Earth’s crust, resulting in a vast flood of lava. Terrence M. Gerlach of Sandia National Laboratory in Albuquerque used one of the Kilauea eruptions as a model and estimated that the Deccan Traps flood basalt injected up to 30 trillion tons of carbon dioxide, six trillion tons of sulfur, and 60 billion tons of halogens (reactive elements such as chlorine and fluorine) into the lower atmosphere over a few hundred years. Since the Deccan and the Siberian Traps were of the same order of magnitude, out-gassing from the Siberian flood basalts would have been more than sufficient to have produced severe and persistent negative effects on plant and animal life, meaning the Permian-Triassic extinction event. For related information, see nuclear/volcanic winter.

            No more than fifteen years ago the Great Dying was assumed by most paleontologists to have been a gradual reduction in species that occurred over several million years, which was consistent with environmental and climatic changes associated with the lava flows from the Siberian Traps. But, today it is commonly accepted that at the very most the event lasted considerably less than a million years, which, geologically speaking, is a very brief period of time.

            Research in South Africa by paleontologist Peter Ward and colleagues directly bears on the length of the extinction. Ward discovered the Permian-Triassic catastrophe had stripped the Earth of many rooted plants, triggering environmental conditions that resulted in severe erosion. From that specific time period sedimentary rocks throughout South Africa’s Karoo Basin show that large rivers that had previously been meandering became braided, with multiple channels as they suddenly became chocked with sediments. The cemented deposits from those braided streams showed that they resembled streams in areas devastated by the 1980 eruption of Mount St. Helens or in areas logged by clear-cutting techniques. Ward’s research found that the rock record indicated rivers had changed from meandering to braided within 50,000 years, then returned to normal meandering courses in another 50,000 to 100,000 years, for a maximum of 150,000 years, which is considerably shorter than several million years. However, in an article in Science published in early 2005, Ward and co-authors used chemical, biological, and magnetic evidence to correlate sedimentary layers in the Karoo Basin of South Africa to similar layers in China that previous research had tied to marine extinction at the end of the Permian. Over seven years, they collected 126 reptile or amphibian skulls from a nearly 1,000-foot thick section of exposed Karoo Basin sediment deposits dating from the time of the extinction and found two patterns. The first showed gradual extinction over about 10 million years leading up to the boundary between the Permian and Triassic periods. The second pattern was a sharp increase in extinction rate at the boundary that then lasted another 5 million years. Evidence from the Karoo Basin is consistent with a mass extinction resulting from catastrophic ecosystem changes over a long time scale, not with catastrophic changes associated with an impact. The scientists stated that they found nothing in the Karoo sediments that would indicate a meteorite struck the Earth around the time of the extinction, although they looked specifically for impact clays or material ejected from a crater.

            However, other recent research has come to the conclusion that the die-off was rapid. Geoscientists at the Smithsonian Institution’s National Museum of Natural History published a 2000 study in Science in which marine rocks from China revealed the Permian-Triassic extinction happened in less than 160,000 years. And in the July 2000 issue of the journal Geology, a study of sea-floor rocks now exposed in the Austrian Alps concluded the extinction happened in less than 60,000 years and perhaps in less than 8,000 years. In addition, recent evidence has been found that links that extinction with the effects of a large bolide impact or massive outpouring of lava from the Siberian Traps or a combination of both. A team of scientists led by Asish Basu (who we met earlier), found dozens of unusual mineral grains from rock samples collected from Graphite Peak, Antarctica, that they believe are tiny fragments of a meteorite 4.56 billion years old that crashed into Earth 250 mya. The researchers also found bits of nearly pure metallic iron in the Antarctic rock that were indicative of being formed by extreme heat, such as that in a severe meteorite impact. The particles resembled those reported by Kunio Kaiho of Tohoku University in Sendai, Japan, from the P-T boundary in Meishan, China, that formed as condensates from a large bolide impact cloud. The very fact that those grains had not deteriorated from weathering meant they must have been buried quickly under sedimentary deposits, another indication of a major impact. A third controversial impact marker was also discovered at Graphite Peak, Antarctica, by Luann Becker, a geo-scientist at the University of California, Santa Barbara, and Robert Poredea of the University of Rochester. That marker consisted of clusters of carbon atoms called buckyballs, materials that are seldom produced in quantity as part of any known terrestrial process except bolide impact.

            In May 2004, Luann Becker and others in her team published an article in the journal, Science, that presented extensive evidence of a 125-mile-wide crater, called Bedout, off the northwestern coast of Australia. Their data included seismic imaging, gravity data and the identification of melt rocks and impact breccias from drill cores. During their recent related research in Antarctica, Becker and her colleagues also found meteoric fragments in a thin claystone breccia layer that indicated an end-Permian event. The breccia in both Antarctica and Australia contained impact debris and evidence of shock metamorphism in a layer of sediment that formed at end-Permian time that matched up with what is called the Great Dying, a period when the Earth was configured as Pangaea and Panthalassa. Whether that specific bolide was responsible for the mass extinction is still up in the air, so to speak, especially since a number of geophysicists, specifically R. D. Müller et al., have determined that the Bedout features failed to pass nearly all unequivocal criteria for impact crater recognition. (R. D. Müller et al., 2005. Geophysical evaluation of the enigmatic Bedout basement high, offshore northwestern Australia Earth and Planetary Science Letters, 237, 264-284).

            Author’s Note: In summary, many scientists believe the cause of the mass die-off included atmospheric and climatic changes initiated by the enormous out-gassing from the Siberian Traps. Others credit a huge comet or asteroid that hit the Earth at or around the Permian-Triassic boundary. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was global not local and that the change was sudden and not gradual. Although many theories have been presented for the cause of the extinction, including plate tectonics, a major impact event, a supernova, and extreme volcanism (flood basalts), no single cause or combination of causes is universally accepted. Many scientists think that the impact of a large bolide could trigger an undersea release of methane that would rob the oceans of oxygen and also trigger massive eruptions from volcanic fissures, specifically the Siberian Traps, which would then pour out lethal levels of carbon dioxide and hydrogen sulfide.

            Although Becker documented ways in which the Chicxulub cores were very similar to the Bedout cores, the scientific jury has just begun to examine and critically discuss the evidence. And research has only started that is intended to shed light on whether the bolide impact and the Siberian Traps fissure flows were related events. The debate between scientists who support the bolide impact mechanism and those supporting the Siberian Traps mechanism continues. Perhaps both groups will be proven partially right and partially wrong in that those events may have been causally related and functioned together to cause the extinction. Consequently, the Permian-Triassic extinction continues to be another of life’s wonderful mysteries. Keep your eyes on the literature over the next few years and you may find how the mystery is resolved.

Mass Extinction: Ordovician-Silurian, Devonian, and Guadalupian

The geologic record documents at least six catastrophic events that brought much of the Earth’s existing life to relatively sudden and untimely ends.

Ordovician-Silurian Extinction

            The first mass extinction occurred 440-450 mya at the Ordovician-Silurian transition. At least two extinction events occurred and together are ranked by many scientists as the second or third largest of the five major extinctions in Earth’s history in terms of percentage of genera that disappeared from the geologic record. Twenty-seven percent of all families and 57 percent of all genera became extinct. The event eliminated many brachiopods and conodonts, which were marine animals with tiny, complex, specialized conical teeth-like structures that appeared long before land animals, fish, or animals with backbones that are now one of the most important biostratigraphic indices available in Paleozoic and Triassic strata. It severely reduced the number of trilobite species and was probably triggered by a major drop in temperature and CO₂ levels, ice sheet formation, and lowering of the ocean level, which combined to adversely affect marine and terrestrial lifeforms. Several scientists have theorized that the leading extinction cause may have been the gradual movement of Gondwana into the South Polar Region and consequent disruption of ocean currents.

Devonian Extinction

            The second extinction event, with effects similar to the first, took place during the late Devonian, about 364 mya in the transition from the late Devonian to the Carboniferous period, and also was probably due to similar causes: decreased temperatures and sea reliction. About 50 percent of all species disappeared. At this time no significant controversy has been generated among geoscientists concerning those events since so little is known of the geological or climatological processes that led to the extinctions.

            Contrary to other mass extinctions, the Devonian event was not sudden; evidence suggests that the extinctions took place in pulses over a period that ranged from about three million to fifteen million years, which is why many paleontologists do not include it as an “event,” since it consisted not only of many independent extinction pulses but perhaps also a multitude of causes including bolide impacts, global anoxia (widespread dissolved oxygen shortages), plate tectonics, sea level changes, global climatic change (cooling rather than warming), and the expansion of terrestrial plants (causing mass extinctions in the tropical oceans). Definitive reasons for the late Devonian extinctions have yet to be demonstrated.

Guadalupian Extinction

            Large-scale extinction event that occurred in the middle Permian around 260 mya that devastated marine life around the world, resulting in a 50 percent extinction in marine genus. Prior to the mid-1990s very little research had been done on this Permian extinction event because it closely preceded the great end-Permian mass extinction peak and for many decades was considered to be part of that larger event. One reason for the prior difficulty in separating these two extinctions was that the duration between them was very short, about eight million years, when compared with durations between the big five mass extinctions, between 41 to 145 million years. Only in 1994 did two research teams  independently identify a separate mass extinction late in the Guadalupian Stage of the Middle Permian followed by a phase of radiation and recovery prior to the end-Permian extinction event. The Guadalupian extinction is best known from evidence provided from shallow marine, equatorial carbonate environments and was particularly severe for brachiopods, corals, echinoderms — blastoids, crinoids, echinoids — foraminifera (especially fusulinids), and reef-forming sponges.
              Researchers led by Paul Wignall of the University of Leeds reported in the journal Science in May 2009 that the Guadalupian Mass Extinction was preceded by massive eruptions in the Emeishan geological province of southwestern China. Since the eruptions occurred in a shallow sea the researchers were able to study both the volcanic rock and the overlying layer of sedimentary carbonates containing fossilized marine life, making it possible to compare dates and directly monitor the relative timing of extinction and volcanism in the same locations. According to Wignall and his colleagues, between the layers of igneous rock are limestone deposits with fossilized evidence of widespread extinction. The injection of hot lava from the volcanoes into the sea would have produced massive cloud formations that likely spread around the world, cooling the planet and producing acid rain. For over half a million years the eruptions discharged about 120,000 cubic miles of lava, killing more than half of the life on Earth and leaving lava deposits 655 feet deep in some locations. Although the evidence discovered by Wignall is not proof of cause-and-effect, the researchers concluded that their study provided solid evidence potentially linking the mass extinction and eruption.