la madre di tutte le estinzioni

[Felix (POL)]

L'ESTINZIONE PERMICA

E' impossibile denumerare la quantita' di vita biologica presente sulla terra oggi.

Gruppi di dozzine di speci e sottospeci fanno parte di un'enorme varieta' di speci di cui molte ancora da scoprire. Che madre natura sia stata sempre cosi' generosa nella fioritura e differenziazione di vita o, diversamente, oggi le condizioni sono tali da favorire l'evoluzione e la diversificazione delle speci?

Secondo paleonteologi e paleobiologi, i modelli di differenziazione vivente sulla terra sono sempre stati cosi' diversi soprattutto nella cosiddetta era glaciale. Si deve quindi concludere che molte speci si siano estinte in seguito a cause diverse e sono state sostituite da altre. Che cosa determina il fenomeno dell'estinzione di una o piu' speci?

Seconda la teoria di Darwin, la sopravvivenza del piu' forte e' alla base di ogni processo d'evoluzione.

Le speci endemiche si possono sia sviluppare in qualcosa di diverso tramite l'evoluzione e l'adattamento, o al contrario possono estinguersi. I fattori climatici contribuiscono maggiormente a cio'; infatti molti animali durante l'era glaciale hanno assunto tecniche particolari per poter sopravvivere in un ambiente cosi' aspramente freddo. Quando il ghiaccio comincio' a sciogliersi, le condizioni non erano piu' favorevoli all'adattamento di tali animali ( mammoth, megalodonte ). L'ultima era glaciale e' ritenuta un esempio imperfetto poiche' vide il sorgere dell'Homo Sapiens che distrusse molte speci tra cui una stessa delle loro (quella Neanderthal ).

In modo alquanto bizzarro molte speci hanno modificato i loro modelli generici e si sono estinte per riapparire dopo alcuni migliaia di anni, il che e' il caso della Tigre dai denti a sciabola. Naturalmente il fatto piu' sorprendente riguardante l'estinzione non e' quello della selezione naturale (per quanto si sappia) ma gli avvenimenti ciclici quali la caduta di asteroidi sulla Terra che spazzano via ogni presenza di vita biologica al momento dell'impatto.

Non si ha ancora alcuna prova sicura che l'impatto proveniente dallo spazio costituisca una selezione naturale e la scienza non accetta la teoria di modelli naturali scritti che per uno scopo maggiore possano creare la giusta condizione in natura per l'avvento della razza umana. Sfortunatamente scienza e filosofia talvolta sono discordi. Tuttavia, quale modello di differenziazione biologica e' stato il prodotto di tali avvvenimenti ciclici?

L'estinzione dei dinosauri e' solo una delle tre estinzioni di massa sinora scoperte causata dall'impatto di asteroidi. L'avvenimento dei dinosauri non e' affatto il piu' tragico avvenimento di estinzione biologica sulla terra; infatti la maggior parte dei mammiferi e rettili sopravvissero a quell'impatto. (ecco il perche' della nostra esistenza).

Se torniamo indietro nel tempo durante l'era permica ed esattamente 260 milioni di anni fa , un'estinzione di massa ebbe luogo a causa di un impato d'asteroide. Cio'e'stata definita dagli scienziati quale madre di ogni estinzione.

Durante l'era permica ci fu una perdita di vita invertebrata pari al 50%, il 75% di nuclei anfibi e l'80% di nuclei rettili. Gruppi paleozoici di certa rilevanza quali i Triboliti, Fusulinid foraminifera, coralli rugosi e tabulati si estinsero. La maggior parte della vita biologica di quella epoca viveva nell'emisfero sud del supercontinente Pangea chiamato Gondwana. Molti scienziati ancor oggi attribuiscono tale estinzione a fattori ambientali oppure ad eruzioni vulcaniche in Siberia. Tuttavia, la teoria degli asteoridi e' quella maggiormente apprezzata.

Se torniamo indietro di 300 milioni di anni esistono altri estinzioni di massa causate da fattori sconosciuti. Tali fattori spazzarono via la maggior parte delle creature viventi dell'epoca. Perche' sono queste estinzioni di massa cosi' importanti?

Semplicemente perche' secondo la teoria di Darwin dell'evoluzione e codice genetico (non dimentichiamoci che ogni cosa vivente di questo pianeta ha in comune un DNA), la vita si evolve in modo da superare l'estinzione, si evolve in qualcosa di diverso, in qualcosa di piu' agile o in una creatura piu' specializzata rispetto a quella originale. Questa evoluzione ciclica implica come accennato l'evoluzione di speci piu' forti. Se la vita viene spazzata via tutta in un baleno in un determinato periodo di tempo, altre speci precedentemente non selezionate per un certo ambiente prendono il soppravento e si sviluppano.

Ma cosa succederebbe se tale impatto non avesse luogo? In quale modo si sviluperebbe la vita e cosa camminerebbe sulla vita ora?

Se l'estinzione permica non fosse avvenuta, i dinosauri non sarebbero apparsi ed altre speci si sarebbero sviluppate e avrebbero governato la terra. Conseguentemente se i dinosauri non si fossero estinti, oggigiorno noi non guideremmo la terra. Percio' e' il periodico impatto di asteroidi da includere nella selezione naturale oppure questi fenomeni sono ancora da considerare accidentali sul processo dell'evoluzione?

Una cosa e' certa: se un asteroide dovesse cadere sulla terra oggi, il genere umano ha grazie al progresso tecnologico ogni mezzo per distruggerlo o per evitare l'impatto in modo che la nostra specie possa sopravvivere. Siamo stati noi i prescelti per governare la terra? Siamo noi il prodotto piu' raffinato o l'evoluzione o la specie piu' fortunata mai esistita?

Il tempo e la speculazione filosofica lo dimostreranno….

Gaetano Cristiano

http://www.utopia-news.co.uk/i/apr2002/6.htm

[Felix (POL)]

Signs of Crater Linked to Mass Extinction Said Found



The world was not a great place to be 250 million years ago. That’s because some 90 percent of the planet’s marine life and 80 percent of life on land had gone extinct at the end of the Permian period. Exactly what caused the mass extinction is a matter of debate, with the two leading theories positing massive volcanism in Siberia or a collision with a meteor much like the one that killed the dinosaurs 65 million years ago. New findings published online today by the journal Science bolster the impact hypothesis and argue that the resulting crater lies buried off the coast of northwest Australia.

Luann Becker of the University of California at Santa Barbara and her colleagues studied two cores drilled by oil companies in the 1970s and 1980s into a geologic structure off the Australian coast known as the Bedout High. "The moment we saw the cores we thought it looked like an impact breccia," Becker says. Specifically, the team found what they say is evidence of a telltale melt layer that formed when a meteor crashed into the earth and created the 125-mile-wide Bedout. Additional support for their contention that Bedout is an impact crater comes from the fact that material from the cores dates to 250 million years ago, give or take 4.5 million years. Together with earlier evidence that Becker and her team collected in Antarctica and Australia--including shocked quartz and molecules called fullerenes containing extraterrestrial helium and argon--the new results provide further evidence that a massive impact brought about the Great Dying, the scientists say. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Becker remarks. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout I don't think we can call such catastrophes occurring together a coincidence anymore."
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The findings do not close the case of exactly what caused the Permian-Triassic (P-T) extinction, however. Some scientists remain unconvinced that Bedout is in fact an impact crater. In addition, although the date given in the new paper is consistent with the timing of the P-T dieout, it is not yet exact enough to be considered simultaneous with the extinction. Becker notes that the team plans to pursue more precise dating. “Evidence for an impact [at the end of the Permian era] has been growing over the last few years,” notes Doug Erwin of the Smithsonian Institution in Washington, D.C. “It’s not yet a slam dunk, but [the new work] makes it a more plausible contender.” --Sarah Graham

[Felix (POL)]

ScienceWeek

EARTH SCIENCE: A POSSIBLE END-PERMIAN IMPACT CRATER

The following points are made by L. Becker et al (Science 2004 304:1469):

1) The cause of the catastrophic mass extinction at the end of the Permian has been the focus of considerable debate. Becker et al. (1-3) and others (4,5) have presented evidence that a major impact was associated with the extinction of > 90% of marine taxa. The evidence includes fullerenes with extraterrestrial helium and argon (1), meteorite fragments, Fe-Ni-Si "metamorphosed grains" of probable meteoritic origin (5), Fe-Ni metals with impact spherules, and shocked quartz (4).

2) Acceptance of the idea that an impact accompanied the Cretaceous-Tertiary (K-T) extinction increased dramatically with the discovery of the Chicxulub crater. The authors searched for a Permian-Triassic (P-T) boundary impact crater in parts of the Southern Hemisphere that once comprised the supercontinent of Gondwana, because the impact evidence is most abundant in continents from this region (such as Australia and Antarctica). Gorter, based on the study of a single seismic line, suggested that the Bedout ("Bedoo") High offshore of northwestern Australia might be the central uplift of a large end-Permian impact crater.

3) The Bedout High is part of the Roebuck basin, which forms the northwestern continental margin of Australia. Existing studies of the structure include two regional seismic surveys conducted by the Australian Geological Survey (AGSO) and the Japan National Oil Company (JNOC), and two exploratory wells drilled 9 km apart on the top and flank of the Bedout High (Bedout-1 and Lagrange-1) that extend to depths of 3052 m (9986 feet) and 3273 m (10,738 feet), respectively. Both wells were drilled through 3 km of marine and fluvial sediments consisting of carbonates with occasional interbeded siltstones and mudstones (Tertiary to Cretaceous) and sandstones interbeded with claystones, siltstones, and coal (Cretaceous to Triassic) before reaching a breccia (Late Permian).

4) In summary: The Bedout High, located on the northwestern continental margin of Australia, has emerged as a prime candidate for an end-Permian impact structure. Seismic imaging, gravity data, and the identification of melt rocks and impact breccias from drill cores located on top of Bedout are consistent with the presence of a buried impact crater. The impact breccias contain nearly pure silica glass (SiO2), fractured and shock-melted plagioclases, and spherulitic glass. The distribution of glass and shocked minerals over hundreds of meters of core material implies that a melt sheet is present. Available gravity and seismic data suggest that the Bedout High represents the central uplift of a crater similar in size to Chicxulub. A plagioclase separate from the Lagrange-1 exploration well has an Ar/Ar age of 250.1 +- 4.5 million years. The location, size, and age of the Bedout crater can account for reported occurrences of impact debris in Permian-Triassic boundary sediments worldwide.

References (abridged):

1. L. Becker et al., Science 291, 1530 (2001)

2. L. Becker, R. J. Poreda, T. E. Bunch, Proc. Natl. Acad. Sci. U.S.A. 97, 2979 (2000)

3. L. Becker, C. Nicholson, R. J. Poreda, American Geophysical Union (AGU) Abstract, December 12 to 17 2002, OS22C-0291 (2002)

4. G. J. Retallack et al., Geology 26, 979 (1998)

5. K. Kaiho et al., Geology 29, 815 (2001)

Science http://www.sciencemag.org

--------------------------------

Related Material:

EVIDENCE FOR AN IMPACT EVENT RELATED TO THE PERMIAN EXTINCTION

Notes by ScienceWeek:

The available evolutionary record on Earth provides evidence of recurrent mass extinctions of biological species. Apparently, environmental catastrophes, occurring for various reasons, have suddenly removed many groups with a resultant collapse of ecosystems. Eventually new forms appear and evolution continues, but the periods of mass extinction apparently are a major factor in the various patterns of evolution.

The geological period ranging approximately from 146 million years ago to 65 million years ago is called the "Cretaceous period", and the geological period comprising the approximate time-frame 65 million to 3 million years ago is called the "Tertiary period". The largest mass extinction of the past 200 million years apparently occurred 65 million years ago at the end of the Cretaceous period, when approximately half of the genera of multicellular organisms on Earth, including all of the dinosaurs, suddenly died off. The geological record indicates that a layer of impact-produced minerals and the element iridium (an element rare in the crust of the Earth but more abundant in primitive meteorites) was deposited at the time the dinosaurs vanished -- the so-called Cretaceous/Tertiary or K/T boundary. In addition to this, the largest known crater on Earth (Chicxulub, Yucatan, MX) to be dated at less than 1 billion years old was apparently formed at this time. Taken together, these data imply that the K/T mass extinction was caused by the impact into the Yucatan peninsula of an asteroid or comet of approximately 10 kilometers in radius.

The "Permian period" comprises the approximate time-frame 286 to 248 million years ago, and the "Triassic period" comprises the approximate time-frame 248 to 213 million years ago. At the end of the Permian period, many groups of animals and plants apparently vanished in the greatest known crisis in the history of life on Earth. This Permian/Triassic mass extinction has traditionally been considered to have had a slow time course, but recent evidence has suggested this event was more abrupt than previously recognized.

"Stony" meteorites (aerolites) are meteorites formed solely of rock-forming silicates, and chondrites are a type of stony meteorite consisting of an agglomeration of millimeter-sized globules (chondrules) that are thought to be unchanged since the original condensation out of the nebula from which the Sun and Solar System formed. A "carbonaceous chondrite" is a chondritic meteorite that contains a relatively large amount of carbon, with a resultant dark appearance. The "Murchison meteorite" is a carbonaceous chondrite that fell in 1969 near Murchison, Australia.

"Fullerenes" are large molecules composed entirely of carbon, with the chemical formula C(n), where n is any even number from 20 to over 100. They apparently have the structure of a hollow spheroidal cage with a surface network of carbon atoms connected in hexagonal and pentagonal rings, and the cage large enough to trap atoms and small molecules. Fullerenes have been identified in the Murchison meteorite.

In this context, the term "planetary" refers to the primordial aggregates (primitive planets) surrounding the Sun from which the current planets were formed.

The following points are made by L. Becker et al (Science 2001 291:1530):

1) The authors point out that the extinction event that marks the Permian/Triassic boundary (251.4 +- 0.3 million years ago) was the most severe in the past 540 million years, killing off over 90 percent of all marine species, approximately 70 percent of terrestrial vertebrate genera, and most land plants. Several new studies have demonstrated that these extinctions were much more abrupt than previously thought, with estimates of the extinction interval ranging from less than 500,000 to approximately 8000 years. Proposed catastrophic hypotheses for the Permian/Triassic boundary extinction include an exploding meteor (bolide) (asteroidal or cometary) and or massive volcanic lava flows (flood basalt volcanism). Other extinctions mechanisms involving ocean anoxia, as well as changes in sea level and climate, have also been proposed.

2) The authors report that fullerenes [C(sub60) to C(sub200)] from sediments of the Permian/Triassic boundary contain trapped helium and argon with isotope ratios similar to the apparent planetary component of carbonaceous chondrites. The authors suggest these data imply that an impact event (asteroidal or cometary) accompanied the extinction, as was the case for the Cretaceous/Tertiary extinction event approximately 65 million years ago.

3) The authors conclude: "Based on the measured helium-3 content for the Permian/Triassic boundary and Murchison fullerenes, the estimated size of the bolide is 9 +- 3 kilometers or comparable to the K/T Chicxulub impactor. Such an event could have caused the severe end-Permian mass extinction. Our results are consistent with recent paleontological studies that now point to a very rapid extinction event. The unique planetary signature measured in fullerenes isolated from the Murchison carbonaceous chondrite and from the Permian/Triassic boundary sediments demonstrates that this distinctive noble gas carrier can survive major impact events and contribute to the unique gas signature of the terrestrial planetary atmospheres."

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

[Felix (POL)]

[b]Mass Extinction Crater Pinpointed[/img]
By Larry O'Hanlon, Discovery News



Mapping the Evidence
This map shows the positions of the continents during the end of the Permian Period. Red dots show where extraterrestrial evidence of a meteorite impace have been reported. Other impact evidence has been found in Permian-Triassic boundary layers at Graphite Peak, Antarctica, Meishan, China and Sasayama, Japan, including meteoritic debris (8), Fe-Ni-Si grains (5,8), shocked quartz (4) and impact spherules (6).

May 13, 2004 — Geologists may have finally found the cause of the greatest mass extinction event in the Earth's history: a gigantic meteorite impact that slammed down 250 million years ago at a location called Bedout in what is now the Indian Ocean.

"We were absolutely flabbergasted when we saw the material from Bedout," said University of Rochester geochemist Robert Poreda, a member of the team that published a paper on the discovery in this week's issue of the journal Science.

In a NASA-sponsored telephone press conference Thursday, researchers from several universities described the discovery of what appears to be the central ridge of an impact crater at Bedout, along with related meteor impact debris in India, Antarctica and Australia.

"This is by far the largest mass extinction in Earth's history," said paleobiologist Douglas Erwin of the National Museum of Natural History.

So big was the event that fossils show it wiping out 90 percent of the species in the Earth's oceans and 70 percent of species on land. That startling fossil evidence of a massive die off caused early geologists to draw a line in the history of the Earth ending the Permian Period and commencing the Triassic Period.

Columns of rock extracted from the Bedout site reveal that buried beneath more recent layers of ocean sediments is a 250-million-year-old meteorite impact crater. The smoking guns of an impact include such things as "shocked" quarts crystals, "glassified" feldspar crystals, pure silica glass and tiny bits of the meteorite itself.

"It is absolutely certain proof of an impact event," said Poreda, who has spent decades studying volcanic rocks that could be mistaken for impact debris by less experienced eyes.

Shocked quartz are crystals of the mineral that are etched by the force of the impact, unlike all other quartz found on Earth. Glassified feldspars are crystals that look normal, until closer inspection reveals that they have been melted and quickly frozen again as non-crystalline glass, as might happen during and after an impact.

The discovery of pure silica glass is another sure sign of a meteorite impact, said Poreda, because ordinary earthbound volcanic processes can't make glass of more than 85 percent silica. "The only way we can imagine this being produced is from impact," he said.

Though the researchers are confident that the evidence points to a meteorite impact, they still have to fully nail down their hypothesis with more evidence and better dating of all the rocks.

"What's going to happen in the next few months is more investigation to try and develop more evidence to support or refute this hypothesis," said Erwin, who was not part of the discovery team.

[agaragar]

qua servono informazioni sulla fine dell'ultima glaciazione e sulla civiltà umana appena precedente altrochè!

la civiltà umana fu allora travolta dal disgelo?

e come era fatta?