Saturday, June 14, 2008

Mass Extinctions

I posted three articles a few days ago about the Younger Dryas extinction event of 12,900 BP and the "smoking gun" represented by the Carolina Bays as evidence of an ET impact.

The fossil record is full of species that no longer exist. For every 100 species that have lived on the Earth, 99 are now extinct. The fossil record indicates that about 2 to 5 families of marine animals disappear every million years, even without a known cataclysmic event. Marine fossils are mostly used to measure extinction rates because they are more plentiful and cover a longer time span than fossils of land organisms.

Since the Cambrian Explosion, around 543 million years ago, when most of the species suddenly appeared on Earth, there have been numerous occasions when a much larger than normal number of species have suddenly (or not so suddenly, as we shall see) disappeared. Mass extinctions probably happened in the Archean and Proterozoic Eons, but before the Phanerozoic there were no animals with hard body parts to leave a significant fossil record. Differences in the statistics reported by different researchers stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.

The five biggest mass extinction events

There have been five extinction events where more than 50% of the species perished. The first date and portion of the explanation for each event is from Wikipedia. This is supplemented, where appropriate, with material from

444 MY ago — at the Ordovician-Silurian transition two extinction events occurred, and together are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct. about 439 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. The toll: 25 percent of marine families and 60 percent of marine genera.

360 MY ago — The Late Devonian extinction, near the Devonian-Carboniferous transition, was a prolonged series of events that eliminated about 70% of all species. This was not a sudden event; it lasted perhaps as long as 20 MY. There is evidence for a series of extinction pulses within this period. It killed 22 percent of marine families and 57 percent of marine genera. Little is known about land organisms at the time.

251 MY ago — The Permian-Triassic transition, Earth's largest extinction event, killed 53% of marine families, 84% of marine genera, about 96% of all marine species, and an estimated 70% of land species (including plants, insects, and vertebrate animals). The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs to become the dominant land vertebrates; in the seas the percentage of sessile animals (animals which are not able to move about) dropped from 67% to 50%. The whole late Permian was a difficult time, at least for marine life. Even before the "Great Dying", there was a level of extinction high enough to be included in the "Big Five".

200 MY ago — The Triassic-Jurassic transition: about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated. Roughly 199 million to 214 million years ago, most likely caused by massive floods of lava erupting from the central Atlantic magmatic province -- an event that triggered the opening of the Atlantic Ocean. The volcanism may have led to deadly global warming. Rocks from the eruptions now are found in the eastern United States, eastern Brazil, North Africa and Spain. The death toll: 22 percent of marine families, 52 percent of marine genera. Vertebrate deaths are unclear.

65 MY ago — at the K/T or Cretaceous–Tertiary extinction event about 50% of all species became extinct. It has great significance for humans because it ended the reign of dinosaurs and opened the way for mammals to become the dominant land vertebrates. In the seas it reduced the percentage of sessile animals to about 33%. The K/T extinction was rather uneven — some groups of organisms became extinct, some suffered heavy losses and some appear to have been only minimally affected. Probably caused or aggravated by impact of several-mile-wide asteroid that created the Chicxulub crater now hidden on the Yucatan Peninsula and beneath the Gulf of Mexico. Some argue for other causes, including gradual climate change or flood-like volcanic eruptions of basalt lava from Indias Deccan Traps. The extinction killed 16 percent of marine families, 47 percent of marine genera (the classification above species) and 18 percent of land vertebrate families, including the dinosaurs.

Not everyone agrees about these mass extinctions. Some researchers think the species extinction estimates are related to the quantity of rock available for sampling from different time periods. Statistical analysis supports the reality of the larger extinctions. Studies of the rock exposure of Western Europe, however, indicate that many of the minor extinction events are most readily explained by sampling bias.

The idea of mass extinctions would tend to change one of the basic beliefs of Darwinism from "survival of the fittest" to "survival of the luckiest". Think about what that means. Survival of the fittest would tend to increase fitness over time. Survival of the luckiest means there should be no increase in fitness over time. Better sight, better brains, better hearing: none of it should have taken place. Another nail in the coffin of Darwinism.

Fewer extinctions now than before?

The graph above indicates the relative intensity of extinctions since the Cambrian Explosion.

The diagram appears to show that the gaps between mass extinctions are becoming longer and the average and background rates of extinction are decreasing. This is speculative because the number of samples is too small to reliably show a trend. One explanation for this, of course, is that there really has been a decrease in both the mass extinction and background extinction rates: the number of large Earth-crossing asteroids has been diminished by collisions.

Another explanation is that there has been no decrease. The farther back you go, the more imperfect the fossil record becomes. Reasonably complete fossils are very rare: most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different genera which were often defined solely to accommodate these finds. The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism.

A third explanation is that the oceans have become more hospitable to life over the last 543M years and less vulnerable to mass extinctions. Dissolved oxygen became more widespread and penetrated to greater depths. The development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events. Marine ecosystems became more diversified so that food chains were less likely to be disrupted.

The accuracy of the graph is probably affected by a combination of these things. Reality isn’t very fond of all Black or all White.

Right now we are in the middle of a sixth mass extinction called the Holocene extinction event. Some scientists, such as Harvard University biologist E. O. Wilson, predict that humanity's destruction of the biosphere could cause the extinction of one-half of all species by the end of the 21st century. In 1993, Wilson estimated that the planet is losing 30,000 species per year - around three species per hour. Some biologists have begun to feel that the biodiversity crisis dubbed the "Sixth Extinction" is even more severe, and more imminent, than Wilson has estimated.
Research and conservation efforts all point to an ongoing period of enhanced extinction, though some offer much lower rates and hence longer time scales before the onset of catastrophic damage. The Younger Dryas extinction event is sometimes considered part of the Holocene extinction event. As is the case with everything else, there is a lot of disagreement. Some paleontologists question whether what is happening now is really as bad as the big five extinctions of the past.

What causes mass extinctions

Any explanation should account for all the losses, not just a few groups like the dinosaurs. It should explain why some species died out and others survived. It should provide a mechanism that could kill some species without causing total extinction. It should be based on events or processes that can be shown to have happened, not just inferred from the extinction.

A combination of causes might be responsible for any given mass extinction. The relevance of a particular cause might be different in different parts of the world. An example of this from my first Younger Dryas article is the North Atlantic Thermohaline Circulation, which regulates temperatures in the Northern Hemisphere, but may have little effect on the climate south of the equator.

Here is a list of the most likely causes.

Flood Basalt events. There are eleven known instances of this kind of event. All are associated with significant extinctions. It looks like in every case the main phase of the extinctions started before the eruptions. Flood basalt events produce dust and particulate aerosols which inhibit photosynthesis and cause food chains to collapse both on land and at sea. They emit sulfur oxides which precipitate as acid rain and poison many organisms, contributing further to the collapse of food chains. They emit carbon dioxide which could cause global warming lasting hundreds of years after the volcanism ends and dust and particulate aerosols dissipate. Massive volcanism is believed to have caused or contributed to the End-Cretaceous, End-Permian, End Triassic and End Jurassic extinctions. Flood basalt events are connected with plate tectonics, which are thought to be the result of plumes of hot rock rising from deep in the mantle and spreading out at the lithosphere boundary to form Large Igneous Provinces. See the map below for the location of LIPs dating from 250 million years ago to the present.

Drops in sea-level. There are twelve known sea-level falls. Seven are associated with significant extinctions. These occur where worldwide sequences of sediments laid down at the same time show all or part of a transition from sea-bed to tidal zone to beach to dry land with no evidence the areas were raised by geologic processes. Sea-level falls could reduce the area of the continental shelf, which is the most productive part of the oceans. This could cause marine mass extinctions and disrupt weather enough to cause extinctions on land. Sea-level falls are probably the result of other things: global cooling or the sinking of the mid-ocean ridges. Sea-level falls are associated with most of the mass extinctions, including all of the big five.

Asteroid impacts. One estimate is that the entire population of Earth-crossing asteroids includes some 1500 larger than one kilometer and 135,000 larger than 100 meters in diameter. Only one crater over 100 km wide is associated with a mass extinction: the Chicxulub crater north of the Yucatan peninsula created during the K-T extinction event. There are more than fifty craters less than 100 km wide. The "great majority" of known craters are not associated with significant extinctions. And don’t forget the lesson of the Carolina Bays: a crater may not be produced in a large ET impact event. Large impacts could produce many of the same effects as Flood Basalt events. They might also produce global firestorms and/or huge tidal waves, although many researchers now regard these as exaggerations. In the painting below an asteroid MUCH larger than any earth-crossers is shown impacting Earth. What would this really look like? It would only take an object half a kilometer in diameter - maybe even smaller - to destroy our civilization. In one of my Carolina Bays posts I showed how an impact can be compared to the energy yield in tons of TNT of a nuclear explosion.

Global cooling. Sustained global cooling, which is distinguished from the temporary climatic effects of flood basalt events or impacts, could kill many polar and temperate species and force others to migrate towards the equator. Cooling would reduce the area available for tropical species. Cooling ties up large amounts of water in ice and snow, causing the climate to become more arid than normal. But the glaciation cycles of the past four million years don’t seem to have had a significant impact on biodiversity. Sustained global cooling may have caused or contributed to the End-Ordovician, Permian-Triassic, Late Devonian extinctions, and possibly others.

Global warming. This would expand the area available for tropical species, kill temperate species, or force them to migrate toward the poles, and might cause extinctions of polar species. Global warming tends to increase the volume of the water cycle, making the climate wetter than normal. It might contribute to anoxic events in the oceans.

The Clathrate gun hypothesis. I rather like this one. Methane clathrates are composites of methane held in a "cage" of water. They exist on the continental shelves and in some deep lakes. There are two kinds of methane clathrate in the oceans, believed to be formed by the microbial reduction of organic matter (accumulating by sedimentation) on the one hand, and by thermal decomposition of organic matter (petroleum) on the other. The two processes probably grade continuously from one to the other. Methane gas formed by either of these processes migrates upward from depth along geological faults and precipitates or crystallizes on contact with cold sea water. Clathrate deposits are located from about 190 meters depth down to 450 meters. Below that the methane exists in gaseous form. The size of the oceanic methane clathrate reservoir was estimated to be very high when clathrates were first discovered in the 1960s and 70s. Since then, the estimates have decreased substantially. At present an estimated 500 to 2,500 gigatons of carbon is thought to be tied up in methane clathrates. This is less than the 5,000 gigatons estimated for all other fossil fuel reserves, but more than the 230 gigatons estimated for other natural gas resources. For comparison, the total carbon in the atmosphere is around 700 gigatons. At present only a limited number of clathrate deposits are thought to be an economically viable resource.

Clathrates are likely to break up rapidly and release their methane if the temperature increases or the pressure decreases beyond a certain threshold. Global warming, a sudden drop in sea level, or an earthquake might trigger a release of methane into the atmosphere. Methane is a much better greenhouse gas than carbon dioxide, so a methane eruption ("clathrate gun") could cause rapid global warming or make it much more severe if the eruption was itself caused by global warming.

Anoxic events. Sometimes a part of the ocean becomes deficient in oxygen. The causes are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by massive sustained volcanism. Widespread black shale beds were formed in the mid-Cretaceous, indicating anoxic events are not always associated with mass extinctions.

Hydrogen sulfide emissions from the seas. It has been proposed that during the Permian-Triassic extinction event global warming upset the oceanic balance between photosynthesizing plankton and deep-water sulfate-reducing bacteria. Massive emissions of hydrogen sulfide resulted. This poisoned life both on land and in the oceans. It also severely weakened the ozone layer, exposing much of the life that still remained to fatal levels of UV radiation.

Oceanic overturn. I’ve already mentioned the North Atlantic Thermohaline Circulation, which warms the north temperate latitudes. When the circulation is interrupted, surface waters, which are denser because of a higher salinity due to evaporation, sink straight down and are replaced by anoxic deep water. This kills most of the oxygen-breathing organisms. This could occur either at the beginning or the end of a glaciation. It would be more serious at the beginning of a glaciation because the preceding warm period would have created a larger volume of anoxic water. Overturns don’t apparently leave easily-identified "signatures" in rocks. They are, at present, purely theoretical.

A nearby nova, supernova, or gamma ray burst. I’ve already mentioned the suggestion by one researcher that the Younger Dryas extinction was the result of a supernova.

Continental drift.. Plate tectonics is responsible for continental drift. The continental plates float around on the lithosphere, driven by material upwelled along the mid-oceanic ridges which is then subducted along the continental margins. Some configurations of the land masses could change ocean and wind currents, altering climate. Seaways could open or close, exposing previously isolated species to competition that would drive them extinct. Most North American marsupials became extinct after the land bridge was created between North and South America. It is widely thought that the creation of the super-continent Pangaea contributed to the End-Permian mass extinction.

Disease or an especially successful biological innovation. This cause has been rejected because it requires events or processes for which there is no evidence.

Are mass extinctions cyclic?

A glance at the graph in this article reveals an apparent periodicity in the major spikes at roughly 50 million year intervals. More detailed studies reveal a 26 million to 30 million year cycle (Raup and Sepkoski – 1986) and a 62 plus or minus 3 million year cycle (Rohde and Muller – 2005). The work in this area suffers from the poor accuracy of geological dating. Notice some of the differences in timing in the "big five" listed above. Errors have been reduced from 10 million years or more down to around 4 million years at present. Here are some proposed causes of cyclic extinctions.

A companion star to the sun. This star, dubbed Nemesis, would periodically perturb the Oort cloud, sending larger numbers of comets and asteroids sunward than normal. Nemesis has been pretty much proved impossible. The close encounter with a random star could have the same effect, but would not be periodic.

Galactic plane oscillations. The solar system travels around the Milky Way galaxy in about 240 million years. It follows an orbit that bobs harmonically up-and-down through the plane of the galaxy about every 52 to 74 million years. The amplitude is ~49 to 93 parsecs out of the galactic plane. (The uncertainties in the estimates of the period and amplitude of the motion are caused by the uncertainty in the amount of dark matter in the galactic disk.) The Sun and planets passed through the galactic plane about 2 to 3 million years ago, moving "northward." During plane passages the solar system might encounter gas clouds or magnetic fields or other possible disruptors of the Oort cloud.

An interesting sidebar is that some researchers believe the velocity of the solar system relative to the plane of the galaxy is so great that it is just passing through the galaxy, not following a sinusoidal path up-and-down through the galactic plane. They believe we are part of the stream of star systems being cannibalized by the Milky Way from the Dwarf Sagittarius galaxy. Note: ABC Science says this is a hoax. Hope you enjoyed it anyway.

Below is a drawing from Astronomy Magazine showing current thinking about the shape of the Milky Way galaxy.

Passage through galactic spiral arms. The solar system is currently positioned on the inside of a minor spiral arm called the Orion Spur. The spiral arms of the galaxy are thought to be density waves, regions of enhanced density that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are the short-lived blue stars that light the arms. Another explanation for spiral arms is the stochastic self-propagating star formation model or SSPSF model. This model proposes that star formation propagates via the action of shock waves produced by stellar winds and supernovae that compose the interstellar medium. These different hypotheses for the spiral arms do not have to be mutually-exclusive. They may work in concert to explain different types of spiral arms. Since the spiral arms move more slowly than the stars around the galactic center, the solar system passes through them periodically – about once every 175 million years.

Geological instabilities. Heat could build up in the Earth and then be released periodically by mantle plumes, periods of major volcanism, and active plate tectonics.

What is your belief?


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