Astonishment, skepticism greet fossils claimed to record dinosaur-killing asteroid impact
Um local fóssil em Dakota do Norte registra uma imagem incrivelmente detalhada da devastação minutos depois que um asteróide atingiu a Terra cerca de 66 milhões de anos atrás, um grupo de paleontólogos argumenta em um documento que deve ser divulgado esta semana. Geólogos teorizaram que o impacto, perto do que hoje é a cidade de Chicxulub na península mexicana de Yucatán, desempenhou um papel na extinção em massa no final do período Cretáceo, quando todos os dinossauros (exceto pássaros) e muita outra vida na Terra desapareceram.
If the team, led by Robert DePalma, a graduate student in
paleontology at the University of Kansas in Lawrence, is correct, it has
uncovered a record of apocalyptic destruction 3000 kilometers from
Chicxulub. At the site, called Tanis, the researchers say they have
discovered the chaotic debris left when tsunamilike waves surged up a
river valley. Trapped in the debris is a jumbled mess of fossils,
including freshwater sturgeon that apparently choked to death on glassy
particles raining out of the sky from the fireball lofted by the impact.
“That’s the first ever evidence of the interaction between life on
the last day of the Cretaceous and the impact event,” says team member
Phillip Manning, a paleontologist at the University of Manchester in the
United Kingdom. The deposit may also provide some of the strongest
evidence yet that nonbird dinosaurs were still thriving on impact day.
“Outcrops like [this] are the reasons many of us are drawn to
geology,” says David Kring, a geologist at the Lunar and Planetary
Institute in Houston, Texas, who wasn’t a member of the research team.
“Those few meters of rock record the wrath of the Chicxulub impact and
the devastation it caused.” But not everyone has fully embraced the
find, perhaps in part because it was first announced to the world last week in an article in The New Yorker. The paper, in the Proceedings of the National Academy of Sciences (PNAS), does not include all the scientific claims mentioned in The New Yorker story, including that numerous dinosaurs as well as fish were buried at the site.
“I hope this is all legit—I’m just not 100% convinced yet,” says
Thomas Tobin, a geologist at the University of Alabama in Tuscaloosa.
Tobin says the PNAS paper is densely packed with detail
from paleontology, sedimentology, geochemistry, and more. “No one is an
expert on all of those subjects,” he says, so it’s going to take a few
months for the research community to digest the findings and evaluate
whether they support such extraordinary conclusions.
In the early 1980s, the discovery of a clay layer rich in
iridium, an element found in meteorites, at the very end of the rock
record of the Cretaceous at sites around the world led researchers to
link an asteroid to the End Cretaceous mass extinction. A wealth of
other evidence has persuaded most researchers that the impact played
some role in the extinctions. But no one has found direct evidence of
its lethal effects.
DePalma’s team says the killing is captured in forensic detail in the
1.3-meter-thick Tanis deposit, which it says formed in just a few
hours, beginning perhaps 13 minutes after impact. Although fish fossils
are normally deposited horizontally, at Tanis, fish carcasses and tree
trunks are preserved haphazardly, some in near vertical orientations,
suggesting they were caught up in a large volume of mud and sand that
was dumped nearly instantaneously. The mud and sand are dotted with
glassy spherules—many caught in the gills of the fish—isotopically dated
to 65.8 million years ago. They presumably formed from droplets of
molten rock launched into the atmosphere at the impact site, which
cooled and solidified as they plummeted back to Earth. A
2-centimeter-thick layer rich in telltale iridium caps the deposit.
Tanis at the time was located on a river that may have drained into
the shallow sea covering much of what is now the eastern and southern
United States. DePalma’s team argues that as seismic waves from the
distant impact reached Tanis minutes later, the shaking generated
10-meter waves that surged from the sea up the river valley, dumping
sediment and both marine and freshwater organisms there. Such waves are
called seiches: The 2011 Tohoku earthquake near Japan triggered
1.5-meter-tall seiches in Norwegian fjords 8000 kilometers away.
DePalma and his colleagues have been working at Tanis since 2012.
“Robert has been meticulous, borderline archaeological in his excavation
approach,” says Manning, who has been working at Tanis from the
beginning.
But others question DePalma’s interpretations. “Capturing the event
in that much detail is pretty remarkable,” concedes Blair Schoene, a
geologist at Princeton University, but he says the site does not
definitively prove that the impact event was the exclusive trigger of
the mass extinction. Schoene and some others believe environmental
turmoil caused by large-scale volcanic activity in what is now central
India may have taken a toll even before the impact.
Other geologists say they can’t shake a sense of suspicion
about DePalma himself, who, along with his Ph.D. work, is also a curator
at the Palm Beach Museum of Natural History in Wellington, Florida. His
reputation suffered when, in 2015, he and his colleagues described a
new genus of dinosaur named Dakotaraptor, found in a site close
to Tanis. Others later pointed out that the reconstructed skeleton
includes a bone that really belonged to a turtle; DePalma and his
colleagues issued a correction.
DePalma may also flout some norms of paleontology, according to The New Yorker,
by retaining rights to control his specimens even after they have been
incorporated into university and museum collections. He reportedly helps
fund his fieldwork by selling replicas of his finds to private
collectors. “His line between commercial and academic work is not as
clean as it is for other people,” says one geologist who asked not to be
named. DePalma did not respond to an email request for an interview.
Manning points out that all fossils described in the PNAS
paper have been deposited in recognized collections and are available
for other researchers to study. “It saddens me that folks are so quick
to knock a study,” he says. “That some competitors have cast Robert in a
negative light is unfortunate and unfair,” says another co-author, Mark
Richards, a geophysicist at the University of California, Berkeley.
Manning confirms rumors that the study was initially submitted to a
journal with a higher impact factor before it was accepted at PNAS.
He says the reviewers for the higher-profile journal made requests that
were unreasonable for a paper that simply outlines the discovery and
initial analysis of Tanis. “After a while, we decided it wasn’t a good
route to go down,” he says. The paper cleared peer review at PNAS within about 4 months.
Several more papers on Tanis are now in preparation, Manning says,
and he expects they will describe the dinosaur fossils that are
mentioned in The New Yorker article. Its author, Douglas
Preston, who learned of the find from DePalma in 2013, writes that
DePalma’s team found dinosaur bones caught up in the 1.3-meter-thick
deposit, some so high in the sequence that DePalma suspects the
carcasses were floating in the roiling water. Such a conclusion might
provide the best evidence yet that at least some dinosaurs were alive to
witness the asteroid impact. But just one dinosaur bone is discussed in
the PNAS study—and it is mentioned in a supplement document
rather than in the paper itself. That “disconnect” bothers Steve
Brusatte, a paleontologist at the University of Edinburgh. “I just hope
this hasn’t been oversensationalized.”
Until a few years ago, some researchers had suspected the last
dinosaurs vanished thousands of years before the catastrophe. If Tanis
is all it is claimed to be, that debate—and many others about this
momentous day in Earth’s history—may be over.
doi:10.1126/science.aax5400
Nenhum comentário:
Postar um comentário
Observação: somente um membro deste blog pode postar um comentário.