When life gives up

Because of its sprawling size and age, the region called Siberian traps, covered by basalt layers one on top of another like a staircase,  which was located at the northern tip of supercontinent Pangaea when all the continets were joined together some 250 million years ago and now at Siberia which makes up the northeastern part of present-day Russia had been drawing suspicions for over twenty years. With an area  covering about  2 million square kilometers, roughly the size of Western Europe and three times that of Turkey, took its name from the Swedish word “trappa” which means stairs.

Radiometric calculations based on the decay of uranium-238 trapped in tiny ctystals of zircon into lead-206 through a slow but smooth working process, had shown that volcanic activity had lasted two million years and roughly coincided with the extinction. But it  was not known for sure, whether volcanic activity had started before the extinction and had triggered the events which nearly wiped off all life from the planet. 

Now, however, measuments taken on the same crystals taken from basalt samples in Siberia and fossil bearing sediments in Southern China with far more sensitive techniques, have provided the answer in sufficent precision: Volcanic activity started in Siberia 252.28 million years ago, say M.I.T. geochronologists Seth Burgess and Samuel Bowring. As for the first series  of extinctions, researchers date them to 251.941 million years ago. That means volcanism had started earlier and was as close to the start of the extinction as to be its cause.

But surprise! As it so often happens for theories probing the dark ages, a new one cropped up just three months after everything looked neatly settled. According to an American-Chinese team led by MIT professor Daniel Rothman, volcanic eruptions that created Siberian traps were not enough to explain the amounts of carbon in deposits dated to the time of extinction. The team proposed, instead, that the sudden and exponential rise in carbon production in oceans had a microbial origin, namely methane-producing archaea dubbed Methanosarcina which multiplied in explosive blooms after a gene transfer from another microbe which enabled them to process large amounts of organic carbon to produce methane But to be able to do that, the microbes needed to dine on a diet of wholesome nutrient, which happens to be nickel – yes, you have guessed it – belched out by the volcanoes in just right amounts on the just right time: The eve of the mass extinction. Nickel turbocharges methane production by the microbes and the potent greenhouse gas causes a steep rise in temperatures combined with the acidification of oceans. 

But when it comes to the factors which have caused mass die-outs, conflicts re-emerge. Shuzhong Shen of the Nanjing Geology and Paleontology Institute in China also rules out  global warming caused by copious amounts of CO₂ pumped to the atmosphere by volcanoes as a possible cause. Although studies conducted on heat-sensitive oxygen isotopes in sediment layers corresponding to the time of mass extinctions show temperature increases of 8-10⁰C, the rise was found to have come just after the mass extinction; that is, it cannot be the cause of the extinction. 

Data from fossil beds in China’s Shensi province indicate that the timespan of extinction was just a couple of thousand years. And this brevity makes other mechanisms for the die-outs more plausible.  Another explanation gaining widespread recognition in recent years (although rejected by the microbes model) blames large volumes of toxic gas supposedly injected into the atmosphere from underground fires ignited when magma, rising from the mantle, infiltrated coal deposits near the surface. Carbon containing microscopic particles which Canadian geologists found at sediments deposited just before the P-T boundary in the country’s arctic regions  are said to be bearing a striking resemblance to the particles in the smoke from the chimneys of coal-burning thermal power plants. Stephen Grasby of the Geological Survey of Canada says the particles in the sediments attest to a great underground fire ejecting ash with dense metal content to stratosphere. The researcher points to the high mercury content of the ash-bearing sediments and emphasises the fact that the amount peaks in sediments at the P-T boundary.

Radiometric dating experts now aim to test the hypothesis that oter mass extinctions were also caused by intense volcanism with state-of-the-art tools of trade at their disposal. For instance, the massive volcanism in the opening of the Atlantic Ocean 201 million years ago, coincides with the mass extinction which paved the way for the dinosaurs to rule the world by eliminating their rivals. But it is not yet known which of the events came first. 

Great Extinctions

But life, continuing until 580 million years ago as single-celled simple forms and the colonies then began to evolve into primitive animals inhabiting shallow waters and seas and then throttled into a sudden and rapid diversification 540 million years ago with a process called “Cambrian explosion.” Until then, life could also have neared extinction due to a variety of causes and recovered again, but fossil evidence gain accuracy only from 500 million years ago.

While numerous mass extinctions are identified conforming to different criteria at different times and dates, five great extinctions achieve largest consensus among the paleontologists. Furthermore, paleontologists generally lean towards the view that instead of a single trigger, great extinctions occur as a result of processes involving the interplay of several causes, at the end of which one gains dominancy. Although protracted volcanic activity is generally seen as having played an important role, researchers refrain from passing a final judgement before investigating possible effects of other factors.  Still, the five biggest mass extinctions and their likely causes are listed as the following, which line up from the oldest to the latest.

Ordovician-Silurian Extinction

It is calculated to have occured 450-to-440 million years ago at a time coinciding with the end of the Ordovician period and the transition into Silurian. The extinction has occured in two peaks separated by hundreds of thousands of years. Since the bulk of life was in seas during the Ordovician, worst hit were marine lifeforms like trilobites, cephalopods and graptolites. According to paleontologists 27% of animal genera, 57% of phyla and 60-to-70% of all animal species disappeared in this event.   (Taxonomic classification of living things from top to bottom are as follows: Kingdom (animal – plants – fungi) Phylum   (eg. Vertaebrates) – class  (eg. Mammals) – Order (eg. carnivores) – Family (eg. Feliciae [cats]) –  Genus (eg. Panthera) – Species(eg. Panthera pardus [Anatolian leopard]. Although not firmly nailed, the cause is speculated to be a major drop in sea levels as a major glaciation trapped the Earth’s water in ice.

Late Devonian Extinction

This drawn-out extinction thought to have taken place 375-355 million years ago lasted almost 20 million years as separate pulses, wiping off 19 percent of the families, 50 percent of the genera and 70 percent of the species.

The extinction, in which plants which had climbed on land, arthropods, insects and first amphibians, along with marine species, all adapted to a warm environment saw their numbers decimated, is believed to have been caused  by an asteroid impact or extensive volcanic activity which ejected prodigious amounts of ash and dust into atmosphere which blocked the sunlight and caused the air and sea temperatures to plunge. 

Permian-Triassic Extinction

 It was determined that this mother-of-all-extinctions, which paleontologists date to 252 million years ago, was caused by intense volcanic activity which lasted two million years at Siberia, which, at the time, was off the northern tip of the supercontinent Pangaea as detailed above and that 96 percent of all marine and terrestrial species had vanished. The mechanism for the extinction is believed to be the collapse of the food chain under acid rains caused by sulphur dioxide spewed by volcanoes. Toxic gases produced by subterranean fires caused by lava infiltrating and igniting coal deposits are also seen to have contributed to the extinction.

Triassic-Jurassic Extinction

As to what caused this extinction which occured some 200 million years ago, clashing theses blame the dispersion of Pangea; large scale volcanic activity which filled an 11 million square kilometer area larger than Canada called Central Atlantic Magmatic Province with basalt and triggered the opening of the Atlantic; and an asteroid impact. The relics of this area now line the shores of Europe, Africa, and North and South America which were joined at the time.

Cretaceous-Tertiary Extinction  This event, dubbed K-T or K-Pg (paleogene) extinction for short, occured about 65-66 million years ago, and once again wiped out half of existing species and all of the dinosaurs except the birds, leaving the stage this time to mammals.

Its cause is again a subject of controversy. While some paleontologists put the blame on volcanism as it  coincides with runaway volcanic activity that created the Dekkan Traps lava region of India, the theory which identifies the real culprit as a 20-km asteroid impacting seems more popular.

 Sixth Great Extinction?

A view steadily gaining converts among paleontologists is that the fastest extinction is the one we are currently living. Large numbers of species have gone extinct or tethering on the brink due to global warming and climate change. And everyone agrees on the culprit: We, humans.

The proposal made by four scientists, including German Nobel laureate  Paul Crutzen for naming of our current era as Antropocene (new human) in view of the prediction that man-made changes will leave their mark on life on Earth for millions of years, is gaining converts among nthropologists, geologists and athmosphere scientists.  

Because the already visible and potential future effects of rapid population increase, proliferation of megacities, the unbridled rises in the use of fossil fuels and ensuing developments like global warming and climate change are anthropogenic, New Human is seen as a fitting name for the new era.

 According to pioneers of the movement, humans have changed the world in such broad scope and depth within just two centuries, that we may be witnessing the start of a new geologic era which will change our planet’s outlook for millions of years and steer it to the sixth and biggest mass extinction.  

Causes of Mass Extinctions

The widely accepted view among the paleontologists is that mass extinctions occur when species exposed to long term, interrelated environmental stresses are pushed over by a single, short term trigger.  The following are listed as chief mass extinction causes. 

Basalt (lava) floods resulting from volcanic activity: While some paleontologists go as far as blaming them for 11 mass extinctions, general view is that they have played major roles in at least five. Ejected in extended volcanic activities,  the lava  which then cool and form basaltic rock, can spread to areas hundreds of thousands and even millions of square kilometers wide. What’s more, dust and ash spewed by volcanoes rise to the atmosphere where they mask the sunlight, preventing photosynthesis by the plants and thereby causing the collapse of the foodchain on which life depends. While sulphur oxides, likewise belched by volcanoes contribute to the same effect by causing acid rains, large scale carbondioxide emissions cause global warming. 

Drops in sea levels: Seven of the 12 such events recorded over the past 500 million years are held responsible for seven extincions of considerable scope. While causing the mass extinction of marine lifeforms by shrinking the continental shelves which are the most productive sections of the ocean floors and harboring the bulk of marine life, dropping sea levels also cause mass extinctions on land by triggering large scale changes in global weather systems. 

Asteroid impacts on oceans: Carbondioxide dissolves in ocean waters and is stored as the bicarbonate radical (─HCO₃) which is stable only under temperatures below 50⁰C. With surface waters heated above this critical value by the thermal shock caused by an asteroid hitting the ocean, huge amounts of carbon dioxide may spurt out of the ocean to spread to the world, and being a heavy gas, may cause the extinction of air-breathing life, especially in low-lying areas.

Long term, large scale global cooling: Such a process causes the deaths of many species inhabiting the polar and temperate regions and cause others to migrate to the tropics. It also shrinks the habitats of tropical species. . By trapping the most of the world’s water in ice and snow, it makes the Earth’s climate more arid. Global cooling is believed to have played roles in end Ordovician, Permian – Triassic transition and late Devonian extinctions. 

Long term, large scale global warming: With an effect opposite to that of cooling, opens up new habitats for tropical species while causing die offs in temperate climes or migration to polar regions, extinction of polar species, turning Earth’s climate to a more humid one with the melting of glaciers and snow cover and a changing regime of precipitation. It may also cause depletion of oxygen in world seas. In an extinction called Paleocene – Eocene Thermal Maximum (PETM) occuring  some 55 million years ago, this route to extinction is believed to have caused an average 6 ⁰C rise in global temperatures. Global warming is also believed to have caused the extinction of one-fifths of marine life in the Triaasic-Jurassic transition extinction.  As it is the closest model showing the effects of global warming we are currently experiencing, PETM has become a focal point of interest for paleontologists in recent years.

Clathrate gun hypothesis: The structures in which a compound encases another compound in a cage are called clathrates. Methane clathrates, or methane (CH₄) trapped inside ice crystals abound on continental shelves. Sudden dips in pressures on them due to abrupt spikes in air temperatures or earthquakes, may destabilise these structures and result in methane injection into the atmosphere. (See: Methane Bomb in the Arctic.) Since methane is a far more potent greenhouse gas than carbon dioxide, such a dissolution (of clathrate cages) may cause a rapid global warming, or, if its presence was due to the global warming, may exacerbate its effects. Researchers think clathrate bomb effect might have come into play in end-Permian extinction and the PETM event.

Oxygen crisis in oceans: Heavy or total depletion of oxygen in mid layers or even nearsurface layers of ocean waters is also seen as a factor leading to the  extinction of marine life. Although its causes is  still a subject of controversy, paleontologists at least concur on the assumtion that known examples had been chiefly due toextended global warming caused by intense volcanic activity. Oceanic oxygen crises are believed to have contributed to the Ordovician-Silurian, late Devonian, Permian-Triassic and Triassic-Jurassic extinctions.

Hydrogen sulfide emissions: There are theses which propose that in Permian-Triassic extinction, global warming upset the balance between the photosynthesising plankton in oceans and the sulphate-reducing bacteria in depths, and the resultant hydrogen sulphide (H2S) poisoned life in seas and on land and as an additional blow, caused extensive damage to  the atmosphere’s ozone layer to expose surviving species to deadly ultraviolet radiation from the sun. 

Oceanic overturn: A process called thermohaline circulates the relatively warm surface waters of oceans on the planet’s surface by means of major currents, thus affecting climate regimes. When this cycle is disrupted for some reason or other, surface waters, more saline (and heavy) because of vaporisation, dive to depths where they push up anoxic waters which cause the death of oxygen-breathing marine life on the surface and mid layers. This overturning usually occurs at the start and end of ice ages. Its occurence midway through the ice age is even more dangerous,as the earlier warm period has increased the volume of anoxic waters in oceans. It is believed to have played roles in late Devonian and Permian-Triassic extinctions.

Nearby gamma ray bursts and supernovae: Gamma ray bursts (GRB) are the most violent events in the universe. They occur when a star far more massive than the sun collapses at the end of its brief life to form a black hole. If one of the reletivistic particle jets blown out from the poles of the collapsing star is aligned with our line of sight, these burst can be detected by the sensors of orbiting gamma ray observatories. A  GRB occuring within 6000 light years of Earth can destroy the ozone layer around the planet, exposing all life to the destructive effects of the sun’s ultraviolet radiation. Supernova explosions mark the end of stars of at least eight solar masses, whose cores collapse to turn into either a black hole, or a city-sized neutron star. The rebounding shock wave tears through the star’s collapsing outer layers, flinging them to space in a gargantuan explosion. Gamma rays from a supernova going off within 30 light years of earth are enough to destroy half of its ozone layer. Some paleontologists tie the end-Ordovician extinction to a GRB.  At the close of the 20th century, researchers of the University of Munich traced the Iron-60 isotopes they collected from the bottom of the Pacific to a supernova explosion five million years ago.

Plate tectonics: Movements of parts of Earth’s broken crust, dubbed plates, some of which carry continents above them while others shoulder oceans, bring the continents together in one great mass and then take them away again. This process can configure the locations of the continents in forms which can cause mass extinctions in several ways. For, instance, it can start and end ice ages, cause climate changes by altering regimes of ocean currents and winds, can expose the species inhabiting isolated regions and thus unprepared for adaptation to the competition of invasive species by opening sea routes or forming land bridges. When the continents assmble into one supercontinent, the total area of the continental shelves rich in life shrinks. Inland areas, meanwhile, become arid and exposed to the effects of big seasonal changes. The sprouting and diversification of life once again may tak five-to-ten million years after mass extinctions and 30 million years following great extinctions.