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Extinctia dinozaurilor, la 300.000 ani dupa impact
Este provocarea la care cercetatorii americani ai Universitatii Princeton supun clasica teorie conform careia extinctia de acum 65 milioane de ani a dinozaurilor s-ar datora impactului devastator dintre un asteroid si planeta noastra.
Se crede despre craterul Chicxulub din Mexico ca reprezinta “arma fumeganda” a cataclismului aducator de moarte din Cretacic. Ca atare, arheologii au desfasurat excavari la fata locului si au identificat fragmente ale unui atare impact chiar sub stratul geologic relationat cu cea de-a cincea extinctie, consumata la granita dintre Cretacic si Tertiar.
Mostrele de sol colectate din craterul cu o deschidere de 180 kilometri sugereaza ca impactul de acum 65 milioane de ani cu un asteroid ar fi precedat disparitia dinozaurilor cu aproximativ 300.000 ani. Ca urmare a acestei concluzii, profesoara Gerta Keller de la Universitatea Princeton a inaintat posibilitatea ca eruptiile vulcanice uriase de la Deccan Traps din India sa fie mai degraba fenomenul raspunzator de disparitia uriaselor reptile, prin emisii masive de gaze si praf, capabile sa invaluie Pamantul si sa produca un efect de sera ucigator.
Ca raspuns, sustinatorii vechii teorii sunt de parere ca extinctia in masa si impactul par departate in mostrele sedimentare doar din cauza unor perturbari provocate de seisme sau tsunamiuri, rezultate in urma ciocnirii cu asteroidul. In contrapartida, prof Keller sustine: “Problema interpretarii tsunamiului este aceea ca stratul de pietris descoperit nu a fost depozitat in ore sau zile de catre un astfel de val, ci intr-o perioada extrem de lunga”.
Mai mult, cercetatorii au descoperit ca impactul de la Chicxulub nici macar nu a avut o rasfrangere atat de dramatica asupra diversitatii speciilor, dupa cum se credea. Ca dovada, la un sit din El Penon, arheologii au descoperit urmele a 52 de specii prezente in sedimentele de sub stratul impactului si au numarat tot atatea in straturi de deasupra fragmentelor asteroidului, ceea ce indica o supravietuire post-catastrofica totala a speciilor.
ENG
Bang goes that theory: Dinosaur extinction 'occurred 300,000 years AFTER asteroid impact '
The popular theory that dinosaurs were wiped out by an asteroid 65million years ago has been challenged.
It was believed the Chicxulub crater in Mexico was the 'smoking gun' of the mass extinction event.
Study author Professor Gerta Keller from Princeton University suggested that the massive volcanic eruptions at the Deccan Traps in India may be responsible for the extinction, releasing huge amounts of dust and gases that could have blocked out sunlight and brought about a significant greenhouse effect
Supporters of the Chicxulub impact theory suggest the impact crater and the mass extinction event only appear far apart in the sedimentary record because of earthquake or tsunami disturbance that resulted from the impact of the asteroid.
But Professor Keller said: 'The problem with the tsunami interpretation is that this sandstone complex was not deposited over hours or days by a tsunami.
'Deposition occurred over a very long time period.'
The scientists also found evidence that the Chicxulub impact didn't have the dramatic impact on species diversity that has been suggested.
At one site at El Penon, the researchers found 52 species present in sediments below the impact spherule layer, and counted all 52 still present in layers above the molten droplets or spherules.
'We found that not a single species went extinct as a result of the Chicxulub impact,' said Professor Keller.
This conclusion should not come as too great a surprise, she says.
None of the other great mass extinctions are associated with an impact, and no other large craters are known to have caused a significant extinction event.
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